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Cambridge Botanical Handbooks

Edited by A. C. SEWARD and A. G. TANSLEY

LICHENS




  CAMBRIDGE UNIVERSITY PRESS
  C. F. CLAY, MANAGER
  LONDON: FETTER LANE, E.C.4

  [Illustration]

  LONDON: H. K. LEWIS AND CO., LTD.,
  136, Gower Street, W.C.1

  LONDON: WHELDON & WESLEY, LTD.,
  28, Essex Street, Strand, W.C.2

  NEW YORK: THE MACMILLAN CO.

  BOMBAY   }
  CALCUTTA } MACMILLAN AND CO., LTD.
  MADRAS   }

  TORONTO: THE MACMILLAN CO. OF CANADA, LTD.

  TOKYO: MARUZEN-KABUSHIKI-KAISHA

  ALL RIGHTS RESERVED




                                 LICHENS

                                   BY
                       ANNIE LORRAIN SMITH, F.L.S.
         ACTING ASSISTANT, BOTANICAL DEPARTMENT, BRITISH MUSEUM

                               CAMBRIDGE:
                         AT THE UNIVERSITY PRESS
                                  1921

                        PRINTED IN GREAT BRITAIN




PREFACE


The publication of this volume has been delayed owing to war conditions,
but the delay is the less to be regretted in that it has allowed the
inclusion of recent work on the subject. Much of the subject-matter is
of common knowledge to lichenologists, but in the co-ordination and
arrangement of the facts the original papers are cited throughout. The
method has somewhat burdened the pages with citations, but it is hoped
that, as a book of reference, its value has been enhanced thereby.
The Glossary includes terms used in lichenology, or those with a
special lichenological meaning. The Bibliography refers only to works
consulted in the preparation of this volume. To save space, etc., the
titles of books and papers quoted in the text are generally translated
and curtailed: full citations will be found in the Bibliography.
Subject-matter has been omitted from the index: references of importance
will be found in the Table of Contents or in the Glossary.

I would record my thanks to those who have generously helped me during
the preparation of the volume: to Lady Muriel Percy for taking notes of
spore production, and to Dr Cavers for the loan of reprints. Prof. Potter
and Dr Somerville Hastings placed at my disposal their photographs of the
living plants. Free use has been made of published text-figures which are
duly acknowledged.

I have throughout had the inestimable advantage of being able to consult
freely the library and herbarium of the British Museum, and have thus
been able to verify references to plants as well as to literature.
A special debt of gratitude is due to my colleagues Mr Gepp and Mr
Ramsbottom for their unfailing assistance and advice.

                                                                 A. L. S.

LONDON, _February, 1920_




CONTENTS


                                                                     PAGE

  GLOSSARY                                                            xix

  ERRATA                                                             xxii

  INTRODUCTION                                                      xxiii

                                CHAPTER I
                         HISTORY OF LICHENOLOGY

  A. INTRODUCTORY                                                        1

  B. PERIOD I. PREVIOUS TO 1694                                          2

  C. PERIOD II. 1694-1729                                                5

  D. PERIOD III. 1729-1780                                               6

  E. PERIOD IV. 1780-1803                                                9

  F. PERIOD V. 1803-1846                                                10

  G. PERIOD VI. 1846-1867                                               15

  H. PERIOD VII. 1867 AND AFTER                                         18

                               CHAPTER II
                   CONSTITUENTS OF THE LICHEN THALLUS

                            I. LICHEN GONIDIA

                  1. GONIDIA IN RELATION TO THE THALLUS

  A. HISTORICAL ACCOUNT OF LICHEN GONIDIA                               21

  B. GONIDIA CONTRASTED WITH ALGAE                                      22

  C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS                        24

  D. THEORIES AS TO THE ORIGIN OF GONIDIA                               25

  E. MICROGONIDIA                                                       26

  F. COMPOSITE NATURE OF THALLUS                                        27

  G. SYNTHETIC CULTURES                                                 27

  H. HYMENIAL GONIDIA                                                   30

  I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS                      31
      _a._ Consortium and symbiosis
      _b._ Different forms of association

  J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM                           36

                     2. PHYSIOLOGY OF THE SYMBIONTS

  A. NUTRITION OF LICHEN ALGAE                                          39
      _a._ Character of algal cells
      _b._ Supply of nitrogen
      _c._ Effect on the alga
      _d._ Supply of carbon
      _e._ Nutrition within the symbiotic plant
      _f._ Affinities of lichen gonidia

  B. NUTRITION OF LICHEN FUNGI                                          44

  C. SYMBIOSIS OF OTHER PLANTS                                          45

                            II. LICHEN HYPHAE

  A. ORIGIN OF HYPHAE                                                   46

  B. DEVELOPMENT OF LICHENOID HYPHAE                                    47

  C. CULTURE OF HYPHAE WITHOUT GONIDIA                                  49

  D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS                           51

                            III. LICHEN ALGAE

  A. TYPES OF ALGAE                                                     51
      _a._ Myxophyceae associated with Phycolichens
      _b._ Chlorophyceae associated with Archilichens

  B. CHANGES INDUCED IN THE ALGA                                        60
      _a._ Myxophyceae
      _b._ Chlorophyceae

  C. CONSTANCY OF ALGAL CONSTITUENTS                                    63

  D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS                           64
      _a._ Normal displacement
      _b._ Local displacement

  E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH LICHEN HYPHAE               65

  F. PARASITISM OF ALGAE ON LICHENS                                     65

                               CHAPTER III
                               MORPHOLOGY

                 I. GENERAL ACCOUNT OF LICHEN STRUCTURE

                       ORIGIN OF LICHEN STRUCTURES

  A. FORMS OF CELL-STRUCTURE                                            67

  B. TYPES OF THALLUS                                                   68
      _a._ Endogenous thallus
      _b._ Exogenous thallus

                          II. STRATOSE THALLUS

                         1. CRUSTACEOUS LICHENS

  A. GENERAL STRUCTURE                                                  70

  B. SAXICOLOUS LICHENS                                                 70
      _a._ Epilithic lichens
          _aa._ Hypothallus or protothallus
          _bb._ Formation of crustaceous tissues
          _cc._ Formation of areolae
      _b._ Endolithic lichens
      _c._ Chemical nature of the substratum

  C. CORTICOLOUS LICHENS                                                77
      _a._ Epiphloeodal lichens
      _b._ Hypophloeodal lichens

                          2. SQUAMULOSE LICHENS

  A. DEVELOPMENT OF THE SQUAMULE                                        79

  B. TISSUES OF SQUAMULOSE THALLUS                                      81

                           3. FOLIOSE LICHENS

  A. DEVELOPMENT OF FOLIOSE THALLUS                                     82

  B. CORTICAL TISSUES                                                   82
     _a._ Types of cortical structure
     _b._ Origin of variation in cortical structure
     _c._ Loss and renewal of cortex
     _d._ Cortical hairs

  C. GONIDIAL TISSUES                                                   87

  D. MEDULLA AND LOWER CORTEX                                           88
     _a._ Medulla
     _b._ Lower cortex
     _c._ Hypothallic structures

  E. STRUCTURES FOR PROTECTION AND ATTACHMENT                           91
     _a._ Cilia
     _b._ Rhizinae
     _c._ Haptera

  F. STRENGTHENING TISSUES OF STRATOSE LICHENS                          95
     _a._ Produced by development of cortex
     _b._ Produced by development of veins or nerves

                          III. RADIATE THALLUS

                    1. CHARACTERS OF RADIATE THALLUS

                    2. INTERMEDIATE TYPES OF THALLUS

                  3. FRUTICOSE AND FILAMENTOUS THALLUS

  A. GENERAL STRUCTURE OF THALLUS                                      101

       Cortical Structures
        _a._ The fastigiate cortex
        _b._ The fibrous cortex

  B. SPECIAL STRENGTHENING STRUCTURES                                  103
         _a._ Sclerotic strands
         _b._ Chondroid axis

  C. SURVEY OF MECHANICAL TISSUES                                      105

  D. RETICULATE FRONDS                                                 106

  E. ROOTING BASE IN FRUTICOSE LICHENS                                 108

                      IV. STRATOSE-RADIATE THALLUS

                     1. STRATOSE OR PRIMARY THALLUS

  A. GENERAL CHARACTERISTICS                                           111

  B. TISSUES OF PRIMARY THALLUS                                        112
      _a._ Cortical tissue
      _b._ Gonidial tissue
      _c._ Medullary tissue
      _d._ Soredia

                     2. RADIATE OR SECONDARY THALLUS

  A. ORIGIN OF THE PODETIUM                                            114

  B. STRUCTURE OF THE PODETIUM                                         114
      _a._ General structure
      _b._ Gonidial tissue
      _c._ Cortical tissue
      _d._ Soredia

  C. DEVELOPMENT OF THE SCYPHUS                                        117
      _a._ From abortive apothecia
      _b._ From polytomous branching
      _c._ From arrested growth
      _d._ Gonidia of the scyphus
      _e._ Species without scyphi

  D. BRANCHING OF THE PODETIUM                                         119

  E. PERFORATIONS AND RETICULATION OF THE PODETIUM                     120

  F. ROOTING STRUCTURES OF CLADONIAE                                   121

  G. HAPTERA                                                           122

  H. MORPHOLOGY OF THE PODETIUM                                        122

  I. PILOPHORUS AND STEREOCAULON                                       125

                    V. STRUCTURES PECULIAR TO LICHENS

                         1. AERATION STRUCTURES

  A. CYPHELLAE AND PSEUDOCYPHELLAE                                     126
      _a._ Historical
      _b._ Development of cyphellae
      _c._ Pseudocyphellae
      _d._ Occurrence and distribution

  B. BREATHING-PORES                                                   129
      _a._ Definite breathing-pores
      _b._ Other openings in the thallus

  C. GENERAL AERATION OF THE THALLUS                                   132

                              2. CEPHALODIA

  A. HISTORICAL AND DESCRIPTIVE                                        133

  B. CLASSIFICATION                                                    135
       I. CEPHALODIA VERA
      II. PSEUDOCEPHALODIA

  C. ALGAE THAT FORM CEPHALODIA                                        136

  D. DEVELOPMENT OF CEPHALODIA                                         137
      _a._ Ectotrophic
      _b._ Endotrophic
      _c._ Pseudocephalodia

  E. AUTOSYMBIOTIC CEPHALODIA                                          140

                               3. SOREDIA

  A. STRUCTURE AND ORIGIN OF SOREDIA                                   141
      _a._ Scattered soredia
      _b._ Isidial soredia
      _c._ Soredia as buds

  B. SORALIA                                                           144
      _a._ Form and occurrence of soralia
      _b._ Position of soraliferous lobes
      _c._ Deep-seated soralia

  C. DISPERSAL AND GERMINATION OF SOREDIA                              147

  D. EVOLUTION OF SOREDIA                                              148

                                4. ISIDIA

  A. FORM AND STRUCTURE OF ISIDIA                                      149

  B. ORIGIN AND FUNCTION OF ISIDIA                                     151

                            VI. HYMENOLICHENS

  A. AFFINITY WITH OTHER PLANTS                                        152

  B. STRUCTURE OF THALLUS                                              153

  C. SPORIFEROUS TISSUES                                               154

                               CHAPTER IV
                              REPRODUCTION

                      I. REPRODUCTION BY ASCOSPORES

  A. HISTORICAL SURVEY                                                 155

  B. FORMS OF REPRODUCTIVE ORGANS                                      156
      _a._ Apothecia
      _b._ Perithecia

  C. DEVELOPMENT OF REPRODUCTIVE ORGANS                                159

                             1. DISCOLICHENS

      _a._ Carpogonia of gelatinous lichens
      _b._ Carpogonia of non-gelatinous lichens
      _c._ General summary
      _d._ Hypothecium and paraphyses
      _e._ Variations in apothecial development
          _aa._ Parmeliae
          _bb._ Pertusariae
          _cc._ Graphideae
          _dd._ Cladoniae

                            2. PYRENOLICHENS

      _a._ Development of the perithecium
      _b._ Formation of carpogonia

  D. APOGAMOUS REPRODUCTION                                            174

  E. DISCUSSION OF LICHEN REPRODUCTION                                 177
      _a._ The Trichogyne
      _b._ The Ascogonium

  F. FINAL STAGES OF APOTHECIAL DEVELOPMENT                            181
      _a._ Open or closed apothecia
      _b._ Emergence of ascocarp

  G. LICHEN ASCI AND SPORES                                            184
      _a._ Historical
      _b._ Development of the ascus
      _c._ Development of the spores
      _d._ Spore germination
      _e._ Multinucleate spores
      _f._ Polaribilocular spores

                          II. SECONDARY SPORES

  A. REPRODUCTION BY OIDIA                                             189

  B. REPRODUCTION BY CONIDIA                                           190
      _a._ Rare instances of conidial formation
      _b._ Comparison with Hyphomycetes

  C. CAMPYLIDIUM AND ORTHIDIUM                                         191

                      III. SPERMOGONIA OR PYCNIDIA

  A. HISTORICAL ACCOUNT OF SPERMOGONIA                                 192

  B. SPERMOGONIA AS MALE ORGANS                                        193

  C. OCCURRENCE AND DISTRIBUTION                                       193
      _a._ Relation to thallus and apothecia
      _b._ Form and size
      _c._ Colour

  D. STRUCTURE                                                         196
      _a._ Origin and growth
      _b._ Form and types of spermatiophores
      _c._ Periphyses and sterile filaments

  E. SPERMATIA OR PYCNIDIOSPORES                                       201
      _a._ Origin and form
      _b._ Size and structure
      _c._ Germination
      _d._ Variation in pycnidia

  F. PYCNIDIA WITH MACROSPORES                                         204

  G. GENERAL SURVEY                                                    205
      _a._ Sexual or asexual
      _b._ Comparison with fungi
      _c._ Influence of symbiosis
      _d._ Value in diagnosis

                                CHAPTER V
                                PHYSIOLOGY

                       I. CELLS AND CELL PRODUCTS

  A. CELL-MEMBRANES                                                    209
      _a._ Chitin
      _b._ Lichenin and allied carbohydrates
      _c._ Cellulose

  B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS                         213
      _a._ Cell-substances
      _b._ Calcium Oxalate
      _c._ Importance of calcium oxalate

  C. OIL-CELLS                                                         215
      _a._ Oil-cells of endolithic lichens
      _b._ Oil-cells of epilithic lichens
      _c._ Significance of oil-formation

  D. LICHEN-ACIDS                                                      221
      _a._ Historical
      _b._ Occurrence and examination of acids
      _c._ Character of acids
      _d._ Causes of variation in quantity and quality
      _e._ Distribution of acids

  E. CHEMICAL GROUPING OF ACIDS                                        225
       I. ACIDS OF THE FAT SERIES
      II. ACIDS OF THE BENZOLE SERIES
            SUBSERIES I. ORCINE DERIVATIVES
            SUBSERIES II. ANTHRACENE DERIVATIVES

  F. CHEMICAL REAGENTS AS TESTS FOR LICHENS                            228

  G. CHEMICAL REACTIONS IN NATURE                                      229

                          II. GENERAL NUTRITION

  A. ABSORPTION OF WATER                                               229
      _a._ Gelatinous lichens
      _b._ Crustaceous lichens
      _c._ Foliose lichens
      _d._ Fruticose lichens

  B. STORAGE OF WATER                                                  232

  C. SUPPLY OF INORGANIC FOOD                                          232
      _a._ In foliose and fruticose lichens
      _b._ In crustaceous lichens

  D. SUPPLY OF ORGANIC FOOD                                            235
      _a._ From the substratum
      _b._ From other lichens
      _c._ From other vegetation

                    III. ASSIMILATION AND RESPIRATION

  A. INFLUENCE OF TEMPERATURE                                          238
      _a._ High temperature
      _b._ Low temperature

  B. INFLUENCE OF MOISTURE                                             239
      _a._ On vital functions
      _b._ On general development

                       IV. ILLUMINATION OF LICHENS

  A. EFFECT OF LIGHT ON THE THALLUS                                    240
      _a._ Sun lichens
      _b._ Colour-changes due to light
      _c._ Shade lichens
      _d._ Varying shade conditions

  B. EFFECT OF LIGHT ON REPRODUCTIVE ORGANS                            244
      _a._ Position and orientation of fruits with regard to light
      _b._ Influence of light on colour of fruits

                          V. COLOUR OF LICHENS

  A. ORIGIN OF LICHEN-COLOURING                                        245
      _a._ Colour given by the algal constituent
      _b._ Colour due to lichen-acids
      _c._ Colour due to amorphous substances
      _d._ Enumeration of amorphous pigments
      _e._ Colour due to infiltration

                               CHAPTER VI
                                BIONOMICS

  A. GROWTH AND DURATION OF LICHENS                                    252

  B. SEASON OF FRUIT FORMATION                                         255

  C. DISPERSAL AND INCREASE                                            256
      _a._ Dispersal of crustaceous lichens
      _b._ Dispersal of foliose lichens
      _c._ Dispersal of fruticose lichens

  D. ERRATIC LICHENS                                                   258

  E. PARASITISM                                                        260
      _a._ General statement
      _b._ Antagonistic symbiosis
      _c._ Parasymbiosis
      _d._ Parasymbiosis of fungi
      _e._ Fungi parasitic on lichens
      _f._ Mycetozoa parasitic on lichens

  F. DISEASES OF LICHENS                                               268
      _a._ Caused by parasitism
      _b._ Caused by crowding
      _c._ Caused by adverse conditions

  G. HARMFUL EFFECT OF LICHENS                                         269

  H. GALL-FORMATION                                                    270

                               CHAPTER VII
                                PHYLOGENY

                          I. GENERAL STATEMENT

  A. ORIGIN OF LICHENS                                                 272

  B. ALGAL ANCESTORS                                                   273

  C. FUNGAL ANCESTORS                                                  273
      _a._ Basidiolichens
      _b._ Ascolichens

                       II. THE REPRODUCTIVE ORGANS

  A. THEORIES OF DESCENT IN ASCOLICHENS                                273

  B. RELATION OF LICHENS TO FUNGI                                      275
      _a._ Pyrenocarpineae
      _b._ Coniocarpineae
      _c._ Graphidineae
      _d._ Cyclocarpineae

                            III. THE THALLUS

  A. GENERAL OUTLINE OF DEVELOPMENT OF THALLUS                         281
      _a._ Preliminary considerations
      _b._ Course of evolution in Hymenolichens
      _c._ Course of evolution in Ascolichens

  B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS                          282

  C. EVOLUTION OF PHYCOLICHENS                                         283
      _a._ Gloeolichens
      _b._ Ephebaceae and Collemaceae
      _c._ Pyrenidiaceae
      _d._ Heppiaceae and Pannariaceae
      _e._ Peltigeraceae and Stictaceae

  D. EVOLUTION OF ARCHILICHENS                                         287
      _a._ Thallus of Pyrenocarpineae
      _b._ Thallus of Coniocarpineae
      _c._ Thallus of Graphidineae
      _d._ Thallus of Cyclocarpineae
          _AA._ LECIDEALES
              _aa._ Coenogoniaceae
              _bb._ Lecideaceae and Gyrophoraceae
              _cc._ Cladoniaceae
                  1. Origin of Cladonia
                  2. Evolution of the primary thallus
                  3. Evolution of the secondary thallus
                  4. Course of podetial development
                  5. Variation in Cladonia
                  6. Causes of variation
                  7. Podetial development and spore-dissemination
                  8. Pilophorus, Stereocaulon and Argopsis
          _BB._ LECANORALES
              _aa._ Course of Development
              _bb._ Lecanoraceae
              _cc._ Parmeliaceae
              _dd._ Usneaceae
              _ee._ Physciaceae

                              CHAPTER VIII
                               SYSTEMATIC

                            I. CLASSIFICATION

  A. WORK OF SUCCESSIVE SYSTEMATISTS                                   304
      _a._ Dillenius and Linnaeus
      _b._ Acharius
      _c._ Schaerer
      _d._ Massalongo and Koerber
      _e._ Nylander
      _f._ Müller-Argau
      _g._ Reinke
      _h._ Zahlbruckner

  B. FAMILIES AND GENERA OF ASCOLICHENS                                311

  C. HYMENOLICHENS                                                     342

                       II. NUMBER AND DISTRIBUTION

                         1. ESTIMATES OF NUMBER

                      2. GEOGRAPHICAL DISTRIBUTION

  A. GENERAL SURVEY                                                    343

  B. LICHENS OF POLAR REGIONS                                          345

  C. LICHENS OF THE TEMPERATE ZONES                                    348

  D. LICHENS OF TROPICAL REGIONS                                       352

                           III. FOSSIL LICHENS

                               CHAPTER IX
                                 ECOLOGY

  A. GENERAL INTRODUCTION                                              356

  B. EXTERNAL INFLUENCES                                               357
      _a._ Temperature
      _b._ Humidity
      _c._ Wind
      _d._ Human Agency

  C. LICHEN COMMUNITIES                                                362

      1. ARBOREAL                                                      363
          _a._ Epiphyllous
          _b._ Corticolous
          _c._ Lignicolous

      2. TERRICOLOUS                                                   367
          _a._ On calcareous soil
          _b._ On siliceous soil
          _c._ On bricks
          _d._ On humus
          _e._ On peaty soil
          _f._ On mosses
          _g._ On fungi

      3. SAXICOLOUS                                                    371
          _a._ Characters of mineral substrata
          _b._ Colonization on rocks
          _c._ Calcicolous
          _d._ Silicicolous

      4. OMNICOLOUS LICHENS                                            376

      5. LOCALIZED COMMUNITIES                                         378
          _a._ Maritime lichens
          _b._ Sand-dune lichens
          _c._ Mountain lichens
          _d._ Tundra lichens
          _e._ Desert lichens
          _f._ Aquatic lichens

  D. LICHENS AS PIONEERS                                               392
      _a._ Soil-formers
      _b._ Outposts of vegetation

                                CHAPTER X
                         ECONOMIC AND TECHNICAL

  A. LICHENS AS FOOD                                                   395
      _a._ Food for insects
      _b._ Insect mimicry of lichens
      _c._ Food for the higher animals
      _d._ Food for man

  B. LICHENS AS MEDICINE                                               405
      _a._ Ancient remedies
      _b._ Doctrine of “signatures”
      _c._ Cure for hydrophobia
      _d._ Popular remedies

  C. LICHENS AS POISONS                                                410

  D. LICHENS USED IN TANNING, BREWING AND DISTILLING                   411

  E. DYEING PROPERTIES OF LICHENS                                      411
      _a._ Lichens as dye-plants
      _b._ The orchil lichen, Roccella
      _c._ Purple dyes: orchil, cudbear and litmus
      _d._ Other orchil lichens
      _e._ Preparation of orchil
      _f._ Brown and yellow dyes
      _g._ Collecting of dye-lichens
      _h._ Lichen colours and spectrum characters

  F. LICHENS IN PERFUMERY                                              418
      _a._ Lichens as perfumes
      _b._ Lichens as hair-powder

  G. SOME MINOR USES OF LICHENS                                        420

  APPENDIX                                                             421

  ADDENDUM                                                             422

  BIBLIOGRAPHY                                                         423

  INDEX                                                                448




GLOSSARY


=Acrogenous=, borne at the tips of hyphae; see spermatium, 312.

=Allelositismus=, Norman’s term to describe the thallus of Moriolaceae
(mutualism), 313.

=Amorphous cortex=, formed of indistinct hyphae with thickened walls; cf.
decomposed cortex.

=Amphithecium=, thalline margin of the apothecium, 157.

=Antagonistic symbiosis=, hurtful parasitism of one lichen on another,
261 _et seq._

=Apothecium=, open or disc-shaped fructification, 11, 156 _et passim_.
Veiled apothecium, 169. Closed or open at first, 182.

=Archilichens=, lichens in which the gonidia are bright green
(Chlorophyceae), 52, 55 _et passim_.

=Ardella=, the small spot-like apothecium of Arthoniaceae, 158.

=Areola= (areolate), small space marked out by lines or chinks on the
surface of the thallus, 73 _et passim_.

=Arthrosterigma=, septate tissue-like sterigma (spermatiophore), 197.

=Ascogonium=, the cell or cells that produce ascogenous hyphae, 180 _et
seq._

=Ascolichens=, lichens in which the fungus is an Ascomycete, 159, 173 _et
passim_.

=Ascus=, enlarged cell in which a definite number of spores (usually 8)
are developed; cf. theca, 157, 184.

=Ascyphous=, podetia without scyphi, 119 _et passim_.

=Biatorine=, apothecia that are soft or waxy, and often brightly
coloured, as in _Biatora_, 158.

=Blasteniospore=, see _polarilocular spore_.

=Byssoid=, slender, thread-like, as in the old genus _Byssus_.

=Campylidium=, supposed new type of fructification in lichens, 191.

=Capitulum=, the globose apical apothecium of Coniocarpineae; cf.
mazaedium, 319.

=Carpogonium=, primordial stage of fructification, 160, 164 _et passim_.

=Cephalodium=, irregular outgrowth from the thallus enclosing mostly
blue-green algae; or intruded packet of algae within the thallus, 11, 133
_et passim_.

=Chrondroid=, hard and tough like cartilage, a term applied to
strengthening strands of hyphae, 104, 114.

=Chroolepoid=, like the genus _Chroolepis_ (_Trentepohlia_).

=Chrysogonidia=, yellow algal cells (_Trentepohlia_).

=Cilium=, hair-like outgrowth from surface or margin of thallus, or
margin of apothecium, 91.

=Consortium= (consortism), mutual association of fungus and alga
(Reinke); also termed “mutualism,” 31, 313.

=Corticolous=, living on the bark of trees, 363.

=Crustaceous=, crust-like closely adhering thallus, 70-79.

=Cyphella=, minute cup-like depression on the under surface of the
thallus (_Sticta_, etc.), 11, 126.

=Decomposed=, term applied to cortex formed of gelatinous indistinct
hyphae (amorphous), 73-81 _et passim_, 357.

=Determinate=, thallus with a definite outline, 72.

=Dimidiate=, term applied to the perithecium, when the outer wall covers
only the upper portion, 159.

=Discoid=, disc-like, an open rounded apothecium, 156.

=Discolichens=, in which the fructification is an apothecium, 160 _et
seq._

=Dual hypothesis=, the theory of two organisms present in the lichen
thallus, 27 _et seq._

=Effigurate=, having a distinct form or figure; cf. placodioid, 80, 201.

=Endobasidial=, Steiner’s term for sporophore with a secondary
sporiferous branch, 200.

=Endogenous=, produced internally, as spores in an ascus, 179; see also
under thallus.

=Endolithic=, embedded in the rock, 75.

=Endosaprophytism=, term used by Elenkin for destruction of the algal
contents by enzymes of the fungus, 36.

=Entire=, term applied to the perithecium when completely surrounded by
an outer wall, 159.

=Epilithic=, growing on the rock surface, 70.

=Epiphloeodal=, thallus growing on the surface of the bark, 77.

=Epiphyllous=, growing on leaves, 363.

=Epithecium=, upper layer of thecium (hymenium), 158.

=Erratic lichens=, unattached and drifting, 259.

=Exobasidial=, Steiner’s term for sporophore without a secondary
sporiferous branch, 200.

=Exogenous=, produced externally, as spores on tips of hyphae; see also
under thallus.

=Fastigiate cortex=, formed of clustered parallel hyphal branches
vertical to long axis of thallus, 82.

=Fat-cells=, specialized hyphal cells containing fat or oil, 75, 215 _et
passim_.

=Fibrous cortex=, formed of hyphae parallel with long axis of thallus, 82.

=Filamentous=, slender thallus with radiate structure, 101 _et seq._

=Foliose=, lichens with a leafy form and stratose in structure, 82-97.

=Foveolae=, =Foveolate=, pitted, 373.

=Fruticose=, upright or pendulous thallus, with radiate structure, 101
_et seq._

=Fulcrum=, term used by Steiner for sporophore, 200.

=Gloeolichens=, lichens in which the gonidia are _Gloeocapsa_ or
_Chroococcus_, 284, 373, 389.

=Gonidium=, the algal constituent of the lichen thallus, 20-45 _et
passim_.

=Gonimium=, blue-green algal cell (Myxophyceae), constituent of the
lichen thallus, 52.

=Goniocysts=, nests of gonidia in Moriolaceae, 313.

=Gyrose=, curved backward and forward, furrowed fruit of _Gyrophora_, 184.

=Hapteron=, aerial organ of attachment, 94, 122.

=Haustorium=, outgrowth or branch of a hypha serving as an organ of
suction, 32.

=Helotism=, state of servitude, term used to denote the relation of alga
to fungus in lichen organization, 38, 40.

=Heteromerous=, fungal and algal constituents of the thallus in definite
strata, 13, 68, 305 _et passim_.

=Hold-fast=, rooting organ of thallus, 109, 122 _et passim_.

=Homobium=, interdependent association of fungus and alga, 31.

=Homoiomerous=, fungal and algal constituents more or less mixed in the
thallus, 13, 68, 305 _et passim_.

=Hymenial gonidia=, algal cells in the hymenium, 30, 314, 315, 327.

=Hymenium=, apothecial tissue consisting of asci and paraphyses; cf.
thecium, 157.

=Hymenolichens=, lichens of which the fungal constituent is a
Hymenomycete, 152-154, 342.

=Hypophloeodal=, thallus growing within the bark, 78, 364.

=Hypothallus=, first growth of hyphae (proto- or pro-thallus) persisting
as hyphal growth at base or margin of the thallus, 70, 257 _et passim_.

=Hypothecium=, layer below the thecium (hymenium), 157.

=Intricate cortex=, composed of hyphae densely interwoven but not
coalescent, 83.

=Isidium=, coral-like outgrowth on the lichen thallus, 149-151.

=Lecanorine=, apothecium with a thalline margin as in _Lecanora_, 158.

=Lecideine=, apothecium usually dark-coloured or carbonaceous and without
a thalline margin, 158.

=Leprose=, mealy or scurfy, like the old form genera, _Lepra_,
_Lepraria_, 191.

=Lichen-acids=, organic acids peculiar to lichens, 221 _et seq._

=Lignicolous=, living on wood or trees, 366.

=Lirella=, long narrow apothecium of Graphideae, 158.

=Mazaedium=, fructification of Coniocarpineae, the spores lying as a
powdery mass in the capitulum, 176.

=Medulla=, the loose hyphal layer in the interior of the thallus, 88 _et
passim_.

=Meristematic=, term applied by Wainio to growing hyphae, 48.

=Microgonidia=, term applied by Minks to minute greenish bodies in lichen
hyphae, 26.

=Multi-septate=, term applied to spores with numerous transverse septa,
316 _et seq._

=Murali-divided=, =Muriform=, term applied to spores divided like the
masonry of a wall, 187.

=Oidium=, reproductive cell formed by the breaking up of the hyphae, 189.

=Oil-cell=, hyphal cell containing fat globules, 215.

=Orculiform=, see polarilocular.

=Orthidium=, supposed new type of fructification in lichens, 192.

=Palisade-cells=, the terminal cells of the hyphae forming the fastigiate
cortex, 82, 83.

=Panniform=, having a felted or matted appearance, 260.

=Paraphysis=, sterile filament in the hymenium, 157.

=Parasymbiosis=, associated harmless but not mutually useful growth of
two organisms, 263.

=Parathecium=, hyphal layer round the apothecium, 157.

=Peltate=, term applied to orbicular and horizontal apothecia in the form
of a shield, 336.

=Perithecium=, roundish fructification usually with an apical opening
(ostiole) containing ascospores, 158 _et passim_.

=Pervious=, referring to scyphi with an opening at the base (_Perviae_),
118.

=Phycolichens=, lichens in which the gonidia are blue-green
(Myxophyceae), 52 _et passim_.

=Placodioid=, thallus with a squamulose determinate outline, generally
orbicular; cf. effigurate, 80.

=Placodiomorph=, see polarilocular.

=Plectenchyma= (=Plectenchymatous=), pseudoparenchyma of fungi and
lichens, 66 _et passim_.

=Pleurogenous=, borne laterally on hyphal cells; see spermatium, 312.

=Pluri-septate=, term applied to spores with several transverse septa,
321 _et seq._

=Podetium=, stalk-like secondary thallus of Cladoniaceae, 114, 293 _et
seq._

=Polarilocular=, =Polaribilocular=, two-celled spores with thick median
wall traversed by a connecting tube, 188, 340-341.

=Polytomous=, arising of several branches of the podetium from one level,
118.

=Proper margin=, the hyphal margin surrounding the apothecium, 157.

=Prothallus=, =Protothallus=, first stages of hyphal growth; cf.
hypothallus, 71.

=Pycnidiospores=, stylospores borne in pycnidia, 198 _et passim_.

=Pycnidium=, roundish fructification, usually with an opening at the
apex, containing sporophores and stylospores; cf. spermogonium, 192 _et
seq._

=Pyrenolichens=, in which the fructification is a closed perithecium, 173
_et passim_.

=Radiate thallus=, the tissues radiate from a centre, 98 _et seq._

=Rhagadiose=, deeply chinked, 74; cf. rimose.

=Rhizina=, attaching “rootlet,” 92-94.

=Rimose=, =Rimulose=, cleft or chinked into areolae, 73.

=Rimose-diffract=, widely cracked or chinked, 74.

=Scutellate=, shaped like a platter, 156.

=Scyphus=, cup-like dilatation of the podetium, 111, 117.

=Signature=, a term in ancient medicine to signify the resemblance of a
plant to any part of the human body, 406, 409.

=Soralium=, group of soredia surrounded by a definite margin, 144.

=Soredium=, minute separable particle arising from the gonidial tissue of
the thallus, and consisting of algae and hyphae, 141.

=Spermatium=, spore-like body borne in the spermogonium, regarded as a
non-motile male cell or as a pycnidiospore, 201.

=Spermogonium=, roundish closed receptacle containing spermatia, 192.

=Sphaeroid-cell=, swollen hyphal cell, containing fat globules, 215.

=Squamule=, a small thalline lobe or scale, 74 _et passim_.

=Sterigma=, Nylander’s term for the spermatiophore, 197.

=Stratose thallus=, where the tissues are in horizontal layers, 70.

=Stratum=, a layer of tissue in the thallus, 70.

=Symbiont=, one of two dissimilar organisms living together, 32.

=Symbiosis=, a living together of dissimilar organisms, also termed
commensalism, 31, 32 _et seq._

=Tegulicolous=, living on tiles, 369.

=Terebrator=, boring apparatus, term used by Lindau for the lichen
“trichogyne,” 179.

=Thalline margin=, an apothecial margin formed of and usually coloured
like the thallus; cf. amphithecium.

=Thallus=, vegetative body or soma of the lichen plant, 11, 421.
Endogenous thallus in which the alga predominates, 68. Exogenous thallus
in which the fungus predominates, 69.

=Theca=, enlarged cell containing spores; cf. ascus.

=Thecium=, layer of tissue in the apothecium consisting of asci and
paraphyses; cf. hymenium, 157.

=Trichogyne=, prolongation of the egg-cell in Florideae which acts as a
receptive tube; septate hypha in lichens arising from the ascogonium,
160, 177-181, 273.

=Woronin’s hypha=, a coiled hypha occurring in the centre of the fruit
primordium, 159, 163.




ERRATA


  p. 24. _For_ Baranetsky _read_ Baranetzky.
  p. 277. For _Ascolium_ read _Acolium_.
  p. 318. For _Lepolichen coccophora_ read _coccophorus_.

Transcriber’s Note: The errata have been corrected.




INTRODUCTION


Lichens are, with few exceptions, perennial aerial plants of somewhat
lowly organization. In the form of spreading encrustations, horizontal
leafy expansions, of upright strap-shaped fronds or of pendulous
filaments, they take possession of the tree-trunks, palings, walls,
rocks or even soil that afford them a suitable and stable foothold. The
vegetative body, or thallus, which may be extremely long-lived, is of
varying colour, white, yellow, brown, grey or black. The great majority
of lichens are Ascolichens and reproduction is by ascospores produced
in open or closed fruits (apothecia or perithecia) which often differ
in colour from the thallus. There are a few Hymenolichens which form
basidiospores. Vegetative reproduction by soredia is frequent.

Lichens abound everywhere, from the sea-shore to the tops of high
mountains, where indeed the covering of perpetual snow is the only
barrier to their advance; but owing to their slow growth and long
duration, they are more seriously affected than are the higher plants
by chemical or other atmospheric impurities and they are killed out by
the smoke of large towns: only a few species are able to persist in
somewhat depauperate form in or near the great centres of population or
of industry.

The distinguishing feature of lichens is their composite nature: they
consist of two distinct and dissimilar organisms, a fungus and an alga,
which, in the lichen thallus, are associated in some kind of symbiotic
union, each symbiont contributing in varying degree to the common
support: it is a more or less unique and not unsuccessful venture in
plant-life. The algae—Chlorophyceae or Myxophyceae—that become lichen
symbionts or “gonidia” are of simple structure, and, in a free condition,
are generally to be found in or near localities that are also the
customary habitats of lichens. The fungus is the predominant partner
in the alliance as it forms the fruiting bodies. It belongs to the
Ascomycetes[1], except in a few tropical lichens (Hymenolichens), in
which the fungus is a Basidiomycete. These two types of plants (algae
and fungi) belonging severally to many different genera and species
have developed in their associated life this new lichen organism,
different from themselves as well as from all other plants, not only
morphologically but physiologically. Thus there has arisen a distinct
class, with families, genera and species, which through all their varying
forms retain the characteristics peculiar to lichens.

In the absence of any “visible” seed, there was much speculation in
early days as to the genesis of all the lower plants and many opinions
were hazarded as to their origin. Luyken[2], for instance, thought that
lichens were compounded of air and moisture. Hornschuch[3] traced their
origin to a vegetable infusorium, _Monas Lens_, which became transformed
to green matter and was further developed by the continued action of
light and air, not only to lichens, but to algae and mosses, the type
of plant finally evolved being determined by the varying atmospheric
influences along with the chemical nature of the substratum. An
account[4] is published of Nees von Esenbeck, on a botanical excursion,
pointing out to his students the green substance, _Lepraria botryoides_,
which covered the lower reaches of walls and rocks, while higher up it
assumed the grey lichen hue. This afforded him sufficient proof that
the green matter in that dry situation changed to lichens, just as in
water it changed to algae. An adverse criticism by Dillenius[5] on a
description of a lichen fructification is not inappropriate to those
early theorists: “Ex quo apparet, quantum videre possint homines, si
imaginatione polleant.”

A constant subject of speculation and of controversy was the origin of
the green cells, so dissimilar to the general texture of the thallus. It
was thought finally to have been established beyond dispute that they
were formed directly from the colourless hyphae and, as a corollary,
_Protococcus_ and other algal cells living in the open were considered
to be escaped gonidia or, as Wallroth[6] termed them, “unfortunate
brood-cells,” his view being that they were the reproductive organs of
the lichen plant that had failed to develop.

It was a step forward in the right direction when lichens were regarded
as transformed algae, among others by Agardh[7], who believed that he
had followed the change from _Nostoc lichenoides_ to the lichen _Collema
limosum_. Thenceforward their double resemblance, on the one hand to
algae, on the other to fungi, was acknowledged, and influenced strongly
the trend of study and investigation.

The announcement[8] by Schwendener[9] of the dual hypothesis solved the
problem for most students, though the relation between the two symbionts
is still a subject of controversy. The explanation given by Schwendener,
and still held by some[10], that lichens were merely fungi parasitic on
algae, was indeed a very inadequate conception of the lichen plant, and
it was hotly contested by various lichenologists. Lauder Lindsay[11]
dismissed the theory as “merely the most recent instance of German
transcendentalism applied to the Lichens.” Earlier still, Nylander[12],
in a paper dealing with cephalodia and their peculiar gonidia, had
denounced it: “Locum sic suum dignum occupat algolichenomachia inter
historias ridiculas, quae hodie haud paucae circa lichenes, majore
imaginatione quam scientia, enarrantur.” He never changed his attitude
and Crombie[13], wholly agreeing with his estimate of these “absurd
tales,” translates a much later pronouncement by him[14]: “All these
allegations belong to inept Schwendenerism and scarcely deserve even
to be reviewed or castigated so puerile are they—the offspring of
inexperience and of a light imagination. No true science there.”
Crombie[15] himself in a first paper on this subject declared that “the
new theory would necessitate their degradation from the position they
have so long held as an independent class.” He scornfully rejected the
whole subject as “a Romance of Lichenology, or the unnatural union
between a captive Algal damsel and a tyrant Fungal master.” The nearest
approach to any concession on the algal question occurs in a translation
by Crombie[16] of one of Nylander’s papers. It is stated there that a
saxicolous alga (_Gongrosira_ Kütz.) had been found bearing the apothecia
of _Lecidea herbidula_ n. sp. Nylander adds: “This algological genus is
one which readily passes into lichens.” At a later date, Crombie[17]
was even more comprehensively contemptuous and wrote: “whether viewed
anatomically or biologically, analytically or synthetically, it is
instead of being true science, only the Romance of Lichenology.” These
views were shared by many continental lichenologists and were indeed, as
already stated, justified to a considerable extent: it was impossible
to regard such a large and distinctive class of plants as merely fungi
parasitic on the lower algae.

Controversy about lichens never dies down, and that view of their
parasitic nature has been freshly promulgated among others by the
American lichenologist Bruce Fink[18]. The genetic origin of the gonidia
has also been restated by Elfving[19]: the various theories and views are
discussed fully in the chapter on the lichen plant.

Much of the interest in lichens has centred round their symbiotic growth.
No theory of simple parasitism can explain the association of the two
plants: if one of the symbionts is withdrawn—either fungus or alga—the
lichen as such ceases to exist. Together they form a healthy unit
capable of development and change: a basis for progress along new lines.
Permanent characters have been formed which are transmitted just as in
other units of organic life.

A new view of the association has been advanced by F. and Mme Moreau[20].
They hold that the most characteristic lichen structures—more
particularly the cortex—have been induced by the action of the alga on
the fungus. The larger part of the thallus might therefore be regarded as
equivalent to a gall: “it is a cecidium, an algal cecidium, a generalized
biomorphogenesis.”

The morphological characters of lichens are of exceptional interest,
conditioned as they are by the interaction of the two symbionts, and new
structures have been evolved by the fungus which provides the general
tissue system. Lichens are plants of physiological symbiotic origin,
and that aspect of their life-history has been steadily kept in view in
this work. There are many new requirements which have had to be met by
the lichen hyphae, and the differences between them and the true fungal
hyphae have been considered, as these are manifested in the internal
economy of the compound plant, and in its reaction to external influences
such as light, heat, moisture, etc.

The pioneers of botanical science were of necessity occupied almost
exclusively with collecting and describing plants. As the number of
known lichens gradually accumulated, affinities were recognized and
more or less successful efforts were made to tabulate them in classes,
orders, etc. It was a marvellous power of observation that enabled the
early workers to arrange the first schemes of classification. Increasing
knowledge aided by improved microscopes has necessitated changes, but the
old fundamental “genus” _Lichen_ is practically equivalent to the Class
_Lichenes_.

The study of lichens has been a slow and gradual process, with a
continual conflict of opinion as to the meaning of these puzzling
plants—their structure, reproduction, manner of subsistence and
classification as well as their relation to other plants. It has been
found desirable to treat these different subjects from a historical
aspect, as only thus can a true understanding be gained, or a true
judgment formed as to the present condition of the science. It is the
story of the evolution of lichenology as well as of lichens that has
yielded so much of interest and importance.

The lichenologist may claim several advantages in the study of his
subject: the abundant material almost everywhere to hand in country
districts, the ease with which the plants are preserved, and, not least,
the interest excited by the changes and variations induced by growth
conditions; there are a whole series of problems and puzzles barely
touched on as yet that are waiting to be solved.

In field work, it is important to note accurately and carefully the
nature of the substratum as well as the locality. Crustaceous species
should be gathered if possible along with part of the wood or rock to
which they are attached; if they are scraped off, the pieces may be
reassembled on gummed paper, but that is less satisfactory. The larger
forms are more easily secured; they should be damped and then pressed
before being laid away: the process flattens them, but it saves them
from the risk of being crushed and broken, as when dry they are somewhat
brittle. Moistening with water will largely restore their original form.
All parts of the lichen, both thallus and fruit, can be examined with
ease at any time as they do not sensibly alter in the herbarium, though
they lose to some extent their colouring: the blue-grey forms, for
instance, often become a uniform dingy brownish-grey.

Microscopic examination in the determination of species is necessary in
many instances, but that disability—if it ranks as such—is shared by
other cryptogams, and may possibly be considered an inducement rather
than a deterrent to the study of lichens. For temporary examination
of microscopic preparations, the normal condition is best observed by
mounting them in water. If the plants are old and dry, the addition of a
drop or two of potash—or ammonia—solution is often helpful in clearing
the membranes of the cells and in restoring the shrivelled spores and
paraphyses to their natural forms and dimensions.

If serial microtome sections are desired, more elaborate methods are
required. For this purpose Peirce[21] has recommended that “when
dealing with plants that are dry but still alive, the material should
be thoroughly wetted and kept moist for two days, then killed and
fixed in a saturated solution of corrosive sublimate in thirty-five
per cent. alcohol.” The solution should be used hot: the usual methods
of dehydrating and embedding in paraffin are then employed with extra
precautions on account of the extremely brittle nature of lichens.

Another method that also gave good results has been proposed by
French[22]: “first the lichen is put into 95 per cent. alcohol for 24
hours, then into thin celloidin and thick celloidin 24 hours each. After
this the specimens are embedded in thick celloidin which is hardened in
70 per cent. alcohol for 24 hours and then cut.” French advises staining
with borax carmine: it colours the fungal part pale carmine and the algal
cells a greenish-red shade.

Modern research methods of work are generally described in full in the
publications that are discussed in the following chapters. The student
is referred to these original papers for information as to fixing,
embedding, staining, etc.

Great use has been made of reagents in determining lichen species.
They are extremely helpful and often give the clinching decision when
morphological characters are obscure, especially if the plant has been
much altered by the environment. It must be borne in mind, however,
that a species is a morphological rather than a physiological unit,
and it is not the structures but the cell-products that are affected by
reagents. Those most commonly in use are saturated solutions of potash
and of bleaching-powder (calcium hypochlorite). The former is cited
in text-books as KOH or simply as K, the latter as CaCl or C. The C
solution deteriorates quickly and must, therefore, be frequently renewed
to produce the required reaction, _i.e._ some change of colour. These
two reagents are used singly or, if conjointly, K followed by C. The
significance of the colour changes has been considered in the discussion
on lichen-acids.

Iodine is generally cited in connection with its staining effect on the
hymenium of the fruit; the blue colour produced is, however, more general
than was at one time supposed and is not peculiar to lichens; the asci of
many fungi react similarly though to a less extent. The medullary hyphae
in certain species also stain blue with iodine.




CHAPTER I

HISTORY OF LICHENOLOGY


A. INTRODUCTORY

The term “lichen” is a word of Greek origin used by Theophrastus in
his _History of Plants_ to signify a superficial growth on the bark of
olive-trees. The name was given in the early days of botanical study not
to lichens, as we understand them, but to hepatics of the _Marchantia_
type. Lichens themselves were generally described along with various
other somewhat similar plants as “Muscus” (Moss) by the older writers,
and more definitely as “Musco-fungus” by Morison[23]. In a botanical work
published in 1700 by Tournefort[24] all the members of the vegetable
kingdom then known were for the first time classified in genera, and the
genus _Lichen_ was reserved for the plants that have been so designated
since that time, though Dillenius[25] in his works preferred the
adjectival name _Lichenoides_.

A painstaking historical account of lichens up to the beginning of modern
lichenology has been written by Krempelhuber[26], a German lichenologist.
He has grouped the data compiled by him into a series of Periods, each
one marked by some great advance in knowledge of the subject, though,
as we shall see, the advance from period to period has been continuous
and gradual. While following generally on the lines laid down by
Krempelhuber, it will be possible to cite only the more prominent writers
and it will be of much interest to British readers to note especially the
work of our own botanists.

Krempelhuber’s periods are as follows:

  I. From the earliest times to the end of the seventeenth century.

  II. Dating from the arrangement of plants into classes called
  genera by Tournefort in 1694 to 1729.

  III. From Micheli’s division of lichens into different orders in
  1729 to 1780.

  IV. The definite and reasoned establishment of lichen genera
  based on the structure of thallus and fruit by Weber in 1780 to
  1803.

  V. The arrangement of all known lichens under their respective
  genera by Acharius in 1803 to 1846.

  VI. The recognition of spore characters in classification by De
  Notaris in 1846 to 1867.

A seventh period which includes modern lichenology, and which dates
after the publication of Krempelhuber’s _History_, was ushered in
by Schwendener’s announcement in 1867 of the hypothesis as to the
dual nature of the lichen thallus. Schwendener’s theory gave a new
impulse to the study of lichens and strongly influenced all succeeding
investigations.


B. PERIOD I. PREVIOUS TO 1694

Our examination of lichen literature takes us back to Theophrastus, the
disciple of Plato and Aristotle, who lived from 371 to 284 B.C., and
who wrote a _History of Plants_, one of the earliest known treatises on
Botany. Among the plants described by Theophrastus, there are evidently
two lichens, one of which is either an _Usnea_ or an _Alectoria_, and
the other certainly _Roccella tinctoria_, the last-named an important
economic plant likely to be well known for its valuable dyeing
properties. The same or somewhat similar lichens are also probably
alluded to by the Greek physician Dioscorides, in his work on _Materia
Medica_, A.D. 68. About the same time Pliny the elder, who was a soldier
and traveller as well as a voluminous writer, mentions them in his
_Natural History_ which was completed in 77 A.D.

During the centuries that followed, there was little study of Natural
History, and, in any case, lichens were then and for a long time after
considered to be of too little economic value to receive much attention.

In the sixteenth century there was a great awakening of scientific
interest all over Europe, and, after the printing-press had come into
general use, a number of books bearing on Botany were published. It will
be necessary to chronicle only those that made distinct contributions to
the knowledge of lichens.

The study of plants was at first entirely from a medical standpoint and
one of the first works, and the first book on Natural History, printed
in England, was the _Grete Herball_[27]. It was translated from a French
work, _Hortus sanitatis_, and published by Peter Treveris in Southwark.
One of the herbs recommended for various ailments is “Muscus arborum,”
the tree-moss (_Usnea_). A somewhat crude figure accompanies the text.

Ruel[28] of Soissons in France, Dorstenius[29], Camerarius[30] and
Tabernaemontanus[31] in Germany followed with works on medical or
economic botany and they described, in addition to the tree-moss, several
species of reputed value in the art of healing now known as _Sticta_
(_Lobaria_) _pulmonaria_, _Lobaria laetevirens_, _Cladonia pyxidata_,
_Evernia prunastri_ and _Cetraria islandica_. Meanwhile L’Obel[32], a
Fleming, who spent the latter part of his life in England and is said to
have had charge of a physic garden at Hackney, was appointed botanist to
James I. He published at Antwerp a large series of engravings of plants,
and added a species of _Ramalina_ to the growing list of recognized
lichens. Dodoens[33], also a Fleming, records not only the _Usnea_
of trees, but a smaller and more slender black form which is easily
identifiable as _Alectoria jubata_. He also figures _Lichen pulmonaria_
and gives the recipe for its use.

The best-known botanical book published at that time, however, is the
_Herball_ of John Gerard[34] of London, Master in Chirurgerie, who had
a garden in Holborn. He recommends as medicinally valuable not only
_Usnea_, but also _Cladonia pyxidata_, for which he coined the name
“cuppe- or chalice-moss.” About the same time Schwenckfeld[35] recorded,
among plants discovered by him in Silesia, lichens now familiar as
_Alectoria jubata_, _Cladonia rangiferina_ and a species of _Peltigera_.

Among the more important botanical writers of the seventeenth century may
be cited Colonna[36] and Bauhin[37]. The former, an Italian, contributes,
in his _Ecphrasis_, descriptions and figures of three additional species
easily recognized as _Physcia ciliaris_, _Xanthoria parietina_ and
_Ramalina calicaris_. Kaspar Bauhin, a professor in Basle, who was one of
the most advanced of the older botanists, was the first to use a binomial
nomenclature for some of his plants. He gives a list in his _Pinax_
of the lichens with which he was acquainted, one of them, _Cladonia
fimbriata_, being a new plant.

John Parkinson’s[38] _Herball_ is well known to English students; he
adds one new species for England, _Lobaria pulmonaria_, already recorded
on the Continent. Parkinson was an apothecary in London and held the
office of the King’s Herbarist; his garden was situated in Long Acre.
How’s[39] _Phytographia_ is notable as being the first account of
British plants compiled without reference to their healing properties.
Five of the plants described by him are lichen species: “Lichen arborum
sive pulmonaria” (_Lobaria pulmonaria_), “Lichen petraeus tinctorius”
(_Roccella_), “Muscus arboreus” (_Usnea_), “Corallina montana” (_Cladonia
rangiferina_) and “Muscus pixoides” (_Cladonia_). Several other British
species were added by Merrett[40], who records in his Pinax, “Muscus
arboreus umbilicatus” (_Physcia ciliaris_), “Muscus aureus tenuissimus”
(_Teloschistes flavicans_), “Muscus caule rigido” (_Alectoria_) and
“Lichen petraeus purpureus” (_Parmelia omphalodes_), the last-named, a
rock lichen, being used, he tells us, for dyeing in Lancashire.

Merret or Merrett was librarian to the Royal College of Physicians.
His _Pinax_ was undertaken to replace How’s _Phytographia_ published
sixteen years previously and then already out of print. Merrett’s work
was issued in 1666, but the first impression was destroyed in the great
fire of London and most of the copies now extant are dated 1667. He
arranged the species of plants in alphabetical order, but as the work
was not critical it fell into disuse, being superseded by John Ray’s
_Catalogus_ and _Synopsis_. To Robert Plot[41] we owe the earliest
record of _Cladonia coccifera_ which had hitherto escaped notice; it was
described and figured as a new and rare plant in the _Natural History
of Staffordshire_[41]. Plot was the first Custos of Ashmole’s Museum in
Oxford and he was also the first to prepare a County Natural History.

The greatest advance during this first period was made by Robert
Morison[42], a Scotsman from Aberdeen. He studied medicine at Angers in
France, superintended the Duke of Orleans’ garden at Blois, and finally,
after his return to this country in 1669, became Keeper of the botanic
garden at Oxford. In the third volume of his great work[42] on Oxford
plants, which was not issued till after his death, the lichens are put
in a separate group—“Musco-fungus”—and classified with some other plants
under “Plantae Heteroclitae.” The publication of the volume projects into
the next historical period.

Long before this date John Ray had begun to study and publish books on
Botany. His _Catalogue of English Plants_[43] is considered to have
commenced a new era in the study of the science. The _Catalogue_ was
followed by the _History of Plants_[44], and later by a _Synopsis of
British Plants_[45], and in all of these books lichens find a place. Two
editions of the _Synopsis_ appeared during Ray’s lifetime, and to the
second there is added an Appendix contributed by Samuel Doody which is
entirely devoted to Cryptogamic plants, including not a few lichens—still
called “Mosses”—discovered for the first time. Doody, himself an
apothecary, took charge of the garden of the Apothecaries’ Society at
Chelsea, but his chief interest was Cryptogamic Botany, a branch of the
subject but little regarded before his day. Pulteney wrote of him as the
“Dillenius of his time.”

Among Doody’s associates were the Rev. Adam Buddle, James Petiver and
William Sherard. Buddle was primarily a collector and his herbarium is
incorporated in the Sloane Herbarium at the British Museum. It contains
lichens from all parts of the world, many of them contributed by Doody,
Sherard and Petiver. Only a few of them bear British localities: several
are from Hampstead where Buddle had a church.

The Society of Apothecaries had been founded in 1617 and the members
acquired land on the river-front at Chelsea, which was extended later
and made into a Physick Garden. James Petiver[46] was one of the first
Demonstrators of Plants to the Society in connection with the garden,
and one of his duties was to conduct the annual herborizing tours of the
apprentices in search of plants. He thus collected a large herbarium
on the annual excursions, as well as on shorter visits to the more
immediate neighbourhood of London. He wrote many tracts on Natural
History subjects, and in these some lichens are included. He was one of
the best known of Ray’s correspondents, and owing to his connection with
the Physic Garden received plants from naturalists in foreign countries.

Sherard, another of Doody’s friends, had studied abroad under Tournefort
and was full of enthusiasm for Natural Science. It was he who brought
Dillenius to England and finally nominated him for the position of the
first Sherardian Professor of Botany at Oxford. Another well-known
contemporary botanist was Leonard Plukenet[47] who had a botanical garden
at Old Palace Yard, Westminster. He wrote several botanical works in
which lichens are included.

Morison is the only one of all the botanists of the time who recognized
lichens as a group distinct from mosses, algae or liverworts, and even
he had very vague ideas as to their development. Malpighi[48] had noted
the presence of soredia on the thallus of some species, and regarded them
as seeds. Porta[49], a Neapolitan, has been quoted by Krempelhuber as
probably the first to discover and place on record the direct growth of
lichen fronds from green matter on the trunks of trees.


C. PERIOD II. 1694-1729

The second Period is ushered in with the publication of a French work,
_Les Élémens de Botanique_ by Tournefort[50], who was one of the greatest
botanists of the time. His object was—“to facilitate the knowledge of
plants and to disentangle a science which had been neglected because it
was found to be full of confusion and obscurity.” Up to this date all
plants were classified or listed as individual species. It was Tournefort
who first arranged them in groups which he designated “genera” and he
gave a careful diagnosis of each genus.

_Les Élémens_ was successful enough to warrant the publication a few
years later of a larger Latin edition entitled _Institutiones_[51]
and thus fitted for a wider circulation. Under the genus _Lichen_, he
included plants “lacking flowers but with a true cup-shaped shallow
fruit, with very minute pollen or seed which appeared to be subrotund
under the microscope.” Not only the description but the figures prove
that he was dealing with ascospores and not merely soredia, though
under _Lichen_ along with true members of the “genus” he has placed a
_Marchantia_, the moss _Splachnum_ and a fern. A few lichens were placed
by him in another genus _Coralloides_.

Tournefort’s system was of great service in promoting the study of
Botany: his method of classification was at once adopted by the German
writer Rupp[52] who published a Flora of plants from Jena. Among these
plants are included twenty-five species of lichens, several of which he
considered new discoveries, no fewer than five being some form of _Lichen
gelatinosus (Collema)_. Buxbaum[53], in his enumeration of plants from
Halle, finds place for forty-nine lichen species, with, in addition,
eleven species of _Coralloides_; and Vaillant[54] in listing the plants
that grew in the neighbourhood of Paris gives thirty-three species for
the genus _Lichen_ of which a large number are figured, among them
species of _Ramalina_, _Parmelia_, _Cladonia_, etc.

In England, however, Dillenius[55], who at this time brought out a third
edition of Ray’s _Synopsis_ and some years later his own _Historia
Muscorum_, still described most of his lichens as “Lichenoides” or
“Coralloides”; and no other work of note was published in our country
until after the Linnaean system of classification and of nomenclature was
introduced.


D. PERIOD III. 1729-1780

Lichens were henceforth regarded as a distinct genus or section of
plants. Micheli[56], an Italian botanist, Keeper of the Grand Duke’s
Gardens in Florence, realized the desirability of still further
delimitation, and he broke up Tournefort’s large comprehensive genera
into numerical Orders. In the genus _Lichen_, he found occasion for 38 of
these Orders, determined mainly by the character of the thallus, and the
position on it of apothecia and soredia. He enumerates the species, many
of them new discoveries, though not all of them recognizable now. His
great work on Plants is enriched by a series of beautiful figures. It was
published in 1729 and marks the beginning of a new Period—a new outlook
on botanical science. Micheli regarded the apothecia of lichens as
“floral receptacles,” and the soredia as the seed, because he had himself
followed the development of lichen fronds from soredia.

The next writer of distinction is the afore-mentioned Dillen or
Dillenius. He was a native of Darmstadt and began his scientific career
in the University of Giessen. His first published work[57] was an account
of plants that were to be found near Giessen in the different months of
the year. Mosses and lichens he has assigned to December and January.
Sherard induced him to come to England in 1721, and at first engaged his
services in arranging the large collections of plants which he, Sherard,
had brought from Smyrna or acquired from other sources.

Three years after his arrival Dillenius had prepared the third edition
of Ray’s _Synopsis_ for the press, but without putting his name on the
title-page[58]. Sherard explained, in a letter to Dr Richardson of Bierly
in Yorkshire, that “our people can’t agree about an editor, they are
unwilling a foreigner should put his name to it.” Dillenius, who was
quite aware of the prejudice against aliens, himself writes also to Dr
Richardson: “there being some apprehension (me being a foreigner) of
making natives uneasy if I should publicate it in my name.” Lichens were
already engaging his attention, and descriptions of 91 species were added
to Ray’s work. So well did this edition meet the requirements of the age,
that the _Synopsis_ remained the text-book of British Botany until the
publication of Hudson’s _Flora Anglica_ in 1762.

William Sherard died in 1728. He left his books and plates to the
University of Oxford with a sum of money to endow a Professorship of
Botany. In his will he had nominated Dr Dillenius for the post. The great
German botanist was accordingly appointed and became the first Sherardian
Professor of Botany, though he did not remove to Oxford till 1734. The
following years were devoted by him to the preparation of _Historia
Muscorum_, which was finally published in 1741. It includes an account
of the then known liverworts, mosses and lichens. The latter—still
considered by Dillenius as belonging to mosses—were grouped under three
genera, _Usnea_, _Coralloides_ and _Lichenoides_. The descriptions and
figures are excellent, and his notes on occasional lichen characteristics
and on localities are full of interest. His lichen herbarium, which still
exists at Oxford, mounted with the utmost care and neatness, has been
critically examined by Nylander and Crombie[59] and many of the species
identified.

Dillenius was ignorant of, or rejected, Micheli’s method of
classification, adopting instead the form of the thallus as a guide to
relationship. He also differed from him in his views as to propagation,
regarding the soredia as the pollen of the lichen, and the apothecia as
the seed-vessels, or even in certain cases as young plants.

Shortly after the publication of Dillenius’ _Historia_, appeared
Haller’s[60] _Systematic and Descriptive list of plants indigenous
to Switzerland_. The lichens are described as without visible leaves
or stamens but with “corpuscula” instead of flowers and leaves. He
arranged his lichen species, 160 in all, under seven different Orders:
1. “Lichenes Corniculati and Pyxidati”; 2. “L. Coralloidei”; 3. “L.
Fruticosi”; 4. “L. Pulmonarii”; 5. “L. Crustacei” (with flower-shields);
6. “L. Scutellis” (with shields but with little or no thallus); and 7.
“L. Crustacei” (without shields).

This period extends till near the end of the eighteenth century, and
thus includes within its scope the foundation of the binomial system of
naming plants established by Linnaeus[61]. The renowned Swedish botanist
rather scorned lichens as “rustici pauperrimi,” happily translated by
Schneider[62] as the “poor trash of vegetation,” but he named and listed
about 80 species. He divided his solitary genus _Lichen_ into sections:
1. “Leprosi tuberculati”; 2. “Leprosi scutellati”; 3. “Imbricati”;
4. “Foliacei”; 5. “Coriacei”; 6. “Scyphiferi”; 7. “Filamentosi.” By
this ordered sequence Linnaeus showed his appreciation of development,
beginning, as he does, with the leprose crustaceous thallus and
continuing up to the most highly organized filamentous forms. He and his
followers still included the genus _Lichen_ among Algae.

A voluminous _History of Plants_ had been published in 1751 by Sir John
Hill[63], the first superintendent to be appointed to the Royal Gardens,
Kew. In the _History_ lichens are included under the Class “Mosses,” and
are divided into several vaguely limited “genera”—_Usnea_, tree mosses,
consisting of filaments only; _Platysma_, flat branched tree mosses,
such as lung-wort; _Cladonia_, the orchil and coralline mosses, such
as _Cladonia furcata_; _Pyxidium_, the cup-mosses; and _Placodium_,
the crustaceous, friable or gelatinous forms. A number of plants are
somewhat obscurely described under each genus. Not only were these new
_Lichen_ genera suggested by him, but among his plants are such binomials
as _Usnea compressa_, _Platysma corniculatum_, _Cladonia furcata_ and
_Cladonia tophacea_; other lichens are trinomial or are indicated, in
the way then customary, by a whole sentence. Hill’s studies embraced
a wide variety of subjects; he had flashes of insight, but not enough
concentration to make an effective application of his ideas. In his
_Flora Britannica_[64], which was compiled after the publication of
Linnaeus’s _Species Plantarum_, he abandoned his own arrangement in
favour of the one introduced by Linnaeus and accepted again the single
genus _Lichen_.

Sir William Watson[65], a London apothecary and physician of scientific
repute at this period, proposed a rearrangement and some alteration of
Linnaeus’s sections. He had failed to grasp the principle of development,
but he gives a good general account of the various groups. Watson was the
progenitor of those who decry the makers and multipliers of species. So
in regard to Micheli, who had increased the number to “298,” he writes:
“it is to be regretted, that so indefatigable an author, one whose
genius particularly led him to scrutinize the minuter subjects of the
science, should have been so solicitous to increase the number of species
under all his genera: an error this, which tends to great confusion and
embarrassment, and must retard the progress and real improvement of the
botanic science.” Linnaeus however in redressing the balance earned his
full approbation: “He has so far retrenched the genus (_Lichen_) that in
his general enumeration of plants he recounts only 80 species belonging
to it.”

Linnaeus’s binomial system was almost at once adopted by the whole
botanical world and the discovery and tabulation of lichens as well as
of other plants proceeded apace. Scopoli’s[66] _Flora Carniolica_, for
instance, published in 1760, still adhered to the old descriptive method
of nomenclature, but a second edition, issued twelve years later, is
based on the new system: it includes 54 lichen species.

About this time Adanson[67] proposed a new classification of plants,
dividing them into families, and these again into sections and genera. He
transferred the lichens to the Family “Fungi,” and one of his sections
contains a number of lichen genera, the names of these being culled from
previous workers, Dillenius, Hill, etc. A few new ones are added by
himself, and one of them, _Graphis_, still ranks as a good genus.

In England, Hudson[68], who was an apothecary and became sub-librarian
of the British Museum, followed Linnaeus both in the first and later
editions of the _Flora Anglica_. He records 102 lichen species.
Withering[69] was also engaged, about this time, in compiling his
_Arrangement of Plants_. He translated Linnaeus’s term “Algae” into the
English word “Thongs,” the lichens being designated as “Cupthongs.” In
later editions, he simply classifies lichens as such. Lightfoot[70],
whose descriptive and economic notes are full of interest, records 103
lichens in the _Flora Scotica_, and Dickson[71] shortly after published
a number of species from Scotland, some of them hitherto undescribed.
Dickson was a nurseryman who settled in London, and his avocations kept
him in touch with plant-lovers and with travellers in many lands.


E. PERIOD IV. 1780-1803

The inevitable next advance was made by Weber[72] who at the time was a
Professor at Kiel. In a first work dealing with lichens he had followed
Linnaeus; then he published a new method of classification in which the
lichens are considered as an independent Order of Cryptogamia, and that
Order, called “Aspidoferae,” he subdivided into genera. His ideas had
been partly anticipated by Hill and by Adanson, but the work of Weber
indicates a more correct view of the nature of lichens. He established
eight fairly well-marked genera, viz. _Verrucaria_, _Tubercularia_,
_Sphaerocephalum_ and _Placodium_, which were based on fruit-characters,
the thallus being crustaceous and rather insignificant, and a second
group _Lichen_, _Collema_, _Cladonia_ and _Usnea_, in which the thallus
ranked first in importance. Though Weber’s scheme was published in
1780, it did not at first secure much attention. The great authority of
Linnaeus dominated so strongly the botany of the period that for a long
time no change was welcomed or even tolerated.

In our own country Relhan at Cambridge and Sibthorp[73] at Oxford were
making extensive studies of plants. The latter was content to follow
Linnaeus in his treatment of lichens. Relhan[74] also grouped his lichens
under one genus though, in a second edition of his _Flora_, he broke away
from the Linnaean tradition and adopted the classification of Acharius.

Extensive contributions to the knowledge of English plants generally were
made by Sir James Edward Smith[75] who, in 1788, founded the Linnean
Society of London of which he was President until his death in 1828. He
began his great work, _English Botany_, in 1790 with James Sowerby as
artist. Smith’s and Sowerby’s part of the work came to an end in 1814;
but a supplement was begun in 1831 by Hooker who had the assistance of
Sowerby’s sons in preparing the drawings. Nearly all the lichens recorded
by Smith are published simply as _Lichen_, and his _Botany_ thus belongs
to the period under discussion, though in time it stretches far beyond.

Continental lichenologists had been more receptive to new ideas,
and other genera were gradually added to Weber’s list, notably by
Hoffmann[76] and Persoon[77].

For a long time little was known of the lichens of other than European
countries. Buxbaum[78] in the East, Petiver[79] and Hans Sloane[80] in
the West made the first exotic records. The latter notes how frequently
lichens grew on the imported Jesuit’s bark, and he quaintly suggests
in regard to some of these species that they may be identical with the
“hyssop that springeth out of the wall.” It was not however till towards
the end of the eighteenth century that much attention was given to
foreign lichens, when Swartz[81] in the West Indies and Desfontaines[82]
in N. Africa collected and recorded a fair number. Swartz describes about
twenty species collected on his journey through the West Indian Islands
(1783-87).

Interest was also growing in other aspects of lichenology. Georgi[83], a
Russian Professor, was the first to make a chemical analysis of lichens.
He experimented on some of the larger forms and extracted and examined
the mucilaginous contents of _Ramalina farinacea_, _Platysma glaucum_,
_Lobaria pulmonaria_, etc., which he collected from birch and pine trees.
About this time also the French scientists Willomet[84], Amoreux and
Hoffmann jointly published theses setting forth the economic value of
such lichens as were used in the arts, as food, or as medicine.


F. PERIOD V. 1803-1846

The fine constructive work of Acharius appropriately begins a new era in
the history of lichenology. Previous writers had indeed included lichens
in their survey of plants, but always as a somewhat side issue. Acharius
made them a subject of special study, and by his scientific system of
classification raised them to the rank of the other great classes of
plants.

Acharius was a country doctor at Wadstena on Lake Mälar in Sweden, as
he himself calls it, “the country of lichens.” He was attracted to the
study of them by their singular mode of growth and organization, both of
thallus and reproductive organs, for which reason he finally judged that
lichens should be considered as a distinct Order of Cryptogamia.

In his first tentative work[85] he had followed his great compatriot
Linnaeus, classifying all the species known to him under the one genus
_Lichen_, though he had progressed so far as to divide the unwieldy Genus
into Families and these again into Tribes, these latter having each a
tribal designation such as _Verrucaria_, _Opegrapha_, etc. He established
in all twenty-eight tribes which, at a later stage, he transformed into
genera after the example of Weber.

Acharius, from the beginning of his work, had allowed great importance
to the structure of the apothecia as a diagnostic character though
scarcely recognizing them as true fruits. He gave expression to his more
mature views first in the _Methodus Lichenum_[86], then subsequently in
the larger _Lichenographia Universalia_[87]. In the latter work there
are forty-one genera arranged under different divisions; the species
are given short and succinct descriptions, with habitat, locality and
synonymy. No material alteration was made in the _Synopsis Lichenum_[88],
a more condensed work which he published a few years later.

The Cryptogamia are divided by Acharius into six “Families,” one
of which, “Lichenes,” is distinguished, he finds, by two methods
of propagation: by propagula (soredia) and by spores produced in
apothecia. He divides the family into classes characterized solely by
fruit characters, and these again into orders, genera and species,
of which diagnoses are given. With fuller knowledge many changes and
rearrangements have been found necessary in the application and extension
of the system, but that in no way detracts from the value of the work as
a whole.

In addition to founding a scientific classification, Acharius invented
a terminology for the structures peculiar to lichens. We owe to him
the names and descriptions of “thallus,” “podetium,” “apothecium,”
“perithecium,” “soredium,” “cyphella” and “cephalodium,” the last word
however with a different meaning from the one now given to it. He
proposed several others, some of which are redundant or have fallen into
disuse, but many of his terms as we see have stood the test of time and
have been found of service in allied branches of botany.

Lichens were studied with great zest by the men of that day. Hue[89]
recalls a rather startling incident in this connection: Wahlberg, it is
said, had informed Dufour that he had sent a large collection of lichens
from Spain to Acharius who was so excited on receiving them, that he
fell ill and died in a few days (Aug. 14th, 1819). Dufour, however, had
added the comment that the illness and death might after all be merely a
coincidence.

Among contemporary botanists, we find that De Candolle[90] in the volume
he contributed to Lamarck’s _French Flora_, quotes only from the earlier
work of Acharius. He had probably not then seen the _Methodus_, as he
uses none of the new terms; the lichens of the volume are arranged under
genera which are based more or less on the position of the apothecia on
the thallus. Flörke[91], the next writer of consequence, frankly accepts
the terminology and the new view of classification, though differing on
some minor points.

Two lists of lichens, neither of particular note, were published at this
time in our country: one by Hugh Davies[92] for Wales, which adheres
to the Linnaean system, and the other by Forster[93] of lichens round
Tonbridge. Though Forster adopts the genera of Acharius, he includes
lichens among algae. A more important publication was S. F. Gray’s[94]
_Natural Arrangement of British Plants_. Gray, who was a druggist
in Walsall and afterwards a lecturer on botany in London, was only
nominally[95] the author, as it was mainly the work of his son John
Edward Gray[96], sometime Keeper of Zoology in the British Museum.
Gray was the first to apply the principles of the Natural System of
classification to British plants, but the work was opposed by British
botanists of his day. The years following the French Revolution and
the Napoleonic wars were full of bitter feeling and of prejudice, and
anything emanating, as did the Natural System, from France was rejected
as unworthy of consideration.

In the _Natural Arrangement_, Gray followed Acharius in his treatment
of lichens; but whereas Acharius, though here and there confusing
fungus species with lichens, had been clear-sighted enough to avoid
all intermixture of fungus genera, with the exception of one only, the
sterile genus _Rhizomorpha_, Gray had allowed the interpolation of
several, such as _Hysterium_, _Xylaria_, _Hypoxylon_, etc. He had also
raised many of Acharius’s subgenera and divisions to the rank of genera,
thus largely increasing their number. This oversplitting of well-defined
genera has somewhat weakened Gray’s work and he has not received from
later writers the attention he deserves.

The lichens of Hooker’s[97] _Flora Scotica_, which is synchronous with
Gray’s work, number 195 species, an increase of about 90 for Scotland
since the publication of Lightfoot’s _Flora_ more than 40 years before.
Hooker also followed Acharius in his classification of lichens both in
the _Flora Scotica_ and in the _Supplement to English Botany_[98], which
was undertaken by the younger Sowerbys and himself. To that work Borrer
(1781-1862), a keen lichenologist, supplied many new and rare lichens
collected mostly in Sussex.

It is a matter of regret that Greville should have so entirely ignored
lichens in his great work on _Scottish Cryptogams_[99]. The two species
of Lichina are the only ones he figured, and these he took to be algae.
He[100] was well acquainted with lichens, for in the _Flora Edinensis_ he
lists 128 species for the Edinburgh district, arranging the genera under
“Lichenes” with the exception of _Opegrapha_ and _Verrucaria_ which are
placed with the fungus genus _Poronia_ in “Hypoxyla.” Though he cites
the publications of Acharius, he does not employ his scientific terms,
possibly because he was writing his diagnoses in English. Two other
British works of this time still remain to be chronicled: Hooker’s[101]
contributions to Smith’s _English Flora_ and Taylor’s[102] work on
lichens in Mackay’s _Flora Hibernica_. Through these the knowledge of the
subject was very largely extended in our country.

The classification of lichens and their place in the vegetable kingdom
were now firmly established on the lines laid down by Acharius.
Fries[103] in his important work _Lichenographia Europaea_ more or
less followed his distinguished countryman. The uncertainty as to the
position and relationship of lichens had rendered the task of systematic
arrangement one of peculiar difficulty and had unduly absorbed attention;
but now that a satisfactory order had been established in the chaos
of forms, the way was clear for other aspects of the study. Several
writers expressed their views by suggesting somewhat different methods
of classification, others wrote monographs of separate groups, or
genera. Fée[104] published an Essay on the Cryptogams (mostly lichens)
that grew on officinal exotic barks; Flörke[105] took up the difficult
genus _Cladonia_; Wallroth[106] also wrote on _Cladonia_; Delise[107] on
_Sticta_, and Chevalier[108] published a long and elaborate account of
Graphideae.

Wallroth and Meyer at this time published, simultaneously, important
studies on the general morphology and physiology of lichens.
Wallroth[109] had contemplated an even larger work on the _Natural
History of Lichens_, but only two of the volumes reached publication.
In the first of these he devoted much attention to the “gonidia” or
“brood-cells” and established the distinction between the heteromerous
and homoiomerous distribution of green cells within the thallus; he also
describes with great detail the “morphosis” and “metamorphosis” of the
vegetative body. In the second volume he discusses their physiology—the
contents and products of the thallus, colouring, nutrition, season of
development, etc.—and finally the pathology of these organisms. He made
no great use of the compound microscope, and his studies were confined to
phenomena that could be observed with a single lens.

Meyer’s[110] work contains a still more exact study of the anatomy and
physiology of lichens; he also devotes many passages to an account of
their metamorphoses, pointing out that species alter so much in varying
conditions, that the same one at different stages may be placed even in
different genera; he however carries his theory of metamorphosis too
far and unites together widely separated plants. Meyer was the first to
describe the growth of the lichen from spores, though his description is
somewhat confused. Possibly the honour of having first observed their
germination should be given to a later botanist, Holle[111]. The works
of both Wallroth and Meyer enjoyed a great and well-merited reputation:
they were standard books of consultation for many years. Koerber[112],
who devoted a long treatise to the study of gonidia, confirmed Wallroth’s
theories: he considered at that time that the gonidia in the soredial
condition were organs of propagation.

Mention should be made here of the many able and keen collectors who,
in the latter half of the eighteenth century and the beginning of the
nineteenth, did so much to further the knowledge of lichens in the
British Isles. Among the earliest of these naturalists are Richard
Pulteney (1730-1801), whose collection of plants, now in the herbarium of
the British Museum, includes many lichens, and Hugh Davies (1739-1821),
a clergyman whose Welsh plants also form part of the Museum collection.
The Rev. John Harriman (1760-1831) sent many rare plants from Egglestone
in Durham to the editors of _English Botany_ and among them were not a
few lichens. Edward Forster (1765-1849) lived in Essex and collected in
that county, more especially in and near Epping Forest, and another East
country botanist, Dawson Turner (1775-1858), though chiefly known as
an algologist, gave considerable attention to lichens. In Scotland the
two most active workers were Charles Lyell (1767-1849), of Kinnordy in
Forfarshire, and George Don (1798-1856), also a Forfar man. Don was a
gardener and became eventually a foreman at the Chelsea Physic Garden.
Sir Thomas Gage of Hengrave Hall (1781-1823) botanized chiefly in his
own county of Suffolk; but most of his lichens were collected in South
Ireland and are incorporated in the herbarium of the British Museum. Miss
Hutchins also collected in Ireland and sent her plants for inclusion
in _English Botany_. But in later years, the principal lichenologist
connected with that great undertaking was W. Borrer, who spent his life
in Sussex: he not only supplied a large number of specimens to the
authors, but he himself discovered and described many new lichens.

American lichenologists were also extremely active all through this
period. The comparatively few lichens of Michaux’s[113] _Flora_ grouped
under “Lichenaceae” were collected in such widely separated regions as
Carolina and Canada. A few years later Mühlenberg[114] included no fewer
than 184 species in his _Catalogue of North American Plants_. Torrey[115]
and Halsey[116] botanized over a limited area near New York, and the
latter, who devoted himself more especially to lichens, succeeded in
recording 176 different forms, old and new. These two botanists were
both indebted for help in their work to Schweinitz, a Moravian brother,
who moved from one country to another, working and publishing, now in
America and now in Europe. His name is however chiefly associated with
fungi. Later American lichenology is nobly represented by Tuckerman[117]
who issued his first work on lichens in 1839, and who continued for
many years to devote himself to the subject. He followed at first the
classification and nomenclature that had been adopted by Fée, but as time
went on he associated himself with all that was best and most enlightened
in the growing science.

Travellers and explorers in those days of high adventure were constantly
sending their specimens to European botanists for examination and
determination, and the knowledge of exotic lichens as of other classes of
plants grew with opportunity. Among the principal home workers in foreign
material, at this time, may be cited Fée[118] who described a very large
series on officinal barks (_Cinchona_, etc.) so largely coming into
use as medicines; he also took charge of the lichens in Martius’s[119]
_Flora of Brazil_. Montagne[120] named large collections, notably those
of Leprieur collected in Guiana, and Hooker[121] and Walker Arnott
determined the plants collected during Captain Beechey’s voyage, which
included lichens from many different regions.


G. PERIOD VI. 1846-1867

The last work of importance, in which microscopic characters
were ignored, was the _Enumeratio critica Lichenum Europaeum_ by
Schaerer[122], a veteran lichenologist, who rather sadly realized at the
end the limitations of that work, as he asks the reader to accept it
“such as it is.” Many years previously, Eschweiler[123] in his _Systema_
and Fée[124] in his account of _Cryptogams on Officinal Bark_, had given
particular attention to the internal structure as well as to the outward
form of the lichen fructification. Fée, more especially, had described
and figured a large number of spores; but neither writer had done more
than suggest their value as a guide in the determination of genera and
species.

It was an Italian botanist, Giuseppe de Notaris[125], a Professor in
Florence, who took up the work where Fée had left it. His comparative
studies of both vegetative and reproductive organs convinced him of
the great importance of spore characters in classification, the spore
being, as he rightly decided, the highest and ultimate product of the
lichen plant. In his microscopic examination of the various recognized
genera, he found that while, in some genera, the spores conformed to one
distinct type, in others their diversities in form, septation or colour
gave a decisive reason for the establishment of new genera, while minor
differences in size, etc. of the spores proved to be of great value in
distinguishing species. The spore standard thus marks a new departure
in lichenology. De Notaris published the results of his researches in a
fragment of a projected larger work that was never completed. Though his
views were overlooked for a time, they were at length fully recognized
and further elaborated by Massalongo[126] in Italy, by Norman[127] in
Norway, by Koerber[128] in Germany and by Mudd[129] in our own country.
Massalongo had drawn up the scheme of a great _Scolia Lichenographica_,
but like de Notaris, he was only able to publish a part. After twelve
years of ill-health, in which he struggled to continue his work, he died
at the early age of 36.

Lindsay[130], Mudd and Leighton[131] were at this time devoting great
attention to British lichens. Lauder Lindsay’s _Popular History of
British Lichens_, with its coloured plates and its descriptive and
economic account of these plants has enabled many to acquire a wide
knowledge of the group. Mudd’s _Manual_, a more complete and extremely
valuable contribution to the subject, followed entirely on the lines
of Massalongo’s work. From his large experience in the examination
of lichens he came to the conclusion that: “Of all organs furnished
by a given group of plants, none offer so many real, constant and
physiological characters as the spores of lichens, for the formation of a
simple and natural classification.”

Meanwhile, a contemporary writer, William Nylander, was rising into fame.
He was born at Uleaborg in Finland[132] in 1822 and became interested in
lichens very early in his career. His first post was the professorship
of botany at Helsingfors; but in 1863 he gave up his chair and removed
to Paris where he remained, except for short absences, until his death.
One of his excursions brought him to London in 1857 to examine Hooker’s
herbarium. He devoted his whole life to the study of lichens, and from
1852, the date of his first lichen publication, which is an account
of the lichens of Helsingfors, to the end of his life he poured out
a constant succession of books or papers, most of them in Latin. One
of his earliest works was an _Essay on Classification_[133] which he
elaborated later, but which in its main features he never altered. He
relied, in his system, on the structure and form of thallus, gonidia and
fructifications, more especially on those of the spermogonia (pycnidia),
but he rejected ascospore characters except so far as they were of use
in the diagnosis of species. He failed by being too isolated and by his
unwillingness to recognize results obtained by other workers. In 1866
he had discovered the staining reactions of potash and hypochlorite of
lime on certain thalli, and though these are at times unreliable owing
to growth conditions, etc., they have generally been of real service.
Nylander, however, never admitted any criticism of his methods; his
opinions once stated were never revised. He rejected absolutely the
theory of the dual nature of lichens propounded by Schwendener without
seriously examining the question, and regarded as personal enemies those
who dared to differ from him. The last years of his life were passed in
complete solitude. He died in March 1899.

Owing to the very inadequate powers of magnification at the service of
scientific workers, the study of lichens as of other plants was for long
restricted to the collecting, examining and classifying of specimens
according to their macroscopic characters; the microscopic details
observed were isolated and unreliable except to some extent for spore
characters. Special interest is therefore attached to the various schemes
of classification, as each new one proposed reflects to a large extent
the condition of scientific knowledge of the time, and generally marks an
advance. It was the improvement of the microscope from a scientific toy
to an instrument of research that opened up new fields of observation and
gave a new impetus to the study of a group of plants that had proved a
puzzle from the earliest times.

Tulasne was one of the pioneers in microscopic botany. He made a
methodical study of a large series of lichens[134] and traced their
development, so far as he was able, from the spore onwards. He gave
special attention to the form and function of spermogonia and spermatia,
and his work is enriched by beautiful figures of microscopic detail.
Lauder Lindsay[135] also published an elaborate treatise on spermogonia,
on their occurrence in the lichen kingdom and on their form and
structure. The paper embodies the results of wide microscopic research
and is a mine of information regarding these bodies.

Much interesting work was contributed at this time by Itzigsohn[136],
Speerschneider[137], Sachs[138], Thwaites[139], and others. They devoted
their researches to some particular aspect of lichen development and
their several contributions are discussed elsewhere in this work.

Schwendener[140] followed with a systematic study of the minute anatomy
of many of the larger lichen genera. His work is extremely important in
itself and still more so as it gradually revealed to him the composite
character of the thallus.

Several important monographs date from this period: Leighton[141]
reviewed all the British “Angiocarpous” lichens with special reference to
their “sporidia” though without treating these as of generic value. He
followed up this monograph by two others, on the _Graphideae_[142] and
the _Umbilicarieae_[143], and Mudd[144] published a careful study of the
_British Cladoniae_. On the Continent Th. Fries[145] issued a revision
of _Stereocaulon_ and _Pilophoron_ and other writers contributed work on
smaller groups.


H. PERIOD VII. 1867 AND AFTER

Modern lichenology begins with the enunciation of Schwendener’s[146]
theory of the composite nature of the lichen plant. The puzzling
resemblance of certain forms to algae, of others to fungi, had excited
the interest of botanists from a very early date, and the similarity
between the green cells in the thallus, and certain lower forms of algae
had been again and again pointed out. Increasing observation concerning
the life-histories of these algae and of the gonidia had eventually piled
up so great a number of proofs of their identity that Schwendener’s
announcement must have seemed to many an inevitable conclusion, though no
one before had hazarded the astounding statement that two organisms of
independent origin were combined in the lichen.

The dual hypothesis, as it was termed, was not however universally
accepted. It was indeed bitterly and scornfully rejected by some of the
most prominent lichenologists of the time, including Nylander[147], J.
Müller and Crombie[148]. Schwendener held that the lichen was a fungus
parasitic on an alga, and his opponents judged, indeed quite rightly,
that such a view was wholly inadequate to explain the biology of
lichens. It was not till a later date that the truer conception of the
“consortium” or “symbiosis” was proposed. The researches undertaken to
prove or disprove the new theories come under review in Chapter II.

Stahl’s work on the development of the carpogonium in lichens gave a new
direction to study, and notable work has been done during the last forty
years in that as in other branches of lichenology.

Exploration of old and new fields furnished the lichen-flora of the world
with many new plants which have been described by various systematists—by
Nylander, Babington, Arnold, Müller, Th. Fries, Stizenberger, Leighton,
Crombie and many others, and their contributions are scattered through
contemporary scientific journals. The number of recorded species is now
somewhere about 40,000, though, in all probability, many of these will be
found to be growth forms. Still, at the lowest computation, the number of
different species is very large.

Systematic literature has been enriched by a series of important
monographs, too numerous to mention here. While treating definite groups,
they have helped to elucidate some of the peculiar biological problems of
the symbiotic growth.

Morphology, since Schwendener’s time, has been well represented by Zukal,
Reinke, Lindau, Fünfstück, Darbishire, Hue, and by an increasing number
of modern writers whose work is duly acknowledged under each subject
of study. Hesse and Zopf, and more recently Lettau, have been engaged
in the examination of those unique products, the lichen acids, while
other workers have investigated lichen derivatives such as fats. Ecology
of lichens has also been receiving increased attention. Problems of
physiology, symbiosis, etc., are not yet considered to be solved and are
being attacked from various sides.

British lichenologists since 1867 have been mainly engaged on field work,
with the exception of Lauder Lindsay who published after that date a
second great paper on the spermogonia of crustaceous lichens. Leighton in
his _Lichen Flora_ and Crombie in numerous publications gave the lead in
systematic work, and with them were associated a band of indefatigable
collectors. Among these may be recalled Alexander Croall (1809-85), a
parish schoolmaster in Scotland whose _Plants of Braemar_ include many
of the rarer mountain lichens. Henry Buchanan Holl (1820-86), a surgeon
in London, collected in the Scottish Highlands as well as in England and
Wales. William Joshua (1828-98) worked mostly in the Western counties of
Somerset and Gloucestershire. Charles Du Bois Larbalestier, who died in
1911, was a keen observer and collector during many years; he discovered
a number of new species in his native Jersey, in Cambridgeshire and also
in Connemara; his plants were generally sent to Nylander to be determined
and described. He issued two sets of lichens, one of Channel Island
plants, the other of more general British distribution, and he had begun
the issue of Cambridgeshire lichens. Isaac Carroll (1828-80), an Irish
botanist, issued a first fascicle of _Lichenes Hibernici_ containing 40
numbers. More recently Lett[149] has reported 80 species and varieties
from the Mourne Mountains in Ireland. Other more extensive sets were
issued by Mudd and by Leighton, and later by Crombie and by Johnson.
All these have been of great service to the study of lichenology in our
country. Other collectors of note are Curnow (Cornwall), Martindale
(Westmoreland), and E. M. Holmes whose valuable herbarium has been
secured by University College, Nottingham.

The publication of the volume dealing with _Lichenes_ in Engler and
Prantl’s _Pflanzenfamilien_ has proved a boon to all who are interested
in the study of lichens. Fünfstück[150] prepared the introduction, an
admirable presentation of the morphological and physiological aspects of
the subject, while Zahlbruckner[151], with equal success, took charge of
the section dealing with classification.




CHAPTER II

CONSTITUENTS OF THE LICHEN THALLUS


I. LICHEN GONIDIA

[Illustration: Fig. 1. _Physcia aipolia_ Nyl. Vertical section of
thallus. _a_, cortex; _b_, algal layer; _c_, medulla; _d_, lower cortex.
× 100 (partly diagrammatic).]

The thallus or vegetative body of lichens differs from that of other
green plants in the sharp distinction both of form and colour between
the assimilative cells and the colourless tissues, and in the relative
positions these occupy within the thallus: in the greater number of
lichen species the green chlorophyll cells are confined to a narrow zone
or band some way beneath and parallel with the surface (Fig. 1); in a
minority of genera they are distributed through the entire thallus (Fig.
2); but in all cases the tissues remain distinct. The green zone can be
easily demonstrated in any of the larger lichens by scaling off the outer
surface cells, or by making a vertical section through the thallus. The
colourless cells penetrate to some extent among the green cells; they
also form the whole of the cortical and medullary tissues.

[Illustration: Fig. 2. _Collema nigrescens_ Ach. Vertical section of
thallus. _a_, chains of the alga _Nostoc_; _b_, fungal filaments. × 600.]

These two different elements we now know to consist of two distinct
organisms, a fungus and an alga. The green algal cells were at one time
considered to be reproductive bodies, and were called “gonidia,” a term
still in use though its significance has changed.


1. GONIDIA IN RELATION TO THE THALLUS


A. HISTORICAL ACCOUNT OF LICHEN GONIDIA

There have been few subjects of botanical investigation that have roused
so much speculation and such prolonged controversy as the question
of these constituents of the lichen plant. The green cells and the
colourless filaments which together form the vegetative structure are so
markedly dissimilar, that constant attempts have been made to explain
the problem of their origin and function, and thereby to establish
satisfactorily the relationship of lichens to other members of the Plant
Kingdom.

In gelatinous lichens, represented by _Collema_, of which several species
are common in damp places and grow on trees or walls or on the ground,
the chains of green cells interspersed through the thallus have long been
recognized as comparable with the filaments of _Nostoc_, a blue-green
gelatinous alga, conspicuous in wet weather in the same localities as
those inhabited by _Collema_. So among early systematists, we find
Ventenat[152] classifying the few lichens with which he was acquainted
under algae and hazarding the statement that a gelatinous lichen such
as _Collema_ was only a _Nostoc_ changed in form. Some years later
Cassini[153] in an account of _Nostoc_ expressed a somewhat similar view,
though with a difference: he suggested that _Nostoc_ was but a monstrous
form of _Collema_, his argument being that, as the latter bore the fruit,
it was the normal and perfect condition of the plant. A few years later
Agardh[154] claimed to have observed the metamorphosis of _Nostoc_ up
to the fertile stage of a lichen, _Collema limosum_. But long before
this date, Scopoli[155] had demonstrated a green colouring substance
in non-gelatinous lichens by rubbing a crustaceous or leprose thallus
between the fingers; and Persoon[156] made use of this green colour
characteristic of lichen crusts to differentiate these plants from fungi.
Sprengel[157] went a step further in exactly describing the green tissue
as forming a definite layer below the upper cortex of foliaceous lichens.

The first clear description and delimitation of the different elements
composing the lichen thallus was, however, given by Wallroth[158]. He
drew attention to the great similarity between the colourless filaments
of the lichen and the hyphae of fungi. The green globose cells in the
chlorophyllaceous lichens he interpreted as brood-cells or gonidia,
regarding them as organs of reproduction collected into a “stratum
gonimon.” To the same author we owe the terms “homoiomerous” and
“heteromerous,” which he coined to describe the arrangement of these
green cells in the tissue of the thallus. In the former case the gonidia
are distributed equally through the structure; in the latter they are
confined to a distinct zone.

Wallroth’s terminology and his views of the function of the gonidia
were accepted as the true explanation for many years, the opinion that
they were solely reproductive bodies being entirely in accordance with
the well-known part played by soredia in the propagation of lichens—and
soredia always include one or more green cells.


B. GONIDIA CONTRASTED WITH ALGAE

In describing the gonidia of the Graphideae Wallroth[159] had pointed
out their affinity with the filaments of _Chroolepus_ (_Trentepohlia_)
_umbrina_. He considered these and other green algae when growing loose
on the trunks of trees to be but “unfortunate brood-cells” which had
become free and, though capable of growth and increase, were unable to
form again a lichen plant.

Further observations on gonidia were made by E. Fries[160]: he found
that the green cells escaped from the lichen matrix and produced new
individuals; and also that the whole thallus in moist localities might
become dissolved into the alga known as _Protococcus viridis_; but, he
continues, “though these _Protococcus_ cells multiplied exceedingly,
they never could rise again to the perfect lichen.” Kützing[161], in a
later account of _Protococcus viridis_, also recognized its affinity with
lichens; he stated that he could testify from observation that, according
to the amount of moisture present, it would develop, either in excessive
moisture to a filamentous alga, or in drier conditions “to lichens such
as _Lecanora subfusca_ or _Xanthoria parietina_.”

[Illustration: Fig. 3. _Coenogonium ebeneum_ A. L. Sm. Tip of lichen
filament, the alga overgrown by dark fungal hyphae × 600.]

A British botanist, G. H. K. Thwaites[162], at one time superintendent
of the botanical garden at Peradeniya in Ceylon, published a notable
paper on lichen gonidia in which he pointed out that as in _Collema_ the
green constituents of the thallus resembled the chains of _Nostoc_, so in
the non-gelatinous lichens, the green globose cells were comparable or
identical with _Pleurococcus_, and Thwaites further observed that they
increased by division within the lichen thallus. He insisted too that
in no instance were gonidia reproductive organs: they were essential
component parts of the vegetative body and necessary to the life of the
plant. In a further paper on _Chroolepus ebeneus_ Ag., a plant consisting
of slender dark-coloured felted filaments, he described these filaments
as being composed of a central strand which closely resembled the alga
_Chroolepus_, and of a surrounding sheath of dark-coloured cells (Fig.
3): “occasionally,” he writes, “the internal filament protrudes beyond
the investing sheath, and may then be seen to consist of oblong cells
containing the peculiar reddish oily-looking endochrome of _Chroolepus_.”
Thwaites placed this puzzling plant in a new genus, _Cystocoleus_, at the
same time pointing out its affinity with the lichen genus _Coenogonium_.
The plant is now known as _Coenogonium ebeneum_. Thwaites was on the
threshold of the discovery as to the true nature of the relationship
between the central filament and the investing sheath, but he failed to
take the next forward step.

Very shortly after, Von Flotow[163] published his views on some other
lichen gonidia. He had come to the conclusion that the various species
of the alga, _Gloeocapsa_, so frequently found in damp places, among
mosses and lichens, were merely growth stages of the gonidia of _Ephebe
pubescens_, and bore the same relation to _Ephebe_ as did _Lepra viridis_
(_Protococcus_) to _Parmelia_. The gonidium of _Ephebe_ is the gelatinous
filamentous blue-green alga _Stigonema_ (Fig. 4), and the separate cells
are not unlike those of _Gloeocapsa_. Flotow had also demonstrated
that the same type of gonidium was enclosed in the cephalodia of
_Stereocaulon_. Sachs[164], too, gave evidence as to the close connection
between _Nostoc_ and _Collema_. He had observed numerous small clumps of
the alga growing in proximity to equally abundant thalli of _Collema_,
with every stage of development represented from one to the other. He
found cases where the gelatinous coils of _Nostoc_ chains were penetrated
by fine colourless filaments “as if invaded by a parasitic fungus.” Later
these threads were seen to be attached to some cell of the _Nostoc_
trichome. Sachs concluded, however, from very careful examination at the
time, that the colourless filaments were produced by the green cells.
As growth proceeded, the coloured _Nostoc_ chains became massed towards
the upper surface, while the colourless filaments tended to occupy the
lower part of the thallus. He calculated that during the summer season
the metamorphosis from _Nostoc_ to a fertile _Collema_ thallus took from
three to four months. He judged that in favourable conditions the change
would inevitably take place, though if there should be too great moisture
no _Collema_ would be formed. His study of _Cladonia_ was less successful
as he mistook some colonies of _Gloeocapsa_ for a growth condition of
_Cladonia_ gonidia, an error corrected later by Itzigsohn[165].

[Illustration: Fig. 4. _Ephebe pubescens_ Nyl. Tip of lichen filament ×
600.]

But before this date Itzigsohn[166] had published a paper setting forth
his views on thallus formation, which marked a distinct advance. He did
not hazard any theory as to the origin of gonidia, but he had observed
spermatia growing, much as did the cells of _Oscillaria_: by increase
in length, and, by subsequent branching, filaments were formed which
surrounded the green cells; these latter had meanwhile multiplied by
repeated division till finally a complete thallus was built up, the
filamentous tissue being derived from the spermatia, while the green
layer came from the original gonidium. In contrasting the development
with that of _Collema_, he represents _Nostoc_ as a sterile product of a
lichen and, like the gonidia of other lichens, only able to form a lichen
thallus when it encounters the fructifying spermatia.

Braxton Hicks[167], a London doctor, some time later, made experiments
with _Chroococcus_ algae which grew in plenty on the bark of trees, and
followed their development into a lichen thallus. He further claimed to
have observed a _Chlorococcus_, which was associated with a _Cladonia_,
divide and form a _Palmella_ stage.


C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS

It had been repeatedly stated that the gonidia might become independent
of the thallus, but absolute proof was wanting until Speerschneider[168],
who had turned his attention to the subject, made direct culture
experiments and was able to follow the liberation of the green cells.
He took a thinnish section of the thallus of _Hagenia_ (_Physcia_)
_ciliaris_, and, by keeping it moist, he was able to observe that
the gonidial cells increased by division; the moist condition at the
same time caused the colourless filaments to die away. This method of
investigation was to lead to further results. It was resorted to by
Famintzin and Baranetzky[169] who made cultures of gonidia extracted
from three different lichens, _Physcia_ (_Xanthoria_) _parietina_,
_Evernia furfuracea_ and _Cladonia_ sp. They were able to observe the
growth and division of the green cells and, in addition, the formation
of zoospores. They recognized the development as entirely identical
with that of the unicellular green alga, _Cystococcus humicola_ Naeg.
Baranetzky[170] continued the experiments and made cultures of the
blue-green gonidia of _Peltigera canina_ and of _Collema pulposum_. In
both instances he succeeded in isolating them from the thallus and in
growing them in moist air as separate organisms. He adds that “many forms
reckoned as algae, may be considered as vegetating lichen gonidia such
as _Cystococcus_, _Polycoccus_, _Nostoc_, etc.” Meanwhile Itzigsohn[171]
had further demonstrated by similar culture experiments that the gonidia
of _Peltigera canina_ corresponded with the algae known as _Gloeocapsa
monococca_ Kütz., and as _Polycoccus punctiformis_ Kütz.


D. THEORIES AS TO THE ORIGIN OF GONIDIA

Though the relationship between the gonidia within the thallus and
free-living algal organisms seemed to be proved beyond dispute, the
manner in which gonidia first originated had not yet been discovered.
Bayrhoffer[172] attacked this problem in a study of foliose and other
lichens. According to his observations, certain colourless cells or
filaments, belonging to the “gonimic” layer, grew in a downward direction
and formed at their tips a faintly yellowish-green cell; it gradually
enlarged and was at length thrown off as a free globose gonidium, which
represented the female cell. Other filaments from the “lower fibrous
layer” of the thallus at the same time grew upwards and from them were
given off somewhat similar gonidia which functioned as male cells. His
observations and deductions were fanciful, but it must be remembered
that the attachment between hypha and alga in lichens is in many cases
so close as to appear genetic, and also it often happens that as the
gonidium multiplies it becomes free from the hypha.

In his _Mémoire sur les Lichens_, Tulasne[173] described the colourless
filaments as being fungal in appearance. The green cells he recognized
as organs of nutrition, and once and again in his paper he states that
they arose directly by a sort of budding process from the medullary or
cortical filaments, either laterally or at the apex. This apparently
reasonable view of their origin was confirmed by other writers on the
subject: by Speerschneider[174] in his account of the anatomy of _Usnea
barbata_, by de Bary[175], and by Schwendener[176] in their earlier
writings. But even while de Bary accepted the hyphal origin of the
gonidia, he noted[177] that, accompanying _Opegrapha atra_ and other
Graphideae, on the bark were to be found free _Chroolepus_ cells similar
to the gonidia in the lichen thallus. He added that gonidia of certain
other lichens in no way differed from _Protococcus_ cells; and as for
the gelatinous lichens he declared that “either they were the perfect
fruiting form of Nostocaceae and Chroococcaceae—hitherto looked on
as algae—or that these same Nostocaceae and Chroococcaceae are algae
which take the form of _Collema_, _Ephebe_, etc., when attacked by an
ascomycetous fungus.”

All these investigators, and other lichenologists such as Nylander[178],
still regarded the free-living organisms identified by them as similar to
the green cells of the thallus, as only lichen gonidia escaped from the
matrix and vegetating in an independent condition.

The old controversy has in recent years been unexpectedly reopened
by Elfving[179] who has sought again to prove the genetic origin of
the green cells. His method has been to examine a large series of
lichens by making sections of the growing areas, and he claims to have
observed in every case the hyphal origin of the gonidia: not only of
_Cystococcus_ but also of _Trentepohlia_, _Stigonema_ and _Nostoc_.
In the case of _Cystococcus_, the gonidium, he says, arises by the
swelling of the terminal cell of the hypha to a globose form, and by the
gradual transformation of the contents to a chlorophyll-green colour,
with power of assimilation. In the case of filamentous gonidia such
as _Trentepohlia_, the hyphal cells destined to become gonidia are
intercalary. In _Peltigera_ the cells of the meristematic plectenchyma
become transformed to blue-green _Nostoc_ cells.

A study was also made by him of the formation of cephalodia[180], the
gonidia of which differ from those of the “host” thallus. In _Peltigera
aphthosa_ he claims to have traced the development of these bodies to
the branching and mingling of the external hairs which, in the end, form
a ball of interwoven hyphae. The central cells of the ball are then
gradually differentiated into _Nostoc_ cells, which increase to form
the familiar chains. Elfving allows that the gonidia mainly increase by
division within the thallus, and that they also may escape and live as
free organisms. His views are unsupported by direct culture experiments
which are the real proof of the composite nature of the thallus.


E. MICROGONIDIA

Another attempt to establish a genetic origin for lichen gonidia was made
by Minks[181]. He had found in his examination of _Leptogium myochroum_
that the protoplasmic contents of the hyphae broke up into a regular
series of globular corpuscles which had a greenish appearance. These
minute bodies, called by him microgonidia, were, he states, at first few
in number, but gradually they increased and were eventually set free by
the mucilaginous degeneration of the cell wall. As free thalline gonidia,
they increased in size and rapidly multiplied by division. Minks was
at first enthusiastically supported by Müller[182] who had found from
his own observations that microgonidia might be present in any of the
lichen hyphae and in any part of the thallus, even in the germinating
tube of the lichen spore, and was in that case most easily seen when the
spores germinated within the ascus. He argued that as spores originated
within the ascus, so microgonidia were developed within the hyphae.
Minks’s theories were however not generally accepted and were at last
wholly discredited by Zukal[183] who was able to prove that the greenish
bodies were contracted portions of protoplasm in hyphae that suffered
from a lowered supply of moisture, the green colour not being due to any
colouring substance, but to light effect on the proteins—an outcome of
special conditions in the vegetative life of the plant. Darbishire[184]
criticized Minks’s whole work with great care and he has arrived at the
conclusion that the microgonidium may be dismissed as a totally mistaken
conception.


F. COMPOSITE NATURE OF THALLUS

Schwendener[185] meanwhile was engaged on his study of lichen anatomy.
Though at first he adhered to the then accepted view of the genetic
connection between hyphae and gonidia, his continued examination of the
vegetative development led him to publish a short paper[186] in which
he announced his opinion that the various blue-green and green gonidia
were really algae and that the complete lichen in all cases represented
a fungus living parasitically on an alga: in _Ephebe_, for example, the
alga was a form of _Stigonema_, in the Collemaceae it was a species of
_Nostoc_. In those lichens enclosing bright green cells, the gonidia were
identical with _Cystococcus humicola_, while in _Graphideae_ the brightly
coloured filamentous cells were those of _Chroolepus_ (_Trentepohlia_).
This statement he repeated in an appendix to the larger work on
lichens[187] and again in the following year[188] when he described more
fully the different gonidial algae and the changes produced in their
structure and habit by the action of the parasite: “though eventually the
alga is destroyed,” he writes, “it is at first excited to more vigorous
growth by contact with the fungus, and in the course of generations may
become changed beyond recognition both in size and form.” In support of
his theory of the composite constitution of the thallus, Schwendener
pointed out the wide distribution and frequent occurrence in nature
of the algae that become transformed to lichen gonidia. He claimed as
further proof of the presence of two distinct organisms that, while the
colourless filaments react in the same way as fungi on the application of
iodine, the gonidia take the stain of algal membranes.


G. SYNTHETIC CULTURES

Schwendener’s “dual hypothesis,” as it was termed, excited great interest
and no little controversy, the reasons for and against being debated
with considerable heat. Rees[189] was the first who attempted to put the
matter to the proof by making synthetic cultures. For this purpose he
took spores from the apothecium of a _Collema_ and sowed them on pure
cultures of _Nostoc_, and as a result obtained the formation of a lichen
thallus, though he did not succeed in producing any fructification. He
observed further that the hyphal filaments from the germinating spore
died off when no _Nostoc_ was forthcoming.

Bornet[190] followed with his record of successful cultures. He selected
for experiment the spores of _Physcia_ (_Xanthoria_) _parietina_ and was
able to show that hyphae produced from the germinating spore adhered
to the free-growing cells of _Protococcus[191] viridis_ and formed
the early stages of a lichen thallus. Woronin[192] contributed his
observations on the gonidia of _Parmelia_ (_Physcia_) _pulverulenta_
which he isolated from the thallus and cultivated in pure water. He
confirmed the occurrence of cell division in the gonidia and also the
formation of zoospores, these again forming new colonies of algae
identical in all respects with the thalline gonidia. He was able to see
the germinating tube from a lichen spore attach itself to a gonidium,
though he failed in his attempts to induce further growth. In our own
country Archer[193] welcomed the new views on lichens, and attempted
cultures but with very little success. Further synthetic cultures were
made by Bornet[194], Treub[195] and Borzi[196] with a series of lichen
spores. They also were able to observe the first stages of the thallus.
Borzi observed spores of _Physcia_ (_Xanthoria_) _parietina_ scattered
among _Protococcus_ cells on the branch of a tree. The spores had
germinated and the first branching hyphae had already begun to encircle
the algae.

[Illustration: Fig. 5. _Endocarpon pusillum_ Hedw. Asci and spores, with
hymenial gonidia × 320 (after Stahl).]

[Illustration: Fig. 6. _Endocarpon pusillum_ Hedw. Spore germinating in
contact with hymenial gonidia × 320 (after Stahl).]

Additional evidence in favour of the theory of the independent origin
of the colourless filaments and the green cells was furnished by
Stahl’s[197] research on hymenial gonidia in _Endocarpon_ (Fig. 5). By
making synthetic cultures of the mature spores with these bodies, he was
able to observe not only the germination of the spores and the attachment
of the filaments to the gonidia (Fig. 6), but also the gradual building
up of a complete lichen thallus to the formation of perithecia and spores.

[Illustration: Fig. 7. Germination of spore of _Physcia parietina_ De
Not. in contact with _Protococcus viridis_ Ag. × 950 (after Bornet).]

[Illustration: Fig. 8. _Physcia parietina_ De Not. Vertical section of
thallus obtained by synthetic culture × 130 (after Bonnier).]

Some years later Bonnier[198] made an interesting series of synthetic
cultures between the spores of lichens germinated in carefully sterilized
conditions, and algae taken from the open (Figs. 7 and 8). Separate
control cultures of spores and algae were carried on at the same time,
with the result that in one case lichen hyphae alone, in the other algae
were produced. The various lichen spores with which he experimented were
sown in association with the following algae:

(1) PROTOCOCCUS.

Pure synthetic cultures of _Physcia_ (_Xanthoria_) _parietina_ were begun
in August 1884 on fragments of bark. In October 1886 the thallus was
several centimetres in diameter, and some of the lobes were fruited.

_Physcia stellaris_ was also grown on bark; in one case both thallus and
apothecia were developed.

_Parmelia acetabulum_, another corticolous species, formed only a minute
thallus about 5 mm. in diameter, but entirely identical with normally
growing specimens.

(2) PLEUROCOCCUS.

_Lecanora_ (_Rinodina_) _sophodes_, sown on rock in 1883, reached in 1886
a diameter of 13 mm. with fully developed apothecia.

_Lecanora ferruginea_ and _L. subfusca_ after three years’ culture formed
sterile thalli only.

_Lecanora coilocarpa_ in four years, and _L. caesio-rufa_ in three years
formed very small thalli without fructification.

(3) TRENTEPOHLIA (Chroolepus).

_Opegrapha vulgata_ in two years had developed thallus and apothecia. The
control culture of the spores formed, as in nature, a considerable felt
of mycelium in the interstices of the bark, but no pycnidia or apothecia.

_Graphis elegans._ Only the beginning of a differentiated thallus was
obtained with this species.

_Verrucaria muralis_ (?)[199] gave in less than a year a completely
developed thallus.

Bonnier also attempted cultures with species of _Collema_ and _Ephebe_,
but was unsuccessful in inducing the formation of a lichen plant.


H. HYMENIAL GONIDIA

Reference has already been made to the minute green cells which were
originally described by Nylander[200] as occurring in the perithecia
of a few Pyrenolichens as free gonidia, _i.e._ unentangled with lichen
hyphae. Fuisting[201] found them in the perithecium of _Polyblastia_
(_Staurothele_) _catalepta_ at a very early stage of its development
when the perithecial tissues were newly differentiated from those of the
surrounding thallus. The gonidia enclosed in the perithecium differed in
no wise from those of the thallus: they had become mechanically enclosed
in the new tissue; and while those in the outer compact layers died off,
those in the centre of the structure, where a hollow space arises, were
subject to very active division, becoming smaller in the process and
finally filling the cavity. Winter’s[202] researches on similar lichens
confirmed Fuisting’s conclusions: he described them as similar to the
thalline gonidia but lighter in colour and of smaller size, measuring
frequently only 2·3 µ in diameter, though this size increased to about 7
µ when cultivated outside the perithecium.

Stahl[203] sufficiently demonstrated the importance of these gonidia in
supplying the germinating spores with the necessary algae. They come to
lie in vertical rows between the asci and, owing to pressure, assume
an elongate form[204] (Figs. 5 and 6). They have been seen in very few
lichens, in _Endocarpon_ and _Staurothele_, both rather small genera of
Pyrenolichens, and, so far as is known, in two Discolichens, _Lecidea
phylliscocarpa_ and _L. phyllocaris_, the latter recorded from Brazil by
Wainio[205], and, on account of the inclusion of gonidia in the hymenium,
placed by him in a section, _Gonothecium_.


I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS

_a._ CONSORTIUM AND SYMBIOSIS. These cultures had established
convincingly the composite nature of the lichen thallus, and
Schwendener’s opinion, that the relationship between the two organisms
was some varying degree of parasitism, was at first unhesitatingly
accepted by most botanists. Reinke[206] was the first to point out
the insufficiency of this view to explain the long continued healthy
life of both constituents, a condition so different from all known
instances of the disturbing or fatal parasitism of one individual on
another. He recognized in the association a state of mutual growth and
interdependence, that had resulted in the production of an entirely new
type of plant, and he suggested _Consortium_ as a truer description of
the connection between the fungus and the alga. This term had originally
been coined by his friend Grisebach in a paper[206] describing the
presence of actively growing _Nostoc_ algae in healthy _Gunnera_ stems;
and Reinke compared that apparently harmless association with the similar
phenomenon in the lichen thallus. The comparison was emphasized by him
in a later paper[207] on the same subject, in which he ascribes to
each “consort” its function in the composite plant, and declares that
if such a mutual life of Alga and Ascomycete is to be regarded as one
of parasitism, it must be considered as reciprocal parasitism; and he
insists that “much more appropriate for this form of organic life is the
conception and title of _Consortium_.” In a special work on lichens,
Reinke[208] further elaborated his theory of the physiological activity
and mutual service of the two organisms forming the consortium.

Frank[209] suggested the term _Homobium_ as appropriate, but it is faulty
inasmuch as it expresses a relationship of complete interdependence, and
it has been proved that the fungus partly, and the alga entirely, have
the power of free growth.

A wider currency was given to this view of a mutually advantageous growth
by de Bary[210]. He followed Reinke in refusing to accept as satisfactory
the theory of simple parasitism, and adduced the evident healthy life of
the algal cells—the alleged victims of the fungus—as incompatible with
the parasitic condition. He proposed the happily descriptive designation
of a _Symbiosis_ or conjoint life which was mostly though not always, nor
in equal degree, beneficial to each of the partners or symbionts.

_b._ DIFFERENT FORMS OF ASSOCIATION. The type of association between the
two symbionts varies in different lichens. Bornet[211], in describing the
development of the thallus in certain members of the Collemaceae, found
that though as a rule the two elements of the thallus, as in some species
of _Collema_ itself, persisted intact side by side, there was in other
members of the genus an occasional parasitism: short branches from the
main hyphae applied themselves by their tips to some cell of the _Nostoc_
chain (Fig. 9). The cell thus seized upon began to increase in size, and
the plasma became granular and gathered at the side furthest away from
the point of attachment. Finally the contents were used up, and nothing
was left but an empty membrane adhering to the fungus hypha. In another
species the hypha penetrated the cell. These instances of parasitism are
most readily seen towards the edge of the thallus where growth is more
active; towards the centre the attached cells have become absorbed, and
only the shortened broken chains attest their disappearance. The other
cells of the chains remain uninjured.

[Illustration: Fig. 9. _Physma chalazanum_ Arn. Cells of _Nostoc_ chains
penetrated and enlarged by hyphae × 950 (after Bornet).]

In _Synalissa_, a small shrubby gelatinous genus, the hypha, as described
by Bornet and by Hedlund[212], pierces the outer wall of the gelatinous
alga (_Gloeocapsa_) and swells inside to a somewhat globose haustorium
which rests in a depression of the plasma (Fig. 10). The alga, though
evidently undamaged, is excited to a division which takes place on a
plane that passes through the haustorium; the two daughter-cells then
separate, and in so doing free themselves from the hypha.

[Illustration: Fig. 10. _Synalissa symphorea_ Nyl. Algae (_Gloeocapsa_)
with hyphae from the internal thallus × 480 (after Bornet).]

Hedlund followed the process of association between the two organisms
in the lichens _Micarea_ (_Biatorina_) _prasina_ and _M. denigrata_
(_Biatorina synothea_), crustaceous species which inhabit trunks of trees
or palings. In these the alga, one of the Chlorophyceae, has assumed the
character of a _Gloeocapsa_ but on cultivation it was found to belong
to the genus _Gloeocystis_. The cells are globose and rather small;
they increase by the division of the contents into two or at most four
portions which become rounded off and covered with a membrane before
they become free from the mother-cell. The lichen hypha, on contact with
any one of the green cells, bores through the outer membrane and swells
within to a haustorium, as in the gonidia of _Synalissa_.

[Illustration: Fig. 11. Gonidia from _Ramalina reticulata_ Nyl. A,
gonidium pierced and cell contents shrinking × 560; B, older stage, the
contents of gonidium exhausted × 900 (after Peirce).]

[Illustration: Fig. 12. _Pertusaria globulifera_ Nyl. Fungus and gonidia
from gonidial zone × 500 (after Darbishire).]

Penetrating haustoria were demonstrated by Peirce[213] in his study of
the gonidia of _Ramalina reticulata_. In the first stage the tip of a
hypha had pierced the outer wall of the alga, causing the protoplasm to
contract away from the point of contact (Fig. 11). More advanced stages
showed the extension of the haustorium into the centre of the cell, and,
finally, the complete disappearance of the contents. In many cases it was
found that penetration equally with clasping of the alga by the filament
sets up an irritation which induces cell-division, and the alga, as in
_Synalissa_, thus becomes free from the fungus. Hue[214] has recorded
instances of penetration in an Antarctic species, _Physcia puncticulata_.
It is easy, he says, to see the tips of the hyphae pierce the sheath of
the gonidium and penetrate to the nucleus.

Lindau[215] has described the association between fungus and alga in
_Pertusaria_ and other crustaceous forms as one of contact only (Fig.
12). He found that the cell-membrane of the two adhering organisms was
unbroken. Occasionally the algal cell showed a slight indentation, but
was otherwise unchanged. The hyphal branch was somewhat swollen at the
tip where it touched the alga, and the wall was slightly thinner. The
attachment between the two cells was so close, however, that pressure on
the cover-glass failed to separate them.

Generally the hypha simply surrounds the gonidium with clasping branches.
Many algae also lie free in the gonidial zone, and Peirce[216] claims
that these are larger, more deeply coloured and in every way healthier
looking than those in the grasp of the fungus. He ignores, however, the
case of the soredial algae which though very closely invested by the
fungus are yet entirely healthy, since on their future increase depends
in many cases the reproduction of new individual lichens.

In a recent study of a crustaceous sandstone lichen, “_Caloplaca
pyracea_,” Claassen[217] has sought to prove a case of pure parasitism.
The rock was at first covered with the green cells of _Cystococcus_ sp.
Later there appeared greyish-white patches on the green, representing the
invasion of the lichen fungus. These patches increased centrifugally,
leaving in time a bare patch in the centre of growth which was again
colonized by the green alga. The lichen fruited abundantly, but wherever
it encroached the green cells were more or less destroyed. The true
explanation seems to be that the green cells were absorbed into the
lichen thallus, though enough of them persisted to start new colonies
on any bare piece of the stone. In the same way large patches of
_Trentepohlia aurea_ have been observed to be gradually invaded by the
dark coloured hyphae of _Coenogonium ebeneum_. In time the whole of the
alga is absorbed and nothing is to be seen but the dark felted lichen.
The free alga as such disappears, but it is hardly correct to describe
the process as one of destruction.

This algal genus _Trentepohlia_ (_Chroolepus_) forms the gonidia of
the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which
increases by apical growth. In the Graphideae, many of which grow on
trees beneath the outer bark (hypophloeodal), the association between the
two symbionts may be of the simplest character, but was considered by
Frank[218] to be of an advanced type. According to his observations and
to those of Lindau[219], the fungal hyphae penetrate first between the
cells of the periderm. The alga, frequently _Trentepohlia umbrina_, tends
to grow down into any cracks of the surface. It goes more deeply in when
preceded by the hyphae. In some species both organisms maintain their
independent growth, though each shows increased vigour when it comes into
contact with the other. In some instances the cells of the alga are
clasped by the fungus which causes the disintegration of the filament.
The cells lose their bright yellow or reddish colour and are changed in
appearance to greenish lichen gonidia; but no penetration by haustoria
has ever been observed in _Trentepohlia_.

Bachmann’s[220] study of a similar gonidium in a calcicolous species of
_Opegrapha_ confirms Frank’s results. The algae had pierced not only
between the looser lime granules but also through a crystal of calcium
carbonate, and occupied nests scooped out in the rock by means of acid
formed and excreted by their filaments. When association took place with
the fungus, the algal cells were more restricted to a gonidial zone; but
some of the cells, having been pushed aside by the hyphae, had started
new centres of gonidia. On contact with the hyphae there was a tendency
to bud out in a yeast-like growth.

In the thallus of the Roccelleae, the algal filament, also a
_Trentepohlia_, is broken up into separate cells, but in the
Coenogoniaceae, whether the gonidium be a _Cladophora_ as in _Racodium_,
or a _Trentepohlia_ as in _Coenogonium_, the filaments remain intact and
are invested more or less closely by the hyphae.

[Illustration: Fig. 13. Outer edge of _Phycopeltis expansa_ Jenn., the
alga attacked by hyphae and passing into separate gonidia × 500 (after
Vaughan Jennings).]

A somewhat different type of association takes place between alga and
fungus in _Strigula complanata_, an epiphyllous lichen more or less
common in tropical regions. Cunningham[221], who found it near Calcutta,
described the algal constituent and placed it in a new genus, _Mycoidea_
(_Cephaleuros_). It forms small plate-like expansions on the surface
of the leaf, and also penetrates below the cuticle, burrowing between
that and the epidermal cells; occasionally, as observed by Cunningham,
rhizoid-like growths pierce deeper into the tissue—into and below the
epidermal layer. Very frequently, in the wet season, a fungus takes
possession of the alga and slender colourless hyphae creep along its
surface by the side of the cell rows, sending out branches which grow
downwards. Marshall Ward[222] described the same lichen from Ceylon. He
states that the alga may be attacked at any stage, and if it is in a very
young condition it is killed by the fungus; at a more advanced period of
growth it continues to develop as an integral part of the lichen thallus,
but with more frequently divided and smaller cells. Vaughan Jennings[223]
observed _Strigula complanata_ in New Zealand associated with a closely
allied chroolepoid alga _Phycopeltis expansa_. He also noted the growth
of the fungus over the alga breaking up the plates of tissue and
separating the cells which, from yellow, change to a green colour and
become rounded off (Fig. 13). The mature lichen, a white thallus dotted
with black fruits, contrasts strikingly with the yellow membranous alga.
Lichen formation usually begins near the edge of the leaf and the margin
of the thallus itself is marked by a green zone showing where the fungus
has recently come into contact with the alga.

More recently Hans Fitting[224] has described “_Mycoidea parasitica_”
as it occurs on evergreen leaves in Java. The alga, a species of
_Cephaleuros_, though at first an epiphyte, becomes partially parasitic
at maturity. It penetrates below the cuticle to the outer epidermal cells
and may even reach the tissue below. When it is joined by the lichen
fungus, both constituents grow together to form the lichen. Fitting adds
that the leaf is evidently but little injured. In this lichen the alga in
the grip of the fungus loses its independence and may be killed off: it
is an instance of something like intermittent parasitism.


J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM

No hyphal penetration of the bright-green algal cell by means of
haustoria had been observed by the earlier workers, Bornet[225],
Bonnier[226] and others, though they followed Schwendener[227] in
regarding the relationship as one of host and parasite. Lindau, also,
after long study accepted parasitism as the only adequate explanation of
the associated growth, though he never found the fungus actually preying
on the alga.

In recent years interest in the subject has been revived by the
researches of Elenkin[228], a Russian botanist who claims to have
established a case for parasitism or rather “endosaprophytism.” He has
demonstrated by means of staining reagents the presence in the thallus
of large numbers of dead algal cells. A few empty membranes are to be
found in the cortex and in the gonidial zone, but the larger proportion
occur below the gonidial zone and partly in the medulla. He describes
the lower layer as a “necral” or “hyponecral” zone, and he considers
that the hyphae draw their nourishment chiefly from dead algal material.
The fungus must therefore be regarded in this case as a saprophyte
rather than a parasite. The algae, he considers, may have perished
from want of sufficient light and air or they may have been destroyed
by an enzyme produced by the fungus. The latter he thinks is the more
probable, as dead cells are frequently present among the living algae
of the gonidial zone. To the action of the enzyme he also attributes
the angular deformed appearance of many gonidia and the paler colour
and gradual disintegration of their contents which are finally used up
as endosaprophytic nourishment by the fungus. Dead algal cells were
more easily seen, he tells us, in crustaceous lichens associated with
“_Pleurococcus_” or “_Cystococcus_”; they were much less frequent in the
larger foliose or fruticose lichens. Dead cells of _Trentepohlia_ were
also difficult to find.

In a second paper Elenkin records one clear instance of a haustorium
entering an algal cell, and says he found some evidence of hyphal
branches penetrating otherwise uninjured gonidia, round holes being
visible in their outer wall, but he holds that it is the cell-wall of the
alga that is mainly dissolved by the ferment and then used as food by the
hyphae.

No allowance has been made by Elenkin for the normal wasting common to
all organic beings: the lichen fungus is continually being renewed,
especially in the cortical structures, and the alga must also be subject
to change. He[229] claims, nevertheless, that his observations have
proved that the one symbiont is always preying on the other, either as a
parasite or as a saprophyte. He has likened the conception of symbiosis
to that of a balance between two organisms, “a moveable equilibrium
of the symbionts.” If, he says, we could conceive a state where the
conditions of life would be equally favourable for both partners there
would be true mutualism, but in practice one only is favoured and gains
the upper hand, using its advantage to prey on the other. Unless the
balance is redressed, the complete destruction of the weaker is certain,
and is followed in time by the death of the stronger. The fungus being
the dominant partner, the balance, he considers, is tipped in its favour.

Elenkin’s conclusions are not borne out by the long continued and
healthy life of the lichen. There is no record of either symbiont having
succumbed to the other, and the alga, when set free, is unchanged and
able to resume its normal development. Without the alga the fungus cannot
form the ascigerous fruit. Is that because as a parasite within the
lichen it has degenerated past recovery, or has it become so adapted to
symbiosis that in saprophytic conditions it fails to develop?

Another Russian lichenologist, U. N. Danilov[230], records results which
would seem to support the theory of parasitism. He found that from the
clasping hyphae minute haustoria were produced, which penetrate the algal
cell-wall, and branch when within the outer membrane, thus forming a
delicate network over the plasma; secondary haustoria arising from this
network protrude into the interior and rob the cell-contents. He observed
gonidia filled with well-developed hyphae and these, after having
exhausted one cell, travel onwards to others. Some gonidia under the
influence of the fungus had become deformed and were finally killed. As
a proof of this latter statement he adduces the presence in the thallus
of some gonidia containing shrivelled protoplasm, of others entirely
empty. He considers, as further evidence in favour of parasitism, the
finding of empty membranes as well as of colourless gonidia filled
with the hyphal network. This description hardly tallies with the usual
healthy appearance of the gonidial zone in the normal thallus, and it has
been suggested that where the fungus filled the algal cell, it was as a
saprophyte preying on dead material.

The gradual perishing of algal cells in time by natural decay and their
subsequent absorption by the fungus is undisputed. It is open to question
whether the varying results recorded by these workers have any further
significance.

These observations of Elenkin and Danilov have been proved to be
erroneous by Paulson and Somerville Hastings[231]. They examined the
thalli of several lichens (_Xanthoria parietina_, _Cladonia_ sp., etc.)
collected in early spring when vegetative growth in these plants was
found to be at its highest activity. They found an abundant increase of
gonidia within the thallus, which they regarded as sporulation of the
algae, and the most careful methods of staining failed to reveal any case
of penetration of the gonidia by the hyphae.

Nienburg[232] has published some recent observations on the association
of the symbionts. In the wide cortex of a _Pertusaria_ he found not only
the densely compact hyphae, but also isolated gonidia. In front of these
latter there was a small hollow cavity and, behind, parallel hyphae
rich in contents. These gonidia had originated from the normal gonidial
zone. They were moved upward by special hyphae called by Nienburg
“push-hyphae.” After their transportation, the algae at once divide and
the products of division pass to a resting stage and become the centre
of a new thalline growth. A somewhat similar process was noted towards
the apex of _Evernia furfuracea_. Radial hyphae pushed up the cortex,
leaving a hollow space over the gonidial zone. Into the space isolated
algae were thrust by “push-hyphae.” In this lichen he also observed the
penetration of the algal cell by haustoria of the fungus. He considers
that the alga reaps advantage but also suffers harm, and he proposes the
term _helotism_ to express the relationship.

An instructive case of the true parasitism of a fungus on an alga has
been described by Zukal[233] in the case of _Endomyces scytonemata_ which
he calls a “half-lichen.” The mature fungus formed small swellings on the
filaments of the _Scytonema_ and, when examined, the hyphae were seen to
have attacked the alga, penetrating the outer gelatinous sheath and then
using up the contents of the green cells. It is only after the alga has
been destroyed and absorbed, that asci are formed by the fungus. Zukal
contrasts the development of this fungus with the symbiotic growth of the
two constituents in _Ephebe_ where both grow together for an indefinite
time.

Mere associated growth however even between a fungus and an alga does
not constitute a lichen. An instance of such growth is described by
Sutherland[234] in an account of marine microfungi. One of these, a
species of _Mycosphaerella_, was found on _Pelvetia canaliculata_, and
Sutherland claims that as no apparent injury was done to the alga, it
was a case of symbiosis and that there was formed a new type of lichen.
The mycelium, always intercellular, pervaded the whole host-plant, and
the fungal fruits were invariably formed on the algal receptacles close
to the oogonia. Their position there is, of course, due to the greater
food supply at that region. Both fungus and alga fruited freely. A closer
analogy could have been found by the writer in the smut fungus which
grows with the host-cereal until fruiting time; or with the mycorrhiza
of _Calluna_ which also pervades every part of the host-plant without
causing any injury. In the true lichen, the alga, though constituting an
important part of the vegetative body, takes no part in reproduction,
except by division and increase of the vegetative cells within the
thallus. The fruiting bodies are always of a modified fungal nature.


2. PHYSIOLOGY OF THE SYMBIONTS

The occurrence of isolated cases of parasitism—the fungus preying on the
alga—in any case leaves the general problem unsolved. The whole question
turns on the physiological activity and requirements of the two component
elements of the thallus. From what sources do they each procure the
materials essential to them as living organisms? It is chiefly a question
of nutrition.


A. NUTRITION OF ALGAE

_a._ CHARACTER OF ALGAL CELLS. Gonidia are chlorophyll-containing bodies
and assimilate carbon-dioxide from the atmosphere by photosynthesis
as do the chlorophyll cells of other plants. They also require water
and mineral salts which, in a free condition, they absorb from their
immediate surroundings, but which, in the lichen thallus, they must
obtain from the fungal hyphae. If the nutriment supplied to them in
their inclosed position be greater or even equal to what the cells could
procure as free-living algae, then they undoubtedly gain rather than lose
by their association with the fungus, and are not to be considered merely
as victims of parasitism.

_b._ SUPPLY OF NITROGEN. Important contributions on the subject of algal
nutrition have been made by Beyerinck[235] and Artari[236]. The former
conducted a series of culture experiments with green algae, including the
gonidia of _Physcia_ (_Xanthoria_) _parietina_. He successfully isolated
the lichen gonidia and, at first, attempted to grow them on gelatine with
an infusion of the Elm bark from which he had taken the lichen. Growth
was very slow and very feeble until he added to the culture-medium a
solution of malt-extract which contains peptones and sugar. Very soon
he obtained an active development of the gonidia, and they multiplied
rapidly by division[237] as in the lichen thallus. This proved to him
conclusively the great advantage to the algae of an abundant supply of
nitrogen.

Artari in his work has demonstrated that there are two different
physiological races of green algae: (1) those that absorb peptones—which
he designates peptone-algae—and (2) those that do not so absorb peptones.
He tested the cells of _Cystococcus humicola_ taken from the thallus
of _Physcia parietina_, and found that they belonged to the peptone
group and were therefore dependent on a sufficiency of nitrogenous
material to attain their normal vigorous growth. It was also discovered
by Artari that the one race can be made by cultivation to pass over to
the other: that ordinary algae can be educated to live on peptones, and
peptone-algae to do without.

We learn further from Beyerinck’s researches that Ascomycetes, the group
of fungi from which the hyphae of most lichens are derived, are what
he terms ammonia-sugar fungi; that is to say, the hyphae can abstract
nitrogen from ammonia salts and, with the addition of sugar, can form
peptones. The lichen peptone-algae are thus evidently, by their contact
with such fungi, in a favourable position for securing the nitrogenous
food supply most suited to their requirements. In their deep-seated
layers, they are to a large extent deprived of light, but it has been
proved by Artari[238] in a series of culture experiments extending over
a long period, that the gonidia of _Xanthoria parietina_ remain green in
the dark under very varied conditions of nutriment, though the colour is
distinctly fainter.

Recently Treboux[239] has revised the work done by Artari and
Beyerinck in reference to _Cystococcus humicola_. He denies that two
physiological races are represented in this alga, the lichen gonidia,
in regard to the nitrogen that they absorb, behaving exactly as do the
free-living forms of the species. He finds that the gonidium is not a
peptone-carbohydrate organism in the sense that it requires nitrogen in
the form of peptones, inorganic ammonia salts being a more acceptable
food supply. Treboux concludes that his results favour the view that
the gonidia are in an unfavourable situation for receiving the kind of
nitrogenous compound most advantageous to them, that they are therefore
in a sense “victims” of parasitism, though he qualifies the condition as
being a lichen-parasitism or helotism. This view does not accord with
Chodat’s[240] results: in his cultures of gonidia he observed that with
glycocoll or peptone, which are nearly equivalent, they developed four
times better than with potassium nitrate as their nitrogenous food, and
he concluded that they assimilated nitrogen better from bodies allied to
peptides.

_c._ EFFECT OF SYMBIOSIS ON THE ALGA. Treboux’s observations however
convinced him that the alga leads but a meagre existence within the
thallus. Cell-division—the expression of active vitality—was, he held,
of rare occurrence in the slowly growing lichen-plant, and zoospore
formation in entire abeyance. He contrasts this sluggish increase[241]
with the rapid multiplication of the free-living algal cells which cover
whole tree-trunks with their descendants in a comparatively short time.
These latter cells, he finds, are indeed rather smaller, being generally
the products of recent division, but mixed with them are numbers of
larger resting cells, comparable in size with the lichen gonidia. He
states further, that the gonidia are less brightly green and, as he
judges, less healthy, though in soredial formation or in the open they
at once regain both colour and power of division. Treboux had entirely
failed to observe the sporulation which is so abundant at certain seasons.

Their quick recovery seems also a strong argument in favour of the
absolutely normal condition of metabolism within the gonidial cell;
and the paler appearance of the chlorophyll is doubtless associated
with the acquisition of carbohydrates from other sources than by
photosynthesis. There is a wide difference between any degree of
unfavourable life-conditions and parasitism however slight, even
though the balance of gain is on the side of the fungus. It is not too
fanciful to conclude that the demand for nitrogen on the part of the
alga has influenced its peculiar association with the fungus. In the
thallus of hypophloeodal lichens it has been proved indeed that the alga
_Trentepohlia_ with apical growth is an active agent in the symbiotic
union. _Cystococcus_ and other green algal cells are stationary, but they
are doubtless equally ready for—as many of them are equally benefited
by—the association. Keeble[242] has pointed out in the case of _Convoluta
roscoffensis_ that nitrogen-hunger induces the green algae to combine
forces with an animal organism, though the benefit to them is only
temporary and though they are finally sacrificed. The lichen gonidia, on
the contrary, persist for a long time, probably far beyond their normal
period of existence as free algae.

Examples of algal association with other plants might be cited here: of
_Nostoc_ in the roots of _Cycas_ and in the cells of _Anthoceros_, and
of _Anabæna_ in the leaf-cells of _Azolla_, but in these instances it is
generally held that the alga secures only shelter. It was by comparing
the lichen-association with the harmless invasion of _Gunnera_ cells by
_Nostoc_ that Reinke[243] arrived at his conception of “consortism.”

_d._ SUPPLY OF CARBON. Carbon, the essential constituent of all organic
life, is partly drawn from the carbon-dioxide of the air, and assimilated
by the green cells; it is also partly contributed by the fungus as a
product of its metabolism. A proof of this is afforded by Dufrenoy[244]:
he found a _Parmelia_ growing closely round pine needles and even sending
suckers into the stomata. He covered the lichen with a black cloth and
after seven weeks found that the gonidia had remained very green. That
growth had not been checked was evidenced by an unusual development
of soredia and of spermogonia. Dufrenoy describes the condition as
a parasitism of the algae on the fungus which in turn was drawing
nourishment from the pine needles.

Artari[245] has proved that lichen gonidia can obtain carbohydrates
from the substratum as well as by photosynthesis. He cultivated the
gonidia of _Xanthoria parietina_ and _Placodium murorum_ on media which
contained organic substances as well as mineral salts, while depriving
them of atmospheric carbon-dioxide and in some cases of light also. The
gonidia not only grew well but, even in the dark, they remained normally
green, a phenomenon coinciding with Etard and Bouilhac’s[246] experience
in growing _Nostoc_ in the dark: with suitable culture media the alga
retained its colour. _Nostoc_ also grows in the dark in the rhizome of
_Gunnera_. Radais’[247] experiments with _Chlorella vulgaris_ confirmed
these results. On certain organic media growth and cell-division were as
rapid in the dark as in the light, and chlorophyll was formed. The colour
was at first yellowish and the full green arrived slowly, especially on
sugar media, but in ten days it was uniform and normal.

When making further experiments with the alga, _Stichococcus bacillaris_,
Artari[248] found that it also grew well on an organic medium and that
grape sugar was the most valuable carbonaceous food supply. Chodat[249]
also found that sugar or glucose was a desirable ingredient of culture
media.

Treboux[250], in his work on organic acids, has also proved by
experimental cultures with a large series of algae, including the gonidia
of _Peltigera_, that these green plants in the absence of light and in
pure cultures would grow and form carbohydrates if the culture medium
contained a small percentage of organic acids. The acids he employed
were combined with potassium and were thus rendered neutral or slightly
alkaline; acetate of potash proved to be the most advantageous compound
of any that was tested. Amino-acids and ammonia salts were added to
provide the necessary nitrogen. Oxalic acid and other organic acids of
varying composition are peculiarly abundant in lichen tissues and may be
a source of carbon supply. Marshall Ward[251] has found calcium carbonate
crystals in the lower air-containing tissues of _Strigula complanata_.

Treboux finally concluded from his researches that just as fungi can
extract carbohydrates from many sources, so algae can secure their carbon
supply in a variety of ways. He affirms that the metabolic activity of
the alga in these cultural conditions is entirely normal, and the various
cell-contents are formed as in the light. Whether, in this case, starch
is formed directly from the acids or through a series of combinations
has not been determined. Uhlir[252], with electric lighting, made
successful cultures of _Nostoc_ isolated from Collemaceae on silicic
acid, proving thereby that these gonidia do not require a rich nutriment.
A certain definite humidity was however essential, and bacteria were
never eliminated as they are associated with the gelatinous membranes of
Nostocaceae.

_e._ NUTRITION WITHIN THE SYMBIOTIC PLANT. Culture experiments bearing
more directly on the nutrition of lichens as a whole were carried out
by F. Tobler[253]. He proved that the gonidia had undoubtedly drawn on
the calcium oxalate secreted by the hyphae for their supply of carbon.
In a culture medium of poplar-bark gelatine he grew hyphae of _Xanthoria
parietina_, and noted an abundant deposit of oxalate crystals on their
cell-walls. A piece of the lichen thallus including both symbionts and
grown on a similar medium formed no crystals, and microscopic examination
showed that crystals were likewise absent from the hyphae of the thallus
that had grown normally on the tree, the inference being that the gonidia
used them up as quickly as they were deposited. It must be remembered in
this connection, however, that Zopf[254] has stated that where lichen
acids are freely formed as, for instance, in _Xanthoria parietina_, there
is always less formation and deposit of calcium oxalate crystals, which
may partly account for their absence in the normal thallus so rich in
parietin.

Tobler next introduced lichen gonidia into a culture medium in which the
isolated hyphal constituent of a thallus had been previously cultivated,
and placed the culture in the dark. In these circumstances he found that
the gonidia were able to thrive but formed no colour: they were obtaining
their carbohydrates, he decided, not from photosynthesis, but from the
excretory products such as calcium oxalate that had been deposited in the
culture medium by the lichen hyphae. We may conclude with more or less
certainty that the loss of carbohydrates, due to the partial deprivation
of light and air suffered by the alga owing to its position in the
lichen thallus, is more than compensated by a physiological symbiosis
with the fungus[255]. It has indeed been proved that in the absence of
free carbon-dioxide, algae may utilize the half-bound CO₂ of carbonates,
chiefly those of calcium and magnesium, dissolved in water.

_f._ AFFINITIES OF LICHEN GONIDIA. Chodat[256] has, in recent years, made
cultures of lichen gonidia with a view to discovering their relation to
free-living algae and to testing at the same time their source of carbon
supply. He has come to the conclusion that lichen gonidia are probably in
no instance the normal _Protococcus viridis_: they differ from that alga
in the possession of a pyrenoid and in their reproduction by zoospores
when free.

Careful cultures were made of different _Cladonia_ gonidia which were
morphologically indistinguishable, and which varied in size from 10 to
16µ in diameter, though smaller ones were always present. He recognized
them to be species of _Cystococcus_: they have a pyrenoid[257] in the
centre and a disc-like chromatophore more or less starred at the edge.
These gonidia grew well on agar, still better on agar-glucose, but best
of all with an addition of peptone to the culture. There was invariably
at first a slight difference in form and colour in the mass between the
gonidia of one species and those of another, but as growth continued they
became alike.

In testing for carbon supply, he found that gonidia grew slowly without
sugar (glucose), and that, as sources of carbon, organic acids could not
entirely replace glucose though, in the dark, the gonidia used them to
some extent; the colony supplied with potassium nitrate, and grown in the
dark, had reached a diameter of only 2 mm. in three months. With glucose,
it measured 5 mm. in three weeks, while in three months it formed large
culture patches.

A further experiment was made to test their absorption of peptones by
artificial cultures carried out both in the light and the dark. The
gonidia grew poorly in all combinations of organic nitrogen compounds.
When combined with glucose, growth was at once more vigorous though only
half as much in the dark as in the light, the difference in this respect
being especially noticeable in the gonidia from _Cladonia pyxidata_. He
concludes that as gonidia in these cultures are saprophytic, so in the
lichen thallus also they are probably more or less saprophytic, obtaining
not only their nitrogen in organic form but also, when possible, their
carbon material as glucose or galactose from the hyphal symbiont which in
turn is saprophytic on humus, etc.


B. NUTRITION OF FUNGI

Fungi being without chlorophyll are always indebted to other organisms
for their supply of carbohydrates. There has never therefore been any
question as to the advantage accruing to the hyphal constituent in the
composite thallus. The gonidia, as various workers have proved, have also
a marked preference for organized nourishment, and, in addition, they
obtain carbon by photosynthesis. Chodat[258] considers that probably
they are thus able to assimilate carbon-dioxide in excess, a distinct
advantage to the hyphae. In some instances the living gonidium is invaded
and the contents used up by the fungus and any dead gonidia are likewise
utilized for food supply. It is also taken for granted that the fungus
takes advantage of the presence of humus whether in the substratum or
in aerial dust. In such slow growing organisms, there is not any large
demand for nourishment on the part of the hyphae: for many lichens it
seems to be mere subsistence with a minimum of growth from year to year.


C. SYMBIOSIS OF OTHER PLANTS

The conception of an advantageous symbiosis of fungi with other
plants has become familiar to us in Orchids and in the mycorhizal
formation on the roots of trees, shrubs, etc. Fungal hyphae are also
frequent inhabitants of the rhizoids of hepatics though, according to
Gargeaune[259], the benefit to the hepatic host-plant is doubtful.

An association of fungus and green plant of great interest and bearing
directly on the question of mutual advantage has been described
by Servettaz[260]. In his study of mosses, he was able to confirm
Bonnier’s[261] account of lichen hyphae growing over such plants as
_Vaucheria_ and the protonema of mosses, which is undoubtedly hurtful;
but he also found an association of a moss with one of the lower fungi,
_Streptothrix_ or _Oospora_, which was distinctly advantageous. In
separate cultivation the fungus developed compact masses and grew well in
peptone agar broth.

Cultures of the moss, _Phascum cuspidatum_, were also made from the
spores on a glucose medium. The specimens in association with the fungus
were fully grown in two months, while the control cultures, without any
admixture of the fungus, had not developed beyond the protonema stage.
Servettaz draws attention to the proved fact that, in certain instances,
plants benefit when provided with substances similar to their own decay
products, and he considers that the fungus, in addition to its normal
gaseous products, has elaborated such substances, as acid products, from
the glucose medium to the great advantage of the moss plant.

A symbiotic association of _Nostoc_ with another alga, described by
Wettstein[262], is also of interest. The blue-green cells were lodged in
the pyriform outgrowths of the siphoneous alga, _Botrydium pyriforme_
Kütz., which the author of the paper places in a new genus, _Geosiphon_.
The sheltering _Nostoc symbioticum_ fills all of the host left vacant
by the plasma, and when the season of decay sets in, it forms resting
spores which migrate into the rhizoids of the host, so that both plants
regenerate together.

Wettstein has compared this symbiotic association with that of lichens,
and finds the analogy all the more striking in that the membrane of his
new alga had become chitinous, which he thinks may be due to organic
nutrition.


II. LICHEN HYPHAE


A. ORIGIN OF HYPHAE

Lichen hyphae form the ground tissue of the thallus apart from the
gonidia or algal cells. They are septate branched filaments of single
cell rows and are colourless or may be tinged by pigments or lichen acids
to some shade of yellow, brown or black. They are of fungal nature, and
are produced by the mature lichen spore.

The germination of the spore was probably first observed by Meyer[263].
His account of the actual process is somewhat vague, and he
misinterpreted the subsequent development into thallus and fruit entirely
for want of the necessary magnification; but that he did succeed in
germinating the spores is unquestionable. He cultivated them on a smooth
surface and they grew into a “dendritic formation”—a true hypothallus.
Many years later the development of hyphae from lichen spores was
observed by Holle[264] who saw and figured the process unmistakably in
_Borrera_ (_Physcia_) _ciliaris_.

A series of spore cultures was undertaken by Tulasne[265] with the
twofold object of discovering the exact origin of hyphae and gonidia
and of their relationship to each other. The results of his classical
experiment with the spores of _Verrucaria muralis_—as interpreted by
him—were accepted by the lichenologists of that time as conclusive
evidence of the genetic origin of the gonidia within the thallus.

[Illustration: Fig. 14. Germinating spores of _Verrucaria muralis_ Ach.
after two months’ culture × ca. 500 (after Tulasne).]

The spores of the lichen in large numbers had been sown by Tulasne in
early spring on the smooth polished surface of a piece of limestone,
and were covered with a watch-glass to protect them from dust, etc.
At irregular intervals they were moistened with water, and from time
to time a few spores were abstracted from the culture and examined
microscopically. Tulasne observed that the spore did not increase or
change in volume in the process of germination, but that gradually the
contents passed out into the growing hyphae, till finally a thin membrane
only was left and still persisted after two months (Fig. 14). For a
considerable time there was no septation; at length cross-divisions were
formed, at first close to the spore, and then later in the branches. The
hyphae meanwhile increased in dimension, the cells becoming rounder and
somewhat wider, though always more slender than the spore which had given
rise to them. In time a felted tissue was formed with here and there
certain cells, filled with green colouring matter, similar to the gonidia
of the lichen and thus the early stages at least of a new thallus were
observed. The green cells, we now know, must have gained entrance to the
culture from the air, or they may have been introduced with the water.


B. DEVELOPMENT OF LICHENOID HYPHAE

Lichen hyphae are usually thick-walled, thus differing from those of
fungi generally, in which the membranes, as a rule, remain comparatively
thin. This character was adduced by the so-called “autonomous” school as
a proof of the fundamental distinction between the hyphal elements of
the two groups of plants. It can, however, easily be observed that, in
the early stages of germination, the lichen hyphae, as they issue from
the spore, are thin-walled and exactly comparable with those of fungi.
Growth is apical, and septation and branching arise exactly as in fungi,
and, in certain circumstances, anastomosis takes place between converging
filaments. But if algae are present in the culture the peculiar lichen
characteristics very soon appear.

Bonnier[266], who made a large series of synthetic cultures,
distinguishes three types of growth in lichenoid hyphae (Fig. 15):

1. Clasping filaments, repeatedly branched, which attach and surround the
algae.

2. Filaments with rather short swollen cells which ultimately form the
hyphal tissues of cortex and medulla.

3. Searching filaments which elongate towards the periphery and go to the
encounter of new algae.

In five days after germination of the spores, the clasping hyphae had
laid hold of the algae which meanwhile had increased by division;
the swollen cells had begun to branch out and ten days later a
differentiation of tissue was already apparent. The searching filaments
had increased in number and length, and anastomosis between them had
taken place when no further algae were encountered. The cell-walls of
the swollen hyphae and their branches had begun to thicken and to become
united to form a kind of cellular tissue or “paraplectenchyma[267].” At
a later date, about a month after the sowing of the spores, there was a
definite cellular cortex formed over the thallus. The hyphal cells are
uninucleate, though in the medulla they may be 1-2-nucleate.

[Illustration: Fig. 15. Synthetic culture of _Physcia parietina_ spores
and _Protococcus viridis_ five days after germination. _s_, lichen-spore;
_a_, septate filaments; _b_, clasping filaments; _c_, searching
filaments. × 500 (after Bonnier).]

The hyphae in close contact with the gonidia remain thin-walled, and
have been termed by Wainio[268] “meristematic.” They furnish the growing
elements of the lichen either apical or intercalary. In most genera the
organs of fructification take rise from them, or in their immediate
neighbourhood, and isidia and soredia also originate from these gonidial
hyphae.

As the filaments pass from the gonidial zone to other layers, the
cell-walls become thicker with a consequent reduction of the cell-lumen,
very noticeable in the pith, but carried to its furthest extent in the
“decomposed” cortex where the cells in the degenerate tissue often become
reduced to disconnected streaks indicating the cell-lumen, and the outer
cortical layer is merely a continuous mass of mucilage.

All lichen tissues arise from the branching and septation of the hyphae,
the septa always forming at right angles to the long axis of the
filaments. There is no instance of longitudinal cell-division except in
the spores of certain genera (_Collema_, _Urceolaria_, _Polyblastia_,
etc.). The branching of the hypha is dichotomous or lateral, and very
irregular. Frequent septation and coherent growth result in the formation
of plectenchyma.


C. CULTURE OF HYPHAE WITHOUT GONIDIA

Artificial cultures had demonstrated the germination of lichen spores,
with the formation of hyphae, and from synthetic cultures of fungus and
alga complete lichen plants had been produced. To Möller[269] we owe
the first cultures of a thalline body from the fungus alone, both from
spermatia and from ascospores. The germination of the spermatia has a
direct bearing on their function as spores or as sexual organs and is
described in a later chapter.

The ascospores of _Lecanora subfusca_ were caught in a drop of water on
a slide as they were ejaculated from the ascus, and, on the following
day, a very fine germinating tube was seen to have pierced the exospore.
The hypha became slightly thicker, and branching began on the third day.
If in water alone the culture soon died off, but in a nutrient solution
growth slowly continued. The hyphae branched out in all directions from
the spore as a centre and formed an orbicular expansion which in fourteen
days had reached a size of ·1 mm. in diameter. After three weeks’ growth
it was large enough to be visible without a lens; the mycelial threads
were more crowded, and certain terminal hyphae had branched upwards in
an aerial tuft, this development taking place from the centre outwards.
Möller marked this stage as the transition from a mere protothallus to a
thallus formation. In three months a diameter of 1·5-2 mm. was reached;
a transverse section gave a thickness of ·86 mm. and from the under
side loose hyphae branched downwards and attached the thallus, when it
had been transferred to a solid substratum such as cork. Above these
rhizoidal hyphae, a stratum of rather loose mycelium represented the
medulla, and, surmounting that, a cortical layer in which the hyphae were
very closely compacted. Delicate terminal branches rose into the air over
the whole surface, very similar in character to hypothallic hyphae at the
margin of the thallus.

_Lecanora subfusca_ has a rather small simple spore; it emitted
germinating tubes from each end, and a septum across the middle of the
spore appeared after germination had taken place. Another experiment
was with a much larger muriform spore measuring 80 µ in length and 20 µ
in thickness. On germination about 20 tubes were formed, some of them
rising into the air at once, the others encircling the spore, so that the
thallus took form immediately; growth in this case also was centrifugal.
In three months a diameter of 6 mm. was reached with a thickness of 1 to
2 mm. and showing a differentiation into medulla and cortex. The hyphae
did not increase in width, but frequently globose or ovate swellings
arose in or at the ends, a character which recurs in the natural growth
of hyphae both of lichens and of Ascomycetes. These swellings depend on
the nutrition.

_Pertusaria communis_ possesses a very large simple spore, but it is
multinucleate and germinates with about 100 tubes which reach their
ultimate width of 3 to 4 µ before they emerge from the exospore. The
hyphae encircle the spore, and an opaque thalline growth is quickly
formed from which rise terminal hyphal branches. In ten weeks the
differentiation into medulla and cortex was reached, and in five months
the hyphal thallus measured 4 mm. in diameter and 1 to 2 mm. in thickness.

Möller instituted a comparison between the thalli he obtained from the
spores and those from the spermatia of another crustaceous lichen,
_Buellia punctiformis_ (_B. myriocarpa_). After germination had taken
place the hyphae from the spermatia grew at first more quickly than those
from the ascospores, but as soon as thallus formation began the latter
caught up and, in eight weeks, both thalli were of equal size.

Another comparative culture with the spermatia and ascospores of
_Opegrapha subsiderella_ gave similar results: the spores of that species
are elongate-fusiform and 6-to 8-septate; germination took place from
the end cells in two to three days after sowing. The germinating hyphae
corresponded exactly with those from the spermatia and growth was equally
slow in both. The middle cells of the spores may also produce germinating
tubes, but never more than about five were observed from any one spore.
A browning of the cortical layer was especially apparent in the hyphal
culture from another lichen, _Graphis scripta_: a clear brown colour
gradually changing to black appeared about the same period in all the
cultures.

The hyphae from the spores of _Arthonia_ developed quickest of all:
the hyphae were very slender, but in three to four months the growth
had reached a diameter of 8 mm. In this plant there was the usual
outgrowth of delicate hyphae from the surface; no definite cortical layer
appeared, but only a very narrow line of more closely interwoven somewhat
darker hyphae. Frequently, from the surface of the original thallus,
excrescences arose which were the beginnings of further thalli.

Tobler[270] experimenting with _Xanthoria parietina_ gained very similar
results. The spores were grown in malt extract for ten days, then
transferred to gelatine. In three to five weeks there was formed an
orbicular mycelial felt about 3 mm. in diameter and 2 mm. thick. The
mycelium was frequently brownish even in healthy cultures, but the aerial
hyphae which, at first, rose above the surface were always colourless.
After these latter disappeared a distinct brownish tinge of the thallus
was visible. In seven months it had increased in size to 15 mm. in
length, 7 mm. in width and 3 mm. thick with a differentiation into three
layers: a lower rather dense tissue representing the pith, above that a
layer of loose hyphae where the gonidial zone would normally find place,
and above that a second compact tissue, or outer cortex, from which arose
the aerial hyphae. The culture could not be prolonged more than eight
months.


D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS

Wahrlich[271] demonstrated that continuity of protoplasm was as constant
between the cells of fungi as it has been proved to be between the cells
of the higher plants. His researches included the hyphae of the lichens,
_Cladonia fimbriata_ and _Physcia_ (_Xanthoria_) _parietina_.

Baur[272] and Darbishire[273] found independently that an open connection
existed between the cells of the carpogonial structures in the lichens
they examined. The subject as regards the thalline hyphae was again
taken up by Kienitz-Gerloff[274] who obtained his best results in the
hypothecial tissue of _Peltigera canina_ and _P. polydactyla_. Most of
the cross septa showed one central protoplasmic strand traversing the
wall from cell to cell, but in some instances there were as many as four
to six pits in the walls. The thickening of the cell-walls is uneven
and projects variously into the cavity of the cell. Meyer’s[275] work
was equally conclusive: all the cells of an individual hypha, he found,
are in protoplasmic connection; and in plectenchymatous tissue the side
walls are frequently perforated. Cell-fusions due to anastomosis are
frequent in lichen hyphae, and the wall at or near the point of fusion
is also traversed by a thread of protoplasm, though such connections are
regarded as adventitious. Fusions with plasma connections are numerous
in the matted hairs on the upper surface of _Peltigera canina_ and they
also occur between the hyphae forming the rhizoids of that lichen. The
work of Salter[276] may also be noted. He claimed that his researches
tended to show complete anatomical union between all the tissues of the
lichen plant, not only between the hyphae of the various tissues but also
between hyphae and gonidia.


III. LICHEN ALGAE


A. TYPES OF ALGAE

The algal constituents of the lichen thallus belong to the two classes,
Myxophyceae, generally termed blue-green algae, and Chlorophyceae
which are coloured bright-green or yellow-green. Most of them are
land forms, and, in a free condition, they inhabit moist or shady
situations, tree-trunks, walls, etc. They multiply by division or by
sporulation within the thallus; zoospores are never formed except in open
cultivation. The determination of the genera and species to which the
lichen algae severally belong is often uncertain, but their distribution
within the lichen kingdom is as follows:

_a._ MYXOPHYCEAE ASSOCIATED WITH PHYCOLICHENS. The blue-green algae
are characterized by the colour of their pigments which persists in
the gonidial condition giving various tints to the component lichens,
and by the gelatinous sheath in which most of them are enclosed. This
sheath, both in the lichen gonidia[277] and in free-living forms, imbibes
and retains moisture to a remarkable extent and the thallus containing
blue-green algae profits by its power of storing moisture. Myxophyceae
form the gonidia of the gelatinous lichens as well as of some other
non-gelatinous genera. Several families are represented[278]:

Fam. CHROOCOCCACEAE. This family includes unicellular algae with thick
gelatinous sheaths. They increase normally by division, and colonies
arise by the cohesion of the cells. Several genera form gonidia:

1. CHROOCOCCUS Naeg. Solitary or forming small colonies of 2-4-8 cells
(Fig. 16) generally surrounded by firm gelatinous colourless sheaths
in definite layers (lamellate). _Chroococcus_ is considered by some
lichenologists to form the gonidium of _Cora_, a genus of Hymenolichens.

2. MICROCYSTIS Kütz. Globose or subglobose cells forming large colonies
surrounded by a common gelatinous layer (gonidia of _Coriscium_).

[Illustration: Fig. 16. Examples of _Chroococcus_. A, _Ch. giganteus_
West; B, _Ch. turgidus_ Naeg.; C and D, _Ch. schizodermaticus_ West × 450
(after West).]

[Illustration: Fig. 17. _Gloeocapsa magma_ Kütz. × 450 (after West).]

3. GLOEOCAPSA Kütz. (including _Xanthocapsa_). Globose cells with a
lamellate gelatinous wall, forming colonies enclosed in a common sheath
(Fig. 17); the inner integument is often coloured red or orange. These
two genera form the gonidia in the family Pyrenopsidaceae. _Gloeocapsa
polydermatica_ Kütz. has been identified as a lichen gonidium.

Fam. NOSTOCACEAE. Filamentous algae unbranched and without base or apex.

NOSTOC Vauch. Composed of flexuous trichomes, with intercalary
heterocysts (colourless cells) (Fig. 18). Dense gelatinous colonies of
definite form are built up by cohesion. In some lichens the trichomes
retain their chain-like appearance, in others they are more or less
broken up and massed together, with disappearance of the gelatinous
sheath (as in _Peltigera_); colour mostly dark blue-green.

[Illustration: Fig. 18. Examples of _Nostoc_. _N. Linckia_ Born. A, nat.
size; B, small portion × 340; C, _N. coerulescens_ Lyngbye, nat. size
(after West).]

[Illustration: Fig. 19. Example of _Scytonema_ alga. _S. mirabile_ Thur.
C, apex of a branch; D, organ of attachment at base of filament. × 440
(after West).]

_Nostoc_ occurs in a few or all of the genera of Pyrenidiaceae,
Collemaceae, Pannariaceae, Peltigeraceae and Stictaceae, and _N.
sphaericum_ Vauch. (_N. lichenoides_ Kütz.) has been determined as
the lichen gonidium. When the chains are broken up it has been wrongly
classified as another alga, _Polycoccus punctiformis_.

Fam. SCYTONEMACEAE. Trichomes of single-cell rows, differentiated into
base and apex. Pseudo-branching arises at right angles to the main
filament.

SCYTONEMA Ag. Pseudo-branches piercing the sheath and passing out as twin
filaments (Fig. 19); colour, golden-brown. This alga occurs in genera of
Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in _Petractis_ a
genus of Gyalectaceae, and in _Dictyonema_ one of the Hymenolichens.

Fam. STIGONEMACEAE. Trichomes of several-cell rows with base and apex;
colour, golden-brown.

STIGONEMA Ag. Stouter than _Scytonema_, with transverse and vertical
division of the cells, and generally copious branching (Fig. 20). This
alga occurs only in a few genera of Ephebaceae. _S. panniforme_ Kirchn.
(_Sirosiphon pulvinatus_ Bréb.) has been determined as forming the
gonidium.

Fam. RIVULARIACEAE. Trichomes with a heterocyst at the base and tapering
upwards, enclosed in mucilage (Fig. 21).

[Illustration: Fig. 20. _Stigonema_ sp. × 200 (after Comère).]

[Illustration: Fig. 21. Examples of _Rivularia_; A, B, C, R.
_Biasolettiana_ Menegh.; D and E, _R. minutula_ Born. and Fl. A and D
nat. size; B, C and E × 480 (after West).]

RIVULARIA Thuret. In tufts fixed at the base and forming roundish
gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has
been identified as _R. nitida_ Ag.

Algae belonging to one or other of these genera of Myxophyceae also
combine with the hyphae of Archilichens to form cephalodia[279] and
Krempelhuber[280] has recorded and figured a blue-green alga, probably
_Gloeocapsa_, in _Baeomyces paeminosus_ from the South Sea Islands. They
also form the gonidia in a few species and genera of such families as
Stictaceae and Peltigeraceae.

_b._ CHLOROPHYCEAE ASSOCIATED WITH ARCHILICHENS. The lichens of this
group are by far the most numerous both in genera and species, though
fewer algal families are represented.

Fam. PROTOCOCCACEAE. Consisting of globular single cells, aggregated in
loose colonies, dividing variously.

[Illustration: Fig. 22. _Pleurococcus vulgaris_ Menegh. (_Protococcus
viridis_ Ag.). _chl._ chloroplast; _p._ protoderma stage; _pa_,
palmelloid stage; _py_, pyrenoid. × 520 (after West).]

1. PROTOCOCCUS VIRIDIS Ag. (_Pleurococcus vulgaris_ Menegh.,
_Cystococcushumicola_ Naeg.). Cells dividing into 2, 4 or 8
daughter-cells and not separating readily; in excessive moisture forming
short filaments. The cells contain parietal chloroplasts, and, according
to Chodat[281], are without a pyrenoid (Fig. 22). This alga, and allied
species, forms the familiar green coating of tree-trunks, walls etc.,
and, in lichenological literature, are quoted as the gonidia of most
of the crustaceous foliose and fruticose lichens. Chodat[281], who has
recently made comparative artificial cultures of algae, throws doubt
on the identity of many such gonidia. He lays great emphasis on the
presence or absence of a pyrenoid in algal cells. West, on the contrary,
considers the pyrenoid as an inconstant character. Chodat insists that
the gonidia that contain pyrenoids belong to another genus, _Cystococcus_
Chod. (_non_ Naeg.), a pyrenoid-containing alga, which, in addition to
multiplying by division of the cells, also forms spores and zoospores
when cultivated. He further records the results of his cultures of
gonidia, and finds that those taken from closely related lichens, such as
different species of _Cladonia_, though they are alike morphologically,
yet show constant variations in the culture colonies. These, he holds,
are sufficient to indicate difference of race if not of species and he
designates the algae, according to the lichen in which they occur, as
_Cystococcus Cladoniae pyxidatae_, _C. Cladoniae fimbriatae_, etc.

[Illustration: Fig. 23. _Cystococcus Cladoniae pyxidatae_ Chod. from
culture × 800 (after Chodat).]

[Illustration: Fig. 23 A. A, C, _Chlorella vulgaris_ Beyer. B and C,
stages in division × about 800 (after Chodat); E, _Chl. faginea_ Wille
× 520 (after Gerneck); F-I _Chl. miniata_; F, vegetable cell; G-I,
formation and escape of gonidia × 1000 (after Chodat).]

Meanwhile Paulson and Somerville Hastings[282] by their careful research
on the growing thallus have thrown considerable light on the identity
of the Protococcaceous lichen gonidium. They selected such well-known
lichens as _Xanthoria parietina, Cladonia_ spp. and others, which they
collected during the spring months, February to April, the period of
most active growth. Many of the gonidia, they found, were in a stage of
reproduction, that showed a simultaneous rounding off of the gonidium
contents into globose bodies varying in number up to 32. Chodat had
figured this method of “sporulation” in his cultures of the lichen
gonidium both in _Chlorella_ Beij. and in _Cystococcus_ Chod. (Fig.
23). It has now been abundantly proved that this form of increase is
of frequent occurrence in the thallus itself. _Chlorella_ has been
suggested as probably the alga forming these gonidia and recently West
has signified his acquiescence in this view[283].

2. CHLORELLA Beij. Occurring frequently on damp ground, bark of trees,
etc., dividing into numerous daughter-cells, probably reduced zoogonidia
(Fig. 23).

Chodat distinguishes between _Cystococcus_ and _Chlorella_ in that
_Cystococcus_ may form zoospores (though rarely), _Chlorella_ only
aplanospores. He found three gonidial species, _Chlorella lichina_
in _Cladonia rangiferina, Ch. viscosa_ and _Ch. Cladoniae_ in other
_Cladonia_ spp.

3. COCCOBOTRYS Chod. The cells of this new algal genus are smaller than
those of _Cystococcus_ or _Protococcus_ and have no pyrenoid. They were
isolated by Chodat from the thallus of _Verrucaria nigrescens_ (Fig. 24),
and, as they have thick membranes, they adhere in a continuous layer or
thallus. Chodat also claims to have isolated a species of _Coccobotrys_
from _Dermatocarpon miniatum_, a foliose Pyrenolichen.

4. COCCOMYXA Schmidle. Cells ellipsoid, also without a pyrenoid. Two
species were obtained by Chodat from the thallus of _Solorinae_ and are
recorded as _Coccomyxa Solorinae croceae_ and _C. Solorinae saccatae_.

_Coccomyxa subellipsoidea_ is given[284] as the gonidium of the primitive
lichen _Botrydina vulgaris_ (Fig. 25). The cells are surrounded by a
common gelatinous sheath.

[Illustration: Fig. 24. _Coccobotrys Verrucariae_ Chod. from culture ×
800 (after Chodat).]

[Illustration: Fig. 25. _Coccomyxa subellipsoidea_ Acton. Actively
dividing cells, the dark portions indicating the chloroplasts × 1000
(after Acton).]

5. DIPLOSPHAERA Bial.[285] _D. Chodati_ was taken from the thallus
of _Lecanora tartarea_ and successfully cultivated. It resembles
_Protococcus_, but has smaller cells and grows more rapidly; it is
evidently closely allied to that genus, if not merely a form of it.

6. UROCOCCUS Kütz. Cells more or less globose, rather large, and coloured
with a red-brown pigment, with the cell-wall thick and lamellate,
forming elongate strands of cells (Fig. 26). Recorded by Hue[286] in the
cephalodium of _Lepolichen coccophorus_, a Chilian lichen.

Fam. TETRASPORACEAE. Cells in groups of 2 or 4 surrounded by a gelatinous
sheath.

1. PALMELLA Lyngb. Cells globose, oblong or ellipsoid, grouped without
order in a formless mucilage (Fig. 27). Among lichens associated with
_Palmella_ are the Epigloeaceae and Chrysothricaceae.

[Illustration: Fig. 26. _Urococcus_ sp. Group of cells much magnified
(after Hassall).]

[Illustration: Fig. 27. _Palmella_ sp. × 400 (after Comère).]

[Illustration: Fig. 28. _Gloeocystis_ sp. × 400 (after Comère).]

2. GLOEOCYSTIS Naeg. Cells oblong or globose with a lamellate sheath
forming small colonies; colour, red-brown (Fig. 28). This alga along
with _Urococcus_ was found by Hue in the cephalodia of _Lepolichen
coccophora_, but whereas _Gloeocystis_ frequently occupies the
cephalodium alone, _Urococcus_ is always accompanied by _Scytonema_, the
normal gonidium of the cephalodium.

[Illustration: Fig. 29. A, _Trentepohlia umbrina_ Born.; B, _T. aurea_
Mart. × 300 (after Kütz.).]

[Illustration: Fig. 30. Example of _Cladophora_. _Cl. glomerata_ Kütz. A,
nat. size; B, × 85 (after West).]

Fam. TRENTEPOHLIACEAE. Filamentous and branched, the filaments short
and creeping or long and forming tufts and felts or cushions; colour,
brownish-yellow or reddish-orange.

TRENTEPOHLIA Born. Branching alternate; cells filled with red or orange
oil; no pyrenoids (Fig. 29). A large number of lichens are associated
with this genus: Pyrenulaceae, Arthoniaceae, Graphidaceae, Roccellaceae,
Thelotremaceae, Gyalectaceae and Coenogoniaceae, etc., in whole or in
part. Two species have been determined, _T. umbrina_ Born., the gonidium
of the Graphidaceae, and _T. aurea_ which is associated with the only
European _Coenogonium, C. ebeneum_ (Fig. 3). Deckenbach[287] claimed that
he had proved by cultures that _T. umbrina_ was a growth stage of _T.
aurea._

Fam. CLADOPHORACEAE. Filamentous, variously and copiously branched, the
cells rather large and multinucleate.

CLADOPHORA Kütz. Filaments branching, of one-cell rows, attached at the
base; colour, bright or dark green; mostly aquatic and marine (Fig. 30).
Only one lichen, _Racodium rupestre_, a member of the Coenogoniaceae,
is associated with _Cladophora_. It is a British lichen, and is always
sterile.

Fam. MYCOIDEACEAE. Epiphytic algae consisting of thin discs which are
composed of radiating filaments.

1. MYCOIDEA Cunningh. (Cephaleuros Kunze). In _Mycoidea parasitica_ the
filaments of the disc are partly erect and partly decumbent, reddish
to green (Fig. 31). It forms the gonidium of the parasitic lichen,
_Strigula complanata_, which was studied by Marshall Ward in Ceylon[288].
Zahlbruckner gives _Phyllactidium_ as an alternative gonidium of
Strigulaceae.

2. PHYCOPELTIS Millard. Disc a stratum one-cell thick, bearing seta,
adnate to the lower surface of the leaf, yellow-green in colour.
_Phycopeltis_ (Fig. 32) has been identified as the gonidium of _Strigula
complanata_ in New Zealand and of _Mazosia_ (Chiodectonaceae), a leaf
lichen from tropical America.

[Illustration: Fig. 31. _Mycoidea parasitica_ Cunningh. much magnified
(after Marshall Ward).]

There is some confusion as to the genera of algae that form the gonidia
of these epiphyllous lichens. _Phyllactidium_ given by Zahlbruckner as
the gonidium of all the Strigulaceae (except _Strigula_ in part) is
classified by de Toni[289] as probably synonymous with _Phycopeltis_
Millard, and as differing from _Mycoidea parasitica_ in the mode of
growth.

Fam. PRASIOLACEAE. Thallus filamentous, often expanded into broad sheets
by the fusion of the filaments in one plane.

PRASIOLA Ag. Thallus filamentous, of one-to many-cell rows, or widely
expanded (Fig. 33). The gonidium of Mastoidiaceae (Pyrenocarpeae).

[Illustration: Fig. 32. _Phycopeltis expansa_ Jenn. much magnified (after
Vaughan Jennings).]

[Illustration: Fig. 33. _Prasiola parietina_ Wille × 500 (after West).]


B. CHANGES INDUCED IN THE ALGA

_a_. MYXOPHYCEAE. Though, as a general rule, the alga is less affected
by its altered life-conditions than the fungus, yet in many instances
it becomes considerably modified in appearance. In species of the
genus _Pyrenopsis_—small gelatinous lichens—the alga is a _Gloeocapsa_
very similar to _G. magma_. In the open it forms small colonies of
blue-green cells surrounded by a gelatinous sheath which is coloured
red with gloeocapsin. As a gonidium lying towards or on the outside
of the granules composing the thallus, the red sheath of the cells
is practically unchanged, so that the resemblance to _Gloeocapsa_ is
unmistakable. In the inner parts of the thallus, the colonies are
somewhat broken up by the hyphae and the sheaths are not only less
evident but much more faintly coloured. In _Synalissa_, a minute shrubby
lichen which has the same algal constituent, the tissue of the thallus
is more highly evolved, and in it the red colour can barely be seen and
then only towards the outside; at the centre it disappears entirely. The
long chaplets of _Nostoc_ cells persist almost unchanged in the thallus
of the Collemaceae, but in heteromerous genera such as _Pannaria_ and
_Peltigera_ they are broken up, or they are coiled together and packed
into restricted areas or zones. The altered alga has been frequently
described as _Polycoccus punctiformis_. A similar modification occurs
in many cephalodia, so that the true affinity of the alga, in most
instances, can only be ascertained after free cultivation.

Bornet[290] has described in _Coccocarpia molybdaea_ the change that
the alga _Scytonema_ undergoes as the thallus develops: in very young
fronds the filaments of _Scytonema_ are unchanged and are merely enclosed
between layers of hyphae. At a later stage, with increase of the thallus
in thickness, the algal filaments are broken up, their covering sheath
disappears, and the cells become rounded and isolated. _Petractis_
(_Gyalecta_) _exanthematica_ has also a _Scytonema_ as gonidium, and
equally exact observations have been made by Fünfstück[291] on the
way it is transformed by symbiosis: with the exception of a very thin
superficial layer, the thallus is immersed in the rock and is permeated
by the alga to its lowest limits, 3 to 4 mm. below the surface,
_Petractis_ being a homoiomerous lichen. The _Scytonema_ trichomes
embedded in the rock become narrower, and the sheath, which in the
epilithic part of the thallus is 4µ wide, disappears almost entirely. The
green colour of the cells fades and septation is less frequent and less
regular. The filaments in that condition are very like oil-hyphae and can
only be distinguished as algal by staining reagents such as alkanna. They
never seem to be in contact with the fungal elements: there is no visible
appearance of parasitism nor even of consortism.

_b._ CHLOROPHYCEAE. As a rule the green-celled gonidium such as
_Protococcus_ is not changed in form though the colour may be less vivid,
but in certain lichens there do occur modifications in its appearance. In
_Micarea_ (_Biatorina_) _prasina_, Hedlund[292] noted that the gonidium
was a minute alga possessing a gelatinous sheath similar to that of a
_Gloeocapsa_. He isolated the alga, made artificial cultures and found
that, in the altered conditions, it gradually increased in size, threw
off the gelatinous sheath and developed into normal _Protococcus_ cells,
measuring 7 to 10µ in diameter. The gelatinous sheath was thus proved to
be merely a biological variation, probably of value to the lichen owing
to its capacity to imbibe and retain moisture. Zukal[293] also made
cultures of this alga, but wrongly concluded it was a _Gloeocystis_.

Moebius[294] has described the transformation from algae to lichen
gonidia in a species epiphytic on Orchids in Porto Rico. He had observed
that most of the leaves were inhabited by a membranaceous alga,
_Phyllactidium_, and that constantly associated with it were small scraps
of a lichen thallus containing isolated globose gonidia. The cells of
the alga, under the influence of the invading fungus, were, in this
case, formed into isolated round bodies which divided into four, each
daughter-cell becoming surrounded by a membrane and being capable, in
turn, of further division.

Frank[295] followed the change from a free alga to a gonidium in
_Chroolepus_ (_Trentepohlia_) _umbrinum_, as shown in the hypophloeodal
thalli of the Graphideae. The alga itself is frequent on beech bark,
where it forms wide-spreading brownish-red incrustations consisting of
short chains occasionally branched. The individual cells have thick
laminated membranes and vary in width from 20 µ to 37 µ. The free alga
constantly tends to penetrate below the cortical layers of the tree
on which it grows, and the immersed cells become not only longer and
of a thinner texture, but the characteristic red colour so entirely
disappears, that the growing penetrating apical cell may be light green
or almost colourless. As a lichen gonidium the alga undergoes even more
drastic changes: the red oily granules gradually vanish and the cells
become chlorophyll-green or, if any retain a bright colour, they are
orange or yellow. The branching of the chains is more regular, the cells
more elongate and narrower; usually they are about 13 to 21 µ long and
8 µ wide, or even less. Deeper down in the periderm, the chains become
disintegrated into separate units. Another notable alteration takes
place in the cell-membrane which becomes thin and delicate. It has,
however, been observed that if these algal cells reach the surface, owing
to peeling of the bark, etc., they resume the appearance of a normal
_Trentepohlia_.

In certain cases where two kinds of algae were supposed to be present in
some lichens, it has been proved that one species only is represented,
the difference in their form being caused by mechanical pressure of
the surrounding hyphae, as in _Endocarpon_ and _Staurothele_ where the
hymenial gonidia are cylindrical in form and much smaller than those of
the thallus. They were on this account classified by Stahl[296] under
a separate algal genus, _Stichococcus_, but they are now known to be
growth forms of _Protococcus_, the alga that is normally present in the
thallus. Similar variations were found by Neubner[297] in the gonidia
of the Caliciaceae, but, by culture experiments with the gonidia apart
from the hyphae, he succeeded in demonstrating transition forms in all
stages between the “_Pleurococcus_” cells and those of “_Stichococcus_,”
though the characters acquired by the latter are transmitted to following
generations. The transformation from spherical to cylindrical algal
cells had been also noted by Krabbe[298] in the young podetia of some
species of _Cladonia_, the change in form being due to the continued
pressure in one direction of the parallel hyphae.

Isolated algal cells have been observed within the cortex of various
lichens. They are carried thither by the hyphae from the gonidial zone
in the process of cortical formation, but they soon die off as in that
position they are deprived of a sufficiency of air and of moisture.
Forssell[299] found _Xanthocapsa_ cells embedded in the hymenium of
_Omphalaria Heppii_. They were similar to those of the thallus, but they
were not associated with hyphae and had undergone less change than the
thalline algae.


C. CONSTANCY OF ALGAL CONSTITUENTS

Lichen hyphae of one family or genus, as a rule, combine with the same
species of alga, and the continuity of genera and species is maintained.
There are, however, related lichens that differ chiefly or only in the
characters of the gonidia. Among such closely allied genera or sections
of genera may be cited _Sticta_ with bright-green algae and the section
_Stictina_ with blue-green; _Peltidea_ similarly related to _Peltigera_
and _Nephroma_ to _Nephromium_. In the genus _Solorina_, some of the
species possess bright-green, others blue-green algae, while in one, _S.
crocea_[300], there is an upper layer of small bright-green gonidia that
project in irregular pyramids into the upper cortex; while below these
there stretches a more or less interrupted band of blue-green _Nostoc_
cells. The two layers are usually separated by strands of hyphae, but
occasionally they come into close contact, and the hyphal filaments
pass from one zone to the other. In this genus cephalodia containing
blue-green _Nostoc_ are characteristic of all the “bright-green” species.
Harmand[301] has recorded the presence of two different types of gonidia
in _Lecanora atra_ f. _subgrumosa_; one of them, the normal _Protococcus_
alga of the species, the other, pale-blue-green cells of _Nostoc_
affinity.

Forssell[302] states that in _Lecanora_ (_Psoroma_) _hypnorum_, the
normal bright-green gonidia of some of the squamules may be replaced
by _Nostoc_. In that case they are regarded as cephalodia, though in
structure they exactly resemble the squamules of _Pannaria pezizoides_,
and Forssell considers that there is sufficient evidence of the identity
of the hyphal constituent in these two lichens, the alga alone being
different.

It may be that in Archilichens with a marked capacity to form a second
symbiotic union with blue-green algae, a tendency to revert to a
primitive condition is evident—a condition which has persisted wholly in
_Peltigera_ with its _Nostoc_ zone, but is manifested only by cephalodia
formation in the _Peltidea_ section of the genus. In this connection,
however, we must bear in mind Forssell’s view that it is the Archilichens
that are the more primitive[303].

The alien blue-green algae with their gelatinous sheaths are adapted
to the absorption and retention of moisture, and, in this way, they
doubtless render important service to the lichens that harbour them in
cephalodia.


D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS

_a._ NORMAL DISPLACEMENT. Lindau[304] has contrasted the advancing
apical growth of the creeping alga _Trentepohlia_ with the stationary
condition of the unicellular species that multiply by repeated division
or by sporulation, and thus form more or less dense zones and groups of
gonidia in most lichens. The fungus in the latter case pushes its way
among the algae and breaks up the compact masses by a shoving movement,
thus letting in light and air. The growing hypha usually applies
itself closely round an algal cell, and secondary branches arise which
in time encircle it in a network of short cells. In the thallus of
_Variolaria_[305] the hyphae from the lower tissues, termed push-hyphae
by Nienburg[306], push their way into the algal groups and filaments
composed of short cells come to lie closely round the individual gonidia.
Continued growth is centrifugal, and the algae are carried outward with
the extension of the hyphae (Fig. 12). Cell-division is more active at
the periphery, that being the area of vigorous growth, and the algal
cells are, in consequence, generally smaller in that region than those
further back, the latter having entered more or less into a resting
condition, or, as is more probable, these smaller cells are aplanospores
not fully mature.

_b._ LOCAL DISPLACEMENT. Specimens of _Parmelia physodes_ were found
several times by Bitter, the grey-green surface of which was marbled
with whitish lines, caused by the absence of gonidia under these
lighter-coloured areas. The thallus was otherwise healthy as was
manifested by the freely fruiting condition: no explanation of the
phenomenon was forthcoming. Bitter compared the condition with the
appearance of lighter areas on the thallus of _Parmelia obscurata_.

Something of the same nature was observed on the thallus of a _Peltigera_
collected by F. T. Brooks near Cambridge. The marking took the form of a
series of concentric circles, starting from several centres. The darker
lines were found on examination to contain the normal blue-green algal
zone, while the colour had faded from the lighter parts. The cause of the
difference in colouration was not apparent.


E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH LICHEN HYPHAE

Bonnier[307] made a series of cultures with lichen spores and green
cells other than those that form lichen gonidia. In one instance he
substituted _Protococcus botryoide_s for the normal gonidia of _Parmelia_
(_Xanthoria_) _parietina_; in another of his cultures he replaced
_Protococcus viridis_ by the filamentous alga _Trentepohlia abietina_.
In both cases the hyphae attached themselves to the green cells and a
certain stage of thallus formation was reached, though growth ceased
fairly early. Another experiment made with the large filaments of
_Vaucheria sessilis_ met with the same amount of success (Fig. 34). The
germinating hyphae attached themselves to the alga and grew all round it,
but there was no advance to tissue formation.

Cultures were also made with the protonema of mosses. Either spores of
mosses and lichens were germinated together, or lichen spores were sown
in close proximity to fully formed protonemata. The developing hyphae
seized on the moss cells and formed a network of branching anastomosing
filaments along the whole length of the protonema without, however,
penetrating the cells. If suitable algae were encountered, proper thallus
formation commenced, and Bonnier considers that the hyphae receive
stimulus and nourishment from the protonema sufficient to tide them
over a considerable period, perhaps until the algal symbiont is met. An
interesting variation was noted in connection with the cultures of _Mnium
hornum_[308]. If the protonema were of the usual vigorous type, the whole
length was encased by the hyphal network; but if it were delicate and
slender, the protoplasm collected in the cell that was touched by hyphae
and formed a sort of swollen thick-walled bud (Fig. 35). This new body
persisted when the rest of the filament and the hyphae had disappeared,
and, in favourable conditions, grew again to form a moss plant.

[Illustration: Fig. 34. Germinating hyphae of _Lecanora subfusca_ Ach.,
growing over the alga _Vaucheria sessilis_ DC., much magnified (after
Bonnier).]


F. PARASITISM OF ALGAE ON LICHENS

A curious instance of undoubted parasitism by an alga, not as in
_Strigula_ on one of the higher plants, but on a lichen thallus,
is recorded by Forssell[309]. A group of _Protococcus_-like cells
established on the thallus of _Peltigera_ had found their way into the
tissue, the underlying cortical cells having degenerated. The blue-green
cells of the normal gonidial layer had died off before their advance
but no zone was formed by the invading algae; they simply withdrew
nourishment and gave seemingly no return. The phenomenon is somewhat
isolated and accidental but illustrates the capacity of the alga to
absorb food supply from lichen hyphae.

[Illustration: Fig. 35. Pure culture of protonema of _Mnium hornum_ L.
with spores and hyphae of _Lecidea vernalis_ Ach. _a_,_a_,_a_, buds
forming × 150 (after Bonnier).]

An instance of epiphytic growth has also been recorded by
Zahlbruckner[310]. He found an alga, _Trentepohlia abietina_, covering
the thallus of a Brazilian lichen, _Parmelia isidiophora_, and growing so
profusely as to obscure the isidiose character towards the centre of the
thallus. There was no genetic connection of the alga with the lichen as
the former was not that of the lichen gonidium. Lichen thalli are indeed
very frequently the habitat of green algae, though their occurrence may
be and probably is accidental.




CHAPTER III

MORPHOLOGY

GENERAL ACCOUNT OF LICHEN STRUCTURE


I. ORIGIN OF LICHEN STRUCTURES

The two organisms, fungus and alga, that enter into the composition
of the lichen plant are each characterized by the simplicity of their
original structure in which there is little or no differentiation into
tissues. The gonidia-forming algae are many of them unicellular, and
increase mainly by division or by sporulation into daughter-cells which
become rounded off and repeat the life of the mother-cell; others,
belonging to different genera, are filaments, mostly of single cell-rows,
with apical growth. The hyphal elements of the lichen are derived from
fungi in which the vegetative body is composed of branching filaments, a
character which persists in the lichen thallus.

The union of the two symbionts has stimulated both, but more especially
the fungus, to new developments of vegetative form, in which the fungus,
as the predominant partner, provides the framework of the lichen
plant-body. Varied structures have been evolved in order to secure life
conditions favourable to both constituents, though more especially to
the alga; and as the close association of the assimilating and growing
tissues is maintained, the thallus thus formed is capable of indefinite
increase.


A. FORMS OF CELL-STRUCTURE

There is no true parenchyma or cellular structure in the lichen thallus
such as forms the ground tissue of the higher plants. The fungal hyphae
are persistently filamentous and either simple or branched. By frequent
and regular cell-division—always at right angles to the long axis—and
by coherent growth, a pseudoparenchyma may however be built up which
functions either as a protective or strengthening tissue (Fig. 36).

[Illustration: Fig. 36. Vertical section of young stage of stratose
thallus (_Xanthoria parietina_ Th. Fr.). _a_, plectenchyma of cortex;
_b_, medullary hyphae; _c_, gonidial zone. × 500 (after Schwendener).]

Lindau[311] proposed the name “plectenchyma” for the tangled weft of
hyphae that is the principal tissue system in fungi as well as lichens.
The more elaborated pseudoparenchyma he designates as “paraplectenchyma,”
while the term “prosoplectenchyma” he reserved for the fibrous or
chondroid strands of compact filaments that occur frequently in
the thallus of the larger fruticose lichens, and are of service in
strengthening the fronds. The term plectenchyma is now generally used for
pseudoparenchyma.


B. TYPES OF THALLUS

Three factors, according to Reinke[312], have been of influence in
determining the thalline development. The first, and most important, is
the necessity to provide for the work of photosynthesis on the part of
the alga. There is also the building up of a tissue that should serve
as a storage of reserve material, essential in a plant the existence
of which is prolonged far beyond the natural duration of either of the
component organisms; and, finally, there is the need of protecting the
long-lived plant as a whole though more particularly the alga.

Wallroth was the first to make a comparative study of the different
lichen thalli. He distinguished those lichens in which the green cells
and the colourless filaments are interspersed equally through the entire
thallus as “homoiomerous” (Fig. 2), and those in which there are distinct
layers of cortex, gonidia, and medulla, as “heteromerous” (Fig. 1),
terms which, though now considered of less importance in classification,
still persist and are of service in describing the position of the alga
with regard to the general structure. A less evident definition of the
different types of thallus has been proposed by Zukal[313] who divides
them into “endogenous” and “exogenous.”

_a._ ENDOGENOUS THALLUS. The term has been applied to a comparatively
small number of homoiomerous lichens in which the alga predominates in
the development, and determines the form of the thallus. These algae,
members of the Myxophyceae, are extremely gelatinous, and the hyphae
grow alongside or within the gelatinous sheath. In the simpler forms the
vegetative structure is of the most primitive type: the alga retains its
original character almost unchanged, and the ascomycetous fungus grows
along with and beside it (Fig. 4). Such are the minutely tufted thalli
of _Thermutis_ and _Spilonema_ and the longer strands of _Ephebe_, in
which the associated _Scytonema_ or _Stigonema_, filamentous blue-green
algae, though excited to excessive growth, scarcely lose their normal
appearance, making it difficult at times to recognize the lichenoid
character unless the fruits also are present.

Equally primitive in most cases is the structure of the thallus
associated with _Gloeocapsa_. The resulting lichens, _Pyrenopsis_,
_Psorotichia_, etc. are simply gelatinous crusts of the alga with a more
or less scanty intermingling of fungal hyphae.

In the Collemaceae, the gonidial cells of which are species of _Nostoc_
(Fig. 2), there appears a more developed thallus; but in general,
symbiosis in _Collema_ has wrought the minimum of change in the habit
of the alga, hence the indecision of the earlier botanists as to the
identification and classification of _Nostoc_ and _Collema_. Though
in many of the species of the genus _Collema_ no definite tissue is
formed, yet, under the influence of symbiosis, the plants become moulded
into variously shaped lobes which are specifically constant. In some
species there is an advance towards more elaboration of form in the
protective tissues of the apothecia, a layer of thin-walled plectenchyma
being occasionally formed beneath or around the fruit as in _Collema
granuliferum_.

In all these lichens, it is only the thallus that can be considered as
primitive: the fruit is a more or less open apothecium—more rarely a
perithecium—with a fully developed hymenium. Frequently it is provided
with a protective thalline margin.

_b._ EXOGENOUS THALLUS. In this group, composed almost exclusively of
heteromerous lichens, Zukal includes all those in which the fungus takes
the lead in thalline development. He counts as such _Leptogium_, a genus
closely allied to _Collema_ but with more membranous lobes, in which
the short terminal cells of the hyphae have united to form a continuous
cortex. A higher development, therefore, becomes at once apparent, though
in some genera, as in _Coenogonium_, the alga still predominates, while
the simplest forms may be merely a scanty weft of filaments associated
with groups of algal cells. Such a thallus is characteristic of the
Ectolechiaceae, and some Gyalectaceae, etc., which have, indeed, been
described by Zahlbruckner[314] as homoiomerous though their gonidia
belong to the non-gelatinous Chlorophyceae.

Heteromerous lichens have been arranged by Hue[315] according to their
general structure in three great series:

1. _Stratosae._ Crustaceous, squamulose and foliose lichens with a
dorsiventral thallus.

2. _Radiatae._ Fruticose, shrubby or filamentous lichens with a
strap-shaped or cylindrical thallus of radiate structure.

3. _Stratosae-Radiatae._ Primary dorsiventral thallus, either crustaceous
or squamulose, with a secondary upright thallus of radiate structure
called the podetium (Cladoniaceae).


II. STRATOSE THALLUS


1. CRUSTACEOUS LICHENS


A. GENERAL STRUCTURE

In the series “Stratosae,” the plant is dorsiventral, the tissues
forming the thallus being arranged more or less regularly in strata one
above the other (Fig. 37). On the upper surface there is a hyphal layer
constituting a cortex, either rudimentary or highly elaborated; beneath
the cortex is situated the gonidial zone composed of algae and hyphae in
close association; and deeper down the medulla, generally a loose tissue
of branching hyphae. The lower cortex which abuts on the medulla may be
as fully developed as the upper or it may be absent.

[Illustration: Fig. 37. Vertical section of crustaceous lichen (_Lecanora
subfusca_ var. _chlarona_ Hue) on bark. _a_, lichen cortex; _b_, gonidia;
_c_, cells of the periderm. × 100.]

The growing tissue is chiefly marginal; the hyphae on the outer edge
remain “meristematic”[316] and provide for horizontal as well as vertical
extension; and there is also continual increase of the algal cells. There
is in addition a certain amount of intercalary growth due to the activity
of the gonidial tissue, both algal and fungal, providing for the renewal
of the cortex, and even interposing new tissue.


B. SAXICOLOUS LICHENS

_a._ EPILITHIC LICHENS. The crustaceous lichens forming this group spread
over the rock surfaces. The support must be stable to allow the necessary
time for the slowly developing organism, and therefore rocks that are
friable or subject to continual weathering are bare of lichens.

_aa._ =Hypothallus or Prothallus.= The first stage of growth in the
lichen thallus can be most easily traced in epilithic crustaceous
species, especially in those that inhabit a smooth rock surface. The
spore, on germination, produces a delicate branching septate mycelium
which radiates on all sides, as was so well observed and recorded by
Tulasne[317] in _Verrucaria muralis_ (Fig. 14). Zukal[318] has called
this first beginning the prothallus. In time the cell-walls of the
filaments become much thicker and though, in some species, they remain
colourless, in others they become dark-coloured, all except the extreme
tips, owing to the presence of lichen pigments—a provision, Zukal[319]
considers, to protect them against the ravages of insects, etc. The
prothallic filaments adhere closely to the substratum and the branching
becomes gradually more dendroid in form, though sometimes hyphae are
united into strands, or even form a kind of plectenchymatous tissue.
This purely hyphal stage may persist for long periods without much
change. In time there may be a fortuitous encounter with the algae (Fig.
38 A) which become the gonidia of the plant. Either these have been
already established on the substratum as free-growing organisms, or, as
accidentally conveyed, they alight on the prothallus. The contact between
alga and hypha excites both to active growth and to cell-division;
and the rapidly multiplying gonidia are as speedily surrounded by the
vigorously growing hyphal filaments.

[Illustration: Fig. 38 A. Hypothallus of _Rhizocarpon confervoides_ DC.,
from the extreme edge, with loose gonidia × 600.]

[Illustration: Fig. 38 B. Young thallus of _Rhizocarpon confervoides_
DC., with various centres of gonidial growth on the hypothallus × 30.]

Schwendener[320] has thus described the origin and further development
of prothallus and gonidia: on the dark-coloured proto- or prothallus,
he noted small nestling groups of green cells which he, at that time,
regarded as direct outgrowths from the lichen hyphae. These gonidial
cells, increasing by division, multiplied gradually and gathered into
a connected zone. He also observed that the hyphae in contact with the
gonidia became more thin-walled and produced many new branches. Some of
these newly formed branches grow upwards and form the cortex, others
grow downwards and build up the medulla or pith; the filaments at the
circumference continue to advance and may start new centres of gonidial
activity (Fig. 38 B). In many species, however, this prothallus or, as
it is usually termed at this stage, the hypothallus, becomes very soon
overgrown and obscured by the vigorous increase of the first formed
symbiotic tissue and can barely be seen as a white or dark line bordering
the thallus (Fig. 39). Schwendener[321] has stated that probably only
lichens that develop from the spore are distinguished by a protothallus,
and that those arising from soredia do not form these first creeping
filaments.

[Illustration: Fig. 39. _Lecanora parella_ Ach. Determinate thallus with
white bordering hypothallus, reduced (M. P., _Photo._).]

_bb._ =Formation of crustaceous tissues.= Some crustaceous lichens have a
persistently scanty furfuraceous crust, the vegetative development never
advancing much beyond the first rather loose association of gonidia and
hyphae; but in those in which a distinct crust or granules are formed,
three different strata of tissue are discernible:

1st. An upper cortical tissue of interlaced hyphae with frequent
septation and with swollen gelatinous walls, closely compacted and with
the lumen of the cells almost obliterated, not unfrequently a layer of
mucilage serving as an outer cuticle. This type of cortex has been called
by Hue[322] “decomposed.” It is subject to constant surface weathering,
thin layers being continually peeled off, but it is as continually being
renewed endogenously by the upward growth of hyphae from the active
gonidial zone. Exceptions to this type of cortex in crustaceous lichens
are found in some _Pertusariae_ where a secondary plectenchymatous cortex
is formed, and in _Dirina_ where it is fastigiate[323] as in _Roccella_.

2nd. The gonidial zone—a somewhat irregular layer of algae and hyphae
below the cortex—which varies in thickness according to the species.

3rd. The medullary tissue of somewhat loosely intermingled branching
hyphae, with generally rather swollen walls and narrow lumen. It rests
directly on the substratum and follows every inequality and crack so
closely, even where it does not penetrate, that the thallus cannot be
detached without breaking it away.

In _Verrucaria mucosa_, a smooth brown maritime lichen found on rocks
between tide-levels, the thallus is composed of tightly packed vertical
rows of hyphae, slender, rather thin-walled, and divided into short
cells. The gonidia are chiefly massed towards the upper surface, but they
also occur in vertical rows in the medulla. One or two of the upper cells
are brown and form an even cortex. The same formation occurs in some
other sea-washed species; the arrangement of the tissue elements recalls
that of crustaceous Florideae such as _Hildenbrandtia_, _Cruoria_, etc.

[Illustration: Fig. 40. Young thallus of _Rhizocarpon geographicum_ DC.,
with primary and subsequent (dotted lines) areolation × 5.]

_cc._ =Formation of areolae.= An “areolate” thallus is seamed and
scored by cracks of varying width and depth which divide it into minute
compartments. These cracks or fissures or chinks originate in two ways
depending on the presence or absence of hypothallic hyphae. Where the
hypothallus is active, new areolae arise when the filaments encounter new
groups of algae. More vigorous growth starts at once and proceeds on all
sides from these algal centres, until similarly formed areolae are met, a
more or less pronounced fissure marking the limits of each. This primary
areolation, termed rimose or rimulose, is well seen in the thin smooth
thallus of _Rhizocarpon geographicum_ (Fig. 40); but the first-formed
areolae are also very frequently slightly marked by subsequent cracks
due to unequal growth. The areolation caused by primary growth conditions
tends to become gradually less obvious or to disappear altogether.

Secondary areolation is due to unequal intercalary growth of the
otherwise continuous thallus[324]. A more active increase of any minute
portions provokes a tension or straining of the cortex between the
swollen areas and the surrounding more sluggish tissues; the surface
layers give way and chinks arise, a condition described by older
lichenologists as “rimose-diffract” or sometimes as “rhagadiose.” The
thallus is generally thicker, more broken and granular in the older
central parts of the lichen. Towards the circumference, where the tissue
is thinner and growth more equal, the chinks are less evident. Sometimes
the more vigorously growing areolae may extend over those immediately
adjoining, in which case the covered portions become brown and their
gonidia gradually disappear.

Strongly marked intersecting lines, similar to those round the margin
of the thallus, are formed when hypothalli that have themselves started
from different centres touch each other. A large continuous patch of
crustaceous thallus may thus be composed of many individuals (Fig. 41).

[Illustration: Fig. 41. _Rhizocarpon geographicum_ DC. on boulder,
reduced (M. P., _Photo._).]

_b._ ENDOLITHIC LICHENS. In many species, only the lower hyphae penetrate
the substratum either of rock or soil. In a few, more especially
those growing on limestone, the greater part or even the whole of the
vegetative thallus and sometimes also the fruits are, to some extent,
immersed in the rock. It has now been demonstrated that a number
of lichens, formerly described as athalline, possess a considerable
vegetative body which cannot be examined until the limestone in which
they are embedded is dissolved by acids. One such species, _Petractis_
(_Gyalecta_) _exanthematica_, studied by Steiner[325] and later by
Fünfstück[326], is associated with the blue-green filamentous alga,
_Scytonema_, and is homoiomerous in structure, the alga growing through
and permeating the whole of the embedded thallus. A partly homoiomerous
thallus, associated with _Trentepohlia_, has been described by
Bachmann[327]. He found the bright-yellow filaments of the alga covering
the surface of a calcareous rock. By reason of their apical growth,
they pierced the rock and dissolved a way for themselves, not only
among the loose particles, but right through a clear calcium crystal
reaching generally to a depth of about 200µ, though isolated threads
had gone 350µ below the surface. Near the outside the tendency was for
the algae to become stouter and to increase by intercalary growth and
by budded yeast-like outgrowths; lower down they were somewhat smaller.
The hyphae that became united with the algae were unusually slender and
were characterized by frequent anastomoses. They closely surrounded the
gonidia and also filled the loose spaces of the limestone with their fine
thread-like strands. Though oil was undoubtedly present in the lower
hyphae there were no swollen nor sphaeroid cells[328]. Some interesting
experiments with moisture proved that the part of the rock permeated with
the lichen absorbed much more water and retained it longer than the part
that was lichen-free.

Generally the embedded tissues follow the same order as in other
crustaceous lichens: an upper layer of cortical hyphae, next a gonidial
zone, and beneath that an interlaced tissue of medullary or rhizoidal
hyphae which often form fat-cells[328]. Friedrich[329] has given
measurements of the immersed thallus of _Lecanora_ (_Biatorella_)
_simplex_: under a cortical layer of hyphae there was a gonidial zone
600-700µ thick, while the lower hyphae reached a depth of 12 mm.; he has
also recorded an instance of a thallus reaching a depth of 30 mm.

On siliceous rocks such as granite, rhizoidal hyphae penetrate the rock
chiefly between the thin separable flakes of mica. Bachmann[330] has
recognized in these conditions three distinct series of cell-formations:
(1) slender long-celled sparsely branched hyphae which form a network
by frequent anastomoses; (2) further down, though only occasionally,
hyphae with short thick-walled bead-like cells; and (3) beneath these,
but only in or near mica crystals, spherical cells containing oil or some
albuminous substance.

_c._ CHEMICAL NATURE OF THE SUBSTRATUM. Lichens growing on calcareous
rocks or soils are more or less endolithic, those on siliceous rocks
are largely epilithic, but Bachmann[331] found that the mica crystals
in granite were penetrated, much in the same way as limestone, by the
lichen hyphae. These travel through the mica in all directions, though
they tend to follow the line of cleavage, thus taking the direction of
least cohesion. He found that oil-hyphae were formed, and also certain
peculiar bristle-like terminal branches; in other cases there were thin
layers of plectenchyma, and gonidia were also present. If however felspar
or quartz crystals, no matter how thin, blocked the way, further growth
was arrested, the hyphae being unable to pierce through or even to leave
any trace on the quartz[332]. On granite containing no mica constituents
the hyphae can only follow the cracks between the different impenetrable
crystals.

Stahlecker[333] has confirmed Bachmann’s observations, but he considers
that the difference in habit and structure between the endolithic and
epilithic series of lichens is due rather to the chemical than to the
physical nature of the substratum. Thus in a rock of mixed composition
such as granite, the more basic constituents are preferred by the hyphae,
and are the first to be surrounded: mica, when present, is at once
penetrated; particles of hornblende, which contain 40 to 50 per cent.
only of silicic acid, are laid hold of by the filaments of the lichen
before the felspar, of which the acid content is about 60 per cent.;
quartz grains which are pure silica are attacked last of all, though in
the course of time they also become corroded.

The character of the substratum also affects to a great extent the
comparative development of the different thalline layers: the hyphal
tissues in silicicolous lichens are much thinner than in lichens on
limestone, and the gonidial zone is correspondingly wider. In a species
of _Staurothele_ on granite, Stahlecker[333] estimated the gonidial
zone to be about 600 µ thick, while the lower medullary hyphae, partly
burrowing into the rock, measured about 6 mm. Other measurements at
different parts of the thallus gave a rhizoidal depth of 3 mm., while on
a more finely granular substratum, with a gonidial zone of 350 µ, the
rhizoidal hyphae measured only 1-1/2 mm. On calcareous rocks, on the
contrary, with a gonidial zone that is certainly no larger, the hyphal
elements penetrate the rock to varying depths down to 15 mm. or even more.

Lang[334] has recorded equally interesting measurements for _Sarcogyne_
(_Biatorella_) _latericola_: on slaty rock which contained no mixture
of lime, the gonidial zone had a thickness of 80 µ, a considerable
proportion of the very thin thallus. Fünfstück[335] has indeed suggested
that this lichen on acid rocks is only a starved condition of
_Sarcogyne_ (_Biatorella_) _simplex_, which on calcareous rocks, though
with a broader gonidial zone, has, as noted above, a correspondingly much
larger hyphal tissue.

Stahlecker’s theory is that the hyphae require more energy to grow in the
acid conditions that prevail in siliceous rocks, and therefore they make
larger demands on the algal symbionts. It follows that the latter must be
stimulated to more abundant growth than in circumstances favourable to
the fungus, such as are found in basic (calcareous) rocks; he concludes
that on the acid (siliceous) rocks, the epilithic or superficial
condition is not only a physical but a biological necessity, to enable
the algae to grow and multiply in a zone well exposed to light with full
opportunity for active photosynthesis and healthy increase.


C. CORTICOLOUS LICHENS

The crustaceous lichens occurring on bark or on dead wood, like those
on rocks, are either partly or wholly immersed in the substratum
(hypophloeodal), or they grow on the surface (epiphloeodal); but even
those with a superficial crust are anchored by the lower hyphae which
enter any crack or crevice of wood or bark and so securely attach the
thallus, that it can only be removed by cutting away the underlying
substance.

_a._ EPIPHLOEODAL LICHENS. These lichens originate in the same way as the
corresponding epilithic series from soredia or from germinating spores,
and follow the same stages of growth; first a hypothallus with subsequent
colonization of gonidia, the formation of granules, areolae, etc. The
small compartments are formed as primary or secondary areolae; the larger
spaces are marked out by the encounter of hypothalli starting from
different centres.

The thickness of the thallus varies considerably according to the
species. In some _Pertusariae_ with a stoutish irregular crust there is
a narrow amorphous cortical layer of almost obliterated cells, a thin
gonidial zone about 35 µ in width and a massive rather dense medulla
of colourless hyphae. Darbishire[336] has described and figured in
_Varicellaria microsticta_, one of the Pertusariaceae, single hyphae
that extend like beams across the wide medulla and connect the two
cortices. In some _Lecanorae_ and _Lecideae_ there is, on the contrary,
an extremely thin thallus consisting of groups of algae and loose fungal
filaments, which grow over and between the dead cork cells of the
outer bark. On palings, there is often a fairly substantial granular
crust present, with a gonidial zone up to about 80 µ thick, while the
underlying or medullary hyphae burrow among the dead wood fibres.

_b._ HYPOPHLOEODAL LICHENS. These immersed lichens are comparable
with the endolithic species of the rock formations, as their thallus
is almost entirely developed under the outer bark of the tree. They
are recognizable, even in the absence of any fructification, by the
somewhat shining brownish, white or olive-green patches that indicate
the underlying lichen. This type of thallus occurs in widely separated
families and genera, _Lecidea_, _Lecanora_, etc., but it is most constant
in Graphideae and in those Pyrenolichens of which the algal symbiont
belongs to the genus _Trentepohlia_. The development of these lichens is
of peculiar interest as it has been proved that though both symbionts
are embedded in the corky tissues, the hyphae arrive there first, and,
at some later stage, are followed by the gonidia. There is therefore no
question of the alga being a “captured slave” or “unwilling mate.”

Frank[337] made a thorough study of several subcortical forms. He found
that in _Arthonia radiata_, the first outwardly visible indication of the
presence of the lichen on ash bark was a greenish spot quite distinct
from the normal dull-grey colour of the periderm. Usually the spots are
round in outline, but they tend to become ellipsoid in a horizontal
direction, being influenced by the growth in thickness of the tree.
At this early stage only hyphae are present; Bornet[338] as well as
Frank described the outer periderm cells as penetrated and crammed with
the colourless slender filaments. Lindau[339], in a more recent work,
disputes that statement: he found that the hyphae invariably grew between
the dead cork cells, splitting them up and disintegrating the bark, but
never piercing the membranes. The purely prothallic condition, as a weft
of closely entangled hyphae, may last, Frank considers, for a long period
in an almost quiescent condition—possibly for several years—before the
gonidia arrive.

It is always difficult to observe the entrance of the gonidia but they
seem to spread first under the second or third layers of the periderm.
With care it is possible to trace a filament of _Trentepohlia_ from
the surface downwards, and to see that the foremost cell is really the
growing and advancing apex of the creeping alga. Both symbionts show
increased vigour when they encounter each other: the thallus at once
develops in extent and in depth, and, ultimately, reproductive bodies are
formed. In some species the apothecia or perithecia alone emerge above
the bark, in others the outer peridermal cells are thrown off, and the
thallus thus becomes superficial to some extent as a white scurfy or
furfuraceous crust.

The change from a hypophloeodal to a partly epiphloeodal condition
depends largely on the nature of the bark. Frank[337] found that
_Lecanora pallida_ remained for a long time immersed when growing on the
thick rugged bark of oak trunks. When well lighted, or on trees with a
thin periderm, such as the ash, the lichen emerges much earlier and
becomes superficial.

Black (or occasionally white) lines intersect the thallus and mark, as
in saxicolous lichens (Fig. 41), the boundary lines between different
individuals or different species. The pioneer hyphae of certain lichens
very frequently become dark-coloured, and Bitter[340] has suggested
as the reason for this that in damp weather the hypothallic growth is
exceptionally vigorous. When dry weather supervenes, with high winds or
strong sunshine, the outlying hyphae, unprotected by the thallus, become
dark-coloured. On the return of more normal conditions the blackened
tips are thrown off. Bitter further states that species of Graphideae do
not form a permanent black limiting line when they grow in an isolated
position: it is only when their advance is checked by some other thallus
that the dark persistent edge appears, a characteristic also to be seen
in the crust of other lichens. The dark boundary is always more marked
in sunny exposed situations: in the shade, the line is reduced to a mere
thread.

Bitter’s restriction of black boundary lines to cases of encountering
thalli only, would exclude the comparison one is tempted to make between
the advancing hyphae of lichens and those of many woody fungi where the
extreme edge of the white invaded woody tissue is marked by a dark line.
In the latter case however it is the cells of the host that are stained
black by the fungus pigment.


2. SQUAMULOSE LICHENS


A. DEVELOPMENT OF THE SQUAMULE

The crustaceous thallus is more or less firmly adherent to, or confused
with, the substratum. Further advance to a new type of thallus is made
when certain hyphal cells of soredium or granule take the lead in
an ascending direction both upwards and outwards. As growth becomes
definitely apical or one-sided, the structure rises free from the
substratum, and small lobules or leaflet-like squamules are formed. Each
squamule in this type of thallus is distinct in origin and not merely the
branch of a larger whole.

In a few lichens the advance from the crustaceous to the squamulose
structure is very slight. The granules seem but to have been flattened
out at one side, and raised into minute rounded projections such as those
that compose the thallus of _Lecanora badia_ generally described as
“subsquamulose.” The squamulose formation is more pronounced in _Lecidea
ostreata_, and in some species of _Pannaria_; and the whole thallus may
finally consist of small separate lobes as in _Lecidea lurida_, _Lecanora
crassa_, _L. saxicola_, species of _Dermatocarpon_ and the primary
thallus of the _Cladoniae_. Most of these squamules are of a firm texture
and more or less round in outline; in some species of _Cladonia_, etc.,
they are variously crenate, or cut into pinnate-like leaflets. Squamulose
lichens grow mostly on rocks or soil, occasionally on dead wood, and are
generally attached by single rhizoidal hyphae, either produced at all
points of the under surface, or from the base only, growth in the latter
case being one-sided. In a few instances, as in _Heppia Guepini_, there
is a central hold-fast.

A frequent type of squamulose thallus is that termed “placodioid,” or
“effigurate,” in which the squamulose character is chiefly apparent at
the circumference. The thallus is more or less orbicular in outline; the
centre may be squamulose or granular and cracked into areolae; the outer
edge is composed of radiating lobules closely appressed to the substratum
(Fig. 42).

[Illustration: Fig. 42. _Placodium murorum_ DC. Part of placodioid
thallus with apothecia × 2.]

All lichens with this type of thallus were at one time included in the
genus _Placodium_, now restricted by some lichenologists to squamulose
or crustaceous species with polarilocular spores. Many of them rival
_Xanthoria parietina_ in their brilliant yellow colouring.

[Illustration: Fig. 43. _Lecania candicans_ A. Zahlbr., with placodioid
thallus, reduced (S. H., _Photo._).]

There are also greyish-white effigurate lichens such as _Lecanora
saxicola_, _Lecania candicans_ (Fig. 43) and _Buellia canescens_,
well-known British species.


B. TISSUES OF SQUAMULOSE THALLUS

The anatomical structure of the squamules is in general somewhat similar
to that of the crustaceous thallus: an upper cortex, a gonidial zone,
and below that a medullary layer of loose hyphae with sometimes a lower
cortex.

1. The upper cortex, as in crustaceous lichens, is generally of
the “decomposed”[341] or amorphous type: interlaced hyphae with
thick gelatinous walls. A more highly developed form is apparent in
_Parmeliella_ and _Pannaria_ where the upper cortex is formed of
plectenchyma, while in the squamules of _Heppia_ the whole structure is
built up of plectenchyma, with the exception of a narrow band of loose
hyphae in the central pith.

2. The gonidia are Myxophyceae or Chlorophyceae; the squamules in some
instances may be homoiomerous as in _Lepidocollema_, but generally they
belong to the heteromerous series, with the gonidia in a circumscribed
zone, and either continuous or in groups. Friedrich[342] held that,
as in crustaceous lichens the development of the gonidial as compared
with the other tissues depended on the substratum. The squamules of
_Pannaria microphylla_ on sandstone were 100 µ thick, and the gonidial
layer occupied 80 or 90 µ of the whole[343]. With that may be compared
_Placodium Garovagli_ on lime-containing rock: the gonidial layer
measured only 50 µ across, the pith hyphae 280 µ and the rhizoidal hyphae
that penetrated the rock 500 µ.

3. The medullary layer, as a rule, is of closely compacted hyphae which
give solidity to the squamules; in those of _Heppia_ it is almost
entirely formed of plectenchyma.

4. The lower cortex is frequently little developed or absent, especially
when the squamules are closely applied to the support as in some
species of _Dermatocarpon_. In some of the squamulose _Lecanorae_ (_L.
crassa_ and _L. saxicola_) the lowest hyphae are somewhat more closely
interwoven; they become brown in colour, and the lichen is attached to
the substratum by rhizoid-like branches. In _Lecanora lentigera_ there is
a layer of parallel hyphae along the under surface. Further development
is reached when a plectenchyma of thick-walled cells is formed both above
and below, as in _Psoroma hypnorum_, though on the under surface the
continuity is often broken. The squamules of _Cladoniae_ are described
under the radiate-stratose series.


3. FOLIOSE LICHENS


A. DEVELOPMENT OF FOLIOSE THALLUS

The larger leafy lichens are occasionally monophyllous and attached at
a central point as in _Umbilicaria_, but mostly they are broken up into
lobes which are either imbricate and crowded, or represent the dividing
and branching of the expanding thallus at the circumference. They are
horizontal spreading structures, with marginal and apical growth. The
several tissues of the squamule are repeated in the foliose thallus,
but further provision is made to meet the requirements of the larger
organism. There is the greater development of cortical tissue, especially
on the lower surface, and the more abundant formation of rhizoidal organs
to attach the large flat fronds to the support. There are also various
adaptations to secure the aeration of the internal tissues[344].


B. CORTICAL TISSUES

Schwendener[345] was the first who, with the improved microscope, made
a systematic study of the minute structure of lichens. He examined
typical species in genera of widely different groups and described their
anatomy in detail. The most variable and perhaps the most important of
the tissues of lichens is the cortex, which is most fully developed in
the larger thalli, and as the same type of cortical structures recurs in
lichens widely different in affinity as well as in form, it seems well to
group together here the ascertained facts about these covering layers.

_a._ TYPES OF CORTICAL STRUCTURE. Zukal[346], and more recently Hue[347],
have made independent studies in the comparative morphology of the
thallus and have given particular attention to the different varieties of
cortex. They each find that the variations come under a definite series
of types. Zukal recognized five of these:

1. =Pseudoparenchymatous= (plectenchyma): by frequent septation of
regularly arranged hyphae and by coalescence a kind of continuous
cell-structure is formed.

2. =Palisade cells=: the outer elongate ends of the hyphae lie close
together in a direction at right angles to the surface of the thallus and
form a coherent row of parallel cells.

3. =Fibrous=: the cortical hyphae lie in strands of fine filaments
parallel with the surface of the thallus.

4. =Intricate=: hyphae confusedly interwoven and becoming dark in colour
form the lower cortex of some foliose lichens.

These four types, Zukal finds, are practically without interstices in the
tissue and form a perfect protection against excessive transpiration. He
adds yet another form:

5. =A cortex formed of hyphae with dark-coloured swollen cells=, which is
not a protection against transpiration. It occurs among lower crustaceous
forms.

Hue has summed up the different varieties under four types, but as he has
omitted the “fibrous” cortex, we arrive again at five different kinds of
cortical formation, though they do not exactly correspond to those of
Zukal. A definite name is given to each type:

1. =Intricate=: an intricate dense layer of gelatinous-walled hyphae,
branching in all directions, but not coalescent (Fig. 44). This rather
unusual type of cortex occurs in _Sphaerophorus_ and _Stereocaulon_, both
of which have an upright rigid thallus (fruticose).

[Illustration: Fig. 44. _Sphaerophorus coralloides_ Pers. Transverse
section of cortex and gonidial layer near the growing point of a frond ×
600.]

[Illustration: Fig. 45. _Roccella fuciformis_ DC. Transverse section of
cortex near the growing point of a frond × 600.]

2. =Fastigiate=: the hyphae bend outwards or upwards to form the cortex.
A primary filament can be distinguished with abundant branches, all
tending in the same direction; anastomosis may take place between the
hyphae. The end branches are densely packed, though there are occasional
interstices (Fig. 45). Such a cortex occurs in _Thamnolia_; in several
genera of Roccellaceae—_Roccellographa_, _Roccellina_, _Reinkella_,
_Pentagenella_, _Combea_, _Schizopelte_ and _Roccella_—and also in the
crustaceous genus _Dirina_. The fastigiate cortex corresponds with
Zukal’s palisade cells.

3. =Decomposed=: in this, the most frequent type of cortex, the hyphae
that travel up from the gonidial layer become irregularly branched and
frequently septate. The cell-walls of the terminal branches become
swollen into a gelatinous mass, the transformation being brought about by
a change in the molecular constituents of the cell-walls which permits
the imbibition and storage of water. The tissue, owing to the enormous
increase of the wall, is so closely pressed together that the individual
hyphae become indistinct; the cell-lumen finally disappears altogether,
or, at most, is only to be detected in section as a narrow disconnected
dark streak. The decomposed cortex is characteristic of many lichens,
crustaceous (Fig. 46) and squamulose, as well as of such highly developed
genera as _Usnea_, _Letharia_, _Ramalina_, _Cetraria_, _Evernia_ and
certain _Parmeliae_.

[Illustration: Fig. 46. _Lecanora glaucoma_ var. _corrugata_ Nyl.
Vertical section of cortex × 500 (after Hue).]

Zukal took no note of the decomposed cortex but the omission is
intentional and is due to his regarding the structure of the youngest
stages of the thallus near the growing point as the most typical and as
giving the best indication as to the true arrangement of hyphae in the
cortex. He thus describes palisade tissue as the characteristic cortex
of _Evernia_, since the formation near the growing point of the fronds
is somewhat palisade-like and he finds fibrous cortex at the tips of
_Usnea_ filaments. In both these instances Hue has described the cortex
as decomposed because he takes account only of the fully formed thallus
in which the tissues have reached a permanent condition.

[Illustration: Fig. 47. _Peltigera canina_ DC. Vertical section of cortex
and gonidial zone × 600.]

4. =Plectenchymatous=: the last of Hue’s types corresponds with the
first described by Zukal. It is the result of the lateral coherence and
frequent septation of the hyphae into short almost square or rounded
cells (Fig. 47). The simplest type of such a cortex can be studied in
_Leptogium_, a genus of gelatinous lichens in which the tips of the
hyphae are cut off at the surface by one or more septa. The resulting
cells are wider than the hyphae and they cohere together to form, in
some species, disconnected patches of cells; in others, a continuous
cortical covering one or more cells thick, while in the margin of the
apothecium they form a deep cellular layer. The cellular type of cortex
is found also, as already stated, in some crustaceous _Pertusariae_, and
in a few squamulose genera or species. It forms the uppermost layer of
the _Peltigera_ thallus and both cortices of many of the larger foliose
lichens such as _Sticta_, _Parmelia_, etc.

5. The “fibrous” cortex must be added to this series, as was pointed
out by Heber Howe[348] who gave the less appropriate designation of
“simple” to the type. It consists of long rather sparingly branched
slender hyphae that grow in a direction parallel with the surface of the
thallus (Fig. 48). It is characteristic of several fruticose and foliose
lichens with more or less upright growth, such as we find in several of
the _Physciae_, and in the allied genus _Teloschistes_, in _Alectoria_,
several genera of Roccellaceae, in _Usnea_ _longissima_ and in _Parmelia
pubescens_, etc. Zukal would have included all the _Usneae_ as the tips
are fibrous.

[Illustration: Fig. 48. _Physcia ciliaris_ DC. Vertical section of
thallus. _a_, cortex; _b_, gonidial zone; _c_, medulla. × 100.]

More than one type of cortex, as already stated, may appear in a genus: a
striking instance of variability occurs in _Solorina_ where, as Hue[349]
has pointed out, the cortex of _S. octospora_ is fastigiate, that of
all the other species being plectenchymatous. Cortical development is a
specific rather than a generic characteristic.

_b._ ORIGIN OF VARIATION IN CORTICAL STRUCTURE. The immediate causes
making for differentiation in cortical development are: the prevailing
direction of growth of the hyphae as they rise from the gonidial zone;
the amount of branching and the crowding of the filaments; the frequency
of septation; and the thickening or degeneration of the cell-walls which
may become almost or entirely mucilaginous. In the plectenchymatous
cortex, the walls may remain quite thin and the cells small as in
_Xanthoria parietina_, or the walls may be much thickened as in both
cortices of _Sticta_. As a result of stretching the cell may increase
enormously in size: in some instances where the internal hyphae are about
3 µ to 4 µ in width, the cortical cells formed from these hyphae may have
a cell cavity 15 µ to 16 µ in diameter.

_c._ LOSS AND RENEWAL OF CORTEX. Very frequently the cortex is covered
over by a layer of homogeneous mucilage which forms an outer cuticle.
It arises from the continual degeneration of the outer cell-walls and
it is liable to friction and removal by atmospheric agency as was
first described by Schwendener[350] in the weather-beaten cortex of
_Umbilicaria pustulata_. He had noted the irregular jagged outline of
the cross section of the thallus, and he then suggested, as the probable
reason, the decay of the outer rind with the constant renewal of it
by the hyphae from the underlying gonidial zone, though he was unable
definitely to prove his theory. The peeling of the dead outer layer (with
its replacement by new tissue) has however been observed many times
since his day. It has been described by Darbishire[351] in _Pertusaria_:
in that genus there is at first a primary cortex formed of hyphae that
grow in a radial direction, parallel to the surface of the thallus. The
walls of these hyphae become gradually more and more mucilaginous till
the cells are obliterated. Meanwhile short-celled filaments grow up in
serried ranks from the gonidial layer and finally push off the dead
“fibrous” cortex. The new tissue takes on a plectenchymatous character,
and the outer cells in time become decomposed and provide a mucilaginous
cuticle which in turn is also subject to wasting.

The same process of peeling was noted by Rosendahl[352] in some species
of brown _Parmeliae_, where the dead tissues were thrown off in shreds,
though only in isolated patches. But whether in patches or as a
continuous sheath, there is constant degeneration, with continual renewal
of the dead material from the internal tissues.

The cortex is the most highly developed of all the lichen structures
and is of immense importance to the plant as may be judged from the
various adaptations to different needs[353]. The cortical cell-walls
are frequently impregnated with some dark-coloured substance which, in
exposed situations, must counteract the influence of too direct sunlight
and be of service in sheltering the gonidia. Lichen acids—sometimes very
brightly coloured—and oxalic acid are deposited in the cortical tissues
in great abundance and aid in retaining moisture; but the two chief
functions to which the cortex is specially adapted are the checking
of transpiration and the strengthening of the thallus against external
strains.

_d._ CORTICAL HAIRS OR TRICHOMES. Though somewhat rare, cortical hairs
are present on the upper surface of several foliose lichens. They take
rise, in all the instances noted, as a prolongation of one of the
cell-rows forming a plectenchymatous cortex.

In _Peltidea_ (_Peltigera_) _aphthosa_ they are especially evident near
the growing edges of the thallus; and they take part in the development
of the superficial cephalodia[354] which are a constant feature of the
lichen. They tend to disappear with age and leave the central older
parts of the thallus smooth and shining. In several other species of
_Peltigera_ (_P. canina_, etc.) they are present and persist during the
life of the cortex. In these lichens the cells of the cortical tissue are
thin-walled, all except the outer layer, the membranes of which are much
thicker. The hairs rising from them are also thick-walled and septate.
Generally they branch in all directions and anastomose with neighbouring
hairs so that a confused felted tangle is formed; they vary in size but
are, as a rule, about double the width of the medullary hyphae as are the
cortical cells from which they rise. They disappear from the thallus,
frequently in patches, probably by weathering, but over large surfaces,
and especially where any inequality affords a shelter, they persist as a
soft down.

Hairs are also present on the upper surface of some _Parmeliae_.
Rosendahl[355] has described and figured them in _P. glabra_ and _P.
verruculifera_—short pointed unbranched hyphae, two or more septate and
with thickened walls. They are most easily seen near the edge of the
thallus, though they persist more or less over the surface; they also
grow on the margins of the apothecia. In _P. verruculifera_ they arise
from the soredia; in _P. glabra_ a few isolated hairs are present on the
under surface.

In _Nephromium tomentosum_ there is a scanty formation of hairs on the
upper surface. They are abundant on the lower surface, and function as
attaching organs. A thick tomentum of hairs is similarly present on the
lower surface of many of the Stictaceae either as an almost unbroken
covering or in scattered patches. In several species of _Leptogium_ they
grow out from the lower cortical cells and attach the thin horizontal
fronds; and very occasionally they are present in _Collema_.


C. GONIDIAL TISSUES

With the exception of some species of _Collema_ and _Leptogium_ lichens
included under the term foliose, are heteromerous in structure, and the
algae that form the gonidial zone are situated below the upper cortex
and, therefore, in the most favourable position for photosynthesis.
Whether belonging to the Myxophyceae or the Chlorophyceae, they form a
green band, straight and continuous in some forms, in others somewhat
broken up into groups. In certain species they push up at intervals among
the cortical cells, as in _Gyrophora_ and in _Parmelia tristis_. In
_Solorina crocea_ a regular series of gonidial pyramids rises towards the
upper surface. The green cells are frequently more dense at some points
than at others, and they may penetrate in groups well into the medulla.

The fungal tissue of the gonidial zone is composed of hyphae which
have thinner walls, and are generally somewhat loosely interlacing.
In _Peltigera_[356] the gonidial hyphae are so connected by frequent
branching and by anastomosis that a net-like structure is formed, in the
meshes of which the algae—a species of _Nostoc_—are massed more or less
in groups. In lichens with a plectenchymatous cortex, the cellular tissue
may extend downwards into the gonidial zone and the gonidia thus become
enmeshed among the cells, a type of formation well seen in the squamulose
species, _Dermatocarpon lachneum_ and _Heppia Guepini_, where the massive
plectenchyma of both the upper and lower cortices encroaches on the pith.
In _Endocarpon_ and in _Psoroma_ the gonidia are also surrounded by short
cells.

A similar type of structure occurs in _Cora Pavonia_, one of the
Hymenolichenes: the gonidial hyphae in that species form a cellular
tissue in which are embedded the blue-green _Chroococcus_ cells[357].


D. MEDULLA AND LOWER CORTEX

_a._ MEDULLA. The hyphal tissue of the dorsiventral thallus that lies
between the gonidial zone and the lower cortex or base of the plant is
always referred to as the medulla or pith. It is, as a rule, by far the
most considerable portion of the thallus. In _Parmelia caperata_ (Fig.
49), for instance, the lobes of which are about 300 µ thick, over 200 µ
of the space is occupied by this layer. It varies however very largely
in extent in different lichens according to species, and also according
to the substratum. In another _Parmelia_ with a very thin thallus, _P.
alpicola_ growing on quartzite, the medulla measures scarcely twice the
width of the gonidial zone. It forms a fairly massive tissue in some of
the crustaceous lichens—in some _Pertusariae_ and _Lecanorae_—attaining a
width of about 600 µ.

Nylander[358] distinguished three types of medullary tissue in lichens:

(1) _felted_, which includes all those of a purely filamentous structure;

(2) _cretaceous_ or _tartareous_, more compact than the felted, and
containing granular or crystalline substances as in some _Pertusariae_;
and lastly

(3) the _cellular_ medulla in which the closely packed hyphae are
divided into short cells and a kind of plectenchyma is formed, as in
_Lecanora_ (_Psoroma_) _hypnorum_, in _Endocarpon_, etc.

[Illustration: Fig. 49. _Parmelia caperata_ Ach. (S. H., _Photo._).]

The felted medulla is characteristic of most lichens and is formed
of loose slender branching septate hyphae with thickish walls. This
interwoven hyphal texture provides abundant air-spaces.

Hue[359] has noted that the walls of the medullary hyphae in _Parmeliae_
are smooth, unless they have been exposed to great extremes of heat or
cold, when they become wrinkled or scaly. They are very thick-walled in
_Peltigera_ (Fig. 50).

[Illustration: Fig. 50. Hyphae from lower medulla of _Peltigera canina_
DC. × 600.]

_b._ LOWER CORTEX. In some foliose lichens such as _Peltigera_ there is
no special tissue developed on the under surface. In _Lobaria pulmonaria_
large patches of the under surface are bare, and the medulla is exposed
to the outer atmosphere, sheltered only by its position. In some other
lichens the lowermost hyphae lie closer together and a kind of felt of
almost parallel filaments is formed, generally darker in colour, as in
_Lecanora lentigera_, and in some species of _Physcia_.

Most frequently however the tissues of the upper cortex are repeated
on the lower surface, though differing somewhat in detail. In all of
the brown _Parmeliae_, according to Rosendahl[360], the structure is
identical for both cortices, though the upper develops now hairs, now
isidia, breathing pores, etc., while the lower produces rhizinae. The
amorphous mucilaginous cuticle so often present on the upper surface
is absent from the lower, the walls of the latter being often charged
instead with dark-brown pigments.

_c._ HYPOTHALLIC STRUCTURES. An unusual development of hyphae from the
lower cortex occurs in the genera _Anzia_ and _Pannoparmelia_—both
closely related to _Parmelia_—whereby a loose sponge-like hypothallus of
anastomosing reticulate strands is formed. In one of the simpler types,
_Anzia colpodes_, a North American species, the hyphae passing out from
the lower medulla become abruptly dark-brown in colour, and are divided
into short thick-walled cells. Frequent branching and anastomosis of
these hyphae result in the formation of a cushion-like structure about
twice the bulk of the thallus. In another species from Australia (_A.
Japonica_) there is a lower cortex, distinct from the medulla, consisting
of septate colourless hyphae with thick walls. From these branch out free
filaments, similar in structure but dark in colour, which branch and
anastomose as in the previous species.

[Illustration: Fig. 51. _Pannoparmelia anzioides_ Darb. Vertical
section of thallus and hypothallus. _a_, cortex; _b_, gonidial zone;
_c_, medulla; _d_, lower cortex; _e_, hypothallus. × ca. 450 (after
Darbishire).]

In _Pannoparmelia_ the lower cortex and the outgrowths from it are
several cells thick; they may be thick-walled as in _Anzia_, or they
may be thin-walled as described and figured by Darbishire[361] in
_Pannoparmelia anzioides_, a species from Tierra del Fuego (Fig. 51). A
somewhat dense interwoven felt of hyphae occurs also in certain parts of
the under surface of _Parmelia physodes_[362].

This peculiar structure, regarded as a hypothallus, is probably of
service in the retention of moisture. The thick cell-walls in most of the
forms suggest some such function.


E. STRUCTURES FOR PROTECTION AND ATTACHMENT

Such structures are almost wholly confined to the larger foliose and
fruticose lichens and are all of the same simple type; they are fungal in
origin and very rarely are gonidia associated with them.

[Illustration: Fig. 52. _Usnea florida_ Web. Ciliate apothecia (S. H.,
_Photo._).]

_a._ CILIA. In a few widely separated lichens stoutish cilia are borne,
mostly on the margins of the thallus lobes, or on the margins of the
apothecia (Fig. 52). They arise from the cortical cells or hyphae,
several of which grow out in a compact strand which tapers gradually
to a point. Cilia vary in length up to about 1 cm. or even longer. In
some lichens they retain the colour of the cortex and are greyish
or whitish-grey, as in _Physcia ciliaris_ or in _Physcia hispida_
(Fig. 110). They provide a yellow fringe to the apothecia of _Physcia
chrysophthalma_ and a green fringe to those of _Usnea florida_. They
are dark-brown or almost black in _Parmelia perlata_ var. _ciliata_ and
in _P. cetrata_, etc. as also in _Gyrophora cylindrica_. The fronds of
_Cetraria islandica_ and other species of the genus are bordered with
short spinulose brown hairs whose main function seems to be the bearing
of “pycnidia” though in many cases they are barren (Fig. 128).

Superficial cilia are more rarely formed than marginal ones, but they
are characteristic of one not uncommon British species, _Parmelia
proboscidea_ (_P. pilosella_ Hue). Scattered over the surface of that
lichen are numerous crowded groups of isidia which, frequently, are
prolonged upwards as dark-brown or blackish cilia. Nearly every isidium
bears a small brown spot on the apex at an early stage of growth. Similar
cilia are sparsely scattered over the thallus, but their base is always
a rather stouter grey structure, which suggests an isidial origin. Cilia
also occur on the margin of the lobes.

As lichens are a favourite food of snails, insects, etc., it is
considered that these structures are protective in function, and that
they impede, if they do not entirely prevent, the larger marauders in
their work of destruction.

[Illustration: Fig. 53. Rhizoid of _Parmelia exasperata_ Carroll (_P.
aspidota_ Rosend.). A, hyphae growing out from lower cortex × 450. B, tip
of rhizoid with gelatinous sheath × 335 (after Rosendahl).]

_b._ RHIZINAE. Lichen rootlets are mainly for the purpose of attachment
and have little significance as organs of absorption. They have been
noted in only one crustaceous lichen, _Varicellaria microsticta_[363],
an alpine species that spreads over bark or soil, and which is further
distinguished by being provided with a lower cortex of plectenchyma.
In foliose lichens they are frequently abundant, though by no means
universal, and attach the spreading fronds to the support. They
originate, as Schwendener[364] pointed out, from the outer cortical
cells, exactly as do the cilia, and are scattered over the under
surface or are confined to special areas. Rosendahl[365] has described
their development in the brown species of _Parmeliae_: the under cortex
in these lichens is formed of a cellular plectenchyma with thickish
walls; the rootlets arise by the outgrowth of several neighbouring cells
from some slight elevation near the edge of the thallus. Branching and
interlacing of these growing rhizinal hyphae follow, the outermost
frequently spreading outwards at right angles to the axis, and forming
a cellular cortex. The apex of the rhizoid is generally an enlarged
tuft of loose hyphae involved in mucilage (Fig. 53), a provision for
securing firmer cohesion to the support; or the tips spread out as a
kind of sucker. Not unfrequently neighbouring “rootlets” are connected
by mucilage at the tips, or by outgrowths of their hyphae, and a rather
large hold-fast sheath is formed.

[Illustration: Fig. 54. _Peltigera canina_ DC. (S. H., _Photo._).]

[Illustration: Fig. 55. _Peltigera canina_ DC. Under surface with veins
and rhizoids (after Reinke).]

In species of _Peltigera_ (Fig. 54) the rhizinae are confined to the
veins or ridges (Fig. 55); they are thickish at the base, and are
generally rather long and straggling. Meyer[366] states that the central
hyphae are stoutish and much entangled owing to the branching and
frequent anastomosis of one hypha with another; the peripheral terminal
branches are thinner-walled and free. These rhizinae vary in colour from
white in _Peltigera canina_ to brown or black in other species. Most
species of _Peltigera_ spread over grass or mosses, to which they cling
by these long loose “rootlets.”

Lichen rhizinae, distinguished by Reinke[367] as “aerial rhizinae,” are
more or less characteristic of all the species of _Parmelia_ with the
exception of those belonging to the subgenus _Hypogymnia_ in which they
are of very rare occurrence, arising, according to Bitter[368], only in
response to some external friction. They are invariably dark-coloured,
rather short, about one to a few millimetres in length, and are simple
or branched. The branches may go off at any angle and are sometimes
curved back at the ends in anchor-like fashion. The _Parmeliae_ grow on
firm substances, trees, rocks, etc., and the irregularities of their
attaching structures are conditioned by the obstacles encountered on the
substratum. Not unfrequently the lobes are attached by the rhizinae to
underlying portions of the thallus.

In the genus _Gyrophora_, the rhizinae are simple strands of hyphae
(_G. polyrhiza_) or they are corticate structures (_G. murina_, _G.
spodochroa_ and _G. vellea_). They are also present in species of
_Solorina_, _Ricasolia_, _Sticta_ and _Physcia_ and very sparingly in
_Cetraria_ (_Platysma_).

_c._ HAPTERA. Sernander[369] has grouped all the more distinctively
aerial organs of attachment, apart from rhizinae, under the term
“hapteron” and he has described a number of instances in which cilia and
even the growing points of the thallus may become transformed to haptera
or sucker-like sheaths.

The long cilia of _Physcia ciliaris_ occasionally form haptera at their
tips where the hyphae are loose and in active growing condition. Contact
with some substance induces branching by which a spreading sheath arises;
a plug-like process may also be developed which pierces the substance
encountered—not unfrequently another lobe of its own thallus. The long
flaccid fronds of _Evernia furfuracea_ are frequently connected together
by bridge-like haptera which rise at any angle of the thallus or from any
part of the surface.

The spinous hairs that border the thalline margins in _Cetraria_ may
also, in contact with some body—often another frond of the lichen—form a
hapteron, either while the spermogonium, which occupies the tip of the
spine, is still in a rudimentary stage, or after it has discharged its
spermatia. The small sucker sheath may in that case arise either from
the apex of the cilium, from the wall of the spermogonium or from its
base. By means of these haptera, not only different individuals become
united together, but instances are given by Sernander in which _Cetraria
islandica_, normally a ground lichen, had become epiphytic by attaching
itself in this way to the trunk of a tree (_Pinus sylvestris_).

In _Alectoria_, haptera are formed at the tip of the thallus filament as
an apical cone-like growth from which hyphae may branch out and penetrate
any convenient object. A species of this genus was thus found clinging
to stems of _Betula nana_. Apical haptera are very frequent in _Cladonia
rangiferina_ and _Cl. sylvatica_, induced here also by contact. These
two plants, as well as several species of _Cetraria_, tend, indeed, to
become entirely epiphytic on the heaths of the _Calluna_ formations.
Haptera similar to those of _Alectoria_ occur in _Usnea_, _Evernia_,
_Ramalina_ and _Cornicularia_ (_Cetraria_). In _Evernia prunastri_ var.
_stictoceros_, a heath form, the fronds become attached to the stems and
branches of _Erica tetralix_ by hapteroid strands of slender glutinous
hyphae which persist on the frond of the lichen after it is detached
as small very dark tubercles surmounted, as Parfitt[370] pointed out,
by a dark-brown grumous mass of cells. Plug-like haptera may be formed
at the base of _Cladoniae_ which attach them to each other and to the
substratum. The brightly coloured fronds of _Letharia vulpina_ are
attached to each other in somewhat tangled fashion by lateral bridges
or by fascicles of hyphae dark-brown at the base but colourless at the
apices, exactly like aerial adventitious rhizinae. They grow out from the
fronds generally at or near the tips and lay hold of a neighbouring frond
by means of mucilage. These haptera are evidently formed in response
to friction. Haptera along with other lichen attachments have received
considerable attention from Galløe[371]. He finds them arising on various
positions of the lichen fronds and has classified them accordingly.

After the haptera have become attached, they increase in size and
strength and supply a strong anchorage for the plant; the point of
contact frequently forms a basis for renewed growth while the part
beneath the hapteron may gradually die off. Haptera are more especially
characteristic of fruticose lichens, but Sernander considers that the
rhizinae of foliose species may function as haptera. They are important
organs of tundra and heath formations as they enable the lichens to get a
foothold in well-lighted positions, and by their aid the fronds are more
able to resist the extreme tearing strains to which they are subjected in
high and unsheltered moorlands.


F. STRENGTHENING TISSUES OF STRATOSE LICHENS

Squamulose and foliose lichens grow mostly in close relation with the
support, and the flat expanding thallus, as in the _Parmeliae_, is
attached at many points to the substance—tree, rock, etc.—over which the
plants spread. Special provision for support is therefore not required,
and the lobes remain thin and flaccid. Yet, in a number of widely
different genera the attachment to the substratum is very slight, and in
these we find an adaptation of existing tissues fitted to resist tearing
strains, resistance being almost invariably secured by the strengthening
of the cortical layers.

_a._ BY DEVELOPMENT OF THE CORTEX. Such a transformation of tissue is
well illustrated in _Heppia Guepini_. The thallus consists of rigid
squamules which are attached at one point only; the cortex of both
surfaces is plectenchymatous and very thick and even the medulla is
largely cellular.

The much larger but equally rigid coriaceous thallus of _Dermatocarpon
miniatum_ (Fig. 56) has also a single central attachment or umbilicus,
and both cortices consist of a compact many-layered plectenchyma. The
same structure occurs in _Umbilicaria pustulata_ and in some species of
_Gyrophora_, which, having only a single central hold-fast, gain the
necessary stiffening through the increase of the cortical layers.

[Illustration: Fig. 56. _Dermatocarpon miniatum_ Th. Fr. (S. H.,
_Photo._).]

In the Stictaceae there are a large number of widely-expanded forms,
and as the attachment depends mostly on a somewhat short tomentum,
strength is obtained here also by the thick plectenchymatous cortex of
both surfaces. When areas denuded of tomentum and cortex occur, as in
_Lobaria pulmonaria_, the under surface is not sensibly weakened, since
the cortical tissue remains connected in a stout and firm reticulation.

_b._ BY DEVELOPMENT OF VEINS OR NERVES. Certain ground lichens belonging
to the Peltigeraceae have a wide spreading thallus often with very large
lobes. The upper cortex is a many-layered plectenchyma, but the under
surface is covered only by a loose felt of hyphae which branch out into
a more or less dense tomentum. As the firm upper cortex continues to
increase by intercalary growth from the branching upwards of hyphae
from the meristematic gonidial zone, there occurs an extension of the
upper thallus with which the lower cannot keep pace[372]. A little
way back from the edge, the result of the stretching is seen in the
splitting asunder of the felted hyphae of the under surface, and in
the consequent formation of a reticulate series of ridges known as the
veins or nerves; they represent the original tomentose covering, and are
white, black or brown, according to the colour of the tomentum itself.
The naked ellipsoid interstices show the white medulla, and, if the veins
are wide, the colourless areas are correspondingly small. Rhizinae are
formed on the nerves in several of the species, and anchor the thallus to
the support. In _Peltigera canina_, the under surface is almost wholly
colourless, the veins are very prominent (Fig. 55), and are further
strengthened by the growth and branching of the parallel hyphae of which
they are composed. They serve to strengthen the large and flabby thallus
and form a rigid base for the long rhizinae by which the lichen clings to
the grass or moss over which it grows.

The most perfect development of strengthening nerves is to be found in
_Hydrothyria venosa_[373], a rather rare water lichen that occurs in
the streams of North America. It consists of fan-like lobes of thin
structure, the cortex being only about one cell thick. The fronds are
about 3 cm. wide and they are contracted below into a stalk which serves
to attach the plant to the substratum. Several fronds may grow together
in a dense tuft, the expanded upper portion floating freely in the water.
Frequently the plants form a dense growth over the rocky beds of the
stream.

At the point where the stalk expands into the free erect frond, there
arise a series of stout veins which spread upwards and outwards. They
are definitely formed structures and not adaptations of pre-existing
tissues: certain hyphae arise from the medulla at the contracted base of
the frond, take a radial direction and, by increase, become developed
into firm strands. The individual hyphae also increase in size, and the
swelling of the nerve gives rise to a ridge prominent on both surfaces.
They seldom anastomose at first but towards the tips they become smaller
and spread out in delicate ramifications which unite at various points.
There is no doubt, as Bitter[372] points out, that the nerves function as
strengthening tissues and preserve the frond from the strain of the water
currents which would, otherwise, tear apart the delicate texture.


III. RADIATE THALLUS


1. CHARACTERS OF RADIATE THALLUS

In the stratose dorsiventral thallus, there is a widely extended growing
area situated round the free margins of the thallus. In the radiate
thallus of the fruticose or filamentous lichens, growth is confined to an
apical region. Attachment to the substratum is at one point only—the base
of the plant—thus securing the exposure of all sides equally to light.
The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae)
lie in a zone or in groups between the cortex and the medulla. It is
the highest type of vegetative development in the lichen kingdom, since
it secures the widest room for the gonidial layer, and the largest
opportunity for photosynthesis.

[Illustration: Fig. 57. _Roccella fuciformis_ DC.]

Shrubby upright lichens consist mostly of strap-shaped fronds, either
simple or branched, which may be broadened to thin bands (Fig. 57) or may
be narrowed and thickened till they are almost cylindrical. The fronds
vary in length according to the species from a few millimetres upwards:
those of _Roccella_ have been found measuring 30 cm. in length; those of
_Ramalina reticulata_, the largest of all the American lichens, extend to
considerably more.

Lichens of filamentous growth are more or less cylindrical (Fig. 58).
They are in some species upright and of moderate length, but in a
few pendulous forms they grow to a great length: specimens of _Usnea
longissima_ have been recorded that measured 6 to 8 metres from base to
tip.

[Illustration: Fig. 58. _Usnea barbata_ Web. (S. H., _Photo._).]

The radiate type of thallus occurs in most of the lichen groups but
most frequently in the Gymnocarpeae. In gelatinous Discolichens it is
represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is
one very minute British lichen in that series, _Pyrenidium actinellum_,
and one from N. America, _Pyrenothamnia_, that are of fruticose habit.


2. INTERMEDIATE TYPES OF THALLUS

Between the foliose and the fruticose types, there are intermediate forms
that might be, and often are, classified now in one group and now in
the other. These are chiefly: _Physcia_ (_Anaptychia_) _ciliaris_, _Ph.
leucomelas_ and the species of _Evernia_.

In the two former the habit is more or less fruticose as the plants are
affixed to the substratum at a basal point, but the fronds are decumbent
and the internal structure is of the dorsiventral type: there is an
upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial
zone—the gonidia lying partly in the cortex and partly among the loose
hyphae of the medulla—and a lower cortex formed of a weft of hyphae which
also run somewhat parallel to the surface. Both species are distinguished
by the numerous marginal cilia, either pale or dark in colour. These two
lichens are greyish-coloured on the upper surface and greyish or whitish
below.

_Evernia furfuracea_ with a basal attachment[374], and with a partly
horizontal and partly upright growth, has a dorsiventral thallus, dark
greyish-green above and black beneath, with occasional rhizinae towards
the base. The cortex of both surfaces belongs to the “decomposed” type;
the gonidial zone lies below the upper surface, and the medullary tissue
is of loose hyphae. In certain forms of the species isidia are abundant
on the upper surface, a character of foliose rather than of fruticose
lichens. _E. furfuracea_ grows on trees and very frequently on palings.

[Illustration: Fig. 59. _Evernia prunastri_ Ach. (M. P., _Photo._).]

_E. prunastri_, the second species of the genus, is more distinctly
upright in habit, with a penetrating basal hold-fast and upright
strap-shaped branching fronds, light-greyish green on the “upper” surface
and white on the other (Fig. 59). The internal structure is sub-radiate;
both cortices are “decomposed”; the gonidial zone consists of somewhat
loose groups of algae, very constant below the “upper” surface, with an
occasional group in the pith near to the lower cortex in positions that
are more exposed to light. There is also a tendency for the gonidial zone
to pass round the margin and spread some way along the under side. The
medulla is of loose arachnoid texture and the whole plant is very limp
when moist. It grows on trees, often in dense clusters.


3. FRUTICOSE AND FILAMENTOUS


A. GENERAL STRUCTURE OF THALLUS

The conditions of strain and tension in the upright plant are entirely
different from those in the decumbent thallus, and to meet the new
requirements, new adaptations of structure are provided either in the
cortex or in the medulla.

CORTICAL STRUCTURES. With the exception of the distinctly
plectenchymatous cortex, all the other types already described recur in
fruticose lichens; in various ways they have been modified to provide not
only covering but support to the fronds.

_a._ =The fastigiate cortex.= This reaches its highest development in
_Roccella_ in which the branched hyphal tips, slightly clavate and
thick-walled, lie closely packed in palisade formation at right angles
to the main axis (Fig. 45). They afford not only bending power, but give
great consistency to the fronds. The cortex is further strengthened
in _R. fuciformis_[375] by the compact arrangement of the medullary
hyphae that run parallel with the surface, and among which occur single
thick-walled filaments. The plant grows on maritime rocks in very exposed
situations; and the narrow strap-shaped fronds, as stated above, may
attain a length of 30 cm., though usually they are from 10 to 18 cm. in
height. The same type of cortex, but less highly differentiated, affords
a certain amount of stiffness to the cylindrical much weaker fronds of
_Thamnolia_.

_b._ =The fibrous cortex.= This type is found in a number of lichens
with long filamentous hanging fronds. It consists of parallel hyphae,
rarely septate and rarely branched, but frequently anastomosing and
with strongly thickened “sclerotic” walls. Such a cortex is the only
strengthening element in _Alectoria_, and it affords great toughness and
flexibility to the thong-like thallus. It is also present in _Ramalina_
(_Alectoria_) _thrausta_, a species with slender fronds (Fig. 60).

[Illustration: Fig. 60. _Alectoria thrausta_ Ach. A, transverse section
of frond; _a_, cortex; _b_, gonidia; _c_, arachnoid medulla × 37. B,
fibrous hyphae from longitudinal section of cortex. × 430 (after Brandt).]

In _Usnea longissima_ the cortex both of the fibrillose branchlets and of
the main axis is fibrous, and is composed of narrow thick-walled hyphae
which grow in a long spiral round the central strand. The hyphae become
more frequently septate further back from the apex (Fig. 61). Such a type
of cortex provides an exceedingly elastic and efficient protection for
the long slender thallus.

[Illustration: Fig. 61. _Usnea longissima_ Ach. Longitudinal sections of
outer cortex. A, near the apex; B, the middle portion of a fibril. × 525
(after Schulte).]

The same type of cortex forms the strengthening element in the fruticose
or partly fruticose members of the family Physciaceae. One of these,
_Teloschistes flavicans_, is a bright yellow filamentous lichen with a
somewhat straggling habit. The fronds are very slender and are either
cylindrical or slightly flattened. The hyphae of the outer cortex are
compactly fibrous; added toughness is given by the presence of some
longitudinal strands of hyphae in the central pith.

Another still more familiar grey lichen, _Physcia ciliaris_, has long
flat branching fronds which, though dorsiventral in structure, are partly
upright in habit. Strength is secured as in _Teloschistes_ by the fibrous
upper cortex. Other species of _Physciae_ are somewhat similar in habit
and in structure.

In _Dendrographa leucophaea_, a slender strap-shaped rock lichen,
Darbishire[376] has described the outer cortex as composed of closely
compacted parallel hyphae resembling the strengthening cortex of
_Alectoria_ and very different from the fastigiate cortex of the
_Roccellae_ with which it is usually classified.


B. SPECIAL STRENGTHENING STRUCTURES

_a._ SCLEROTIC STRANDS. This form of strengthening tissue is
characteristic of _Ramalina_. With the exception of _R. thrausta_ (more
truly an _Alectoria_) all the species have a rather weak cortical layer
of branching intricate thick-walled hyphae, regarded by Brandt[377] as
plectenchymatous, but more correctly by Hue[378] as “decomposed” on
account of the gelatinous walls and diminishing lumen of the irregularly
arranged cells.

[Illustration: Fig. 62. _Ramalina minuscula_ Nyl. A, transverse section
of frond × 37; B, longitudinal strengthening hyphae of inner cortex × 430
(after Brandt).]

In _R. evernioides_, a plant with very wide flat almost decumbent fronds
of soft texture, in _R. ceruchis_ and in _R. homalea_ there is a somewhat
compact medulla which gives a slight stiffness to the thallus. The
other species of the genus are provided with strengthening mechanical
tissue within the cortex formed of closely united sclerotic hyphae that
run parallel to the surface (Fig. 62). In a transverse section of the
thallus, this tissue appears sometimes as a continuous ring which may
project irregularly into the pith (_R. calicaris_); more frequently it
is in the form of strands or bundles which alternate with the groups of
gonidia (_R. siliquosa_, _R. Curnowii_, etc.). In _R. fraxinea_ these
strands may be scarcely discernible in young fronds, though sometimes
already well developed near the tips. Occasionally isolated strands of
fibres appear in the pith (_R. Curnowii_), or the sclerotic projections
may even stretch across the pith to the other side (_R. strepsilis_)
(Fig. 75 B).

In the _Cladoniae_ support along with flexibility is secured to the
upright podetium by the parallel closely packed hyphae that form round
the hollow cylinder a band called the “chondroid” layer from its
cartilage-like consistency.

_b._ CHONDROID AXIS. The central medullary tissue in _Ramalina_ is, with
few exceptions, a loose arachnoid structure; often the fronds are almost
hollow. In one species of _Usnea_, _U. Taylori_, found in polar regions,
there is a similar loose though very circumscribed medullary and gonidial
tissue in the centre of the somewhat cylindrical thallus, and a wide band
of sclerotic fibres towards the cortex.

[Illustration: Fig. 63 A. A, _Usnea barbata_ Web. Longitudinal section
of filament with young adventitious branch. _a_, chondroid axis;
_b_, gonidial tissue; _c_, cortex. × 100 (after Schwendener). B, _U.
longissima_ Ach. Hyphae from central axis × 525 (after Schulte).]

In all other species of _Usnea_ the medulla itself is transformed into
a strong central strand of long-celled thick-walled hyphae closely knit
together by frequent anastomoses (Fig. 63 A). This central strand of
the Usneas is known as the “chondroid axis.” A narrow band of loose
air-containing hyphae and a gonidial zone lie round the central axis
between it and the outer cortex (Fig. 63 A, _b_). At the extreme apex,
the external cortical hyphae grow in a direction parallel with the long
axis of the plant, but further back, they branch out at right angles and
become swollen and mostly “decomposed” as in the cortex of _Ramalina_.

In _Letharia_ (_L. vulpina_, etc.) the structure is midway between
_Ramalina_ and _Usnea_: the central axis is either a solid strand of
chondroid hyphae or several separate strands.

[Illustration: Fig. 63 B. _Usnea longissima_ Ach. A, transverse section
of fibril × 85. B, _a_, chondroid axis; _b_, gonidial tissue; _c_, cortex
× 525 (after Schulte).]

In three other genera with upright fruticose thalli, _Sphaerophorus_,
_Argopsis_ and _Stereocaulon_, rigidity is maintained by a medulla
approaching the chondroid type. In _Sphaerophorus_ the species may have
either flattened or cylindrical branching stalks, but in all of them,
the centre is occupied by longitudinal strands of hyphae. _Argopsis_,
a monotypic genus from Kerguelen, has a cylindrical branching thallus
with a strong solid axis; it is closely allied to _Stereocaulon_, a
genus of familiar moorland lichens. The central tissue of the stalks in
_Stereocaulon_ is also composed of elongate, thick-walled conglutinate
hyphae, formed into a strand which is, however, not entirely solid.


C. SURVEY OF MECHANICAL TISSUES

Mechanical tissues scarcely appear among fungi, except perhaps as
stoutish cartilaginous hyphae in the stalks of some Agarics (_Collybiae_,
etc.), or as a ring of more compact consistency round the central hyphae
of rhizomorphic strands. It is practically a new adaptation of hyphal
structure confined to lichens of the fruticose group, where there is
the same requirement as in the higher plants for rigidity, flexure and
tenacity.

Rigidity is attained as in other plants by groups or strands of
mechanical tissue situated close to the periphery, as they are so
arranged in _Ramalina_ and _Cladonia_; or the same end is achieved by a
strongly developed fastigiate cortex as in _Roccella_. Bending strains
to which the same lichens are subjected, are equally well met by the
peripheral disposition of the mechanical elements.

Tenacity and elasticity are provided for in the pendulous forms either by
a fibrous cortex as in _Alectoria_, or by the chondroid axis in _Usnea_.
Haberlandt[379] has recorded some interesting results of tests made by
him as to the stretching capacity of a freshly gathered pendulous species
in which the central strand was from ·5 to 1 mm. thick. He found he could
draw it out 100 to 110 per cent. of its normal length before it gave way.
In an upright species the frond broke when stretched 60 to 70 per cent.
In both of the plants tested, the central strand retained its elasticity
up to 20 per cent. of stretching. The outer cortical tissue was cracked
and broken in the experiments. Schulte[380] calculated somewhat roughly
the tenacity of _Usnea longissima_ and found that a piece of the main
axis 8 cm. long carried up to 300 grms. without breaking.


D. RETICULATE FRONDS

In the upright radiate thallus, more especially among the _Ramalinae_,
though also among _Cladoniae_[381], there has appeared a reticulate
thallus resulting from the elongate splitting of the tissues, and due
to unequal growth tension and straining of the gelatinous cortex when
swollen with moisture. In several species of _Ramalina_, the strap-shaped
frond is hollow in the centre; and strands of strengthening fibres
give rise to a series of cortical ridges. The thinner tissue between
is frequently torn apart and ellipsoid openings appear which do not
however pierce beyond the central hollow. Such breaks are irregular
and accidental though occurring constantly in _Ramalina fraxinea_, _R.
dilacerata_, etc.

A more complete type of reticulation is always present in a Californian
lichen, _Ramalina reticulata_, in which the large flat frond is a
delicate open network from tip to base (Fig. 64). It grows on the
branches of deciduous trees and hangs in crowded tufts up to 30 cm. or
more in length. Usually it is so torn, that the real size attainable can
only be guessed at. It is attached at the base by a spreading discoid
hold-fast, and, in mature plants, consists of a stoutish main axis from
which side branches are irregularly given off. These latter are firm at
the base like the parent stalk, but soon they broaden out into very wide
fronds. Splitting begins at the tips of the branches while still young;
they are then spathulate in form with a slightly narrower recurved tip,
below which the first perforations are visible, small at first, but
gradually enlarging with the growth of the frond.

[Illustration: Fig. 64. _Ramalina reticulata_ Krempelh. Portion of frond
(after Cramer).]

_Ramalina reticulata_ is an extremely gelatinous lichen and the formation
of the network was supposed by Lutz[382] to be entirely due to the
swelling of the tissues, or the imbibition of water, causing tension
and splitting. A more exact explanation of the phenomenon is given by
Peirce[383]: he found that it was due to the thickened incurved tip,
which, on the addition of moisture, swells in length, breadth and
thickness, causing it to bend slightly upwards and then curve backwards
over the thallus, thus straining the part immediately behind. These
various movements result in the splitting of the frond while it is young
and the cortices are thin and weak.

Peirce made a series of experiments to test the capacity of the tissues
to support tensile strains. In a dry state, a piece of the lichen held a
weight up to 150 grms.; when wet it broke with a weight of 30 grms. It
was also observed that the thickness of the frond doubled on wetting.


E. ROOTING BASE IN FRUTICOSE LICHENS

Fruticose and filamentous lichens are distinguished by their mode of
attachment to the substratum: instead of a system of rhizinae or of hairs
spread over a large area, there is usually one definite rooting base by
which the plant maintains its hold on the support.

Intermediate between the foliose and fruticose types of thallus are
several species which are decumbent in habit, but which are attached at
one (or sometimes more) definite points, with but little penetration of
the underlying substance. One such lichen, _Evernia furfuracea_, has
been classified now as foliose, and again as fruticose. The earliest
stage of the thallus is in the form of a rosette-like sheath which bears
rhizinae on the under surface, very numerous at the centre of the sheath,
but entirely wanting towards the periphery. A secondary thallus of
strap-shaped rather narrow fronds rises from the sheath and increases by
irregular dichotomous branching. These branches, which are considered by
Zopf[384] as adventitious, may also come into contact with the substratum
and produce a few rhizinae at that point; or if the frond is more closely
applied, the irritation thus produced causes a still greater outgrowth
of rhizinae and the formation of a new base from which other fronds
originate. These renewed centres of growth are not of very frequent
occurrence; they were first observed and described by Lindau[385] in
another species, _Evernia prunastri_, and were aptly compared by him to
the creeping stolons of flowering plants.

_Evernia furfuracea_ grows frequently on dead wood, palings, etc., as
well as on trees. _E. prunastri_ grows invariably on trees, and has a
more constantly upright fruticose habit; in this species also, a basal
sheath is present, and the attachment is secured by means of rhizoidal
hyphae which penetrate deeply into the periderm of the tree, taking
advantage of the openings afforded by the lenticels. The sheath hyphae
are continuous with the medullary hyphae of the frond, and gonidia are
frequently enclosed in the tissues; the sheath spreads to some extent
over the surface of the bark, and round the base of the fronds, thus
rendering the attachment of the lichen to the tree doubly secure.

Among _Ramalinae_, the development of the base was followed by
Brandt[386] in one species, _R. Landroensis_, an arboreal lichen from S.
Tyrol. A rosette-like sheath was formed consisting solely of strands of
thick-walled hyphae which spread over the bark. There were no gonidia
included in the tissue.

A different type of attachment was found by Lilian Porter[387] in
corticolous _Ramalinae_—_R. fraxinea_, _R. fastigiata_, and _R.
pollinaria_. The lichens were anchored to the tree by strands of closely
compacted hyphae longitudinally arranged and continuous with the cortical
hyphae. These enter the periderm of the tree by cracks or lenticels,
and by wedge action cause extensive splitting. The strands may also
spread horizontally and give rise to new plants. The living tissues of
the tree were thus penetrated and injured, and there was evidence that
hypertrophied tissue was formed and caused erosion of the wood.

[Illustration: Fig. 65. _Ramalina siliquosa_ A. L. Sm., on rocks, reduced
(M. P., _Photo._).]

Several _Ramalinae_—_R. siliquosa_, _R. Curnowii_, etc.—grow on rocks,
often in extremely exposed situations, in isolated tufts or in crowded
swards (Fig. 65). The separate tufts are not unfrequently connected at
the base by a crustaceous thallus. It is possible also to see on the
rock, here and there, small areas of compact thalline granules that
have scarcely begun to put out the upright fronds. These granules are
corticate on the upper surface and contain gonidia; from the lower
surface, slender branching hyphae in rhizoid-like strands penetrate
down between the inequalities and separable particles of the rock,
if the formation is granitic. They frequently have groups of gonidia
associated with them, and they continue to ramify and spread, the pure
white filaments often enough enclosing morsels of the rock. The upright
fronds are continuous with the base and are thus securely anchored to the
substratum.

On a smooth rock surface such as quartzite a continuous sward of
_Ramalina_ growth is impossible. The basal hyphae being unable to
penetrate the even surface of the rock, the attachment is slight and
the plants are easily dislodged. They do however succeed, sometimes,
in taking hold, and small groups of fronds arise from a crustaceous
base which varies in depth from ·5 to 1 mm. The tissues of this base
are very irregularly arranged: towards the upper surface loose hyphae
with scattered groups of algae are traversed by strands of gelatinized
sclerotic hyphae similar to the strengthening tissues of the upright
fronds, while down below there are to be found not only slender hyphae,
but a layer of gonidia visible as a white and green film on the rock when
the overlying particles are scaled off.

Darbishire[388] found that attachment to the substratum by means of a
basal sheath was characteristic of all the genera of Roccellaceae. He
looks on this sheath, which is the first stage in the development of the
plant, as a primary or proto-thallus, analogous to the primary squamules
of the _Cladoniae_, and he carries the analogy still further by treating
the upright fronds as podetia. The sheath of the Roccellaceae varies
in size but it is always of very limited extent; it is mainly composed
of medullary hyphae, and gonidia may or may not be present. The whole
structure is permanent and important, and is generally protected by a
well-developed upper cortex similar in structure to that of the upright
thallus, _i.e._ of a fastigiate type. There is no lower cortex.

The two British species of _Roccella_—_R. fuciformis_ and _R.
phycopsis_—grow on maritime rocks, the latter also occasionally on trees.
In _R. fuciformis_, the attaching sheath is a flat structure which slopes
up a little round the base of the upright frond. It is about 2 mm. thick,
the cortex occupying about 40 µ of that space; a few scattered gonidia
are present immediately below. The remaining tissue of the sheath is
composed of firmly wefted slender filaments. Towards the lower surface,
there is a more closely compacted dark brown layer from which pass out
the hyphae that penetrate the rock.

The sheath of _R. phycopsis_ is a small structure about 3 to 4 mm. in
width and 1·5 mm. thick. A few gonidia may be found below the dense
cortical layer, but they tend to disappear as the upright fronds become
larger and the shade, in consequence, more dense. Lower down the hyphae
take an intensely yellow hue; mixed with them are also some brown
filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of
_R. tinctoria_. In structure it corresponds—as do those of the other
species—with the ones already described.

In purely filamentous species such as _Usnea_ there is also primary
sheath formation: the medullary hyphae spread out in radiating strands
which force their way wherever possible into the underlying substance;
on trees they enter into any chink or crevice of the outer bark like
wedges; or they ramify between the cork cells which are split up by the
mere growth pressure. By the vertical increase of the base, the fronds
may be hoisted up and an intercalary basal portion may arise lacking both
gonidia and cortical layer. Very frequently several bases are united and
the lichen appears to be of tufted habit.

A basal sheath provides a similar firm attachment for _Alectoria jubata_
and allied species: these are slender mostly dark brown lichens which
hang in tangled filaments from the branches of trees, rocks, etc.

These attaching sheaths differ in function as well as in structure
from the horizontal thallus of the Cladoniaceae. They may be more
truly compared with the primary thallus of the red algae _Dumontia_
and _Phyllophora_ which are similarly affixed to the substratum, while
upright fronds of subsequent formation bear the fructifications.


IV. STRATOSE-RADIATE THALLUS


1. STRATOSE OR PRIMARY THALLUS


A. GENERAL CHARACTERISTICS

[Illustration: Fig. 66. _Cladonia pyxidata_ Hoffm. Basal squamule and
podetium. _a_, apothecia; _s_, spermogonia (after Krabbe).]

This series includes the lichens of one family only, the Cladoniaceae,
the genera of which are characterized by the twofold thallus, one portion
being primary, horizontal and stratose, the other secondary and radiate,
the latter an upright simple or branching structure termed a “podetium”
which narrows above, or widens to form a trumpet-shaped cup or “scyphus”
(Fig. 66). The apothecia are terminal on the podetium or on the margins
of the scyphi; in a few species they are developed on the primary
thallus. Some degree of primary thallus-formation has been demonstrated
in all the genera, if not in all the species of the family. The genus
_Cladina_ was established to include those species of _Cladonia_ in
which, it was believed, only a secondary podetial thallus was present,
but Wainio[389] found in _Cladonia sylvatica_ a granular basal crust
and, in _Cladonia uncialis_, minute round scales with crenate margins
measuring from ·5 to 1 mm. in width. In some species (subgenus _Cladina_)
the primary thallus is quickly evanescent, in others it is granular or
squamulose and persistent. Where the basal thallus is so much reduced
as to be practically non-existent, apothecia are rarely developed and
soredia are absent. Renewal of growth in these lichens is secured by the
dispersal of fragments of the podetial thallus; they are torn off and
scattered by the wind or by animals, and, if suitable conditions are met,
a new plant arises.

_Cladonia_ squamules vary in size from very small scales as in _Cl.
uncialis_ to the fairly large foliose fronds of _Cl. foliacea_ which
extend to 5 cm. in length and about 1 cm. or more in width. It is
interesting to note that when the primary thallus is well developed, the
podetia are relatively unimportant and frequently are not formed. As a
rule the squamules are rounded or somewhat elongate in form with entire
or variously cut and crenate margins. They may be very insignificant
and sparsely scattered over the substratum, or massed in crowded swards
of leaflets which are frequently almost upright. In colour they are
bluish-grey, yellowish or brownish above, and white beneath (red in
_Cl. miniata_), frequently becoming very dark-coloured towards the
rooting base. These several characteristics are specific and are often
of considerable value in diagnosis. In certain conditions of shade
or moisture, squamules are formed on the podetium; they repeat the
characters of the basal squamules of the species.


B. TISSUES OF THE PRIMARY THALLUS

The stratose layers of tissue in the squamules of _Cladonia_ are arranged
as in other horizontal thalli.

_a._ CORTICAL TISSUE. In nearly all these squamules the cortex is of
the “decomposed” type. In a few species there is a plectenchymatous
formation—in _Cl. nana_, a Brazilian ground species, and in two New
Zealand species, _Cl. enantia_ f. _dilatata_ and _Cl. Neo-Zelandica_. The
principal growing area is situated all round the margins though generally
there is more activity at the apex. Frequently there is a gradual
perishing of the squamule at the base which counterbalances the forward
increase.

The upper surface in some species is cracked into minute areolae; the
cracks, seen in section, penetrate almost to the base of the decomposed
gelatinous cortex. They are largely due to alternate swelling and
contraction of the gelatinous surface, or to extension caused, though
rarely, by intercalary growth from the hyphae below. The surface is
subject to weathering and peeling as in other lichens; but the loss is
constantly repaired by the upward growth of the meristematic hyphae from
the gonidial zone; they push up between the older cortical filaments and
so provide for the expansion as well as for the renewal of the cortical
tissue.

_b._ GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae
form a layer immediately below the cortex. Isolated green cells are not
unfrequently carried up by the growing hyphae into the cortical region,
but they do not long survive in this compact non-aerated tissue. Their
empty membranes can however be picked out by the blue stain they take
with iodine and sulphuric acid.

Krabbe[390] has described the phases of development in the growing
region: he finds that differentiation into pith, gonidial zone and cortex
takes place some little way back from the edge. At the extreme apex the
hyphae lie fairly parallel to each other; further back, they branch
upwards to form the cortex, and to separate the masses of multiplying
gonidia, by pushing between them and so spreading them through the whole
apical tissue. The gonidia immediately below the upper cortex, where
they are well-lighted, continue to increase and gradually form into the
gonidial zone; those that lie deeper among the medullary hyphae remain
quiescent, and before long disappear altogether.

Where the squamules assume the upright position (as in _Cladonia
cervicornis_), there is a tendency for the gonidia to pass round to the
lower surface, and soredia are occasionally formed.

_c._ MEDULLARY TISSUE. The hyphae of the medulla are described by Wainio
as having long cells with narrow lumen, and as being encrusted with
granulations that may coalesce into more or less detachable granules;
in colour they are mostly white, but pale-yellow in _Cl. foliacea_ and
blood-red in _Cl. miniata_, a subtropical species. They are connected at
the base of the squamules with a filamentous hypothallus which penetrates
the substratum and attaches the plant. In a few species rhizinae are
formed, while in others the hyphae of the podetium grow downwards,
towards and into the substratum as a short stout rhizoid.

_d._ SOREDIA. Though frequent on the podetia, soredia are rare on the
squamules, and, according to Wainio[391], always originate at the growing
region, from which they spread over the under surface—rather sparsely in
_Cl. cariosa_, _Cl. squamosa_, etc., but abundantly in _Cl. digitata_
and a few others. In some instances, they develop further into small
corticate areolae on the under surface (_Cl. coccifera_, _Cl. pyxidata_
and _Cl. squamosa_).


2. RADIATE OR SECONDARY THALLUS


A. ORIGIN OF THE PODETIUM

The upright podetium, as described by Wainio[392] and by Krabbe[393], is
a secondary product of the basal granule or squamule. It is developed
from the hyphae of the gonidial zone, generally where a crack has
occurred in the cortex and rather close to the base or more rarely on
or near the edge of the squamule (_Cl. verticillata_, etc.). At these
areas, certain meristematic gonidial hyphae increase and unite to form a
strand of filaments below the upper cortex but above the gonidial layer,
the latter remaining for a time undisturbed as to the arrangement of the
algal cells.

This initial tissue—the primordium of the podetium—continues to grow
not only in width but in length: the basal portion grows downwards
and at length displaces the gonidial zone, while the upper part as a
compact cylinder forces its way through the cortex above, the cortical
tissue, however, taking no part in its formation; as it advances, the
edges of the gonidial and cortical zones bend upwards and form a sheath
distinguishable for some time round the base of the emerging podetium.

Even when the primary horizontal thallus is merely crustaceous, the
podetia take origin similarly from a subcortical weft of hyphae in an
areola or granule.


B. STRUCTURE OF THE PODETIUM

_a._ GENERAL STRUCTURE. In the early stages of development the podetium
is solid throughout, two layers of tissue being discernible—the hyphae
forming the centre of the cylinder being thick-walled and closely
compacted, and the hyphae on the exterior loosely branching with numerous
air-spaces between the filaments.

In all species, with the exception of _Cl. solida_, which remains solid
during the life of the plant, a central cavity arises while the podetium
is still quite short (about 1 to 1·5 mm. in _Cl. pyxidata_ and _Cl.
degenerans_). The first indication of the opening is a narrow split in
the internal cylinder, due to the difference in growth tension between
the more free and rapid increase of the external medullary layer and
the slower elongation of the chondroid tissue at the centre. The cavity
gradually widens and becomes more completely tubular with the upward
growth of the podetium; it is lined by the chondroid sclerotic band which
supports the whole structure (Fig. 67).

_b._ GONIDIAL TISSUE. In most species of Cladoniaceae, a layer of
gonidial tissue forms a more or less continuous outer covering of the
podetium, thus distinguishing it from the purely hyphal stalks of the
apothecia in Caliciaceae. Even in the genus _Baeomyces_, while the
podetia of some of the species are without gonidia, neighbouring species
are provided with green cells on the upright stalks clearly showing their
true affinity with the _Cladoniae_. In one British species of _Cladonia_
(_Cl. caespiticia_) the short podetium consists only of the fibrous
chondroid cylinder, and thus resembles the apothecial stalk of _Baeomyces
rufus_, but in that species also there are occasional surface gonidia
that may give rise to squamules.

[Illustration: Fig. 67. _Cladonia squamosa_ Hoffm. Vertical section of
podetium with early stage of central tube and of podetial squamules × 100
(after Krabbe).]

Krabbe[394] concluded from his observations that the podetial gonidia
of _Cladonia_ arrived from the open, conveyed by wind, water or insects
from the loose soredia that are generally so plentiful in any _Cladonia_
colony. They alighted, he held, on the growing stalks and, being
secured by the free-growing ends of the exterior hyphae, they increased
and became an integral part of the podetium. In more recent times
Baur[395] has recalled and supported Krabbe’s view, but Wainio[396],
on the contrary, claims to have proved that in the earliest stages of
the podetium the gonidia were already present, having been carried up
from the gonidial zone of the primary thallus by the primordial hyphae.
Increase of these green cells follows normally by cell-division or
sporulation.

Algal cells have been found to be common to different lichens, but in
_Cladoniae_ Chodat[397] claims to have proved by cultures that each
species tested has a special gonidium, determined by him as a species of
_Cystococcus_, which would render colonization by algae from the open
much less probable. In addition, the fungal hyphae are specific, and any
soredia (with their combined symbionts) that alighted on the podetium
could only be utilized if they originated from the same species; or, if
they were incorporated, the hyphae belonging to any other species would
of necessity die off and be replaced by those of the podetium.

_c._ CORTICAL TISSUE. In some species a cortex of the decomposed type
of thick-walled conglutinate hyphae is present, either continuous over
the whole surface of the podetium, as in _Cl. gracilis_ (Fig. 68), or in
interrupted areas or granules as in _Cl. pyxidata_ (Fig. 69) and others.
In _Cl. degenerans_, the spaces between the corticated areolae are filled
in by loose filaments without any green cells. _Cl. rangiferina_, _Cl.
sylvatica_, etc. are non-corticate, being covered all over with a loose
felt of intricate hyphae.

[Illustration: Fig. 68. _Cladonia gracilis_ Hoffm. (S. H., _Photo._).]

[Illustration: Fig. 69. _Cladonia pyxidata_ Hoffm. (S. H., _Photo._).]

In the section _Clathrinae_ (_Cl. retepora_, etc.) the cortex is formed
of longitudinal hyphae with thick gelatinous walls.

_d._ SOREDIA. Frequently the podetium is coated in whole or in part by
granules of a sorediate character—coarsely granular in _Cl. pyxidata_,
finely pulverulent in _Cl. fimbriata_. Though fairly constant to type
in the different species, they are subject to climatic influences, and,
when there is abundant moisture, both soredia and areolae develop into
squamules on the podetium. A considerable number of species have thus a
more or less densely squamulose “form” or “variety.”


C. DEVELOPMENT OF THE SCYPHUS

Two types of podetia occur in _Cladonia_: those that end abruptly and
are crowned when fertile by the apothecia or spermogonia (pycnidia), or
if sterile grow indefinitely tapering gradually to a point (Fig. 70);
and those that widen out into the trumpet-shaped or cup-like expansion
called the scyphus (Fig. 69). Species may be constantly scyphiferous or
as constantly ascyphous; in a few species, and even in individual tufts,
both types of podetium may be present.

[Illustration: Fig. 70. _Cladonia furcata_ Schrad. Sterile thallus (S.H.,
_Photo._).]

Wainio[398], who has studied every stage of development in the
_Cladoniae_, has described the scyphus as originating in several
different ways:

_a._ FROM ABORTIVE APOTHECIA. In certain species the apothecium appears
at a very early stage in the development of the podetium of which it
occupies the apical region. Owing to the subsequent formation of the
tubular cavity in the centre of the stalk, the base of the apothecium
may eventually lie directly over the hollow space and, therefore, out
of touch with the growing assimilating tissues; or even before the
appearance of the tube, the wide separation between the primordium of
the apothecium and the gonidia, entailing deficient nutrition, may
have produced a similar effect. In either case central degeneration of
the apothecium sets in, and the hypothecial filaments, having begun to
grow radially, continue to travel in the same direction both outwards
and upwards so that gradually a cup-shaped structure is evolved—the
amphithecium of the fruit without the thecium.

The whole or only a part of the apothecium may be abortive, and the
scyphus may therefore be entirely sterile or the fruits may survive at
the edges. The apothecia may even be entirely abortive after a fertile
commencement, but in that case also the primordial hyphae retain the
primitive impulse not only to radial direction, but also to the more
copious branching, and a scyphus is formed as in the previous case. It
must also be borne in mind that the tendency in many _Cladonia_ species
to scyphiform has become hereditary.

Baur[399], in his study of _Cl. pyxidata_, has taken the view that
the origin of the scyphus was due to a stronger apical growth of the
hyphae at the circumference than over the central tubular portion of the
podetium, and that considerable intercalary growth added to the expanding
sides of the cup.

Scyphi originating from an abortive apothecium are characteristic of
species in which the base is closed (Wainio’s Section _Clausae_), the
tissue in that case being continuous over the inside of the cup as in
_Cl. pyxidata_, _Cl. coccifera_ and many others.

_b._ FROM POLYTOMOUS BRANCHING. Another method of scyphus formation
occurs in _Cl. amaurocrea_ and a few other species in which the branching
is polytomous (several members rising from about the same level).
Concrescence of the tissues at the base of these branches produces
a scyphus; it is normally closed by a diaphragm that has spread out
from the different bases, but frequently there is a perforation due to
stretching. These species belong to the Section _Perviae_.

_c._ FROM ARRESTED GROWTH. In most cases however where the scyphus is
open as in _Cl. furcata_, _Cl. squamosa_, etc., development of the cup
follows on cessation of growth, or on perforation at the summit of the
podetium. Round this quiescent portion there rises a circle of minute
prominences which carry on the apical development. As they increase in
size, the spaces between them are bridged over by lateral growth, and the
scyphus thus formed is large or small according to the number of these
outgrowths. Apothecia or spermogonia may be produced at their tips, or
the vegetative development may continue. Scyphi formed in this manner are
also open or “pervious.”

_d._ GONIDIA OF THE SCYPHUS. Gonidia are absent in the early stages
of scyphus formation when it arises from degeneration of the apical
tissues, either fertile or vegetative; but gradually they migrate from
the podetium, from the base of young outgrowths, or by furrows at the
edge, and so spread over the surface of the cup. Soredia may possibly
alight, as Krabbe insists that they do, and may aid in colonizing the
naked area. Their presence, however, would only be accidental; they are
not essential, and scyphi are formed in many non-sorediate species such
as _Cl. verticillata_. The cortex of the scyphus becomes in the end
continuous with that of the podetium and is always similar in type.

_e._ SPECIES WITHOUT SCYPHI. In species where the whole summit of the
podetium is occupied by an apothecium, as in _Cl. bellidiflora_, no
scyphus is formed. There is also an absence of scyphi in podetia that
taper to a point. In those podetia the hyphae are parallel to the long
axis and remain in connection with the external gonidial layer so that
they are unaffected by the central cavity. Instances of tapering growth
are also to be found in species that are normally scyphiferous such as
_Cl. fimbriata_ subsp. _fibula_, and _Cl. cornuta_, as well as in species
like _Cl. rangiferina_ that are constantly ascyphous.

The scyphus is considered by Wainio[400] to represent an advanced stage
of development in the species or in the individual, and any conditions
that act unfavourably on growth, such as excessive dryness, would also
hinder the formation of this peculiar lichen structure.


D. BRANCHING OF THE PODETIUM

Though branching is a constant feature in many species, regular dichotomy
is rare: more often there is an irregular form of polytomy in which
one of the members grows more vigorously than the others and branches
again, so that a kind of sympodium arises, as in _Cl. rangiferina_, _Cl.
sylvatica_, etc.

Adventitious branches may also arise from the podetium, owing to some
disturbance of the normal growth, some undue exposure to wind or to too
great light, or owing to some external injury. They originate from the
gonidial tissue in the same way as does the podetium from the primary
thallus; the parallel hyphae of the main axis take no part in their
development.

In a number of species secondary podetia arise from the centre of
the scyphus—constantly in _Cl. verticillata_ and _Cl. cervicornis_,
etc., accidentally or rarely in _Cl. foliacea_, _Cl. pyxidata_, _Cl.
fimbriata_, etc. Wainio[401] has stated that they arise when the scyphus
is already at an advanced stage of growth and that they are to be
regarded as adventitious branches.

The proliferations from the borders of the scyphus are in a different
category. They represent the continuity of apical growth, as the edges of
the scyphus are but an enlarged apex. These marginal proliferations thus
correspond to polytomous branching. In many instances their advance is
soon stopped by the formation of an apothecium and they figure more as
fruit stalks than as podetial branches.


E. PERFORATIONS AND RETICULATION OF THE PODETIUM

Perforations in the podetial wall at the axils of the branches are
constant in certain species such as _Cl. rangiferina_, _Cl. uncialis_,
etc. They are caused by the tension of the branches as they emerge from
the main stalk. A tearing of the tissue may also arise in the base of the
scyphus, due to its increase in size, which causes the splitting of the
diaphragm at the bottom of the cup.

Among the _Cladoniae_ the reticulate condition recurs now and again. In
our native _Cladonia cariosa_ the splitting of the podetial wall is a
constant character of the species, the carious condition being caused by
unequal growth which tears apart the longitudinal fibres that surround
the central hollow.

A more advanced type of reticulation arises in the group of the
_Clathrinae_ in which there is no inner chondroid cylinder. In _Cladonia
aggregata_, in which the perforations are somewhat irregular, two types
of podetia have been described by Lindsay[402] from Falkland Island
specimens: those bearing apothecia are short and broad, fastigiately
branched upwards and with reticulate perforations, while podetia
bearing spermogonia are slender, elongate and branched, with fewer
reticulations. An imperfect network is also characteristic of _Cl.
Sullivani_, a Brazilian species. But the most marvellous and regular form
of reticulation occurs in _Cl. retepora_, an Australian lichen (Fig.
71): towards the tips of the podetia the ellipsoid meshes are small, but
they gradually become larger towards the base. In this species the outer
tissue, though of parallel hyphae, is closely interwoven and forms a
continuous growth at the edges of the perforations, giving an unbroken
smooth surface and checking any irregular tearing. The enlargement of the
walls is solely due to intercalary growth. The origin of the reticulate
structure in the _Clathrinae_ is unknown, though it is doubtless
associated with wide podetia and rendered possible by the absence of
an internal chondroid layer. The reticulate structure is marvellously
adapted for the absorption of water: _Cl. retepora_, more especially,
imbibes and holds moisture like a sponge.

[Illustration: Fig. 71. _Cladonia retepora_ Fr. From Tasmania.]


F. ROOTING STRUCTURES OF CLADONIAE

The squamules of the primary thallus are attached, as are most squamules,
to the supporting substance by strands of hyphae which may be combined
into simple or branching rhizinae and penetrate the soil or the wood on
which the lichen grows. There is frequently but one of these rooting
structures and it branches repeatedly until the ultimate branchlets
end in delicate mycelium. Generally they are grey or brown and are
not easily traced, but when they are orange-coloured, as according to
Wainio[403] they frequently are in _Cladonia miniata_ and _Cl. digitata_,
they are more readily observed, especially if the habitat be a mossy one.

In _Cl. alpicola_ it has been found that the rooting structure is
frequently as thick as the podetium itself. If the podetium originates
from the basal portion of the squamule, the hyphae from the chondroid
layer, surrounding the hollow centre, take a downward direction and
become continuous with the rhizoid. Should the point of insertion be near
the apex of the squamule, these hyphae form a nerve within the squamule
or along the under surface, and finally also unite with the rhizoid at
the base, a form of rooting characteristic of _Cl. cartilaginea_, _Cl.
digitata_ and several other species.

Mycelium may spread from the rhizinae along the surface of the substratum
and give rise to new squamules and new tufts of podetia, a method of
reproduction that is of considerable importance in species that are
generally sterile and that form no soredia.

Many species, especially those of the section _Cladina_, soon lose all
connection with the substratum, there being a continual decay of the
lower part of the podetia. As apical growth may continue for centuries,
the perishing of the base is not to be wondered at.


G. HAPTERA

The presence of haptera in _Cladoniae_ has already been alluded to. They
occur usually in the form of cilia or rhizinae[404], but differ from the
latter in their more simple regular growth being composed of conglutinate
parallel hyphae. They arise on the edges of the squamules or of the
scyphus, but in _Cl. foliacea_ and _Cl. ceratophylla_ they are formed at
the points of the podetial branches (more rarely in _Cl. cervicornis_
and _Cl. gracilis_). By the aid of these rhizinose haptera the squamule
or branch becomes attached to any substance within reach. They also aid
in the production of new individuals by anchoring some fragment of the
thallus to a support until it has grown to independent existence and has
produced new rhizinae or hold-fasts. They are a very prominent feature of
_Cl. verticillaris_ f. _penicillata_ in which they form a thick fringe on
the edges of the squamules, or frequently grow out as branched cilia from
the proliferations on the margins of the scyphus.


H. MORPHOLOGY OF THE PODETIUM

In the above account, the podetia have been treated as part of
the vegetative thallus, seeing that, partly or entirely, they are
assimilative and absorptive organs. This view does not, however, take
into account their origin and development, in consideration of which
Wainio[405] and later Krabbe[406] considered them as part of the
sporiferous organ. This view was also held by some of the earliest
lichenologists: Necker[407], for instance, constantly referred to the
upright structure as “stipes”; Persoon[408] included it, under the
term “pedunculus,” as part of the “inflorescence” of the lichen, and
Acharius[409] established the name “podetium” to describe the stalk of
the apothecium in _Baeomyces_.

Later lichenologists, such as Wallroth[410], looked on the podetia as
advanced stages of the thallus, or as forming a supplementary thallus.
Tulasne[411] described them as branching upright processes from the
horizontal form, and Koerber[412] considered them as the true thallus,
the primary squamule being merely a protothallus. By them and by
succeeding students of lichens the twofold character of the thallus
was accepted until Wainio and Krabbe by their more exact researches
discovered the endogenous origin of the podetium, which they considered
was conclusive evidence of its apothecial character: they claimed that
the primordium of the podetium was homologous with the primordium of the
apothecium. Reinke[413] and Wainio are in accord with Krabbe as to the
probable morphological significance of the podetium, but they both insist
on its modified thalline character. Wainio sums up that: “the podetium
is an apothecial stalk, that is to say an elongation of the conceptacle
most frequently transformed by metamorphosis to a vertical thallus,
though visibly retaining its stalk character.” Sättler[414], one of the
most recent students of _Cladonia_, regards the podetium as evolved with
reference to spore-dissemination, and therefore of apothecial character.
His views are described and discussed in the chapter on phylogeny.

Reinke and others sought for a solution of the problem in _Baeomyces_,
one of the more primitive genera of the Cladoniaceae. The thallus, except
in a few mostly exotic species, scarcely advances beyond the crustaceous
condition; the podetia are short and so varied in character that species
have been assigned by systematists to several different genera. In one
of them, _Baeomyces roseus_, the podetium or stalk originates according
to Nienburg[415] deep down in the medulla of a fertile granule as a
specialized weft of tissue; there is no carpogonium nor trichogyne
formed; the hyphae that grow upward and form the podetium are generative
filaments and give rise to asci and paraphyses. In a second species, _B.
rufus_ (_Sphyridium_), the gonidial zone and outer cortex of a thalline
granule swell out to form a thalline protuberance; the carpogonium arises
close to the apex, and from it branch the generative filaments. Nienburg
regards the stalk of _B. roseus_ as apothecial and as representing an
extension of the proper margin[416] (_excipulum proprium_), that of _B.
rufus_ as a typical vegetative podetium.

In the genus _Cladonia_, differentiation of the generative hyphae
may take place at a very early stage. Wainio[417] observed, in _Cl.
caespiticia_, a trichogyne in a still solid podetium only 90µ in height;
usually they appear later, and, where scyphi are formed, the carpogonium
often arises at the edge of the scyphus. Baur[418] and Wolff[419] have
furnished conclusive evidence of the late appearance of the carpogonium
in _Cl. pyxidata_, _Cl. degenerans_, _Cl. furcata_ and _Cl. gracilis_:
in all of these species carpogonia with trichogynes were observed on
the edge of well-developed scyphi. Baur draws the conclusion that the
podetium is merely a vertical thallus, citing as additional evidence that
it also bears the spermogonia (or pycnidia), though at the same time
he allows that the apothecium may have played an important part in its
phylogenetic development. He agrees also with the account of the first
appearance of the podetium as described by Krabbe, who found that it
began with the hyphae of the gonidial zone branching upwards in a quite
normal manner, only that there were more of them, and that they finally
pierced the cortex. Krabbe also asserted that in the early stages the
podetia were without gonidia and that these arrived later from the open
as colonists, in this contradicting Wainio’s statement that gonidia were
carried up from the primary thallus.

It seems probable that the podetium—as Wainio and Baur both have
stated—is homologous with the apothecial stalk, though in most cases it
is completely transformed into a vertical thallus. If the view of their
formation from the gonidial zone is accepted, then they differ widely in
origin from normal branches in which the tissues of the main axis are
repeated in the secondary structures, whereas in this vertical thallus,
hyphae from the gonidial zone alone take part in the development. It must
be admitted that Baur’s view of the podetium as essentially thalline
seems to be strengthened by the formation of podetia at the centre of
the scyphus, as in _Cl. verticillata_, which are new structures and are
not an elongation of the original conceptacular tissue. It can however
equally be argued that the acquired thalline character is complete and,
therefore, includes the possibility of giving rise to new podetia.

The relegation of the carpogonium to a position far removed from the base
or primordium of the apothecium need not necessarily interfere with the
conception of the primordial tissue as homologous with the conceptacle;
but more research is needed, as Baur dealt only with one species, _Cl.
pyxidata_, and Gertrude Wolff confined her attention to the carpogonial
stages at the edge of the scyphus.

The _Cladoniae_ require light, and inhabit by preference open moorlands,
naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with
large and persistent squamules can live in arid situations, probably
because the primary thallus is able to retain moisture for a long
time[420]. When the primary thallus is small and feeble the podetia are
generally much branched and live in close colonies which retain moisture.
Sterile podetia are long-lived and grow indefinitely at the apex though
the base as continually perishes and changes into humus. Wainio[421]
cites an instance in which the bases of a tuft of _Cl. alpestris_ had
formed a gelatinous mass more than a decimetre in thickness.


I. PILOPHORUS AND STEREOCAULON

These two genera are usually included in Cladoniaceae on account of
their twofold thallus and their somewhat similar fruit formation. They
differ from _Cladonia_ in the development of the podetia which are not
endogenous in origin as in that genus, but are formed by the growth
upwards of a primary granule or squamule and correspond more nearly to
Tulasne’s conception of the podetium as a process from the horizontal
thallus. In _Pilophorus_ the primary granular thallus persists during
the life of the plants; the short podetium is unbranched, and consists
of a somewhat compact medulla of parallel hyphae surrounded by a looser
cortical tissue, such as that of the basal granule, in which are embedded
the algal cells. The black colour of the apothecium is due to the thick
dark hypothecium.

_Stereocaulon_ is also a direct growth from a short-lived primary
squamule[422]. The podetia, called “pseudopodetia” by Wainio, are usually
very much branched. They possess a central strand of hyphae not entirely
solid, and an outer layer of loose felted hyphae in which the gonidia
find place. A coating of mucilage on the outside gives a glabrous shiny
surface, or, if that is absent, the surface is tomentose as in _St.
tomentosum_. In all the species the podetia are more or less thickly
beset with small variously divided squamules similar in form to the
primary evanescent thallus. Gall-like cephalodia are associated with most
of the species and aid in the work of assimilation.

_Stereocaulon_ cannot depend on the evanescent primary thallus for
attachment to the soil. The podetia of the different species have
developed various rooting bases: in _St. ramulosum_ there is a basal
sheath formed, in _St. coralloides_ a well-developed system of
rhizoids[423].


V. STRUCTURES PECULIAR TO LICHENS


1. AERATION STRUCTURES


A. CYPHELLAE AND PSEUDOCYPHELLAE

The thallus of Stictaceae has been regarded by Nylander[424] and others
as one of the most highly organized, not only on account of the size
attained by the spreading lobes, but also because in that family are
chiefly found those very definite cup-like structures which were named
“cyphellae” by Acharius[425]. They are small hollow depressions about 1/2
mm. or more in width scattered irregularly over the under surface of the
thallus.

_a._ HISTORICAL. Cyphellae were first pointed out by the Swiss botanist,
Haller[426]. In his description of a lichen referable to _Sticta
fuliginosa_ he describes certain white circular depressions “to be found
among the short brown hairs of the under surface.” At a later date
Schreber[427] made these “white excavated points” the leading character
of his lichen genus _Sticta_.

In urceolate or proper cyphellae, the base of the depression rests on
the medulla; the margin is formed from the ruptured cortex and projects
slightly inwards over the edge of the cup. Contrasted with these are the
pseudocyphellae, somewhat roundish openings of a simpler structure which
replace the others in many of the species. They have no definite margin;
the internal hyphae have forced their way to the exterior and form a
protruding tuft slightly above the surface. Meyer[428] reckoned them all
among soredia; but he distinguished between those in which the medullary
hyphae became conglutinated to form a margin (true cyphellae) and those
in which there was a granular outburst of filaments (pseudocyphellae).
He also included a third type, represented in _Lobaria pulmonaria_ on
the under surface of which there are numerous non-corticate, angular
patches where the pith is laid bare (Fig. 72). Delise[429], writing
about the same time on the _Sticteae_, gives due attention to their
occurrence, classifying the various species of _Sticta_ as cyphellate or
non-cyphellate.

Acharius had limited the name “cyphella” to the hollow urceolate bodies
that had a well-defined margin. Nylander[430] at first included under
that term both types of structure, but later[431] he classified the
pulverulent “soredia-like” forms in another group, the pseudocyphellae.
As a rule they bear no relation to soredia, and algae are rarely
associated with the protruding filaments. Schwendener[432], and later
Wainio[433], in describing _Sticta aurata_ from Brazil, state, as
exceptional, that the citrine-yellow pseudocyphellae of that species are
sparingly sorediate.

[Illustration: Fig. 72. _Lobaria pulmonaria_ Hoffm. Showing pitted
surface. a, under surface. Reduced (S. H., _Photo._).]

[Illustration: Fig. 73. _Sticta damaecornis_ Nyl. Transverse section of
thallus with cyphella × 100.]

_b._ DEVELOPMENT OF CYPHELLAE. The cortex of both surfaces in the thallus
of _Sticta_ is a several-layered plectenchyma of thick-walled closely
packed cells, the outer layer growing out into hairs on the under surface
of most of the species. Where either cyphellae or pseudocyphellae
occur, a more or less open channel is formed between the exterior and
the internal tissues of the lichen. In the case of the cyphellae, the
medullary hyphae which line the cup are divided into short roundish
cells with comparatively thin walls (Fig. 73). They form a tissue
sharply differentiated from the loose hyphae that occupy the medulla.
The rounded cells tend to lie in vertical rows, though the arrangement
in fully formed cyphellae is generally somewhat irregular. The terminal
empty cells are loosely attached and as they are eventually abstricted
and strewn over the inside of the cup they give to it the characteristic
white powdery appearance.

According to Schwendener[434] development begins by an exuberant growth
of the medulla which raises and finally bursts the cortex; prominent
cyphellae have been thus formed in _Sticta damaecornis_ (Fig. 73). In
other species the swelling is less noticeable or entirely absent. The
opening of the cup measures usually about 1/2 mm. across, but it may
stretch to a greater width.

_c._ PSEUDOCYPHELLAE. In these no margin is formed, the cortex is simply
burst by the protruding filaments which are of the same colour—yellow or
white—as the medullary hyphae. They vary in size, from a minute point up
to 4 mm. in diameter.

_d._ OCCURRENCE AND DISTRIBUTION. The genus _Sticta_ is divided into
two sections: (1) _Eusticta_ in which the gonidia are bright-green
algae, and (2) _Stictina_ in which they are blue-green. Cyphellae and
pseudocyphellae are fairly evenly distributed between the sections; they
never occur together. Stizenberger[435] found that 36 species of the
section _Eusticta_ were cyphellate, while in 43 species pseudocyphellae
were formed. In the section _Stictina_ there were 38 of the former
and only 31 of the latter type. Both sections of the genus are widely
distributed in all countries, but they are most abundant south of the
equator, reaching their highest development in Australia and New Zealand.

In the British Isles _Sticta_ is rather poorly represented as follows:

  § _Eusticta_ (with bright-green gonidia).

  Cyphellate: _S. damaecornis._

  Pseudocyphellate: _S. aurata._

  § _Stictina_ (with blue-green gonidia).

  Cyphellate: _S. fuliginosa_, _S. limbata_, _S. sylvatica_, _S.
  Dufourei_.

  Pseudocyphellate: _S. intricata_ var. _Thouarsii_, _S. crocata_.

Structures resembling cyphellae, with an overarching rim, are sprinkled
over the brown under surface of the Australian lichen, _Heterodea
Mülleri_; the thallus is without a lower cortex, the medulla being
protected by thickly woven hyphae. _Heterodea_ was at one time included
among Stictaceae, though now it is classified under Parmeliaceae.
Pseudocyphellae are also present on the non-corticate under surface of
_Nephromium tomentosum_, where they occur as little white pustules among
the brown hairs; and the white impressed spots on the under surface of
_Cetraria islandica_ and allied species, first determined as air pores by
Zukal[436], have also been described by Wainio[437] as pseudocyphellae.

There seems no doubt that the chief function of these various structures
is, as Schwendener[438] suggested, to allow a free passage of air to the
assimilating gonidial zone. Jatta[439] considers them to be analogous
to the lenticels of higher plants and of service in the interchange
of gases—expelling carbonic acid and receiving oxygen from the outer
atmosphere. It is remarkable that such serviceable organs should have
been evolved in so few lichens.


B. BREATHING-PORES

[Illustration: Fig. 74. _Parmelia exasperata_ Carroll. Vertical section
of thallus. _a_, breathing-pores; _b_, rhizoid. × 60 (after Rosendahl).]

_a._ DEFINITE BREATHING-PORES. The cyphellae and pseudocyphellae
described above are confined to the under surface of the thallus in
those lichens where they occur. Distinct breathing-pores of a totally
different structure are present on the upper surface of the tree-lichen,
_Parmelia aspidota_ (_P. exasperata_), one of the brown-coloured species.
They are somewhat thickly scattered as isidia- or cone-like warts over
the lichen thallus (Fig. 74) and give it the characteristically rough or
“exasperate” character. They are direct outgrowths from the thallus, and
Zukal[440], who discovered their peculiar nature and function, describes
them as being filled with a hyphal tissue, with abundant air-spaces,
and in direct communication with the medulla; gonidia, if present, are
confined to the basal part. The cortex covering these minute cones, he
further states, is very thin on the top, or often wanting, so that a true
pore is formed which, however, is only opened after the cortex elsewhere
has become thick and horny. Rosendahl[441], who has re-examined these
“breathing-pores,” finds that in the early stage of their growth, near
the margin or younger portion of the thallus, they are entirely covered
by the cortex. Later, the hyphae at the top become looser and more
frequently septate, and a fine network of anastomosing and intricate
filaments takes the place of the closely cohering cortical cells. These
hyphae are divided into shorter cells, but do not otherwise differ from
those of the medulla. Rosendahl was unable to detect an open pore at any
stage, though he entirely agrees with Zukal as to the breathing function
of these structures. The gonidia of the immediately underlying zone are
sparsely arranged and a few of them are found in the lower half of the
cone; the hyphae of the medulla can be traced up to the apex. Zukal[442]
claims to have found breathing-pores in _Cornicularia_ (_Parmelia_)
_tristis_ and in several other _Parmeliae_, notably in _Parmelia stygia_.
The thallus of the latter species has minute holes or openings in the
upper cortex, but they are without any definite form and may be only
fortuitous.

[Illustration: Fig. 75 A. _Ramalina fraxinea_ Ach. A, surface view of
frond. _a_, air-pores; _b_, young apothecia. × 12. B, transverse section
of part of frond. _a_, breathing-pore; _f_, strengthening fibres. × 37
(after Brandt).]

[Illustration: Fig. 75 B. _Ramalina strepsilis_ Zahlbr. Transverse
section of part of frond showing distribution of: _a_, air-pores, and
_f_, strengthening fibres. × 37 (after Brandt).]

Zukal[442] published drawings of channels of looser tissue between the
exterior and the pith in _Oropogon Loxensis_ and in _Usnea barbata_.
He considered them to be of definite service in aeration. The fronds
of _Ramalina dilacerata_ by stretching develop a series of elongate
holes. Reinke[443] found openings in _Ramalina Eckloni_ which pierced to
the centre of the thallus, and Darbishire[444] has figured a break in
the frond of another species, _R. fraxinea_ (Fig. 75 A), which he has
designated as a breathing-pore. Finally Brandt[445], in his careful study
of the anatomy of _Ramalinae_, has described as breathing-pores certain
open areas usually of ellipsoid form in the compact cortex of several
species: in _R. strepsilis_ (Fig. 75 B) and _R. Landroensis_, and in
the British species, _R. siliquosa_ and _R. fraxinea_. These openings
are however mostly rare and difficult to find or to distinguish from
holes that may be due to any accident in the life of the lichen. It is
noteworthy that Rosendahl found no further examples of breathing-pores in
the brown _Parmeliae_ that he examined in such detail. No other organs
specially adapted for aeration of the thallus have been discovered.

_b._ OTHER OPENINGS IN THE THALLUS. _Lobaria_ is the only genus of
Stictaceae in which neither cyphellae nor pseudocyphellae are formed; but
in two species, _L. scrobiculata_ and _L. pulmonaria_, the lower surface
is marked with oblong or angular bare convex patches, much larger than
cyphellae. They are exposed portions of the medulla, which at these spots
has been denuded of the covering cortex. Corresponding with these bare
spots there is a pitting of the upper surface.

A somewhat similar but reversed structure characterizes _Umbilicaria
pustulata_, which as the name implies is distinguished by the presence
of pustules, ellipsoid swellings above, with a reticulation of cavities
below. Bitter[446] in this instance has proved that they are due to
disconnected centres of intercalary growth which are more vigorous on the
upper surface and give rise to cracks in the less active tissue beneath.
These cracks gradually become enlarged; they are, as it were, accidental
in origin but are doubtless of considerable service in aeration.

In some _Parmeliae_ there are constantly formed minute round holes,
either right through the apothecia (_P. cetrata_, etc.), or through the
thallus (_P. pertusa_). Minute holes are also present in the under cortex
of _Parmelia vittata_ and of _P. enteromorpha_, species of the subgenus
_Hypogymnia_. Nylander[447], who first drew attention to these holes of
the lower cortex, described them as arising at the forking of two lobes;
but though they do occur in that position, they as frequently bear no
relation to the branching. Bitter’s[448] opinion is that they arise
by the decay of the cortical tissues in very limited areas, from some
unknown cause, and that the holes that pierce right through the thallus
in other species may be similarly explained.

Still other minute openings into the thallus occur in _Parmelia vittata_,
_P. obscurata_ and _P. farinacea_ var. _obscurascens_. In the two latter
the openings like pin-holes are terminal on the lobes and are situated
exactly on the apex, between the pith and the gonidial zone; sometimes
several holes can be detected on the end of one lobe. Further growth in
length is checked by these holes. They appear more frequently on the
darker, better illuminated plants. In _Parmelia vittata_ the terminal
holes are at the end of excessively minute adventitious branches which
arise below the gonidial zone on the margin of the primary lobes. All
these terminal holes are directed upwards and are visible from above.

Bitter does not attribute any physiological significance to these very
definite openings in the thallus. It has been generally assumed that they
aid in the aeration of the thallus; it is also possible that they may be
of service in absorption, and they might even be regarded as open water
conductors.


C. GENERAL AERATION OF THE THALLUS

Definite structures adapted to secure the aeration of the thallus
in a limited number of lichens have been described above. These are
the breathing-pores of _Parmelia exasperata_ and the cyphellae and
pseudocyphellae of the Stictaceae, with which also may be perhaps
included the circumscribed breaks in the under cortex in some members of
that family.

Though lichens are composed of two actively growing organisms, the
symbiotic plant increases very slowly. The absorption of water and
mineral salts must in many instances be of the scantiest and the
formation of carbohydrates by the deep-seated chlorophyll cells of
correspondingly small amount. Active aeration seems therefore uncalled
for though by no means excluded, and there are many indirect channels by
which air can penetrate to the deeper tissues.

In crustaceous forms, whether corticate or not, the thallus is often
deeply seamed and cracked into areolae, and thus is easily pervious to
water and air. The growing edges and growing points are also everywhere
more or less loose and open to the atmosphere. In the larger foliose and
fruticose lichens, the soredia that burst an opening in the thallus,
and the cracks that are so frequent a feature of the upper cortex, all
permit of gaseous interchange. The apical growing point of fruticose
lichens is thin and porous, and in many of them the ribs and veins of
their channelled surfaces entail a straining of the cortical tissue that
results in the formation of thinner permeable areas. Zukal[449] devoted
special attention to the question of aeration, and he finds evidence
of air-passages through empty spermogonia and through the small round
holes that are constant in the upper surface of certain foliose species.
He claims also to have proved a system of air-canals right through the
thallus of the gelatinous Collemaceae. Though his proof in this instance
is somewhat unconvincing, he establishes the abundant presence of air in
the massively developed hypothecium of _Collema_ fruits. He found that
the carpogonial complex of hyphae was always well supplied with air, and
that caused him to view with favour the suggestion that the function of
the trichogyne is to provide an air-passage. In foliose lichens, the
under surface is frequently non-corticate, in whole or in part; or the
cortex becomes seamed and scarred with increasing expansion, the growth
in the lower layers failing to keep pace with that of the overlying
tissues, as in _Umbilicaria pustulata_.

It is unquestionable that the interior of the thallus of most lichens
contains abundant empty spaces between the loose-lying hyphae, and that
these spaces are filled with air.


2. CEPHALODIA


A. HISTORICAL AND DESCRIPTIVE

The term “cephalodium” was first used by Acharius[450] to designate
certain globose apothecia (pycnidia). At a later date he applied it
to the peculiar outgrowths that grow on the thallus of _Peltigera
aphthosa_, already described by earlier writers, along with other similar
structures, as “corpuscula,” “maculae,” etc. The term is now restricted
to those purely vegetative gall-like growths which are in organic
connection with the thallus of the lichen, but which contain one or more
algae of a different type from the one present in the gonidial zone. They
are mostly rather small structures, and they take various forms according
to the lichen species on which they occur. They are only found on thalli
in which the gonidia are bright-green algae (Chlorophyceae) and, with a
few exceptions, they contain only blue-green (Myxophyceae). Cephalodia
with bright-green algae were found by Hue[451] on two _Parmeliae_ from
Chili, in addition to the usual blue-green forms; the one contained
_Urococcus_, the other _Gloeocystis_. Several with both types of algae
were detected also by Hue[451] within the thallus of _Aspicilia_ spp.

Flörke[452] in his account of German lichens described the cephalodia
that grow on the podetia of _Stereocaulon_ as fungoid bodies, “corpuscula
fungosa.” Wallroth[453], who had made a special study of lichen gonidia,
finally established that the distinguishing feature of the cephalodia was
their gonidia which differed in colour from those of the normal gonidial
zone. He considered that the outgrowths were a result of changes that had
arisen in the epidermal tissues of the lichens, and, to avoid using a
name of mixed import such as “cephalodia,” he proposed a new designation,
calling them “phymata” or warts.

Further descriptions of cephalodia were given by Th. M. Fries[454]
in his _Monograph of Stereocaulon and Pilophorus_; but the greatest
advance in the exact knowledge of these bodies is due to Forssell[455]
who made a comprehensive examination of the various types, examples
of which occurred, he found, in connection with about 100 different
lichens. Though fairly constant for the different species, they are not
universally so, and are sometimes very rare even when present, and then
difficult to find. A striking instance of variability in their occurrence
is recorded for _Ricasolia amplissima_ (_Lobaria laciniata_) (Fig. 76).
The cephalodia of that species are prominent upright branching structures
which grow in crowded tufts irregularly scattered over the surface. They
are an unfailing and conspicuous specific character of the lichens in
Europe, but are entirely wanting in North American specimens.

[Illustration: Fig. 76. _Ricasolia amplissima_ de Not. (_Lobaria
laciniata_ Wain.) on oak, reduced. The dark patches are tufts of
branching cephalodia (A. Wilson, _Photo._).]

As cephalodia contain rather dark-coloured, blue-green algae, they are
nearly always noticeably darker than the thalli on which they grow,
varying from yellowish-red or brown in those of _Lecanora gelida_ to
pale-coloured in _Lecidea consentiens_[456], a darker red in _Lecidea
panaeola_ and various shades of green, grey or brown in _Stereocaulon_,
_Lobaria_ (_Ricasolia_), etc. They form either flat expansions of varying
size on the upper surface of the thallus, rounded or wrinkled wart-like
growths, or upright branching structures. On the lower surface, where
they are not unfrequent, they take the form of small brown nodules or
swellings. In a number of species packets of blue-green algae surrounded
by hyphae are found embedded in the thallus, either in the pith or
immediately under the cortex. They are of the same nature as the
superficial excrescences and are also regarded as cephalodia.


B. CLASSIFICATION

Forssell has drawn up a classification of these structures, as follows:


I. CEPHALODIA VERA.

1. =Cephalodia epigena= (including =perigena=) developed on the upper
outer surface of the thallus, which are tuberculose, lobulate, clavate or
branched in form. These are generally corticate structures.

2. =Cephalodia hypogena= which are developed on the under surface of the
thallus; they are termed “thalloid” if they are entirely superficial,
and “immersed” when they are enclosed within the tissues. They are
non-corticate though surrounded by a weft of hyphae. Forssell further
includes here certain placodioid (lobate), granuliform and fruticose
forms which develop on the hypothallus of the lichen, and gradually
push their way up either through the host thallus, or, as in _Lecidea
panaeola_, between the thalline granules.

Nylander[457] arranged the cephalodia known to him in three groups: (1)
Ceph. epigena, (2) Ceph. hypogena and (3) Ceph. endogena. Schneider[458]
still more simply and practically describes them as Ectotrophic
(external), and Endotrophic (internal).


II. PSEUDOCEPHALODIA.

These are a small and doubtful group of cephalodia which are apparently
in very slight connection with the host thallus, and show a tendency to
independent growth. They occur as small scales on _Solorina bispora_[459]
and _S. spongiosa_ and also on _Lecidea pallida_. Forssell has suggested
that the cephalodia of _Psoroma hypnorum_ and of _Lecidea panaeola_ might
also be included under this head.

Forssell and others have found and described cephalodia in the following
families and genera:

Sphaerophoraceae.

  _Sphaerophorus_ (_S. stereocauloides_).

Lecideaceae.

  _Lecidea_ (_L. panaeola_, _L. consentiens_, _L. pelobotrya_,
  etc.).

Cladoniaceae.

  _Stereocaulon_, _Pilophorus_ and _Argopsis_.

Pannariaceae.

  _Psoroma_ (_P. hypnorum_).

Peltigeraceae.

  _Peltigera_ (_Peltidea_), _Nephroma_ and _Solorina_.

Stictaceae.

  _Lobaria_, _Sticta_.

Lecanoraceae.

  _Lecania_ (_L. lecanorina_), _Aspicilia_[460].

Physciaceae.

  _Placodium bicolor_[461].


C. ALGAE THAT FORM CEPHALODIA

The algae of the cephalodia belong mostly to genera that form the normal
gonidia of other lichens. They are:

_Stigonema_,—in _Lecanora gelida_, _Stereocaulon_, _Pilophorus robustus_,
and _Lecidea pelobotrya_.

_Scytonema_,—a rare constituent of cephalodia.

_Nostoc_,—the most frequent gonidium of cephalodia. It occurs in those of
the genera _Sticta_, _Lobaria_, _Peltigera_, _Nephroma_, _Solorina_ and
_Psoroma_; occasionally in _Stereocaulon_ and in _Lecidea pallida_.

_Lyngbya_ and _Rivularia_,—rarely present, the latter in _Sticta
oregana_[462].

_Chroococcus_ and _Gloeocapsa_,—also very rare.

_Scytonema_, _Chroococcus_, _Gloeocapsa_ and _Lyngbya_ are generally
found in combination with some other cephalodia-building alga, though
Nylander[463] found _Scytonema_ alone in the lobulate cephalodia of
_Sphaerophorus stereocauloides_, a New Zealand lichen, and the only
species of that genus in which cephalodia are developed; and Hue[460]
records _Gloeocapsa_ as forming internal cephalodia in two species of
_Aspicilia_. Bornet[464] found _Lyngbya_ associated with _Scytonema_ in
the cephalodia of _Stereocaulon ramulosum_, and, in the same lichen,
Forssell[465] found, in the several cephalodia of one specimen, _Nostoc_,
_Scytonema_, and _Lyngbya_, while, in those of another, _Scytonema_ and
_Stigonema_ were present. In the latter instance these algae were living
free on the podetium. Forssell[465] also determined two different algae,
_Gloeocapsa magma_ and _Chroococcus turgidus_, present in a cephalodium
on _Lecidea panaeola_ var. _elegans_.

As a general rule only one kind of alga enters into the formation of the
cephalodia of any species or genus. A form of _Nostoc_, for instance,
is invariably the gonidial constituent of these bodies in the genera,
_Lobaria_, _Sticta_, etc. In other lichens different blue-green algae,
as noted above, may occupy the cephalodia even on the same specimen.
Forssell finds alternative algae occurring in the cephalodia of:

_Lecanora gelida_ and _Lecidea illita_ contain either _Stigonema_ or
_Nostoc_;

_Lecidea panaeola_, with _Gloeocapsa_, _Stigonema_ or _Chroococcus_;

_Lecidea pelobotrya_, with _Stigonema_ or _Nostoc_;

_Pilophorus robustus_, with _Gloeocapsa_, _Stigonema_, or _Nostoc_.

[Illustration: Fig. 77. _Lecanora gelida_ Ach. _a_, lobate cephalodia ×
12 (after Zopf).]

Riddle[466] has employed cephalodia with their enclosed algae as
diagnostic characters in the genus _Stereocaulon_. When the alga is
_Stigonema_, as in _S. paschale_, etc., the cephalodia are generally
very conspicuous, grey in colour, spherical, wrinkled or folded, though
sometimes black and fibrillose (_S. denudatum_). Those containing
_Nostoc_ are, on the contrary, minute and are coloured verdigris-green
(_S. tomentosum_ and _S. alpinum_).

Instances are recorded of algal colonies adhering to, and even
penetrating, the thallus of lichens, but as they never enter into
relationship with the lichen hyphae, they are antagonistic rather than
symbiotic and have no relation to cephalodia.


D. DEVELOPMENT OF CEPHALODIA

_a._ ECTOTROPHIC. Among the most familiar examples of external cephalodia
are the small rather dark-coloured warts or swellings that are scattered
irregularly over the surface of _Peltigera_ (_Peltidea_) _aphthosa_.
This lichen has a grey foliose thallus of rather large sparingly divided
lobes; it spreads about a hand-breadth or more over the surface of the
ground in moist upland localities. The specific name “aphthosa” was given
by Linnaeus to the plant on account of the supposed resemblance of the
dotted thallus to the infantile ailment of “thrush.” Babikoff[467] has
published an account of the formation and development of these _Peltidea_
cephalodia. He determined the algae contained in them to be _Nostoc_ by
isolating and growing them on moist sterilized soil. He observed that
the smaller, and presumably younger, excrescences were near the edges
of the lobes. The cortical cells in that position grow out into fine
septate hairs that are really the ends of growing hyphae. Among the
hairs were scattered minute colonies of _Nostoc_ cells lying loose or so
closely adhering to the hairs as to be undetachable (Fig. 78 A). In older
stages the hairs, evidently stimulated by contact with the _Nostoc_, had
increased in size and sent out branches, some of which penetrated the
gelatinous algal colony; others, spreading over its surface, gradually
formed a cortex continuous with that of the thallus. The alga also
increased, and the structure assumed a rounded or lentiform shape. The
thalline cortex immediately below broke down, and the underlying gonidial
zone almost wholly died off and became absorbed. The hyphae of the
cephalodium had meanwhile penetrated downwards as root-like filaments,
those of the thallus growing upwards into the new overlying tissue (Fig.
78 B). The foreign alga has been described as parasitic, as it draws
from the lichen hyphae the necessary inorganic food material; but it
might equally well be considered as a captive pressed into the service of
the lichen to aid in the work of assimilation or as a willing associate
giving and receiving mutual benefit.

[Illustration: Fig. 78 A. Hairs of _Peltigera aphthosa_ Willd. associated
with _Nostoc_ colony much magnified (after Babikoff).]

[Illustration: Fig. 78 B. _Peltigera aphthosa_ Willd. Vertical section of
thallus and cephalodium × 480 (after Babikoff).]

Th. M. Fries[468] had previously described the development of the
cephalodia in _Stereocaulon_ but failed to find the earliest stages. He
concluded from his observations that parasitic algae were common in the
cortical layer of the lichens, but only rarely formed the “monstrous
growths” called cephalodia.

_b._ ENDOTROPHIC. Winter[469] examined the later stages of internal
cephalodine formation in a species of _Sticta_. The alga, probably a
species of _Rivularia_, which gives origin to the cephalodia, may be
situated immediately below the upper cortex, in the medullary layer
close to the gonidial zone, or between the pith and the under cortex.
The protuberance caused by the increasing tissue, which also contains
the invading alga, arises accordingly either on the upper or the lower
surface. In some cases it was found that the normal gonidial layer had
been pushed up by the protruding cephalodium and lay like a cap over the
top. The cephalodia described by Winter are endogenous in origin, though
the mature body finally emerges from the interior and becomes either
epigenous or hypogenous. Schneider[470] has followed the development of a
somewhat similar endotrophic or endogenous type in _Sticta oregana_ due
also to the presence of a species of _Rivularia_. How the alga attained
its position in the medulla of the thallus was not observed.

[Illustration: Fig. 79. _Nephroma expallidum_ Nyl. Vertical section of
thallus. _a_, endotrophic cephalodium × 100 (after Forssell).]

Both the algal cells of internal cephalodia and the hyphae in contact
with them increase vigorously, and the newly formed tissue curving
upwards or downwards appears on the outside as a swelling or nodule
varying in size from that of a pin-head to a pea. On the upper surface
the gonidial zone partly encroaches on the nodule, but the foreign alga
remains in the centre of the structure well separated from the thalline
gonidia by a layer of hyphae. The group is internally divided into small
nests of dark-green algae surrounded by strands of hyphae (Fig. 79).
The swellings, when they occur on the lower surface of the lichen,
correspond to those of the upper in general structure, but there is no
intermixture of thalline gonidia. That _Nostoc_ cells can grow and retain
the power to form chlorophyll in adverse conditions was proved by Etard
and Bouilhac[471] who made a culture of the alga on artificial media in
the dark, when there was formed a green pigment of chlorophyll nature.

Endotrophic cephalodia occur in many groups of lichens. Hue[472]
states that he found them in twelve species of _Aspicilia_. As packets
of blue-green algae they are a constant feature in the thallus of
_Solorinae_. The species of that genus grow on mossy soil in damp places,
and must come frequently in contact with _Nostoc_ colonies. In _Solorina
crocea_ an interrupted band of blue-green algae lies below the normal
gonidial zone and sometimes replaces it—a connecting structure between
cephalodia and a true gonidial zone.

_c._ PSEUDOCEPHALODIA. Under this section have been classified those
cephalodia that are almost independent of the lichen thallus though
to some extent organically connected with it, as for instance that of
_Lecidea panaeola_ which originate on the hypothallus of the lichen and
maintain their position between the crustaceous granules.

The cephalodia of _Lecanora gelida_, as described by Sernander[473],
might also be included here. He watched their development in their
native habitat, an exposed rock-surface which was richly covered with
the lichen in all stages of growth. Two kinds of thallus, the one
containing blue-green algae (_Chroococcus_), the other bright-green,
were observed on the rock in close proximity. At the point of contact,
growth ceased, but the thallus with bright-green algae, being the more
vigorous, was able to spread round and underneath the other and so
gradually to transform it to a superficial flat cephalodium. All such
thalli encountered by the dominant lichen were successively surrounded in
the same way. The cephalodium, growing more slowly, sent root-like hyphae
into the tissue of the underlying lichen, and the two organisms thus
became organically connected. Sernander considers that the two algae are
antagonistic to each other, but that the hyphae can combine with either.

The pseudocephalodia of _Usnea_ species are abortive apothecia; they are
surrounded at the base by the gonidial zone and cortex of the thallus,
and they contain no foreign gonidia.


E. AUTOSYMBIOTIC CEPHALODIA

Bitter[474] has thus designated small scales, like miniature thalli,
that develop constantly on the upper cortex of _Peltigera lepidophora_,
a small lichen not uncommon in Finland, and first recorded by Wainio as
a variety of _Peltigera canina_. The alga contained in the scales is a
blue-green _Nostoc_ similar to the gonidia of the thallus. Bitter[475]
described the development as similar to that of the cephalodia of
_Peltigera aphthosa_, but the outgrowths, being lobate in form, are
less firmly attached and thus easily become separated and dispersed; as
the gonidia are identical with those of the parent thallus they act as
vegetative organs of reproduction.

Bitter’s work has been criticized by Linkola[476] who claims to have
discovered by means of very thin microtome sections that there is a
genetic connection between the scales and the underlying thallus, not
only with the hyphae, as in true cephalodia, but with the algae as well,
so that these outgrowths should be regarded as isidia.

In the earliest stages, according to Linkola, a small group of algae may
be observed in the cortical tissue of the _Peltigera_ apart from the
gonidial zone and near the upper surface. Gradually a protruding head
is formed which is at first covered over with a brown cortical layer
one cell thick. The head increases and becomes more lobate in form,
being attached to the thallus at the base by a very narrow neck and
more loosely at other parts of the scale. In older scales, the gonidia
are entirely separated from those of the thallus, and a dark-brown
cortex several cells in thickness covers over the top and sides; there
is a colourless layer of plectenchyma beneath. At this advanced stage
the scales are almost completely superficial and correspond with the
cephaloidal rather than with the isidial type of formation. The algae
even in the very early stages are distinct from the gonidial zone and the
whole development, if isidial, must be considered as somewhat abnormal.


3. SOREDIA


A. STRUCTURE AND ORIGIN OF SOREDIA

Soredia are minute separable parts of the lichen thallus, and are
composed of one or more gonidia which are clasped and surrounded by
the lichen hyphae (Fig. 80). They occur on the surface or margins of
the thallus of a fairly large number of lichens either in a powdery
excrescence or in a pustule-like body comprehensively termed a “soralium”
(Fig. 81). The soralia vary in form and dimensions according to the
species. Each individual soredium is capable of developing into a new
plant; it is a form of vegetative reproduction characteristic of lichens.

[Illustration: Fig. 80. Soredia. _a_, of _Physcia pulverulenta_ Nyl.;
_b_, of _Ramalina farinacea_ Ach. × 600.]

Acharius[477] gave the name “soredia” to the powdery bodies with
reference to their propagating function; he also interpreted the soredium
as an “apothecium of the second order.” But long before his time they
had been observed and commented on by succeeding botanists: first by
Malpighi[478] who judged them to be seeds, he having seen them develop
new plants; by Micheli[479] who however distinguished between the true
fruit and those seeds; and by Linnaeus[480] who considered them to be
the female organs of the plant, the apothecia being, as he then thought,
the male organs. Hedwig[481], on the other hand, regarded the apothecia
as the seed receptacles and the soredia as male bodies. Sprengel’s[482]
statement that they were “a subtile germinating powder mixed with
delicate hair-like threads which take the place of seeds” established
finally their true function. Wallroth[483], who was the first really
to investigate their structure and their relation to the parent plant,
recognized them as of the same type as the “brood-cells” or gonidia; and
as the latter, he found, could become free from the thallus and form a
green layer on trees, walls, etc., in shady situations, so the soredia
also could become free, though for a time they remained attached to the
lichen and were covered by a veil, _i.e._ by the surrounding hyphal
filaments. Koerber[484] also gave much careful study to soredia, their
nature and function. As propagating organs he found they were of more
importance than spores, especially in the larger lichens.

[Illustration: Fig. 81. Vertical section of young soralium of _Evernia
furfuracea_ var. _soralifera_ Bitter × 60 (after Bitter).]

According to Schwendener[485], the formation of soredia is due to
increased and almost abnormal activity of division in the gonidial cell;
the hyphal filament attached to it also becomes active and sends out
branches from the cell immediately below the point of contact which force
their way between the newly divided gonidia and finally surround them.
A soredial “head” of smaller or larger size is thus gradually built up
on the stalk filament or filaments, and is ultimately detached by the
breaking down of the slender support.

_a._ SCATTERED SOREDIA. The simplest example of soredial formation may
be seen on the bark of trees or on palings when the green coating of
algal cells is gradually assuming a greyish hue caused by the invasion
of hyphal lichenoid growth. This condition is generally referred to as
“leprose” and has even been classified as a distinct genus, _Lepra_ or
_Lepraria_. Somewhat similar soredial growth is also associated with
many species of _Cladonia_, the turfy soil in the neighbourhood of the
upright podetia being often powdered with white granules. Such soredia
are especially abundant in that genus, so much so, that Meyer[486],
Krabbe[487] and others have maintained that the spores take little part
in the propagation of species. The under side of the primary thallus, but
more frequently the upright podetia, are often covered with a coating of
soredia, either finely furfuraceous, or of larger growth and coarsely
granular, the size of the soredia depending on the number of gonidia
enclosed in each “head.”

Soredia are only occasionally present on the apothecial margins:
the rather swollen rims in _Lobaria scrobiculata_ are sometimes
powdery-grey, and Bitter[488] has observed soredia, or rather soralia,
on the apothecial margins of _Parmelia vittata_; they are very rare,
however, and are probably to be explained by excess of moisture in the
surroundings.

_b._ ISIDIAL SOREDIA. In a few lichens soredia arise by the breaking down
of the cortex at the tips of the thalline outgrowths termed “isidia.”
In _Parmelia verruculifera_, for instance, where the coralloid isidia
grow in closely packed groups or warts, the upper part of the isidium
frequently becomes soredial. In that lichen the younger parts of the
upper cortex bear hairs or trichomes, and the individual soredia are also
adorned with hairs. The somewhat short warted isidia of _P. subaurifera_
may become entirely sorediose, and in _P. farinacea_ the whole thallus
is covered with isidia transformed into soralia. The transformation is
constant and is a distinct specific character. Bitter[488] considers that
it proves that no sharp distinction exists between isidia and soralia, at
least in their initial stages.

[Illustration: Fig. 82. _Usnea barbata_ Web. Longitudinal section of
filament and base of “soredial” branch × 40 (after Schwendener).]

_c._ SOREDIA AS BUDS. Schwendener[489] has described soredia in the
genus _Usnea_ which give rise to new branches. Many of the species in
that genus are plentifully sprinkled with the white powdery bodies. A
short way back from the apex of the filament the separate soredia show a
tendency to apical growth and might be regarded as groups of young plants
still attached to the parent branch. One of these developing more quickly
pushes the others aside and by continued growth fills up the soredial
opening in the cortex with a plug of tissue; finally it forms a complete
lateral branch. Schwendener calls them “soredial” branches (Fig. 82)
to distinguish them from the others formed in the course of the normal
development.


B. SORALIA

In lichens of foliose and fruticose structure, and in a few crustaceous
forms, the soredia are massed together into the compact bodies called
soralia, and thus are confined to certain areas of the plant surface.
The simpler soralia arise from the gonidial zone below the cortex by the
active division of some of the algal cells. The hyphae, interlaced with
the green cells, are thin-walled and are, as stated by Wainio[490], still
in a meristematic condition; they are thus able readily to branch and to
form new filaments which clasp the continually multiplying gonidia. This
growth is in an upward or outward direction away from the medulla, and
strong mechanical pressure is exerted by the increasing tissue on the
overlying cortical layers. Finally the soredia force their way through
to the surface at definite points. The cortex is thrown back and forms a
margin round the soralium, though shreds of epidermal tissue remain for a
time mixed with the powdery granules.

_a._ FORM AND OCCURRENCE OF SORALIA. The term “soralium” was first
applied only to the highly developed soredial structures considered
by Acharius to be secondary apothecia; it is now employed for any
circumscribed group of soredia.[491] The soralia vary in size and
form and in position, according to the species on which they occur;
these characters are constant enough to be of considerable diagnostic
value. Within the single genus _Parmelia_, they are to be found as
small round dots sprinkled over the surface of _P. dubia_; as elongate
furrows irregularly placed on _P. sulcata_; as pearly excrescences at
or near the margins of _P. perlata_, and as swollen tubercles at the
tips of the lobes of _P. physodes_ (Fig. 83). Their development is
strongly influenced and furthered by shade and moisture, and, given such
conditions in excess, they may coalesce and cover large patches of the
thallus with a powdery coating, though only in those species that would
have borne soredia in fairly normal conditions.

Soralia of definite form are of rather rare occurrence in crustaceous
lichens, with the exception of the Pertusariaceae, where they are
frequent, and some species of _Lecanora_ and _Placodium_. They are known
in only two hypophloeodal (subcortical) lichens, _Arthonia pruinosa_ and
_Xylographa spilomatica_. Among squamulose thalli they are typical of
some _Cladoniae_, and also of _Lecidea_ (_Psora_) _ostreata_, where they
are produced on the upper surface towards the apex of the squamule.

[Illustration: Fig. 83. _Parmelia physodes_ Ach. Thallus growing
horizontally; soredia on the ends of the lobes (S. H., _Photo._).]

_b._ POSITION OF SORALIFEROUS LOBES. According to observations made by
Bitter[492], the occurrence of soralia on one lobe or another may depend
to a considerable extent on the orientation of the thallus. He cites the
variability in habit of the familiar lichen, _Parmelia physodes_ and its
various forms, which grow on trees or on soil. In the horizontal thalli
there is much less tendency to soredial formation, and the soredia that
arise are generally confined to branching lobes on the older parts of the
thallus.

That type of growth is in marked contrast with the thallus obliged to
take a vertical direction as on a tree. In such a case the lobes, growing
downward from the point of origin, form soralia at their tips at an early
stage (Fig. 84). The lateral lobes, and especially those that lie close
to the substratum, are the next to become soraliate. Similar observations
have been made on the soraliferous lobes of _Cetraria pinastri_. The
cause is probably due to the greater excess of moisture draining
downwards to the lower parts of the thallus. The lobes that bear the
soralia are generally narrower than the others and are very frequently
raised from contact with the substratum. They tend to grow out from
the thallus in an upright direction and then to turn backwards at the
tip, so that the opening of the soralium is directed downwards. Bitter
says that the cause of this change in direction is not clear, though
possibly on teleological reasoning it is of advantage that the opening
of the soralium should be protected from direct rainfall. The opening
lies midway between the upper and lower cortex, and the upper tissue in
these capitate soralia continues to grow and to form an arched helmet or
hood-covering which serves further to protect the soralium.

[Illustration: Fig. 84. _Parmelia physodes_ Ach. Thallus growing
vertically; soredia chiefly on the lobes directed downwards, reduced (M.
P., _Photo._).]

Similar soralia are characteristic of _Physcia hispida_ (_Ph. stellaris_
subsp. _tenella_), the apical helmet being a specially pronounced feature
of that species, though, as Lesdain[493] has pointed out, the hooded
structures are primarily the work of insects. In vertical substrata they
occur on the lower lobes of the plant.

Apical soralia are rare in fruticose lichens, but in an Alpine variety
of _Ramalina minuscula_ they are formed at the tips of the fronds and
are protected by an extension of the upper cortical tissues. Another
instance occurs in a _Ramalina_ from New Granada referred by Nylander to
_R. calicaris_ var. _farinacea_: it presents a striking example of the
helmet tip.

_c._ DEEP-SEATED SORALIA. In the cases already described Schwendener[494]
and Nilson[495] held that the algae gave the first impulse to the
formation of the soredia; but in the Pertusariaceae[496], a family of
crustaceous lichens, there has been evolved a type of endogenous soralium
which originates with the medullary hyphae. In these, special hyphae rise
from a weft of filaments situated just above the lowest layer of the
thallus at the base of the medulla, the weft being distinguished from the
surrounding tissue by staining blue with iodine. A loose strand of hyphae
staining the usual yellow colour rises from the surface of the “blue”
weft and, traversing the medullary tissue, surrounds the gonidia on the
under side of the gonidial zone. The hyphae continue to grow upward,
pushing aside both the upper gonidial zone and the cortex, and carrying
with them the algal cells first encountered. When the summit is reached,
there follows a very active growth of both gonidia and hyphae. Each
separate soredium so produced consists finally of five to ten algal cells
surrounded by hyphae and measures 8 µ to 13 µ in diameter. The cortex
forms a well-defined wall or margin round the mass of soredia.

A slightly different development is found in _Lecanora tartarea_,
one of the “crottle” lichens, which has been placed by Darbishire in
Pertusariaceae. The hyphae destined to form soredia also start from the
weft of tissue at the base of the thallus, but they simply grow through
the gonidial zone instead of pushing it aside.

In his examination of Pertusariaceae Darbishire found that the apothecia
also originated from a similar deeply seated blue-staining tissue,
and he concluded that the soralia represented abortive apothecia and
really corresponded to Acharius’s “apothecia of the second order.”
His conclusion as to the homology of these two organs is disputed by
Bitter[497], who considers that the common point of origin is explained
by the equal demand of the hyphae in both cases for special nutrition,
and by the need of mechanical support at the base to enable the hyphae to
reach the surface and to thrust back the cortex without deviating from
their upward course through the tissues.


C. DISPERSAL AND GERMINATION OF SOREDIA

Soredia become free by the breaking down of the hyphal stalks at the
septa or otherwise. They are widely dispersed by wind or water and
soon make their appearance on any suitable exposed soil. Krabbe[498]
has stated that, in many cases, the loosely attached soredia coating
some of the _Cladonia_ podetia are of external origin, carried thither
by the air-currents. Insects too aid in the work of dissemination:
Darbishire[499] has told us how he watched small mites and other insects
moving about over the soralia of _Pertusaria amara_ and becoming
completely powdered by the white granules.

Darbishire[499] also gives an account of his experiments in the culture
of soredia. He sowed them on poplar wood about the beginning of February
in suitable conditions of moisture, etc. Long hyphal threads were at once
produced from the filaments surrounding the gonidia, and gonidia that had
become free were seen to divide repeatedly. Towards the end of August
of the same year a few soredia had increased in size to about 450µ in
diameter, and were transferred to elm bark. By September they had further
increased to a diameter of 520µ, and the gonidia showed a tendency
towards aggregation. No further differentiation or growth was noted.

More success attended Tobler’s[500] attempt to cultivate the soredia of
_Cladonia_ sp. He sowed them on soil kept suitably moist in a pot and
after about nine months he obtained fully formed squamules, at first only
an isolated one or two, but later a plentiful crop all over the surface
of the soil. Tobler also adds that soredia taken from a _Cladonia_, that
had been kept for about half a year in a dry room, grew when sown on a
damp substratum. The algae however had suffered more or less from the
prolonged desiccation, and some of them failed to develop.

A suggestion has been made by Bitter[501] that a hybrid plant might
result from the intermingling of soredia from the thallus of allied
lichens. He proposed the theory to explain the great similarity between
plants of _Parmelia physodes_ and _P. tubulosa_ growing in close
proximity. There is no proof that such mingling of the fungal elements
ever takes place.


D. EVOLUTION OF SOREDIA

Soredia have been compared to the gemmae of the Bryophytes and also to
the slips and cuttings of the higher plants. There is a certain analogy
between all forms of vegetative reproduction, but soredia are peculiar
in that they include two dissimilar organisms. In the lichen kingdom
there has been evolved this new form of propagation in order to secure
the continuance of the composite life, and, in a number of species, it
has almost entirely superseded the somewhat uncertain method of spore
germination inherited from the fungal ancestor, but which leaves more or
less to chance the encounter with the algal symbiont.

From a phylogenetic point of view we should regard the sorediate
lichens as the more highly evolved, and those which have no soredia
as phylogenetically young, though, as Lindau[502] has pointed out,
soredia are all comparatively recent. They probably did not appear until
lichens had reached a more or less advanced stage of development, and,
considering the polyphyletic origin of lichens, they must have arisen at
more than one point, and probably at first in circumstances where the
formation of apothecia was hindered by prolonged conditions of shade and
moisture.

That soredia are ontogenetic in character, and not, as Nilson[503] has
asserted, accidental products of excessively moist conditions is further
proved by the non-sorediate character of those species of crustaceous
lichens belonging to _Lecanora_, _Verrucaria_, etc. that are frequently
immersed in water. Bitter[504] found that the soredia occurring on
_Peltigera spuria_ were not formed on the lobes which were more
constantly moist, nor at the edges where the cortex was thinnest: they
always emerged on young parts of the thallus a short way back from the
edge.

Bitter[504] points out that in extremely unfavourable circumstances—in
the polluted atmosphere near towns, or in persistent shade—lichens,
that would otherwise form a normal thallus, remain in a backward
sorediose state. He considers, however, that many of these formless
crusts are autonomous growths with specific morphological and chemical
peculiarities. They hold these outposts of lichen vegetation and are not
found growing in any other localities. The proof would be to transport
them to more favourable conditions, and watch development.


4. ISIDIA


A. FORM AND STRUCTURE OF ISIDIA

Many lichens are rough and scabrous on the surface, with minute simple
or divided coral-like outgrowths of the same texture as the underlying
thallus, though sometimes they are darker in colour as in _Evernia
furfuracea_. They always contain gonidia and are covered by a cortex
continuous with that of the thallus.

This very marked condition was considered by Acharius[505] as of
generic importance and the genus, _Isidium_, was established by him,
with the diagnostic characters: “branchlets produced on the surface,
or coralloid, simple and branched.” In the genus were included the
more densely isidioid states of various crustaceous species such as
_Isidium corallinum_ and _I. Westringii_, both of which are varieties of
_Pertusariae_. Fries[506], with his accustomed insight, recognized them
as only growth forms. The genus was however still accepted in English
Floras[507] as late as 1833, though we find it dropped by Taylor[508] in
the _Flora Hibernica_ a few years later.

The development of the isidial outgrowth has been described by
Rosendahl[509] in several species of _Parmelia_. In one of them, _P.
papulosa_, which has a cortical layer one cell thick, the isidium begins
as a small swelling or wart on the upper surface of the thallus. At that
stage the cells of the cortex have already lost their normal arrangement
and show irregular division. They divide still further, as gonidia and
hyphae push their way up. The full-grown isidia in this species are
cylindrical or clavate, simple or branched. They are peculiar in that
they bear laterally here and there minute rhizoids, a development not
recorded in any other isidia. The inner tissue accords with that of the
normal thallus and there is a clearly marked cortex, gonidial zone and
pith. A somewhat analogous development takes place in the isidia of
_Parmelia proboscidea_; in that lichen they are mostly prolonged into a
dark-coloured cilium.

In _Parmelia scortea_ the cortex is several cells thick, and the
outermost rows are compressed and dead in the older parts of the thallus;
but here also the first appearance of the isidium is in the form of a
minute wart. The lower layers (4 to 6) of living cortical cells divide
actively; the gonidia also share in the new growth, and the protuberance
thus formed pushes off the outer dead cortex and emerges as an isidium
(Fig. 85). They are always rather stouter in form than those of _P.
papulosa_ and may be simple or branched. The gonidia in this case do not
form a definite zone, but are scattered through the pith of the isidium.

Here also should be included the coralloid branching isidia that
adorn the upper surface and margins of the thallus of _Umbilicaria
pustulata_. They begin as small tufts of somewhat cylindrical bodies,
but they sometimes broaden out to almost leafy expansions with crisp
edges. Most frequently they are situated on the bulging pustules where
intercalary growth is active. Owing to their continued development on
these areas, the tissue becomes slack, and the centre of the isidial tuft
may fall out, leaving a hole in the thallus which becomes still more
open by the tension of thalline expansion. New isidia sprout from the
edges of the wound and the process may again be repeated. It has been
asserted that these structures are only formed on injured parts of the
thallus—something like gall-formations—but Bitter[510] has proved that
the wound is first occasioned by the isidial growth weakening the thallus.

[Illustration: Fig. 85. Vertical section of isidia of _Parmelia scortea_
Ach. A, early stage; B. later stage. × 60 (after Rosendahl).]


B. ORIGIN AND FUNCTION OF ISIDIA

Nilson[511] (later Kajanus[512]) insists that isidia and soredia are
both products of excessive moisture. He argues that lichen species, in
the course of their development, have become adapted to a certain degree
of humidity, and, if the optimum is passed, the new conditions entail
a change in the growth of the plant. The gonidia are stimulated to
increased growth, and the mechanical pressure exerted by the multiplying
cells either results in the emergence of isidial structures where the
cortex is unbroken, or, if the cortex is weaker and easily bursts, in the
formation of soralia.

This view can hardly be accepted; isidia as well as soredia are typical
of certain species and are produced regularly and normally in ordinary
conditions; both of them are often present on the same thallus. It is
not denied, however, that their development in certain instances is
furthered by increased shade or moisture. In _Evernia furfuracea_ isidia
are more freely produced on the older more shaded parts of the thallus.
Zopf[513] has described such an instance in _Evernia olivetorina_ (_E.
furfuracea_), which grew in the high Alps on pine trees, and which was
much more isidiose when it grew on the outer ends of the branches,
where dew, rain or snow had more direct influence. He[514] quotes other
examples occurring in forms of _E. furfuracea_ which grew on the branches
of pines, larches, etc. in a damp locality in S. Tyrol. The thalli hung
in great abundance on each side of the branches, and were invariably more
isidiose near the tips, because evidently the water or snow trickled down
and was retained longer there than at the base.

Bitter[515] has given a striking instance of shade influence in
_Umbilicaria_. He found that some boulders on which the lichen grew
freely had become covered over with fallen pine needles. The result was
at first an enormous increase of the coralline isidia, though finally the
lichen was killed by the want of light.

Isidia are primarily of service to the plant in increasing the
assimilating surface. Occasionally they grow out into new thallus
lobes. The more slender are easily rubbed off, and, when scattered,
become efficient organs of propagation. This view of their function is
emphasized by Bitter who points out that both in _Evernia furfuracea_
and in _Umbilicaria pustulata_ other organs of reproduction are rare or
absent. Zopf[513] found new plants of _Evernia furfuracea_ beginning to
grow on the trunk of a tree lower down than an old isidiose specimen.
They had developed from isidia which had been detached and washed down by
rain.


VI. HYMENOLICHENS


A. SUPPOSED AFFINITY WITH OTHER PLANTS

Lichens in which the fungal elements belong to the Hymenomycetes are
confined to three tropical genera. They are associated with blue-green
algae and are most nearly related to the Thelephoraceae among fungi. The
spores are borne, as in that family, on basidia.

[Illustration: Fig. 86. _Cora Pavonia_ Fr. (after Mattirolo).]

The best known Hymenolichen, _Cora Pavonia_ (Fig. 86), was discovered
by Swartz[516] during his travels in the W. Indies (1785-87) growing on
trees in the mountains of Jamaica, and the new plant was recorded by
him as _Ulva montana_. Gmelin[517] also included it in _Ulva_ in close
association with _Ulva_ (_Padina_) _Pavonia_, but that classification was
shortly after disputed by Woodward[518] who thought its affinity was more
nearly with the fungi and suggested that it should be made the type of a
new genus near to _Boletus_ (_Polystictus_) _versicolor_. Fries[519] in
due time made the new genus _Cora_, though he included it among algae;
finally Nylander[520] established the lichenoid character of the thallus
and transferred it to the Lecanorei.

It was made the subject of more exact investigation by Mattirolo[521]
who recognized its affinity with _Thelephora_, a genus of Hymenomycetes.
Later Johow[522] went to the West Indies and studied the Hymenolichens in
their native home. The genera and species described by Johow have been
reduced to _Cora_ and _Dictyonema_; a new genus _Corella_ has since been
added by Wainio[523].

Johow found that _Cora_ grew on the mountains usually from 1000 to 2000
ft. above sea-level. As it requires for its development a cool damp
climate with strong though indirect illumination, it is found neither
in sunny situations nor in the depths of dark woods. It grows most
freely in diffuse light, on the lower trunks and branches of trees in
open situations, but high up on the stem where the vegetation is more
dense. It stands out from the tree like a small thin bracket fungus,
one specimen placed above another, with a dimidiate growth similar to
that of _Polystictus versicolor_. Both surfaces are marked by concentric
zones which give it an appearance somewhat like _Padina Pavonia_. These
zones indicate unequal intercalary growth both above and below. The whole
plant is blue-green when wet, greyish-white when dry, and of a thin
membranaceous consistency.


B. STRUCTURE OF THALLUS

There is no proper cortex in any of the genera, but in _Cora_ there
is a fastigiate branching of the hyphae in parallel lines towards the
upper surface; just at the outside they turn and lie in a horizontal
direction, and, as the branching becomes more profuse, a rather compact
cover is formed. The gonidia, which consist of blue-green _Chroococcus_
cells, lie at the base of the upward branches and they are surrounded
with thin-walled short-celled hyphae closely interwoven into a kind of
cellular tissue. The medulla of loose hyphae passes over to the lower
cortex, also of more or less loose filaments. The outermost cells of the
latter very frequently grow out into short jagged or crenate processes
(Fig. 87).

[Illustration: Fig. 87. _Cora Pavonia_ Fr. Vertical section of thallus.
_a_, upper cortex; _b_, gonidial layer; _c_, medulla and lower cortex
of crenate cells; _d_, tuft of fertile hyphae. × 160. _e_, basidia and
spores × 1000 (after Johow).]

In _Corella_, the mature lichen is squamulose or consists of small lobes;
in _Dictyonema_ there is a rather flat dimidiate expansion; in both the
alga is _Scytonema_, the trichomes of which largely retain their form and
are surrounded by parallel growths of branching hyphae. The whole tissue
is loose and spongy.

_Corella_ spreads over soil on a white hypothallus without rhizinae. In
the other two genera which live on soil, or more frequently on trees,
there is a rather extensive formation of hold-fast tissue. When the
dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae
travel over it and penetrate between the cracks; if the bark is smooth,
there is a more continuous weft of hyphae. In both cases a spongy cushion
of filamentous tissue develops at the base of the lichen between the
tree and the bracket thallus. There is also in both genera an encrusting
form which Johow regarded as representing a distinct genus _Laudatea_,
but which Möller found to be merely a growth stage. Möller[524] judged
from that and from other characteristics that the same fungus enters into
the composition of both _Cora_ and _Dictyonema_ and that only the algal
constituents are different.


C. SPORIFEROUS TISSUES

As in Hymenomycetes, the spores of Hymenolichens are exogenous, and
are borne at the tips of basidia which in these lichens are produced
on the under surface of the thallus. In _Cora_ the fertile filaments
may form a continuous series of basidia over the surface, but generally
they grow out in separate though crowded tufts. As these tufts broaden
outwards, they tend to unite at the free edges, and may finally present a
continuous hymenial layer. Each basidium bears four sterigmata and spores
(Fig. 87 _e_); paraphyses exactly similar to the basidia are abundant in
the hymenium. In _Dictyonema_ the hymenium is less regular, but otherwise
it resembles that of _Cora_. No hymenium has as yet been observed in
_Corella_; it includes, so far as known, one species, _C. brasiliensis_,
which spreads over soil or rocks.




CHAPTER IV

REPRODUCTION


1. REPRODUCTION BY ASCOSPORES


A. HISTORICAL SURVEY

The earliest observations as to the propagation of lichens were made by
Malpighi[525] who recorded the presence of soredia on the lichen plant
and noted their function as reproductive bodies. He was followed after a
considerable interval by Tournefort[526] who placed lichens in a class
apart owing to the form of the fruit: “This fruit,” he writes, “is a
species of bason or cup which seems to take the place of seeds in these
kinds of plants.” He figures _Ramalina fraxinea_ and _Physcia ciliaris_,
both well fruited specimens, and he represents the “minute dust”
contained in the fruits as subrotund in form. The spores of _Physcia
ciliaris_ are of a large size and dark in colour and were undoubtedly
seen by Tournefort. Morison[527], in his _History of Oxford Plants_,
published very shortly after, dismissed Tournefort’s “seeds” as being too
minute to be of any practical interest.

Micheli[528], with truer scientific insight, made the fruiting organs
the subject of special study. He decided that the apothecia were floral
receptacles, _receptacula florum_, and that the spores were the “flowers”
of the lichen. He has figured them in a vertical series in situ, in a
section of the disc of _Solorina_ saccata[529] and also in a species of
_Pertusaria_[529], in both of which plants the ascospores are unusually
large. He adds that he had not so far seen the “semina.”

Micheli’s views were not shared by his immediate successors.
Dillenius[530] scarcely believed that the spores could be “flowers” and,
in any case, he concluded that they were too minute to be of any real
significance in the life of the plant.

Linnaeus[531], and after him Necker[532], Scopoli[533] and others
describe the apothecia as the male, the soredia as the female organs
of lichens. These old time botanists worked with very low powers
of magnification, and easily went astray in the interpretation of
imperfectly seen phenomena.

Koelreuter[534], a Professor of Natural History in Carlsruhe, who
published a work on _The discovered Secret of Cryptogams_, next hazarded
the opinion that the seeds of lichens originated from the substance of
the pith, and that the overlying cortical layer supplied the fertilizing
sap. Hoffmann[535] devoted a great deal of attention to lichen
fructification and he also thought that fertilization must take place
within the tissue of the lichens. He regarded the soredia as the true
seeds, while allowing that a second series of seeds might be contained in
the scutellae (apothecia).

A distinct advance was made by Hedwig[536], a Professor of Botany
in Leipzig, towards the end of the eighteenth century. He followed
Tournefort in selecting _Physcia ciliaris_ for research, and in
that plant he describes and figures not only the apothecia with the
dark-coloured septate spores, but also the pycnidia or spermogonia which
he regarded as male organs. The soredia, typically represented and
figured by him on _Parmelia physodes_, he judged to be “male flowers of a
different type.”

Acharius[537] did not add much to the knowledge of reproduction in
lichens, though he takes ample note of the various fruiting structures
for which he invented the terms _apothecia_, _perithecia_ and _soredia_.
Under still another term _gongyli_ he included not only spores, but the
spore guttulae as well as the gonidia or cells forming the soredia.

Hornschuch[538] of Greifswald described the propagation of the lower
lichens as being solely by means of a germinating “powder”; the more
highly organized forms were provided with receptacles or apothecia
containing spores which he considered as analogous to flowers rather than
to fruits. The important contributions to Lichenology of Wallroth[539]
and Meyer[540] close this period of uncertainty: the former deals almost
exclusively with the form and character of the vegetative thallus and
the function of the “reproductive gonidia.” Meyer, a less prolix writer,
very clearly states that the method of reproduction is twofold: by spores
produced in fruits, or by the germinating granules of the soredia.


B. FORMS OF REPRODUCTIVE ORGANS

From the time of Tournefort, considerable attention had been given to
the various forms of _scutellae_, _tuberculae_, etc., as characters of
diagnostic importance. Sprengel[541] grouped these bodies finally into
nine different types with appropriate names which have now been mostly
superseded by the comprehensive terms, apothecia and perithecia. A
general classification on the lines of fruit development was established
by Luyken[542], who, following Persoon’s[543] classification of fungi,
and thus recognizing their affinity, summed up all known lichens as
_Gymnocarpeae_ with open fruits, and _Angiocarpeae_ with closed fruits.

_a._ APOTHECIA. As in discomycetous fungi, the lichen apothecium is in
the form of an open concave or convex disc, but generally of rather
small size, rarely more than 1 cm. in diameter (Fig. 88); there is no
development in lichen fruits equal to the cup-like ascomata of the larger
_Pezizae_. In most cases the lichen apothecium retains its vitality as
a spore-bearing organ for a considerable period, sometimes for several
years, and it is strengthened and protected by one or more external
margins of sterile tissue. Immediately surrounding the fertile disc there
is a compact wall of interwoven hyphae. In some of the shorter-lived
soft fruits, as in _Biatora_, this hyphal margin may be thin, and may
gradually be pushed aside as the disc develops and becomes convex, but
generally it forms a prominent rim round the disc and may be tough or
even horny, and often hard and carbonaceous. This wall, which is present,
to some extent, in nearly all lichens, is described as the “proper
margin.” A second “thalline margin” containing gonidia is present in many
genera[544]: it is a structure peculiar to the lichen apothecium and
forms the _amphithecium_.

[Illustration: Fig. 88. _Lecanora subfusca_ Ach. A, thallus and apothecia
× 3; B, vertical section of apothecium. _a_, hymenium; _b_, hypothecium;
_c_, thalline margin or amphithecium; _d_, gonidia. × 60 (after Reinke).]

At the base of the apothecium there is a weft of light- or dark-coloured
hyphae called the _hypothecium_, which is continued up and round the
sides as the _parathecium_ merging into the “proper margin.” It forms
the lining of a cup-shaped hollow which is filled by the paraphyses,
which are upright closely packed thread-like hyphae, and by the
spore-containing asci or _thecae_, these together constituting the
thecium or hymenium. The paraphyses are very numerous as compared with
the asci; they are simple or branched, frequently septate, especially
towards the apex, and mostly slender, varying in width from 1-4µ, though
Hue describes paraphyses in _Aspicilia atroviolacea_ as 8-12µ thick. They
may be thread-like throughout their length, or they may widen towards the
tips which are not infrequently coloured. Small apical cells are often
abstricted and lie loose on the epithecium, giving at times a pruinose
or powdered character to the disc. In some genera there is a profuse
branching of the paraphyses to form a dense protective epithecium over
the surface of the hymenium as in the genus _Arthonia_.

The apothecia may be sessile and closely adnate to or even sunk in the
thallus, or they may be shortly stalked. The thalline margin shares
generally the characters of the thallus; the disc is mostly of a firm
consistency and is light or dark in colour according to genus or species;
most frequently it is some shade of brown. Marginate apothecia, _i.e._
those with a thalline margin, are often referred to as “lecanorine,” that
being a distinctive feature of the genus _Lecanora_. In the immarginate
series, with a proper margin only, the texture may be soft and waxy,
termed “biatorine” as in _Biatora_; or hard and carbonaceous as in the
genus _Lecidea_, and is then described as “lecideine.”

In the subseries Graphidineae, the apothecium has the form of a very
flat, roundish or irregular body entirely without a margin, called an
“ardella” as in _Arthonia_; or more generally it is an elongate narrow
“lirella,” in which the disc is a mere slit between two dark-coloured
proper margins. The hypothecium of the lirellae is sometimes much reduced
and in that case the hymenium rests directly on a thin layer above the
thalline tissue as in _Graphis elegans_ (Fig. 89).

[Illustration: Fig. 89. _Graphis elegans_ Ach. A, thallus and lirellae;
B, vertical section of furrowed lirella. × ca. 50.]

Lichen fruits require abundant light, and plants growing in the shade are
mostly sterile. Naturally, therefore, the reproductive bodies are to be
found on the best illuminated parts of the thallus. In crustaceous and
in most foliose forms, they are variously situated on the upper surface,
wherever the light falls most directly. In the genera _Nephromium_
and _Nephromopsis_, on the contrary, they arise on the under surface,
though at the extreme margin, but as the fertile lobes eventually turn
upwards the apothecia as they mature become fully exposed. In shrubby
or fruticose lichens their position is lateral on the fronds, or more
frequently at or near the tips.

_b._ PERITHECIA. The small closed perithecium is characteristic of the
Pyrenocarpeae which correspond with the Pyrenomycetes among fungi. It
is partially or entirely immersed in the thallus or in the substratum on
which the lichen grows, and is either a globose or conical body wholly
surrounded by a hyphal wall, when it is described as “entire” (Fig. 90),
or it is somewhat hemispherical in form and the outer wall is developed
only on the upper exposed part: a type of perithecium usually designated
by the term “dimidiate.” As the perithecial wall gives sufficient
protection to the asci, the paraphyses are of less importance and are
frequently very sparingly produced, or they may even be dissolved and
used up at an early stage. The thallus of the Pyrenocarpeae is often
extremely reduced, and the perithecia are then the only visible portion
of the lichen.

A few lichens among Graphidineae and Pyrenocarpeae grow in a united body
generally looked on as a stroma; but Wainio[545] has demonstrated that as
the fruiting bodies give rise to this structure by agglomeration—by the
cohesion of their margins—it can only be regarded as a pseudostroma. Two
British genera of Pyrenolichens, _Mycoporum_ and _Mycoporellum_, exhibit
this pseudo-stromatoid formation.

[Illustration: Fig. 90. A, entire perithecium of _Porina olivacea_ A. L.
Sm. × ca. 40; B, dimidiate perithecium of _Acrocordia gemmata_ Koerb. ×
ca. 20.]


C. DEVELOPMENT OF REPRODUCTIVE ORGANS

As most known lichens belong to the Ascolichens, the study of development
has been concentrated on that group. Tulasne[546] was the first to make
a microscopic study of lichen tissues and he described in considerable
detail the general anatomical structure of apothecia and perithecia.
Later, Fuisting[547] traced the development of a number of perithecia
through their different stages of growth, but his most interesting
discovery was made in _Lecidea fumosa_, a crustaceous Discolichen with an
areolate thallus in which the apothecia are seated on the fungal hyphae
between the areolae. In the very early stages represented by a complex
of slender hyphae, he observed an unbranched septate filament with short
cuboid cells, richer in contents than the surrounding filaments and
somewhat similar to the structure known to mycologists as “Woronin’s
hypha,” which is an ascogonial structure. These specialized cells
disappeared as the hymenium began to form.


1. DISCOLICHENS

[Illustration: Fig. 91. _Collema microphyllum_ Ach. Vertical section of
thallus. _a_, carpogonium; _b_, trichogyne. × 350 (after Stahl).]

_a._ CARPOGONIA OF GELATINOUS LICHENS. Stahl’s[548] work on various
Collemaceae followed on the same lines as that of Fuisting. The first
species selected by him for examination, _Collema_ (_Leptogium_)
_microphyllum_’ is a gelatinous lichen which grows on old trunks of
poplars and willows. It has a small olive-green thallus which, in autumn,
is crowded with apothecia; the spermogones or pycnidia appear as minute
reddish points on the edge of the thallus. Within the thallus, and midway
between the upper and lower surface, there arises, as a branch from a
vegetative hypha, a many-septate filament coiled in spiral form at the
base, with the free end growing upwards and projecting a short distance
above the surface and occasionally forked (Fig. 91). The tip-cell is
slightly swollen and covered with a mucilaginous coat continuous with
the mucilage of the thallus. The whole structure, characterized by the
larger size and by the richer contents of its cells, was regarded by
Stahl as a carpogonium, the coiled base representing the ascogonium,
the upright hypha functioning as the receptive organ or trichogyne,
comparable to that of the Florideae. The spermatia, which mature at this
early stage of carpogonial development, are expelled from a neighbouring
spermogonium on the addition of moisture and easily reach the protruding
trichogyne. They adhere to the mucilaginous wall of the end-cell, and,
in two or three instances, Stahl found that copulation had taken place.
As the affixed spermatium was empty, he concluded that the contents had
passed over into the trichogyne, and that the nucleus had travelled down
to the ascogonium. Certain degenerative changes that followed seemed to
confirm the view that there had been fertilization: the cells of the
trichogyne had lost their turgidity and at the same time the cross-walls
had swollen considerably and stood out like knots in the hypha (Fig. 92).
The ascogonial cells had also increased not only in size but in number
by intercalary division, so that the spiral arrangement became obscured.
Ascogenous hyphae arose from the ascogonial cells, and asci cut off by a
basal septum were finally formed from these hyphae. Lateral branches from
below the septum also formed asci.

[Illustration: Fig. 92. _Collema microphyllum_ Ach. Carpogonium and
trichogyne after copulation × 500 (after Stahl).]

Stahl’s observations were repeated and extended by Borzi[549] on another
of the Collemaceae, _Collema nigrescens_. In that plant the foliaceous
thallus is of thin texture and has a distinct cellular cortex. The
carpogonia were found at varying depths near to the cortical region;
the ascogonium, of two and a half to four spirals, consisted of ten
to fifteen cells with very thin walls, the trichogyne of five to ten
cells, the terminal cell projecting above the thallus. Borzi also found
spermatia fused with the tip-cell.

A further important contribution was made by Baur[550] in his study of
_Collema crispum_[551]. There occur in nature two forms of this lichen,
one of them crowded with apothecia and spermogonia, the other with a
more luxuriant thallus, but with few apothecia and no spermogonia.
On the latter almost sterile form Baur found in spring and again in
autumn immense numbers of carpogonia—about one thousand in a medium
sized thallus—which nearly all gradually lost the characteristics of
reproductive organs, and, anastomising with other hyphae, became part of
the vegetative system. In a few cases in which, presumably, a spermatium
had fused with a trichogyne, very large apothecia had developed.

As the first-mentioned form was always crowded with apothecia in every
stage of development, as well as with carpogonia and spermogonia, it
seemed natural to conclude that the difference was entirely due to
the presence or absence of spermatia in sufficient numbers to ensure
fertilization. The period during which copulation is possible passes
very rapidly, though subsequent development is slow, occupying about
half-a-year from the time of fertilization to the formation of the first
ascus.

Baur confirmed Stahl’s observations on the various developmental changes.
In several instances he found a spermatium fused with the trichogyne,
though he could not see continuity between the lumina of the fusing
cells. After copulation with the spermatium the trichogyne nucleus,
which occupied the lower third of the terminal cell, had disappeared,
and the plasma contents had acquired a deeper tint; the other trichogyne
cells, which had also lost their nuclei, were partly collapsed owing
to the pressure of the surrounding tissue, and openings were plainly
visible through some of the swollen septa, especially of the lower cells.
In addition the ascogonial cells, all of which were uninucleate, had
increased in number by intercalary division. Plasma connections were
opened from cell to cell, but only in the primary septa, the later formed
cell-membranes being continuous. Ascogenous hyphae had branched out from
the ascogonium as a series of uninucleate cell rows from which the asci
finally arose.

Baur’s interpretation was that the first cell of the ascogonium reached
by the male nucleus after its passage down through the cells of the
trichogyne represented the egg-cell, and that, after fusion, the
resultant nucleus divided, and a daughter nucleus passed on to the other
auxiliary-cells. No male nucleus nor fusion of nuclei was, however,
observed by him, and his deductions rest on conjecture.

Krabbe[552] and after him Mäule[553] found in _Collema pulposum_
reproductive organs similar to those described by Stahl, but in a recent
paper on an American form of that species a peculiar condition has been
described by Freda Bachmann[554]. She[555] found that the spermatia
originated, not in spermogonia, but as groups of cells budded off from
vegetative hyphae within the tissue of the lichen and occupying the same
position as spermogonia, _i.e._ the region close below the upper surface.
The trichogynes, therefore, never emerged into the open, but travelled
towards these internal spermatia, and fusion with them was effected. The
changes that afterwards took place in the carpogonial cells were similar
to those that had been recognized by Stahl and Baur as consequent on
fertilization.

Additional cytological details have been published in a subsequent
paper[556]: after fusion with the spermatium the terminal cell of the
trichogyne collapsed, its nucleus became disintegrated and the cross
septa of the lower trichogyne cells became perforated, these perforations
being closed again at a later stage by a gelatinous plug. The nuclear
history is more doubtful: the disappearance of the nuclei from the
spermatium and from the terminal cell of the trichogyne was noted; two
nuclei were seen to be present in the penultimate cell, and these the
author interpreted as division products of the original cell nucleus.
In the same cell, lying close against the lower septum and partly within
the opening, there was a mass of chromatin material which might be the
male nucleus migrating downwards. The next point of interest was observed
in the twelfth cell from the tip in which there were two nuclei, a
larger and a smaller, the latter judged to be the male cell, the small
size being due to probable division of the spermatium nucleus either
before or after leaving the spermatium. It is stated however that the
spermatium was always uninucleate. Meanwhile the cells of the ascogonium
had increased in size, the perforations of the septa between the cells
became more evident, and their nuclei persisted. In one cell at this
stage two nuclei were present, one of the two presumably a male nucleus;
no fusion of nuclei was observed in the ascogonial cells. Later the cross
walls between the cells were seen to have disappeared more completely and
migration of nuclei had taken place, so that some of the cells appeared
to be empty while others were multinucleate. Considerable multiplication
of the nuclei occurred before the ascogenous hyphae were formed: twelve
nuclei were observed in a part of the ascogonium which was just beginning
to give off a branch. Several branches might arise from one cell, and
their cells were either uni- or binucleate, the nuclei being larger
than those of the vegetative hyphae. The formation of the asci was not
distinctly seen, but young binucleate asci were not uncommon. The fusion
of the two nuclei was followed by the enlargement of the ascus and the
subsequent nuclear division for spore formation. In the first heterotypic
division twelve chromosomes, double the number observed in the vegetative
nucleus, were counted on the equatorial plate. In the third division
they were reduced to the normal number of six, from which F. Bachmann
concludes that a twofold fusion must have taken place—in the ascogonium
and again in the ascus.

Spiral or coiled ascogonia were observed by Wainio[557] in the gelatinous
crustaceous genus _Pyrenopsis_, but the trichogynes did not reach the
surface. In _Lichina_[558], a maritime gelatinous lichen where the
carpogonia occur in groups, trichogynes have not been demonstrated.

A peculiarity of some gelatinous lichens noted by Stahl[559] and others
in species of _Physma_, and by Forssell[560] in _Pyrenopsis_ and
_Psorotichia_, is the development of carpogonia at the base of, and
within the perithecial walls of old spermogonia. No special significance
is however attached to this phenomenon, and it is interesting to note
that a similar growth was observed by Zukal[561] in a pyrenomycetous
fungus, _Pleospora collematum_, a harmless parasite on _Physma compactum_
and other Collemaceae. The structures invaded were true pycnidia of the
fungus as the minute spores were seen to germinate. A “Woronin’s hypha”
at the base of several of these pycnidia developed asci which pushed up
among the spent sporophores.

_b._ CARPOGONIA OF NON-GELATINOUS LICHENS. The soft loose tissue of the
gelatinous lichens is more favourable for the minute study of apothecial
development than the closely interwoven hyphae of non-gelatinous forms,
but Borzi[562] had already extended the study to species of _Parmelia_,
_Anaptychia_, _Sticta_, _Ricasolia_ and _Lecanora_, and in all of them
he succeeded in establishing the presence of ascogonia and trichogynes.
After him a constant succession of students have worked at the problem of
reproduction in lichens.

Lindau[563] published results of the examination of a considerable
series of lichens. In _Anaptychia_ (_Physcia_) _ciliaris_, _Physcia
stellaris_, _Ph. pulverulenta_, _Ramalina fraxinea_, _Placodium_
(_Lecanora_) _saxicolum_, _Lecanora subfusca_ and _Lecidea enteroleuca_
he demonstrated the presence of ascogonia with trichogynes. In _Parmelia
tiliacea_ and in _Xanthoria parietina_ he found ascogonia but failed to
see trichogynes. In none of the species examined by him did he observe
any fusion between the trichogyne and a spermatium.

In _Anaptychia ciliaris_ he was able to pick out extremely early stages
by staining with a solution of chlor-zinc-iodine. Mäule[564] applied the
same test to _Physcia pulverulenta_, but found that to be successful the
reaction required some time. Certain cells of the hyphae—mostly terminal
cells—in the lower area of the gonidial zone and even occasionally in
the pith (according to Lindau) coloured a deep brown, while the ordinary
thalline hyphae were tinted yellow. He assumed that these were initial
ascogonial cells on account of the richer plasma contents, and also
because of the somewhat larger size of the cells. In the same region of
the thallus young carpogonia were observed as outgrowths from vegetative
hyphae, though the trichogynes had not yet reached the surface.

[Illustration: Fig. 93. _Physcia pulverulenta_ Nyl. Vertical section
of thallus and carpogonium before fertilization. _a_, outer cortex;
_b_, inner cortex; _c_, gonidial layer; _d_, medulla. × ca. 540 (after
Darbishire).]

[Illustration: Fig. 94. _Physcia_ (_Anaptychia_) _ciliaris_ DC. Vertical
section of developing ascogonium. _a_, paraphyses; _b_, ascogonial
hyphae; _c_, ascogonial cells. × 800 (after Baur).]

At a more advanced stage the carpogonia were seen to be embedded in
the gonidial zone and occurred in groups. The cells of the ascogonium,
easily recognized by the darker stain, were short and stout, measuring
about 6-8µ in length and 4·4µ in width. They were arranged in somewhat
indistinct spirals; but the crowding of the groups resulted in a confused
intermingling of the various generative filaments. The trichogynes
composed of longer narrower cells rose above the hyphae of the cortex;
they also stained a deep brown and the projecting cell was always
thin-walled. Lindau frequently observed spermatia very firmly attached
to the trichogyne cell but without any plasma connection between the two.
The changes in the trichogyne described by Stahl and Baur in Collemaceae
were not seen in _Anaptychia_; the peculiar swelling of the septa seems
to be a phenomenon confined to gelatinous lichens. During the trichogyne
stage in this lichen the vegetative hyphae from the medulla grow up
and surround the young carpogonia, and, at the same time, very slender
hyphae begin to branch upwards to form the paraphyses. Darbishire’s[565]
examination of _Physcia pulverulenta_ demonstrated the presence of the
coiled ascogonium and the trichogyne in that species (Fig. 93).

Baur[566] has also given the results of an examination of _Anaptychia_.
He frequently observed copulation between the spermatium and the tip
of the trichogyne, but not any passage of nucleus or contents. After
copulation the ascogonial cells increased in size and became irregular
in form, and open communication was established between them (Fig.
94). There was no increase in their number by intercalary division as
in _Collema_. After producing ascogenous hyphae the cells were seen to
have lost their contents and then to have gradually disappeared. The
fertile hyphae, which now took a blue colouration with chlor-zinc-iodine,
gradually spread out and formed a wide-stretching hymenium. Several
carpogonia took part in the formation of one apothecium.

The tissue below the ascogonium meanwhile developed vigorously, forming
a weft of encircling hyphae, while the upper branches grew vertically
towards the cortex. Gonidia in contact with the developing fruit also
increased, and, with the hyphae, formed the exciple or thalline margin.
The growth upward of the paraphyses raises the overlying cortex which in
_Anaptychia_ is “fibrous”; it gradually dies off and allows the exposure
of the disc, though small shreds of dead tissue are frequently left. In
species such as those of _Xanthoria_ where the cortex is of vertical
cell-rows, the apothecial hyphae simply push their way between the
cell-rows and so through to the open.

Baur found the development of the apothecium somewhat similar in the
crustaceous corticolous lichen, _Lecanora subfusca_. After a long period
of sterile growth, spermogonia appeared in great abundance, and, a little
later, carpogonia in groups of five to ten; the trichogynes emerged
very slightly above the cortex; they were now branched. The ascogonia
were frequently a confused clump of cells, though sometimes they showed
distinct spirals. The surrounding hyphae had taken a vertical direction
towards the cortex at a still earlier stage, and the brown tips were
visible on the exterior before the trichogynes were formed. The whole
growth was extremely slow.

In _Physcia stellaris_ the carpogonia occurred in groups also, though
Lindau[567] thinks that, unlike _Anaptychia_ (_Physcia_) _ciliaris_, only
one is left to form the fruit. Only one, according to Darbishire[568],
entered into the apothecium in the allied species, _Physcia
pulverulenta_. In the latter plasma connections were visible from cell
to cell of the trichogyne, and, after copulation with the spermatium,
the ascogonial cells increased in size—though not in number—and the
plasma connections between them became so wide that the ascogonium
had the appearance of an almost continuous multinucleate cell or
coenogamete[569]. As in gelatinous lichens, each of these cells gave rise
to ascogenous hyphae.

_c._ GENERAL SUMMARY. The main features of development described above
recur in most of the species that have been examined.

(1) The carpogonia arise in a complex of hyphae situated on the under
side of, or immediately below the gonidial zone. Usually they vary
in number from five to twenty for each apothecium, though as many as
seventy-two have been computed for _Icmadophila ericetorum_[570], and
Wainio[571] describes them as so numerous in _Coccocarpia pellita_
var., that their trichogynes covered some of the young apothecia with a
hairy pile perceptible with a hand lens, though at the same time other
apothecia on the same specimens were absolutely smooth.

(2) The trichogynes, when present, travel up through the gonidial and
cortical regions of the thallus; Darbishire[572] observes that in
_Physcia pulverulenta_, they may diverge to the side to secure an easier
course between the groups of algae. They emerge above the surface to
a distance of about 15µ or less; after an interval they collapse and
disappear. Their cells, which are longer and narrower than those of the
ascogonium, are uninucleate and vary in number according to species or to
individual lichens. Baur[573] thought that possibly several trichogynes
in succession might arise from one ascogonium.

(3) How many carpogonia share in the development of the apothecium is
still a debated question. In _Collema_ only one is functional. Baur[574]
was unable to decide if one or more were fertilized in _Parmelia
acetabulum_, and in _Usnea_ Nienburg[575] found that, out of several, one
alone survived (Fig. 95). But in _Anaptychia ciliaris_ and in _Lecanora
subfusca_ Baur[574] considers it proved that several share in the
formation of the apothecium. In this connection it is interesting to note
that, according to Harper[576] and others, several ascogonia enter into
one _Pyronema_ fruit.

[Illustration: Fig. 95. _Usnea barbata_ Web. Carpogonium with trichogyne
× 1100 (after Nienburg).]

(4) The ascogonial cells, before and after fertilization, are
distinguished from the surrounding hyphae by a reaction to various
stains, which is different from that of the vegetative hyphae, and also
by the shortness and width of their cells. The whole of the apothecial
primordium is generally recognizable by the clear shining appearance of
the cells.

(5) The ascogonia do not always form a distinct spiral; frequently they
lie in irregular groups. Each cell is uninucleate and may ultimately
produce ascogenous hyphae, though in _Anaptychia_ Baur[574] noted that
some of the cells failed to develop.

(6) The hyphae from the ascogonial cells spread out in a complex layer
at the base of the hymenium, and send up branches which form the asci,
either, as in most Ascomycetes, from the penultimate cell of the
fertile branch, or from the last cell, as in _Sphyridium_ (_Baeomyces
rufus_)[575] and in _Baeomyces roseus_. The same variation has been
observed in fungi—in a species of _Peziza_[577], in which it is the
end-cell of the branch that becomes the mother-cell of the ascus; but
this deviation from the normal is evidently of rare occurrence either in
lichens or fungi.

_d._ HYPOTHECIUM AND PARAPHYSES. The hypothecium is the layer of hyphae
that subtends the hymenium, and is formed from the complex of hyphae
that envelope the first stages of the carpogonia. It is vegetative in
origin and distinct from the generative system.

In lichens belonging to the Collemaceae, the paraphyses rise from
the branching of the carpogonial stalk-cell immediately below the
ascogonium[578], but have no plasma connection with it. They are thus
comparable in origin with the paraphyses of many Discomycetes.

In several genera in which the algal constituents are blue-green, such
as _Stictina_, _Pannaria_, _Nephroma_, _Ricasolia_ and _Peltigera_,
Sturgis[579] found that reproduction was apogamous and also that asci and
paraphyses originated from the same cell-system: a tuft of paraphyses
arose from the basal cell of the ascus, or an ascus from the basal cell
of a paraphysis. These results are at variance with those of most other
workers, but the figures drawn by Sturgis seem to be clear and convincing.

Again in _Usnea barbata_, as described by Nienburg[580], the ascogonial
cells, after the disappearance of the trichogyne, branch profusely not
only upwards towards the cortex but also downwards and to each side.
The upward branches give rise normally to the asci, the lower branches
produce the subhymenium and later the paraphyses, and the two systems are
thus genetically connected, though they remain distinct from each other,
and asci are never formed from the lower cells.

In most heteromerous lichens, however, the origin of the paraphyses
is exclusively vegetative: they arise as branches from the primordial
complex that forms the covering hyphae of the ascogonium both above
and below. Schwendener[581] had already pointed out the difference in
origin between the two constituents of the hymenium in one of his earlier
studies on the development of the apothecium, and this view has been
repeatedly confirmed by recent workers, except by Wahlberg[582] who has
insisted that they rise from the same cells as the asci, a statement
disproved by Baur[583]. The paraphyses originate not only from the
covering hyphae, but from vegetative cells in close connection with the
primordium. In this mode of development, lichens diverge from fungi, but
even in these a vegetative origin for the paraphyses has been pointed
out in _Lachnea scutellata_[584] where they branch from the hyphae lying
round the ascogonium.

There is no general rule for the order of development. In _Lecanora
subfusca_ Baur[583] found that vertical filaments had reached the
surface by the time the trichogyne was formed, and their pointed brown
tips gave a ready clue to the position of the carpogonia. In _Lecidea
enteroleuca_[585] they show their characteristic form and arrangement
before there is any trace of ascus formation. In _Solorina_[585] they
are well formed before the ascogenous hyphae appear. In other lichens
such as _Placodium saxicolum_[586], _Peltigera rufescens_[587] and
_P. malacea_[587] the two systems—paraphyses and ascogonium—grow
simultaneously, though in _P. horizontalis_ the ascogonium has
disappeared by the time the paraphyses are formed. In the genus
_Nephroma_, in _Physcia stellaris_ and in _Xanthorina parietina_ the
paraphyses are also late in making their appearance.

In most instances, the paraphyses push their way up between the cortical
cells which gradually become absorbed, or they may stop short of the
surface as in _Nephromium tomentosum_[587]. The overlying layer of
cortical cells in that case dies off gradually and in time disappears.
Such an apothecium is said to be “at first veiled.” Later formed
paraphyses at the circumference of the apothecium form the parathecium,
which is thus continuous with the hypothecium.

_e._ VARIATIONS IN APOTHECIAL DEVELOPMENT. Lichens are among the least
stereotyped of plants: instances of variation have been noted in several
genera.

_aa._ _PARMELIAE._ A somewhat complicated course of development has
been traced by Baur[588] in _Parmelia acetabulum_. In that lichen
the group of three to six carpogonia do not lie free in the gonidial
tissue, but originate nearer the surface (Fig. 96) and are surrounded
from the first by a tissue connected with, and resembling the tissue
of the cortex. In the several ascogonia, there are more cells and more
spirals than in _Collema_ or in _Physcia_, and all of them are somewhat
confusedly intertwined. The trichogynes are composed of three to five
cells and project 10 to 15µ above the surface. When further development
begins, the ascogonial cells branch out and form a primary darker layer
or hypothecium above which extends the subhymenium, a light-coloured
band of loosely woven hyphae. Branches from the ascogonial hyphae at a
later stage push their way up through this tissue and form above it a
second plexus of hyphae—the base of the hymenium. Baur considers this a
very advanced type of apothecium; he found it also present in _Parmelia
saxatilis_, though, in that species, the further growth of the first
ascogonial layer was more rapid and the secondary plexus and hymenium
were formed earlier in the life of the apothecium. He has also stated
that a similar development occurs in other genera such as _Usnea_, though
Nienburg’s[589] work scarcely confirms that view.

[Illustration: Fig. 96. _Parmelia acetabulum_ Dub. Vertical section of
thallus and carpogonial group × 550 (after Baur).]

In the brown _Parmeliae_, Rosendahl[590] found the same series of
apothecial tissues, but he interprets the course of development somewhat
differently: the basal dark layer or hypothecium he found to be of purely
vegetative origin; above it extended the lighter-coloured subhymenium;
the ascogenous hyphae were present only in the second layer of dark
tissue immediately under the hymenium.

In most lichens the primordium of the apothecium arises towards the lower
side of the gonidial zone, the hyphae of which retain the meristematic
character. In _Parmeliae_, as was noted by Lindau[591] in _P. tiliacea_,
and by Baur[592] and Rosendahl[590] in other species, the carpogonial
groups are formed above the gonidial zone, either immediately below the
cortex as in _P. glabratula_, or in a swelling of the cortex itself as
in _P. aspidota_, in which species the external enlargement is visible
by the time the trichogynes reach the surface. In _P. glabra_, with a
development entirely similar to that of _P. aspidota_, no trichogynes
were seen at any stage. The position of the primordium close under the
cortex is also a feature of _Ramalina fraxinea_ as described by G.
Wolff[593]. The trichogynes in that species are fairly numerous.

A further peculiarity in _Parmelia acetabulum_ attracted Baur’s[592]
attention. Carpogonia with trichogynes are extremely numerous in that
species as are the spermogonia, the open pores of which are to be found
everywhere between the trichogynes, and yet fertilization can occur but
rarely, as disintegrating carpogonia are abundant and very few apothecia
are formed. Baur makes the suggestion that possibly cross-fertilization
may be necessary, or that the spermatia, in this instance, do not
fertilize and that development must therefore be apogamous, in which
case the small number of fruits formed is due to some unknown cause.
Fünfstück[594] thought that degeneration of the carpogonia had not gone
so far, but that a few had acquired the power to develop apogamously.
In _Parmelia saxatilis_ only a small percentage of carpogonia attain to
apothecia, although spermogonia are abundant and in close proximity, but
in that species, unlike _P. acetabulum_, a large number reach the earlier
stages of fruit formation; the more vigorous apothecia seem to inhibit
the growth of those that lag behind.

_bb._ _PERTUSARIAE._ In _Pertusaria_, the apothecial primordium is
situated immediately below the gonidial zone; the cells have a somewhat
larger lumen and thinner walls than those of the vegetative hyphae. In
the ascogonium there are more cells than in _Parmelia acetabulum_; the
trichogynes are short-lived, and several carpogonia probably enter into
the formation of each apothecium; the paraphyses arise from the covering
hyphae. So far the course of development presents nothing unusual. The
peculiar pertusarian feature as described by Krabbe[595], and after him
by Baur[596], does not appear till a later stage. By continual growth
in thickness of the overlying thallus, the apothecia gradually become
submerged and tend to degenerate; meanwhile, however, a branch from
the ascogonial hyphae at the base of the hymenium pushes up along one
side and forms a secondary ascogonial cell-plexus over the top of the
first-formed disc. A new apothecium thus arises and remains sporiferous
until it also comes to lie in too deep a position, when the process
is repeated. Sometimes the regenerating hypha travels to the right or
left away from the original apothecium, it may be to a distance of 2
mm. or according to Fünfstück even considerably farther. Fünfstück[597]
has gathered indeed from his own investigations that such cases of
regeneration are by no means rare: ascogenous hyphae, several centimetres
long, destined to give rise to new apothecia are not unusual, and their
activity can be recognized macroscopically by the linear arrangement
of the apothecia in such lichens as _Rhizocarpon_ (_petraeum_)
_concentricum_ (Fig. 97).

[Illustration: Fig. 97. _Rhizocarpon petraeum_ Massal. Concentrically
arranged apothecia, reduced (J. Adams, _Photo._).]

In _Variolaria_, a genus closely allied to or generally included in
_Pertusaria_, Darbishire[598] has described the primordial tissue as
taking rise almost at the base of the crustaceous thallus: strands of
delicate hyphae, staining blue with iodine, mount upwards from that
region through the medulla and gonidial zone[599]. The ascogonium does
not appear till the surface is almost reached.

_cc._ _GRAPHIDEAE._ Several members of the Graphidaceae were studied by
G. Wolff[600]: she demonstrated the presence of carpogonia with emerging
trichogynes in _Graphis elegans_, a species which is distinguished by the
deeply furrowed margins of the lirellae (Fig. 89). Before the carpogonia
appeared it was possible to distinguish the cushion-like primordial
tissue of the apothecium in the thallus which is almost wholly immersed
in the periderm layers of the bark on which it grows. The trichogynes
were very sparingly septate, and a rather large nucleus occupied a
position near the tip of the terminal cell. The dark carbonaceous
outer wall makes its appearance in this species at an early stage of
development along the sides of the lirellae, but never below, as there
is always a layer of living cells at the base. After the first-formed
hymenium is exhausted, these basal cells develop a new apothecium with
a new carbonaceous wall that pushes back the first-formed, leaving a
cleft between the old and the new. This regenerating process, somewhat
analogous to the formation of new apothecia in _Pertusaria_, may be
repeated in _Graphis elegans_ as many as five times, the traces of the
older discs being clearly seen in the channelled margins of the lirellae.

[Illustration: Fig. 98. _Cladonia decorticata_ Spreng. Vertical section
of squamule and primordium of podetium. _a_, developing podetium; _b_,
probably fertile hyphae; _c_, cortical tissue; _d_, gonidial cells. 1 ×
600 (after Krabbe).]

_dd._ _CLADONIAE._ The chief points of interest in the _Cladoniae_
are the position of the apothecial primordia and the function of the
podetium, which are discussed later[601]. Krabbe[602] determined not
only the endogenous origin of the podetium but also the appearance of
fertile cells in the primordium (Fig. 98). Both frequently take rise
where a crack occurs in the cortex of the primary squamule, the cells of
the gonidial tissue being especially active at these somewhat exposed
places. The fertile hyphae elongate and branch within the stalk of the
developing podetium, sometimes very early, or not until there is a pause
in growth, when carpogonia are formed. As a rule trichogynes emerge in
great numbers[600], generally close to, or rather below, the spermogonia.
In _Cl. pyxidata_[603] the carpogonia are characterized by the large
diameter of the cells—three to five times that of the vegetative hyphae.
Though most of the trichogynes disappear at an early stage, some of them
may persist for a considerable period. As development proceeds, the
vegetative hyphae interspersed among the ascogonial cells grow upwards,
slender branches push up between them and gradually a compact sheath of
paraphyses is built up. The ascogenous hyphae meanwhile spread radially
at the base of the paraphyses and the asci begin to form. The apothecia
may be further enlarged by intercalary growth, and this vigorous
development of vegetative tissue immediately underneath raises the whole
fruit structure well above the surface level.

Sättler[604] in his paper on _Cladoniae_[605] cites as an argument
in favour of fertilization the relative positions of carpogonia
and spermogonia on the podetia. The carpogonia with their emerging
trichogynes being situated rather below the spermogonia. Both organs, he
states, have been demonstrated in eleven species; he himself observed
them in the primordial podetia of _Cladonia botrytes_ and of _Cl.
Floerkeana_.


2. PYRENOLICHENS

_a._ DEVELOPMENT OF THE PERITHECIUM. It is to Fuisting[606] that we
owe the first account of development in the lichen perithecium. Though
he failed to see the earlier stages (in _Verrucaria Dufourii_), he
recognized the primordial complex of hyphae in the gonidial zone of the
thallus, from which originated a vertical strand of hyphae destined to
form the tubular neck of the perithecium. Growth in the lower part is in
abeyance for a time, and it is only after the neck is formed, and the
fruiting body is widened by the ingrowth of external hyphae, that the
asci begin to branch up from the tissue at the base.

[Illustration: Fig. 99. _Dermatocarpon miniatum_ Th. Fr. Vertical section
of thallus and carpogonial group × 600 (after Baur).]

_b._ FORMATION OF CARPOGONIA. Stahl[607] had indicated that not only
in gymnocarpous but also in angiocarpous lichens, it would be found
that carpogonia were formed as in _Collema_. Baur[608] justified this
surmise, and demonstrated the presence of ascogonia in groups of three
to eight, with trichogynes that reached the surface in _Endocarpon_
(_Dermatocarpon_) _miniatum_ (Fig. 99). It is one of the few foliaceous
Pyrenolichens, and the leathery thallus is attached to the substratum
by a central point, thus allowing in the thallus not only peripheral
but also intercalary growth, the latter specially active round the
point of basal attachment; carpogonia may be found in any region where
the tissue is newly formed, and at any season. The upper cortex is
composed of short-celled thick-walled hyphae, with branching vertical
to the surface, and so closely packed that there is an appearance of
plectenchyma; the medullary hyphae are also thick-walled but with
longer cells. The carpogonia of this species arise as a branch from the
vegetative hyphae and are without special covering hyphae, so frequent
a feature in other lichens. The trichogynes bore their way through
the compact cortex and rise well above the surface. After they have
disappeared—presumably after fertilization—the vegetative hyphae round
and between the ascogonia become active and travel upwards slightly
converging to a central point. The asci begin to grow out from the
ascogenous hyphae of the base before the vertical filaments have quite
pierced the cortex.

_Pyrenula nitida_ has also been studied by Baur[609]. It is a very common
species on smooth bark, with a thin crustaceous thallus immersed among
the outer periderm cells. Unlike most other lichens, it forms carpogonia
in spring only, from February to April. A primordial coil of hyphae lies
at the base of the gonidial layer, and, before there is any appearance of
carpogonia, a thick strand of hyphae is seen to be directed upwards, so
that a definite form and direction is given to the perithecium at a very
early stage. The ascogonial cells which are differentiated are extremely
small, and, like those of all other species examined, are uninucleate.
There are five to ten carpogonia in each primordium; the trichogynes grow
up through the hyphal strand and emerge 5-10 µ above the surface. After
their disappearance, a weft of ascogenous tissue is formed at the base,
and, at the same time, the surrounding vegetative tissue takes part in
the building up of a plectenchymatous wall of minute dark-coloured cells.
Further development is rapid and occupies probably only a few weeks.

In many of the pyrenocarpous lichens—_Verrucariae_ and others—the walls
of the paraphyses dissolve in mucilage as the spores become mature, a
character associated with spore ejection and dispersal. In some genera
and species, as in _Pyrenula_, they remain intact.


D. APOGAMOUS REPRODUCTION

Though fertilization by an externally produced male nucleus has not been
definitely proved there is probability that, in some instances, the fruit
may be the product of sexual fusion. There are however a number of genera
and species in which the development is apogamous so far as any external
copulation is possible and the sporiferous tissue seems to be a purely
vegetative product up to the stage of ascus formation.

In _Phlyctis agelaea_ Krabbe[610] found abundant apothecia developing
normally and not accompanied by spermogonia; in _Phialopsis rubra_
studied also by him the primordium arises among the cells of the
periderm on which the lichen grows, and he failed to find any trace
of a sexual act. In his elaborate study of Gloeolichens Forssell[611]
established the presence of carpogonia with trichogynes in two
species—_Pyrenopsis phaeococca_ and _P. impolita_, but without any
appearance of fertilization; in all the others examined, the origin of
the fruit was vegetative. Wainio[612] records a similar observation in a
species of _Pyrenopsis_ in which there was formed a spiral ascogonium and
a trichogyne, but the latter never reached the surface.

Neubner[613] claimed to have proved a vegetative origin for the asci in
the Caliciaceae; but he overlooked the presence of spermogonia and his
conclusions are doubtful.

Fünfstück[614] found apogamous development in _Peltigera_ (including
_Peltidea_) and his results have never been disputed. The ascogonial
cells are surrounded at an early stage by a weft of vegetative hyphae.
No trichogynes are formed and spermogonia are absent or very rare in the
genus, though pycnidia with macrospores occur occasionally.

Some recent work by Darbishire[615] on the genus supplies additional
details. The apothecial primordium always originated near the growing
margin of the thallus, where certain medullary hyphae were seen to swell
up and stain more deeply than others. These at first were uninucleate,
but the nuclei increased by division as the cells became larger, and
in time there was formed a mass of closely interwoven cells full of
cytoplasm. “No coiled carpogonia can be made out, but these darkly
stained cells form part of a connected system of branching hyphae
coming from the medulla further back.” Long unbranched multi-septate
hyphae—evidently functionless trichogynes—travelled towards the cortex
but gradually died off. Certain of the larger cells—the “ascogonia”—grew
out as ascogenous hyphae into which the nuclei passed in pairs and
finally gave rise to the asci.

These results tally well with those obtained by M. and Mme Moreau[616],
though they make no mention of any trichogyne. They found that the
terminal cells of the ascogenous hyphae were transformed into asci, and
the two nuclei in these cells fused—the only fusion that took place. In
_Nephromium_, one of the same family, the case for apogamy is not so
clear; but Fünfstück found no trichogynes, and though spermogonia were
present on the thallus, they were always somewhat imperfectly developed.

Sturgis[617] supplemented these results in his study of other lichens
containing blue-green algae. In species of _Heppia_, _Pannaria_,
_Hydrothyria_, _Stictina_ and _Ricasolia_, he failed to find any evidence
of fertilization by spermatia.

_Solorina_, also a member of Peltigeraceae, was added to the list of
apogamous genera by Metzger[618] and his work was confirmed and amplified
by Baur[619]: certain hyphae of the gonidial zone branch out into
larger ascogonial cells which increase by active intercalary growth, by
division and by branching, and so gradually give rise to the ascogenous
hyphae and finally to the asci. Baur looked on this and other similar
formations as instances of degeneration from the normal carpogonial
type of development. Moreau[620] (Fernand and Mme) have also examined
_Solorina_ with much the same results: the paraphyses rise first from
cells that have been produced by the gonidial hyphae; later, ascogenous
hyphae are formed and spread horizontally at the base of the paraphyses,
finally giving rise at their tips to the asci. Metzger[618] had further
discovered that spermogonia were absent and trichogynes undeveloped
in two very different crustaceous lichens, _Acarospora_ (_Lecanora_)
_glaucocarpa_ and _Verrucaria calciseda_, the latter a pyrenocarpous
species and, as the name implies, found only on limestone.

Krabbe[621] had noted the absence of any fertilization process in
_Gyrophora vellea_. At a later date, _Gyrophora cylindrica_ was made the
subject of exact research by Lindau[622]. In that species the spermogonia
(or pycnidia) are situated on the outer edge of the thallus lobes; a few
millimetres nearer the centre appear the primordia of the apothecia, at
first without any external indication of their presence. The initial
coil which arises on the lower side of the gonidial zone consists of
thickly wefted hyphae with short cells, slightly thicker than those of
the thallus. It was difficult to establish their connection with the
underlying medullary hyphae since these very soon change to a brown
plectenchyma. From about the middle of the ascogonial coil there rises
a bundle of parallel stoutish hyphae which traverse the gonidial zone
and the cortex and slightly overtop the surface. They are genetically
connected at the base with the more or less spirally coiled hyphae, and
are similar to the trichogynes described in other lichens. Lindau did
not find that they had any sexual significance, and ascribed to them
the mechanical function of terebrators or borers. The correctness of
his deductions has been disputed by various workers: Baur[619] looks on
these “trichogynes” as the first paraphyses. The reproductive organs
in _Stereocaulon_ were examined by G. Wolff[623], and the absence of
trichogynes was proved, though spermogonia were not wanting. She also
failed to find any evidence of fertilization in _Xanthoria parietina_,
in which lichen the ascogenous hyphae branch out from an ascogonium that
does not form a trichogyne.

Rosendahl[624], as already stated, could find no trichogynes in _Parmelia
glabra_. In _Parmelia obscurata_, on the contrary, Bitter[625] found that
carpogonia with trichogynes were abundant and spermogonia very rare. In
other species of the subgenus, _Hypogymnia_, he has pointed out that
apothecia are either absent or occur but seldom, while spermogonia are
numerous, and he concludes that the spermatia must function as spores or
conidia. Baur[626] however does not accept that conclusion; he suggests
as probable that the male organs persist longer in a functionless
condition than do the apothecia.

Still more recently Nienburg[627] has described the ascogonium of
_Baeomyces_ sp. and also of _Sphyridium byssoides_ (_Baeomyces rufus_)
as reduced and probably degenerate. His results do not disprove those
obtained by Krabbe[628] on the same lichen (_Sphyridium fungiforme_).
The apothecia are terminal on short stalks in that species. When the
stalk is about ·5mm. in height, sections through the tip show numerous
primordia (12 to 15) ranged below the outer cortex, though only one, or
at most three, develop further. These ascogonial groups are connected
with each other by delicate filaments, and Nienburg concluded that they
were secondary products from a primary group lower down in the tissue.
Spirals were occasionally seen in what he considered to be the secondary
ascogonia, but usually the fertile cells lie in loose uncoiled masses;
isolated hyphae were observed to travel upwards from these cells, but
they never emerged above the surface.

_Usnea macrocarpa_—if Schulte’s[629] work may be accepted—is also
apogamous, though in _Usnea barbata_ Nienburg[627] found trichogynes
(Fig. 95) and the various developments that are taken as evidence of
fertilization. Wainio[630] had demonstrated emergent straight trichogynes
in _Usnea laevis_ but without any sign of fertilization.


E. DISCUSSION OF LICHEN REPRODUCTION

In Ascolichens fertilization by the fusion of nuclei in the ascogonium is
still a debated question. The female organ or carpogonium, as outlined
above, comprises a twisted or spirally coiled multi-septate hypha, with
a terminal branch regarded as a trichogyne which is also multi-septate,
and through which the nucleus of the spermatium must travel to reach the
female cell. It is instructive to compare the lichen carpogonium with
that of other plants.

_a._ THE TRICHOGYNE. In the Florideae, or red seaweeds, in which the
trichogyne was first described, that organ is merely a hair-like
prolongation of the egg-cell and acts as a receptive tube. It contains
granular protoplasm but no nucleus and terminates in a shiny tip covered
with mucilage. The spermatium, unlike that of lichens, is a naked cell,
and being non-motile is conveyed by water to the tip of the trichogyne
to which it adheres; the intervening wall then breaks down and the male
nucleus passes over. After this process of fertilization a plug of
mucilage cuts off the trichogyne, and it withers away.

In _Coleochaete_, a genus of small freshwater green algae, a trichogyne
is also present in some of the species: it is again a prolongation of an
oogonial cell.

In the Ascomycetes, certain cells or cell-processes associated with
the ascogonium have been described as trichogynes or receptive cells.
In one of the simpler types, Monascus[631], the “trichogyne” is a cell
cut off from the ascogonial cell. When fertilization takes place, the
wall between the two cells breaks down to allow the passage of the
male nucleus, but closes up when the process is effected. In _Pyronema
confluens_[632] it is represented by a process from the ascogonial cell
which fuses directly with the male cell. A more elaborate “trichogyne”
has been evolved in _Lachnea stercorea_[633], another Discomycete: in
that fungus it takes the form of a 3-5-septate hypha with a longer
terminal cell; it rises from some part of the ascogonial cell but has
no connection with any process of fertilization, so that the greater
elaboration of form is in this case concomitant with loss of function.

In the Laboulbeniaceae, a numerous and very peculiar series of
Ascomycetes that live on insects, there are, in nearly all of the
reproductive bodies, a carpogonial cell, a trichophoric cell and a
trichogyne. The last-named organ is in some genera a simple continuous
cell, in others it is septate and branched, occasionally it is
absent[634]. The male cells are spermatia of two kinds, exogenous or
endogenous, and the plants are monoecious or dioecious. Laboulbeniaceae
have no connection with lichens. Faull[635], a recent worker on the
group, states that though he observed spermatia attached to the
trichogynes, he was not able to demonstrate copulation (possibly owing to
over-staining), nor could he trace any migration of the nucleus through
the trichophoric cell down to the carpogonial cell. In two species of
_Laboulbenia_ that he examined there were no antheridia, and the egg-cell
acquired its second nucleus from the neighbouring trichophoric cell.
These conjugate nuclei divided simultaneously and the two daughter nuclei
passed on to the ascus and fused, as in other Ascomycetes, to form the
definitive nucleus.

Convincing evidence as to the importance of the trichogyne in fungi
was supposed, until lately, to be afforded by the presence and
functional activity of that organ associated with spermogonia in a few
Pyrenomycetes—in _Poronia_, _Gnomonia_ and _Polystigma_. _Poronia_ was
examined by M. Dawson[636] who found that a trichogyne-like filament
distinct from the vegetative hyphae rose from the neighbourhood
of the ascogonial cells. It took an upward course towards the
exterior, but there was no indication that it was ever receptive. In
_Gnomonia erythrostoma_ and in _Polystigma rubrum_ spermogonia with
spermatia—presumably male organs—are produced in abundance shortly
before the ascosporous fruit is developed. The spermatia in both cases
exhibit the characters of male cells, _i.e._ very little cytoplasm and a
comparatively large nucleus that occupies most of the cell cavity, along
with complete incapacity to germinate. Brooks[637] found in _Gnomonia_
that tufts of the so-called trichogynes originated near the ascogonial
cells, but they were “mere continuations of ordinary vegetative hyphae
belonging to the coil.” They are septate and reach the surface, and the
tip-cell is longer than the others as in the lichen trichogyne.

A somewhat similar arrangement is present in _Polystigma_, in which
Blackman and Welsford[638] have proved that the filaments, considered
as trichogynes by previous workers, are merely vegetative hyphae. A
trichogyne-like structure is also present in _Capnodium_, one of the more
primitive Pyrenomycetes, but it has no sexual significance.

Lindau[639] in his paper on _Gyrophora_ suggested that the trichogyne
in lichens acted as a “terebrator” or boring apparatus, of service to
the deeply immersed carpogonium in enabling it to reach the surface. Van
Tieghem[640] explained its presence on physiological grounds as necessary
for respiration, a view also favoured by Zukal[641], while Wainio[642]
and Steiner[643] see in it only an “end-hypha,” the vigorous growth of
which is due to its connection with the well-nourished cells of the
ascogonium.

Lindau’s view has been rejected by succeeding writers: as has been
already stated, it is the paraphyses that usually open the way outward
for the apothecium. Van Tieghem’s theory has been considered more
worthy of attention and both Dawson and Brooks incline to think that
the projecting filaments described above may perform some service in
respiration, even though primarily they may have functioned as sexual
receptive organs.

There is very little support to be drawn from fungi for the theory
that the presence of a trichogyne necessarily entails fertilization by
spermatia. Lichens in this connection must be judged as a class apart.

It has perhaps been too lightly assumed that the trichogyne in lichens
indicates some relationship with the Florideae[644]. Such a view might
be possible if we could regard lichens and Florideae as derived from
some common remote ancestor, though even then the difference in spore
production—in one case exogenous, and in the other in asci and therefore
endogenous—would be a strong argument against their affinity. But all
the evidence goes to prove that lichens are late derivatives of fungi
and have originated from them at different points. Fungi are interposed
between lichens and any other ancestors, and inherited characters must
have been transmitted through them. F. Bachmann’s suggestion[645] that
_Collema pulposum_ should be regarded “as a link between aquatic red
algae and terrestrial ascomycetes such as _Pyronema_ and the mildews”
cannot therefore be accepted. It seems more probable that the lichen
trichogyne is a new structure evolved in response to some physiological
requirement—either sexual or metabolic—of the deeply embedded fruit
primordium.

_b._ THE ASCOGONIUM. In fungi there is usually one cell forming the
ascogonium, a coenogamete, which after fertilization produces ascogenous
hyphae. There are exceptions, such as Cutting[646] found in _Ascophanus
carneus_, in which it is composed of several cells in open contact by
the formation of wide secondary pores in the cell-walls. In lichens
the ascogonium is divided into a varying number of uninucleate cells.
Darbishire[647] (in _Physcia_) and Baur[648] (in _Anaptychia_) have
described an opening between the different cells, after presumed
fertilization, that might perhaps constitute a coenogamete. Ascogenous
hyphae arise from all, or nearly all the cells, whether fertilized by
spermatia or not, and asci continue to be formed over a long period of
time. There may even be regeneration of the entire fruiting body as
described in _Graphis elegans_ and in _Pertusaria_, apparently without
renewed fertilization.

Spermogonia (or pycnidia) and the ascosporous fruits generally grow on
the same thallus, though not unfrequently only one of the two kinds
is present. As the spermogonia appear first, while the apothecia or
perithecia are still in the initial stages, that sequence of development
seems to add support to the view that their function is primarily sexual;
but it is equally valid as a proof of their pycnidial nature since the
corresponding bodies in fungi precede the more perfect ascosporous fruits
in the life-cycle.

The differences in fertility between the two kinds of thallus in _Collema
crispum_ may be recalled[649]. Baur considered that development of the
carpogonia was dependant on the presence of spermatia: a strong argument
for the necessity of fertilization by these. The conditions in _Parmelia
acetabulum_, also recorded by Baur, lend themselves less easily to any
conclusion. On the thallus of that species the spermogonia and carpogonia
present are out of all proportion to the very few apothecia that are
ultimately formed. Though Baur suggested that cross-fertilization might
be necessary, he admits that the development may be vegetative and so
uninfluenced by the presence or absence of spermatia.

It is the very frequent occurrence of the trichogyne as an integral
part of the carpogonium that constitutes the strongest argument for
fertilization by spermatia. There is a possibility that such an organ
may have been universal at one time both in fungi and in lichens, and
that it has mostly degenerated through loss of function in the former,
as it has disappeared in many instances in lichens. Again, there is but
a scanty and vestigial record of spermogonia in Ascomycetes. They may
have died out, or they may have developed into the asexual pycnidia
which are associated with so many species. If we take that view we may
trace the same tendency in lichens, as for instance in the capacity of
various spermatia to germinate, though in lichen spermogonia there has
been apparently less change from the more primitive condition. It is
also possible that some process of nuclear fusion, or more probably of
conjugation, takes place in the ascogonial cells, and that in the latter
case the only fusion, as in some (or most) fungi, is between the two
nuclei in the ascus.

If it be conceded that fully developed carpogonia with emergent
trichogynes, accompanied by spermogonia and spermatia, represent
fertilization, or the probability of fertilization, then the process may
be assumed to take place in a fairly large and widely distributed series
of lichens. Copulation between the spermatium and the trichogyne has
been seen by Stahl[650], Baur[651] and by F. Bachmann[652] in _Collema_.
In _Physcia pulverulenta_ Darbishire[653] could not prove copulation
in the earlier stages, but he found what he took to be the remains of
emptied spermatia adhering to the tips of old trichogynes. Changes in the
trichogyne following on presumed copulation have been demonstrated by
several workers in the Collemaceae, and open communication as a result of
fertilization between the cells of the ascogonium has been described in
two species. This coenocytic condition of the ascogonium (or archicarp),
considered by Darbishire and others as an evidence of fertilization, has
been demonstrated by Fitzpatrick[654] in the fungus _Rhizina undulata_.
The walls between the cells of the archicarp in that Ascomycete became
more or less open, so that the ascogenous hyphae growing from the central
cells were able easily to draw nutrition from the whole coenocyte, but
no process of fertilization in _Rhizina_ preceded the breaking down of
the septa and no fusion of nuclei was observed until the stage of ascus
formation.

The real distinction between fertile and vegetative hyphae lies,
according to Harper[655], in the relative size of the nuclei. F. Bachmann
speaks of one large nucleus in the spermatium of _Collema pulposum_ which
would indicate sexual function. There is however very little nuclear
history of lichens known at any stage until the beginning of ascus
formation, when fusion of two nuclei certainly take place as in fungi to
form the definitive nucleus of the ascus.

The whole matter may be summed up in Fünfstück’s[656] statement that:
“though research has proved as very probable that fertilization takes
place, it is an undoubted fact that no one has observed any such process.”


F. FINAL STAGES OF APOTHECIAL DEVELOPMENT

The emergence of the lichen apothecium from the thallus, and the form it
takes, are of special interest, as, though it is essentially fungal in
structure, it is subject to various modifications entailed by symbiosis.

_a._ OPEN OR CLOSED APOTHECIA. Schwendener[657] drew attention to two
types of apothecia directly influenced by the thallus: those that are
closed at first and only open gradually, and those which are, as he says,
open from the first. The former occur in genera and species in which the
thallus has a stoutish cortex, as, for instance, in _Lobaria_ where the
young fructification has all the appearance of an opening perithecium.
The open apothecia (_primitus aperta_) are found in non-corticate
lichens, in which case the pioneer paraphyses arrive at the surface
easily and without any converging growth. Similar apothecia are borne
directly on the hypothallus at the periphery, or between the thalline
areolae, and they are also characteristic of thin or slender thalli as in
_Coenogonium_.

In both types of apothecium, the paraphyses pierce the cortex (Fig. 100)
and secure the emergence of the developing ascomata.

[Illustration: Fig. 100. _Physcia ciliaris_ DC. Vertical section
of apothecium still covered by the cortex. _a_, paraphyses; _b_,
hypothecium; _c_, gonidia of thallus and amphithecium. × 150 (after
Baur).]

_b._ EMERGENCE OF THE ASCOCARP. Hue[658] has taken up this subject in
recent years and has described the process by which the vegetative hyphae
surrounding the fruit primordium, excited to active growth by contact
with the generative system, take part in the later stages of fruit
formation. The primordium generally occupies a position near to, or just
within, the upper medulla, and the hyphae in contact with it soon begin
to branch freely in a vertical direction, surrounding the developing
fruit and carrying it upwards generally to a superficial position. The
different methods of the final emergence give two very distinct types of
mature apothecium: the _lecideine_ in which the gonidial zone takes no
part in the upward growth, and the _lecanorine_ into which the gonidia
enter as an integral part.

In the lecideine series (Fig. 101) the encircling hyphae from the upper
medulla rise as a compact column through the gonidial zone to the surface
of the thallus; they then spread radially before curving up to form
the outer wall or “proper margin” round the spore-bearing disc. The
branching of the hyphae is fastigiate with compact shorter branches at
the exterior. In such an apothecium gonidia are absent both below the
hypothecium and in the margins.

[Illustration: Fig. 101. _Lecidea parasema_ Ach. Vertical section of
thallus and apothecium with proper margin only × ca. 50.]

In lecanorine development the ascending hyphae from the medulla, in some
cases, carry with them algal cells which multiply and spread as a second
gonidial layer under the hypothecium (Fig. 102). These hyphae may also
spread in a radial direction while still within the thallus and give
rise to an “immersed” apothecium which is lecanorine as it encloses
gonidia within its special tissues, for example, in _Acarospora_ and
_Solorina_. But in most cases the lecanorine fruit is superficial and
not unfrequently it is raised on a short stalk (_Usnea_, etc.); both the
primary gonidial zone of the thallus and the outer cortex are associated
with the medullary column of hyphae from the first and grow up along
with it, thus providing the outer part of the apothecium, an additional
“thalline margin” continuous with the thallus itself. It is an advanced
type of development peculiar to lichens, and it provides for fertility
of long continuance which is in striking contrast with the fugitive
ascocarps of the Discomycetes.

[Illustration: Fig. 102. _Lecanora tartarea_ Ach. Vertical section of
apothecium. _a_, hymenium; _b_, proper margin or parathecium; _c_,
thalline margin or amphithecium. × 30 (after Reinke).]

The distinction between lecideine and lecanorine apothecia is of great
value in classification, but it is not always easily demonstrable; it
is occasionally necessary to examine the early stages, as in the more
advanced the thalline margin may be pushed aside by the turgid disc and
become practically obliterated.

The “proper margin” reaches its highest development in the lecideine
and graphideine types. It is less prominent or often almost entirely
replaced when the thalline margin is superadded, except in genera such as
_Thelotrema_ and _Diploschistes_ which have distinct “double margins.”

There is an unusual type of apothecium in the genus _Gyrophora_.
The fruit is lecideine, the thalline gonidia taking no part in the
development. The growth of the initial ascogenous tissue, according to
Lindau[659], is constantly towards the periphery of the disc so that a
weak spot arises in the centre which is promptly filled by a vigorous
sterile growth of paraphyses. This process is repeated from new centres
again and again, resulting in the irregularly concentric lines of sterile
and fertile areas of the “gyrose” fruit (Fig. 103). The paraphyses soon
become black at the tips. Asci are not formed until the ascogenous layer
has acquired a certain degree of stability, and spores are accordingly
present only in advanced stages of growth.

[Illustration: Fig. 103. Apothecial gyrose discs of _Gyrophora
cylindrica_ Ach. × 12 (after Lindau).]


G. LICHEN ASCI AND SPORES

_a_. HISTORICAL. The presence of spores, as such, in the lichen fruit was
first established by Hedwig[660] in _Anaptychia_ (_Physcia_) _ciliaris_.
He rightly judged the minute bodies to be the “semina” of the plant. In
that species they are fairly large, measuring about 50µ, long and 24µ
thick, and as they are very dark in colour when mature, they stand out
conspicuously from the surrounding colourless tissue of the hymenium.
Acharius[661] also took note of these “semina” and happily replaced the
term by that of “spores.” They may be produced, he states, in a compact
nucleus (_Sphaerophoron_), in a naked disc (_Calicium_), or they may be
embedded in the disc (_Opegrapha_ and _Lecidea_). Sprengel[662] opined
that the spores—which he figures—were true seeds, though he allows
that there had been no record of their development into new plants.
Luyken[663] made a further contribution to the subject by dividing
lichens into gymnocarpous and angiocarpous forms, according as the
spores, enclosed in cells or vesicles (thecae), were borne in an open
disc or in a closed perithecium.

In his _Systema_ of lichen genera Eschweiler[664], some years later,
described and figured the spores as “thecae” enclosed in cylindrical
asci. Fée[665] in contemporary works gave special prominence to the
colour and form of the spores in all the lichens dealt with.

_b_. DEVELOPMENT OF THE ASCUS. The first attempt to trace the origin
and development of lichen asci and spores was made by Mohl[666]. He
describes the mother-cell (the ascus) as filled at first with a clouded
granular substance changing later into a definite number—usually eight—of
simple or septate spores. Dangeard[667] included the lichens _Borrera_
(_Physcia_) _ciliaris_ and _Endocarpon_ (_Dermatocarpon_) _miniatam_
among the plants that he studied for ascus and spore development. He
found that in lichens, as in fungi, the ascus arose usually from the
penultimate cell of a crooked hypha (Fig. 104) and that it contained
at first two nuclei derived from adjoining cells. These nuclei are
similar in size to those of the vegetative hyphae, and in each there is
a large nucleolus with chromatin material massed on one side. Fusion
takes place, as in fungi, between the two nuclei, and the secondary or
definitive nucleus thus formed divides successively to form the eight
spore-nuclei. Baur[668] and Nienburg[669] have confirmed Dangeard’s
results as regards lichens, and René Maire[670] has also contributed
important cytological details on the development of the spores. In
_Anaptychia_ (_Physcia_) _ciliaris_ he found that the fused nucleus
became larger and that a synapsis stage supervened during which the long
slender chromatin filaments became paired, and at the same time shorter
and thicker. The nuclear membrane disappeared as the chromatin filaments
were united in masses joined together by linin threads which also
disappeared later. At the most advanced stage observed by Maire there
was visible a nucleolus embedded in a condensed plasma and surrounded
by eight medianly constricted filaments destined to form the equatorial
plate. A few isolated observations were also made on the cytology of the
ascus in _Peltigera canina_, in which lichen the preceding ascogonial
development is wholly vegetative. The secondary nucleus was seen to
contain a chromatin mass and a large nucleolus; in addition two angular
bodies of uncertain signification were associated with the nucleolus,
each with a central vacuole. The nucleolus disappeared in the prophase of
the first division and four double chromosomes were then plainly visible.
The succeeding phases of the first and the second nuclear division were
not seen, but in the prophase of the third it was possible to distinguish
four chromatin masses outside the nucleolus. The slow growth of the
lichen plant renders continuous observation extremely difficult.

[Illustration: Fig. 104. Developing asci of _Physcia ciliaris_ DC. × 800
(after Baur).]

F. Bachmann[671] was able to make important cytological observations in
her study of _Collema pulposum_. As regards the vegetative and ascogonial
nuclei, five or perhaps six chromosomes appeared on the spindle when the
nucleus divided. In the asci, the usual paired nuclei were present in the
early stages and did not fuse until the ascus had elongated considerably.
After fusion the definitive nucleus enlarged with the growth of the ascus
and did not divide until the ascus had attained full size. The nucleolus
was large, and usually excentric, and there were at first a number of
chromatin masses on an irregular spirem. In synapsis the spirem was drawn
into a compact mass, but after synapsis, “the chromatin is again in the
form of a knotty spirem.” In late prophases the chromosomes, small ovoid
bodies, were scattered on the spindle; later they were aggregated in the
centre, and, in the early metaphase, about twelve were counted now split
longitudinally. There were thus twice as many chromosomes in the first
division in the ascus as in nuclear divisions of the vegetative hyphae.
F. Bachmann failed to see the second division; there were at least five
chromosomes in the third division.

Considerable importance is given to the number of the chromosomes in
the successive divisions in the ascus since they are considered to be
proof of a previous double fusion—in the ascogonium and again in the
ascus—necessitating, therefore, a double reduction division to arrive at
the gametophytic or vegetative number of five or six chromosomes in the
third division in the ascus. There have been too few observations to draw
any general conclusions.

_c._ DEVELOPMENT OF SPORES. The spore wall begins to form, as in
Ascomycetes, at the apex of the nucleus with the curving over of the
astral threads, the nucleus at that stage presenting the figure of
a flask the neck of which is occupied by the centrosome. The final
spore-nucleus, as observed by Maire, divides once again in _Anaptychia_
and division is followed by the formation of a median septum, the mature
spore being two-celled. In _Peltigera_ the spore is at first ovoid,
but both nucleus and spore gradually elongate. The fully formed spore
is narrowly fusiform and by repeated nuclear division and subsequent
cross-septation it becomes 4- or even 5-6-celled.

The spores of lichens are wholly fungoid, and, in many cases, form a
parallel series with the spores of the Ascomycetes. Markings of the
epispore, such as reticulations, spines, etc., are rarely present
(_Solorina spongiosa_), though thickening of the wall occurs in many
species (_Pertusariae_, etc.), a peculiarity which was first pointed out
by Mohl[672] who contrasted the spore walls with the delicate membranes
of other lichen cells. Some spores, described as “halonate,” have an
outer gelatinous covering which probably prevents the spore from drying
up and thus prolongs the period of possible germination. Both asci and
spores are, as a rule, more sparingly produced than in fungi; in many
instances some or all of the spores in the ascus are imperfectly formed,
and the full complement is frequently lacking, possibly owing to some
occurrence of adverse conditions during the long slow development of the
apothecium. In the larger number of genera and species the spores are
small bodies, but in some, as for instance in the _Pertusariae_ and in
some _Pyrenocarpeae_, they exceed in size all known fungus spores. In
_Varicellaria microsticta_, a rare crustaceous lichen of high mountains,
the solitary 1-septate spore measures up to 350 µ, in length and 115 µ
in width. Most spores contain reserve material in the form of fat, etc.,
many are dark-coloured; Zukal[673] has suggested that the colour may be
protective.

Their ejection from the ascus at maturity is caused by the twofold
pressure of the paraphyses and the marginal hyphae on the addition of
moisture. The spores may be shot up at least 1 cm. from the disc[674].

_d._ SPORE GERMINATION. Meyer[675] was the first who cultivated lichen
spores and the dendritic formation which he obtained by growing them on
a smooth surface was undoubtedly the prothallus (or hypothallus) of the
lichen. Actual germination was however not observed till Holle[676] in
1846 watched and figured the process as it occurs in _Physcia ciliaris_.

Spores divided by transverse septa into two or more cells, as well
as those that have become “muriform” by transverse and longitudinal
septation, may germinate from each cell.

[Illustration: Fig. 105. Multinucleate spore of _Lecidea_ (_Mycoblastus_)
_sanguinaria_ Ach. × 540 (after Zopf).]

[Illustration: Fig. 106. Germination of multinucleate spore of
_Ochrolechia pallescens_ Koerb. × 390 (after de Bary).]

_e._ MULTINUCLEATE SPORES. These spores, which are all very large, occur
in several genera or subgenera: in _Lecidea_ subg. _Mycoblastus_ (Fig.
105), _Lecanora_ subg. _Ochrolechia_ and in Pertusariaceae. Tulasne[677]
in his experiments with germinating spores found that in _Lecanora
parella (Ochrolechia pallescens?)_ germinating tubes were produced all
over the surface of the spore (Fig. 106). De Bary[678] verified his
observations in that and other species and added considerable detail:
about twenty-four hours after sowing spores of _Ochrolechia pallescens_,
numerous little warts arose on the surface of the spore which gradually
grew out into delicate hyphae. All these spores contain fat globules and
finely granular protoplasm with a very large number of minute nuclei; the
presence of the latter has been demonstrated by Haberlandt[679] and later
by Zopf[680] who reckoned about 200 to 300 in the spore of _Mycoblastus
sanguinarius_. These nuclei had continued to multiply during the ripening
of the spore while it was still contained in the ascus[680]. Owing to the
presence of the large fat globules the plasma is confined to an external
layer close to the spore wall; the nuclei are embedded in the plasma and
are connected by strands of protoplasm. The epispore in some of these
large spores is extremely developed: in some _Pertusariae_ it measures
4-5 µ in thickness.

_f._ POLARIBILOCULAR SPORES. The most peculiar of all lichen spores
are those termed _polaribilocular_—signifying a two-celled spore of
which the median septum has become so thickened that the cell-cavities
with their contents are relegated to the two poles of the spore, an
open canal frequently connecting the two cell-spaces (Fig. 107). Other
terms have been suggested and used by various writers to describe this
unusual character such as blasteniospore[681], orculiform[682] and
placodiomorph[683] or more simply polarilocular.

The polarilocular colourless spore is found in a connected series of
lichens—crustaceous, foliose and fruticose (_Placodium_, _Xanthoria_,
_Teloschistes_). In another series with a darker thallus (_Rinodina
and Physcia_) the spore is brown-coloured, and the median septum cuts
across the plasma-connection. In other respects the brown spore is
similar to the colourless one and possesses a thickened wall with reduced
cell-cavities.

[Illustration: Fig. 107. Polarilocular spores. _a_, _Xanthoria parietina_
Th. Fr.; _b_, _Rinodina roboris_ Th. Fr.; _c_, _Physcia pulverulenta_
Nyl.; _d_, _Physcia ciliaris_ DC. × 600.]

The method of cell-division in these spores resembles that known as
“cleavage by constriction,” in which the cross wall arises by an
ingrowth from all sides of the cell; in time the centre is reached
and the wall is complete, or an open pore is left between the divided
cells. Cell “cleavage” occurs frequently among Thallophytes, though it
is unknown among the higher plants. Among Algae it is the normal form
of cell-division in _Cladophora_ and also in _Spirogyra_, though in the
latter the wall passes right across and cuts through the connecting
plasma threads. Harper[684] found “cleavage by constriction” in two
instances among fungi: the conidia of _Erysiphe_ and the gametes of
_Sporodinia_ are cut off by a septum which originates as a circular
ingrowth of the outer wall, comparable, he considers, with the
cell-division of _Cladophora_.

The development of the thickened wall of polarilocular spores has been
studied by Hue[685], who contends however that there is no true septation
in the colourless spores so long as the central canal remains open.
According to his observations the wall of the young spore is formed
of a thin tegument, everywhere equal in thickness, and consisting of
concentric layers. This tegument becomes continually thicker at the
equator of the spore by the addition of new layers from the interior, and
the protoplasmic contents are compressed into a gradually diminishing
space. In the end the wall almost touches at the centre, and the
spore consists of two polar cell-cavities with a narrow open passage
between. A median line pierced by the canal is frequently seen. In a few
species there is a second constriction cleavage and the spore becomes
quadrilocular.

Hue insists that this spore should be regarded as only one-celled; for
though the walls may touch at the centre, he says they never coalesce.
He has unfortunately given no cytological observations as to whether the
spore is uni- or binucleate.

In _Xanthoria parietina_, one of the species with characteristic
polaribilocular spores, germination, it would seem, takes place mostly at
one end only of the spore, though a germinating tube issues at both ends
frequently enough to suggest that the spore is binucleate and two-celled.
The absence of germination from one or other of the cells only may
probably be due to the drain on their small resources. Hue has cited the
rarity of such instances of double germination in support of his view
of the one-celled nature of the spore. He instances that out of fifteen
spores, Tulasne[686] has figured only three that have germinated at each
end; Bornet[687] figures one in seven with the double germination and
Bonnier[688] one in sixteen spores.

Further evidence is wanted as to the nuclear history of these hyaline
spores. In the case of the brown spores, which show the same thickening
of the wall and restricted cell-cavity, though with a distinct median
septum, nuclear division was observed by René Maire[689] before septation
in one such species, _Anaptychia ciliaris_.


II. SECONDARY SPORES


A. REPRODUCTION BY OIDIA

In certain conditions of nutrition, fungal hyphae break up into separate
cells, each of which functions as a reproductive _conidium_ or _oidium_,
which on germination forms new hyphae. Neubner[690] has demonstrated a
similar process in the hyphae of the Caliciaceae and compares it with the
oidial formation described by Brefeld[691] in the Basidiomycetes.

The thallus of this family of lichens is granular or furfuraceous; it
never goes beyond the _Lepra_ stage of development[692]. In some species
it is scanty, in others it is abundant and spreads over large areas of
the trunks of old trees. It is only when growth is especially luxuriant
that oidia are formed. Neubner was able to recognize the oidial condition
by the more opaque appearance of the granules, and under the microscope
he observed the hyphae surrounding the gonidia gradually fall away and
break up into minute cylindrical cells somewhat like spermatia in size
and form. There was no question of abnormal or unhealthy conditions, as
the oidia were formed in a freely fruiting thallus.

The gonidia associated with the oidial hyphae also showed unusual
vitality and active division took place as they were set free by the
breaking up of the encircling hyphae. The germination of the oidia
provides an abundance of hyphal filaments for the rapidly increasing
algal cells, and there follows a wide-spread development of the lichen
thallus.

Oidial formation has not been observed in any other family of lichens.


B. REPRODUCTION BY CONIDIA

_a._ INSTANCES OF CONIDIAL FORMATION. It is remarkable that the type of
asexual reproduction so abundantly represented in fungi by the large
and varied group of the Hyphomycetes is practically absent in lichens.
An exception is to be found in a minute gelatinous lichen that grows
on soil. It was discovered by Bornet[693] and called by him _Arnoldia_
(_Physma_) _minutula_. From the thallus rise up simple or sparingly
branched colourless conidiophores which bear at the tips globose brown
conidia (Fig. 108). Bornet[694] obtained these conidia by keeping very
thin sections of the thallus in a drop of water[693].

[Illustration: Fig. 108. Conidia developed from thallus of _Arnoldia
minutula_ Born. × 950 (after Bornet).]

Yet another instance of conidial growth is given by Steiner[695]. He
had observed that the apothecia on plants of _Caloplaca aurantia_
var. _callopisma_ Stein. differed from those of normal appearance in
the warted unevenness of the disc and also in being more swollen and
convex, the thalline margin being almost obliterated. He found, on
microscopical examination, that the hymenium was occupied by paraphyses
and by occasional asci, the latter seldom containing spores, and being
usually more or less collapsed. The component parts of the apothecium
were entirely normal and healthy, but the paraphyses and the few asci
were crushed aside by the intrusion of numerous slender unbranched
septate conidiophores. Several of these might spring from one base and
the hypha from which they originated could be traced some distance into
the ascogenous layer, though a connection with that cell-system could not
be demonstrated. While still embedded in the hymenium, an ellipsoid or
obovate swelling began to form at the apex of the conidiophore; it became
separated from the stalk by a septum and later divided into a two-celled
conidium. The conidiophore increased in length by intercalary growth and
finally emerged above the disc; the mature conidium was pyriform and
measured 15-20 µ × 9-11 µ.

Steiner regarded these conidia as entirely abnormal; pycnidia with
stylospores are unknown in the genus and they were not, he alleges, the
product of any parasitic growth.

_b._ COMPARISON WITH HYPHOMYCETES. The conidial form of fructification
in fungi, known as a Hyphomycete, is generally a stage in the life-cycle
of some Ascomycete; it represents the rapid summer form of asexual
reproduction. The ascospore of the resting fruit-form in many species
germinates on any suitable matrix and may at once produce conidiophores
and conidia, which in turn germinate, and either continue the conidial
generation or proceed to the formation of the perfect fruiting form with
asci and ascospores.

Such a form of transient reproduction is almost impossible in lichens,
as the hypha produced by the germinating lichen ascospore has little
vitality without the algal symbiont. In natural conditions development
practically ceases in the absence of symbiosis. When union between the
symbionts takes place, and growth becomes active, thallus construction at
once commences. But in certain conditions of shade and moisture, only the
rudiments of a lichen thallus are formed, known as a leprose or sorediose
condition. Soredia also arise in the normal life of many lichens. As
the individual granules or soredia may each give rise to a complete
lichen plant, they may well be considered as replacing the lost conidial
fructification.


C. CAMPYLIDIUM AND ORTHIDIUM

Müller[696] has described under the name _Campylidium_ a supposed new
type of asexual fructification which he found on the thallus of tropical
species of _Gyalecta_, _Lopadium_, etc., and which he considered
analogous to pycnidia and spermogonia. Wainio[697] has however recognized
the cup-like structure as a fungus, _Cyphella aeruginascens_ Karst.,
which grows on the bark of trees and occasionally is parasitic on the
crustaceous thallus of lichens. Wainio has also identified the plant,
_Lecidea irregularis_, first described by Fée[698], as also synonymous
with the fungus.

Another name _Orthidium_ was proposed by Müller[699] for a type of
fructification he found in Brazil which he contrasts or associates with
_Campylidium_. It has an open marginate disc with sporophores bearing
acrogenous spores. He found it growing in connection with a thin lichen
thallus on leaves and considered it to be a form of lichen reproduction.
Possibly _Orthidium_ is also a _Cyphella_.


III. SPERMOGONIA OR PYCNIDIA


A. HISTORICAL ACCOUNT OF SPERMOGONIA

The name spermogonium was given by Tulasne[700] to the “punctiform
conceptacles” that are so plentifully produced on many lichen thalli, on
the assumption that they were the male organs of the plant, and that the
spore-like bodies borne in them were non-motile male cells or spermatia.

The first record of their association with lichens was made by
Dillenius[701], who indicates the presence of black tubercles on the
thallus of _Physcia ciliaris_. He figures them also on several species
of _Cladonia_, on _Ramalina_ and on _Dermatocarpon_, but without any
suggestion as to their function. Hedwig’s[702] study of the reproductive
organs of the Linnaean Cryptogams included lichens. He examined _Physcia
ciliaris_, a species that not only is quite common but is generally
found in a fruiting condition and with very prominent spermogonia, and
has been therefore a favourite lichen for purposes of examination and
study. Hedwig describes and figures not only the apothecia but also those
other bodies which he designates as “punctula mascula,” or again as
“puncta floris masculi.” In his later work he gives a drawing of _Lichen_
(_Gyrophora_) _proboscideus_, with two of the spermogonia in section.

Acharius[703] included them among the lichen structures which he called
“cephalodia”: he described them as very minute tubercles rising up
from the substance of the thallus and projecting somewhat above it. He
also figures a section through two “cephalodia” of _Physcia ciliaris_.
Fries[704] looked on them as being mostly “anamorphoses of apothecia,
the presence of abortive fruits transforming the angiocarpous lichen to
the appearance of a gymnocarpous form.” Wallroth[705] assigned the small
black fruits to the comprehensive fungus genus _Sphaeria_ or classified
lichens bearing spermogonia only as distinct genera and species
(_Pyrenothea_ and _Thrombium_). Later students of lichens—Schaerer[706],
Flotow[707], and others—accepted Wallroth’s interpretation of their
relation to the thallus, or they ignored them altogether in their
descriptions of species.


B. SPERMOGONIA AS MALE ORGANS

Interest in these minute “tubercles” and their enclosed “corpuscles” was
revived by Itzigsohn[708] who examined them with an improved microscope.
He macerated in water during a few days that part of the thallus on which
they were developed, and, at the end of the time, discovered that the
solution contained large numbers of motile bodies which he naturally
took to be the corpuscles from the broken down tubercles. He claimed to
have established their function as male motile cells or spermatozoa.
The discovery seemed not only to prove their sexual nature, but to link
up the reproduction of lichens with that of the higher cryptogams. The
tubercles in which the “spermatozoa” were produced he designated as
antheridia. More prolonged maceration of the tissue to the very verge
of decay yielded still larger numbers of the “spermatozoa” which we now
recognize to have been motile bacilli.

Tulasne[709] next took up the subject, and failing to find the motile
cells, he wrongly insisted that Itzigsohn had been misled by mere
Brownian movement, but at the same time he accepted the theory that
the minute conceptacles were spermogonia or male organs of lichens.
He also pointed out that their constant occurrence on the thallus of
practically every species of lichen, and their definite form, though
with considerable variation, rendered it impossible to regard them as
accidental or of no importance to the life of the plant. He compared them
with fungal pycnidia such as _Phyllosticta_ or _Septoria_ which outwardly
they resembled, but whereas the pycnidial spores germinated freely,
the spermatia of the spermogonia, as far as his experience went, were
incapable of germination.


C. OCCURRENCE AND DISTRIBUTION

_a._ RELATION TO THALLUS AND APOTHECIA. We owe to Tulasne[710] the first
comparative study of lichen spermogonia. He described not only their
outward form, but their minute structure, in a considerable number of
representative species. A few years later Lindsay[711] published a
memoir dealing with the spermogonia of the larger foliose and fruticose
lichens, and, in a second paper, he embodied the results of his study of
an equally extensive selection of crustaceous species. Lindsay’s work is
unfortunately somewhat damaged by faulty determination of the lichens he
examined, and by lack of the necessary discrimination between one thallus
and another of associated and intermingled species. Both memoirs contain,
however, much valuable information as to the forms of spermogonia, with
their spermatiophores and spermatia, and as to their distribution over
the lichen thallus.

Though spermogonia are mostly found associated with apothecia, yet in
some lichens, such as _Cerania_ (_Thamnolia_) _vermicularis_, they are
the only sporiferous organs known. Not unfrequently crustaceous thalli
bear spermogonia only, and in some _Cladoniae_, more especially in
ascyphous species, spermogonia are produced abundantly at the tips of
the podetial branches (Fig. 109), while apothecia are exceedingly rare.
Usually they occur in scattered or crowded groups, more rarely they are
solitary. Very often they are developed and the contents dispersed before
the apothecia reach the surface of the thallus; hence the difficulty
in relating these organisms, since the mature apothecium is mostly of
extreme importance in determining the species.

[Illustration: Fig. 109. _Cladonia furcata_ Schrad. Branched podetium
with spermogonia at the tips (after Krabbe).]

[Illustration: Fig. 110. _Physcia hispida_ Tuckerm. Ciliate frond. _a_,
spermogonia; _b_, apothecia. × ca. 5 (after Lindsay).]

In a very large number of lichens, both crustaceous and foliose, the
spermogonia are scattered over the entire thallus (Fig. 110), covering it
more or less thickly with minute black dots, as in _Parmelia conspersa_.
In other instances, they are to some extent confined to the peripheral
areas as in _Parmelia physodes_; or they occur on the extreme edge of the
thallus as in the crustaceous species _Lecanora glaucoma_ (_sordida_). In
_Pyrenula nitida_ they grow on the marginal hypothallus, usually on the
dark line of demarcation between two thalli.

They tend to congregate on, and indeed are practically restricted to
the better lighted portions of the thallus. On the fronds of foliose
forms, they appear, for instance, on the swollen pustules of _Umbilicaria
pustulata_, while in _Lobaria pulmonaria_, they are mostly lodged in
the ridges that surround the depressions in the thallus. In _Parmelia
conspersa_, _Urceolaria_ (_Diploschistes_) _scruposa_ and some others,
they occasionally invade the margins of the apothecium or even the
apothecial disc as in _Lichina_. Forssell[712] found that a spermogonium
had developed among cells of _Gloeocapsa_ that covered the disc of a
spent apothecium of _Pyrenopsis haematopis_.

In fruticose lichens such as _Usnea_, _Ramalina_, etc. they occur near
the apex of the fronds, and in _Cladonia_ they occupy the tips of the
ascyphous podetia or the margins of the scyphi. In some _Cladoniae_,
however, spermogonia are produced on the basal squamules, more rarely on
the squamules that clothe the podetia.

_b._ FORM AND SIZE. Spermogonia are specifically constant in form, the
same type being found on the same lichen species all over the globe. The
larger number are entirely immersed and are ovoid or roundish (Fig. 111 A)
or occasionally somewhat flattened bodies (_Nephromium laevigatum_), or
again, but more rarely, they are irregular in outline with an infolding
of the walls that gives the interior a chambered form (Fig. 111 B)
(_Lichina pygmaea_); but all of these are only visible as minute points
on the thallus.

[Illustration: Fig. 111. Immersed spermogonia. A, globose in _Parmelia
acetabulum_ Dub. × 600; B, with infolded walls in _Lecidea_ (_Psora_)
_testacea_ Ach. × 144 (after Glück).]

A second series, also immersed, are borne in small protuberances of the
thallus. These very prominent forms are rarely found in crustaceous
lichens, but they are characteristic of such well-known species as
_Ramalina fraxinea_, _Xanthoria parietina_, _Ricasolia amplissima_,
_Baeomyces roseus_, etc. Other spermogonia project slightly above the
level of the thallus, as in _Cladonia papillaria_ and _Lecidea lurida_;
while in a few instances they are practically free, these last strikingly
exemplified in _Cetraria islandica_ where they occupy the small
projections or cilia (Fig. 112) that fringe the margins of the lobes;
they are free also in most species of _Cladonia_.

In size they vary from such minute bodies as those in _Parmelia
exasperata_ which measure 25-35 µ in diam., up to nearly 1 mm. in Lobaria
_laetevirens_. As a rule, they range from about 150 µ to 400 µ across
the widest part, and are generally rather longer than broad. They open
above by a small slit or pore called the ostiole about 20 µ to 100 µ wide
which is frequently dark in colour. In one instance, in _Icmadophila
aeruginosa_, Nienburg[713] has described a spermogonium with a wide
opening, the spermatiophores being massed in palisade formation along the
bottom of a cup-like structure.

_c._ COLOUR OF SPERMOGONIA. Though usually the ostiole is visible as a
darker point than the surrounding tissue, spermogonia are often difficult
to locate unless the thallus is first wetted, when they become visible to
slight magnification. They appear as black points in many _Parmeliae_,
_Physciae_, _Roccellae_, etc., though even in these cases they are often
brown when moistened. They are distinctly brown in some _Cladoniae_, in
_Nephromium_, and in some _Physciae_; orange-red or yellow in _Placodium_
and concolorous with the thallus in _Usnea_, _Ramalina_, _Stereocaulon_,
etc.

[Illustration: Fig. 112. Free spermogonia in spinous cilia of _Cetraria
islandica_ Ach. A, part of frond; B, cilia. × 10.]


D. STRUCTURE

_a._ ORIGIN AND GROWTH. The spermogonia (or pycnidia) of lichens when
mature are more or less hollow structures provided with a distinct wall
or “perithecium,” sometimes only one cell thick and then not easily
demonstrable, as in _Physcia speciosa_, _Opegrapha vulgata_, _Pyrenula
nitida_, etc. More generally the “perithecium” is composed of a layer of
several cells with stoutish walls which are sometimes colourless, but
usually some shade of yellow to dark-brown, with a darker ostiole. The
latter, a small slit or pore, arises by the breaking down of some of the
cells at the apex. After the expulsion of the spermatia, a new tissue is
formed which completely blocks up the empty spermogonium. In filamentous
lichens such as _Usnea_ a dangerous local weakening of the thallus is
thus avoided.

Spermogonia originate from hyphae in or near the gonidial zone. The
earliest stages have not been seen, but Möller[714] noted as the first
recognizable appearance or primordium of the “pycnidia” in cultures of
_Calicium trachelinum_ a ball or coil of delicate yellowish-coloured
hyphae. At a more advanced stage the sporophores (or spermatiophores)
could be traced as outgrowths from the peripheral hyphae, directed in
palisade formation towards the centre of the hyphal coil about 20-30 µ
long and very slender and colourless. They begin to bud off spermatia
almost immediately, as it has been found that these are present in
abundance while the developing spermogonium is still wholly immersed in
the thallus. Meanwhile there is gradually formed on the outside a layer
of plectenchyma which forms the wall. Additional spermatiophores arise
from the wall tissue and push their way inwards between the ranks of the
first formed series. The spermogonium slowly enlarges and stretches and
as the spermatiophores do not grow any longer a central hollow arises
which becomes packed with spermatia (or spores) before the ostiole is
open.

A somewhat similar process of development is described by Sturgis[715]
in the spermogonia of _Ricasolia amplissima_, in which species the
primordium arises by a profuse branching of the medullary hyphae in
certain areas close to the gonidial zone. The cells of these branching
hyphae are filled with oily matter and gradually they build up a dense,
somewhat cylindrical body which narrows above to a neck-like form. The
growth is upwards through the gonidial layer, and the structure widens to
a more spherical outline. It finally reaches the outer cortex when some
of the apical cell membranes are absorbed and a minute pore is formed.
The central part becomes hollow, also by absorption, and the space thus
left is lined and almost filled with multicellular branches of the hyphae
forming the wall; from the cells of this new tissue the spermatia are
abstricted.

_b._ FORMS AND TYPES OF SPERMATIOPHORES. The variations in form of
the fertile hyphae in the spermogonium were first pointed out by
Nylander[716] who described them as sterigmata[717]. He considered the
differences in branching, etc. as of high diagnostic value, dividing
them into two groups: simple “sterigmata” (or spermatiophores), with
non-septate hyphae, and arthrosterigmata, with jointed or septate hyphae.

Simple “sterigmata” comprise those in which the spore or spermatium is
borne at the end of a secondary branch or sterigma, the latter having
arisen from a cell of the upright spermatiophore or from a simple basal
cell. The arthrosterigmata consist of “short cells almost as broad as
they are long, much pressed together, and appearing almost agglutinate
especially toward the base; they fill almost the whole cavity of the
spermogonium.” The arthrosterigmata may grow out into the centre of the
cavity as a single cell-row, as a loose branching network, or, as in
_Endocarpon_, they may form a tissue filling the whole interior. Each
cell of this tissue that borders on a cavity may bud off a spermatium
either directly or from the end of a short process.

[Illustration: Fig. 113 A. Types of lichen “sporophores” and
pycnidiospores. 1, _Peltigera rufescens_ Hoffm. × 910; 2,_Lecidea (Psora)
testacea_ Ach. × 1200; 3,_Cladonia cariosa_ Spreng. × 1000; 4, _Pyrenula
nitida_ Ach. × 1130; 5, _Parmelia tristis_ Nyl. × 700; 6, _Lobaria
pulmonaria_ Hoffm. × 1000 (after Glück).]

The most important contributions on the subject of spermogonia in recent
years are those of Glück[718] and Steiner[719]. Glück, who insisted on
the “pycnidial” non-sexual character of the organs, recognized eight
types of “sporophores” differing in the complexity of their branching or
in the form of the “spores” (Fig. 113 A):

1. The _Peltigera_ type: the sporophores consist of a basal cell bearing
one or more long sterigmata and rather stoutish ellipsoid spores. (These
are true pycnidia.)

2. The _Psora_ type: a more elongate simple sporophore with sterigmata
and oblong spores.

3. The _Cladonia_ type: a branching sporophore, each branch with
sterigmata and oblong spores.

4. The _Squamaria_ type (called by Glück _Placodium_): also a branching
sporophore but with long sickle-like bent spores.

5. The _Parmelia_ type: a more complicated system of branching and
anastomosing of the sporophores, with oblong spores.

6, and 7. The _Sticta_ and _Physcia_ types: in both of these the
sporophores are multi-septate; they consist of a series of radiately
arranged hyphae rising from a basal tissue all round the pycnidium. They
anastomose to form a network and bud off “spermatia” from the free cells
or rather from minute sterigmata. In the _Physcia_ type there is more
general anastomosis of the sporophores and frequently masses of sterile
cells along with the fertile members occupy the centre of the pycnidium.
The spermatia of these and the following _Endocarpon_ type are short
cylindrical bodies (Fig. 113 B).

[Illustration: Fig. 113 B. 7, _Physcia ciliaris_ DC. × 600; 8,
_Endocarpon_ sp. × 600 (after Glück).]

8. _Endocarpon_ type: the pycnidium is filled by a tissue of short broad
cells, with irregular hollow spaces lined by fertile cells similar to
those of the _Sticta_ and _Physcia_ types.

The three last named types of sporophores represent Nylander’s section
of arthrosterigmata. Steiner has followed Nylander in also arranging the
various forms into two leading groups. The first, characterized by the
secondary branch or “sterigma,” he designates “exobasidial”; the second,
comprising the three last types in which the spores are borne directly on
the cells of the sporophore or on very short processes, he describes as
“endobasidial.” Steiner also introduces a new term, _fulcrum_, for the
sporophore.

The pycnidia in which these different sporophores occur are not, as a
rule, characteristic of one family. _Peltigera_ type is found only in
one family and the Cladonia type is fairly constant in _Cladoniae_,
but “_Psora_” pycnidia are found on very varying lichens among the
Lecideaceae, Verrucariaceae and others. The _Squamaria_ type with long
bent spores is found not only in _Squamaria_ (Glück’s _Placodium_) but
also in _Lecidea_, _Roccella_, _Pyrenula_, etc. _Parmelia_ type is
characteristic of many _Parmeliae_ and also of species of _Evernia_,
_Alectoria_, _Platysma_ and _Cetraria_. The _Sticta_ type occurs in
_Gyrophora_, _Umbilicaria_, _Nephromium_ and _Lecanora_ as well as in
_Sticta_ and in one species at least of _Collema_. To the _Physcia_ type
belong the pycnidia of most _Physciaceae_ and of various _Parmeliae_, and
to the closely related _Endocarpon_ type the pycnidia of _Endocarpon_ and
of _Xanthoria parietina_.

[Illustration: Fig. 114. Sterile filaments in spermogonia of _Lecidea
fuscoatra_ Ach. much magnified (after Lindsay).]

_c._ PERIPHYSES AND STERILE FILAMENTS. In a few species, _Roccella
tinctoria_, _Pertusaria globulifera_, etc., short one-celled sterile
hyphae are formed within the spermogonium near the ostiole, towards
which they converge. They correspond to the periphyses in the perithecia
of some Pyrenolichens, Verrucaria, etc. (described by Gibelli[720] as
spermatiophores); they are also present in some of the Pyrenomycetes
(_Sordaria_, etc.), and in many cases replace the paraphyses in function
when these have broken down. Sterile hyphae also occur, towards the
base, mingled with the fertile spermatiophores (Fig. 114). These latter
were first described and figured by Tulasne[721] in the spermogonia
of _Ramalina fraxinea_ as stoutish branching filaments, rising from
the same base as the spermatiophores but much longer, and frequently
anastomosing with each other. They have been noted also in _Usnea
barbata_ and in several species of _Parmelia_, and have been compared by
Nylander[722] to paraphyses. They are usually colourless, but, in the
_Parmeliae_, are often brownish and thus easily distinguished from the
spermatiophores. It has been stated that these filaments are sometimes
fertile. Similar sterile hyphae have been recorded in the pycnidia of
fungi, in _Sporocladus_ (_Hendersonia_) _lichenicola_ (Sphaeropsideae) by
Corda[723] who described them as paraphyses, and also in _Steganosporium
cellulosum_ (Melanconieae). These observations have been confirmed by
Allescher[724] in his recent work on _Fungi Imperfecti_. Keiszler[725]
has described a _Phoma_-like, pycnidium parasitic on the leprose thallus
of _Haematomma elatinum_. It contains short slender sporophores and,
mixed with these, long branched sterile hyphae which reach to the ostiole
and evidently function as paraphyses, though Keiszler suggests that they
may be a second form of sporophore that has become sterile. On account of
their presence he placed the fungus in a new genus _Lichenophoma_.


E. SPERMATIA OR PYCNIDIOSPORES

_a._ ORIGIN AND FORM OF SPERMATIA. Lichen spermatia arise at the tips of
the sterigmata either through simple abstriction or by budding. In the
former case—as in the _Squamaria_ type—a delicate cross-wall is formed
by which the spermatium is separated off. When they arise by budding,
there is first a small clavate sac-like swelling of the end of the short
process or sterigma which gradually grows out into a spermatium on a very
narrow base. This latter formation occurs in the _Sticta_, _Physcia_ and
_Endocarpon_ types.

Nylander[726] has distinguished the following forms of spermatia:

1. Ob-clavate, the broad end attached to the sterigma as in _Usneae_,
_Cetraria glauca_ and _C. juniperina_.

2. Acicular and minute but slightly swollen at each end, somewhat
dumb-bell like, in _Cetraria nivalis_, _C. cucullata_, _Alectoria_,
_Evernia_ and some _Parmeliae_, frequently borne on “arthrosterigmata.”

3. Acicular, cylindrical and straight, the most common form; these
occur in most of the _Lecanorae_, _Cladoniae_, _Lecideae_, Graphideae,
Pyrenocarpeae and occasionally they are budded off from arthrosterigmata.

4. Acicular, cylindrical, bent; sometimes these are very long, measuring
up to 40 µ; they are found in various _Lecideae_, _Lecanorae_,
Graphideae, Pyrenocarpeae, and also in _Roccella_, _Pilophorus_ and
species of _Stereocaulon_.

5. Ellipsoid or oblong and generally very minute; they are borne on
simple sterigmata and are characteristic of the genera _Calicium_,
_Chaenotheca_, _Lichina_, _Ephebe_, of the small genus _Glypholecia_ and
of a few species of _Lecanora_ and _Lecidea_.

In many instances there is more or less variation of form and of size in
the species or even in the individual. There are no spherical spermatia.

_b._ SIZE AND STRUCTURE. The shortest spermatia in any of our British
lichens are those of _Lichina pygmaea_ which are about 1·4 µ in length
and the longest are those of _Lecanora crassa_ which measure up to 39
µ. In width they vary from about 0·5 µ to 2 µ. The mature spermogonium
is filled with spermatia and, generally, with a mass of mucilage that
swells with moisture and secures their expulsion.

The spermatia of lichens are colourless and are provided with a
cell-wall and a nucleus. The presence of a nucleus was demonstrated by
Möller[727] in the spermatia of _Calicium parietinum_, _Opegrapha atra_,
_Collema microphyllum_, _C. pulposum_ and _C. Hildenbrandii_, and by
Istvanffi[728] in those of _Buellia punctiformis_ (_B. myriocarpa_),
_Opegrapha subsiderella_, _Collema Hildenbrandii_, _Calicium
trachelinum_, _Pertusaria communis_ and _Arthonia communis_ (_A.
astroidea_). Istvanffi made use of fresh material, fixing the spermatia
with osmic acid, and in all of these very minute bodies he demonstrated
the presence of a nucleus which stained readily with haematoxylin and
which he has figured in the spermatia of _Buellia punctiformis_ as
an extremely small dot-like structure in the centre of the cell. On
germination, as in the cell-multiplication of other plants, the nucleus
leads the way. Germination is preceded by nuclear division, and each new
hyphal cell of the growing mycelium receives a nucleus.

_c._ GERMINATION OF SPERMATIA (pycnidiospores). The strongest argument
in favour of regarding the spermatia of lichens as male cells had always
been the impossibility of inducing their germination. That difficulty
had at length been overcome by Möller[727] who cultivated them in
artificial solutions, and by that means obtained germination in nine
different lichen species. He therefore rejected the commonly employed
terms spermatia and spermogonia and substituted pycnoconidium and
pycnidia. Pycnidiospore has been however preferred as more in accordance
with modern fungal terminology. His first experiment was with the
“spermatia” of _Buellia punctiformis_ (_B. myriocarpa_) which measure
about 8-10 µ in length and about 3 µ in width, and are borne directly
on the septate spermatiophores (arthrosterigmata). In a culture drop,
the spore had swelled to about double its size by the second or third
day, and germination had taken place at both ends, the membrane of the
spore being continuous with that of the germinating tube. In a short time
cross septa were formed in the hyphae which at first were very close
to each other. While apical growth advanced these first formed cells
increased in width to twice the original size and, in consequence, became
slightly constricted at the septa. In fourteen days a circular patch of
mycelium had been formed about 280 µ in diameter. The development exactly
resembled that obtained from the ascospores of the same species grown in
the absence of gonidia. The largest thallus obtained in either case was
about 2 mm. in diameter after three months’ growth. The older hyphae had
a tendency to become brownish in colour; those at the periphery remained
colourless. In _Opegrapha subsiderella_ the development, though equally
successful, was very much slower. The pycnidiospores (or spermatia) have
the form of minute bent rods measuring 5·7 µ × 1·5 µ. Each end of the
spore produced slender hyphae about the fifth or sixth day after sowing.
In four weeks, the whole length of the filament with the spore in the
middle was 300 µ. In four months a patch of mycelium was formed 2 mm.
in diameter. Growth was even more sluggish with the pycnidiospores of
_Opegrapha atra_. In that species they are rod-shaped and 5-6 µ long.
Germination took place on the fifth or sixth day and in fourteen days a
germination tube was produced about five times the length of the spore.
In four weeks the first branching was noticed and was followed by a
second branching in the seventh week. In three months the mycelial growth
measured 200-300 µ across.

Germination was also observed in a species of _Arthonia_, the spores of
which had begun to grow while still in the pycnidium. The most complete
results were obtained in species of _Calicium_: in _C. parietinum_ the
spores, which are ovoid, slightly bent, and brownish in colour, swelled
to an almost globose shape and then germinated by a minute point at the
junction of spore and sterigma, and also at the opposite end; very rarely
a third germinating tube was formed. Growth was fairly rapid, so that
in four weeks there was a loose felt of mycelium measuring about 2 cm.
× 1 cm. and 1 mm. in depth. Parallel cultures were carried out with the
ascospores and the results in both cases were the same; in five or six
weeks small black points appeared, which gradually developed to pycnidia
with mature pycnidiospores from which further cultures were obtained.

On _C. trachelinum_, which has a thin greyish-white thallus spreading
over old trunks of trees, the pycnidia are usually abundant. Lindsay had
noted two different kinds and his observation was confirmed by Möller.
The spores in one pycnidium are ovoid, measuring 2·5-3 µ × 1·5-2 µ;
in the other rarer form, they are rod-shaped and 5-7 µ long. In the
artificial cultures they both swelled, the rod-like spores to double
their width before germination, and sometimes several tubes were put
forth. Growth was slow, but of exactly the same kind from these two types
of spores as from the ascospores. At the end of the second month pycnidia
appeared on all the cultures, in each case producing the ovoid type of
spore.

In a second paper Möller[729] recorded the partially successful
germination of the “spermatia” of _Collema_ (_Leptogium_) _microphyllum_,
the species in which Stahl had demonstrated sexual reproduction. Growth
was extraordinarily slow: after a month in the culture solution the first
swelling of the spermatium prior to germination took place, and some time
later small processes were formed in two or three directions. In the
fourth month a branched filament was formed.

Möller’s experiments with ascospores and pycnidiospores were primarily
undertaken to prove that the lichen hyphae were purely fungal and
parasitic on the algae. A series of cultures were made by Hedlund[730] in
order to demonstrate that the pycnidiospores were asexual reproductive
bodies; they were grown in association with the lichen alga and their
germination was followed up to the subsequent formation of a lichen
thallus.

_d._ VARIATION IN PYCNIDIA. On the thallus of _Catillaria denigrata_
(_Biatorina synothea_) Hedlund found that there were constantly present
two types of pycnidia: the one with short slightly bent spores 4-8
µ × 1·5 µ, the other with much longer bent spores 10-20 µ × 1·5 µ;
there were numerous transition forms between the two kinds of spores.
Germination took place by the prolongation of the spore; the hypha
produced became septate and branches were soon formed. Hedlund found that
frequently germination had already begun in the spores expelled from the
spermogonium. In newly formed thalline areolae it was possible to trace
back the mycelium to innumerable germinating spores of both types, long
and short.

Lindsay had recorded more than one form of spermogonium on the same
lichen thallus, the spermatia varying considerably in size; but he was
most probably dealing with the mixed growth of more than one species. The
observations of Möller and Hedlund on this point are more exact, but the
limits of variation would very well include the two forms found by Möller
in _Calicium trachelinum_; and in the different pycnidia of _Catillaria
denigrata_ Hedlund not only observed transition stages between the two
kinds of spores, but the longer pycnidiospores, as he himself allows,
indicated the elongation prior to germination: there is no good evidence
of more than one form in any species.


F. PYCNIDIA WITH MACROSPORES

Tulasne[731] records the presence on the lichen thallus of “pycnidia”
as well as of “spermogonia”; the former producing stylospores, larger
bodies than spermatia, occasionally septate and containing oil-drops or
guttulae. These spores are pyriform or ovoid in shape and are always
borne at the tips of simple sporophores. He compared the pycnidia with
the fungus genera _Cytospora_, _Septoria_, etc. As a rule they occur on
lichens with a poorly developed thallus, on some species of _Lecanora_,
_Lecanactis_, _Calicium_, _Porina_, in the family Strigulaceae and in
_Peltigera_.

There is no morphological difference between pycnidia and spermogonia
except that the spermatia of the latter are narrower; but the difference
is so slight that, as Steiner has pointed out, these organs found on
_Lecanora piniperda_, _L. Sambuci_ and _L. effusa_ have been described at
one time as containing microconidia (spermatia), at another macroconidia
(stylospores). He also regards as macrospores those of the pycnidia
of _Calicium trachelinum_ which Möller was able to germinate so
successfully, and all the more so as they were brownish in colour, true
microspores or spermatia being colourless.

Müller[732] has recorded some observations on the pycnidia and
stylospores of the Strigulaceae, a family of tropical lichens inhabiting
the leaves of the higher plants. On the thallus of _Strigula elegans_
var. _tremula_ from Madagascar and from India, he found pycnidia with
stylospores of abnormal dimensions measuring 18-26 µ in length and 3 µ
in width, and with 1 to 7 cross septa. In _Strigula complanata_ var.
_genuina_ the stylospores were 2-8-septate and varied from 7-65 µ, in
length, some of the spores being thus ten times longer than others, while
the width remained the same. Müller considers that in these cases the
stylospore has already grown to a septate hypha while in the pycnidium.
As in the pycnidiospores, described later by Hedlund, the spores had
germinated by increase in length followed by septation.

The spermogonia of _Strigula_, which are exactly similar to the pycnidia
in size and structure, produce spermatia, measuring about 3 µ × 2 µ,
and it is suggested by Müller that the stylospores may represent merely
an advanced stage of development of these spermatia. Both organs were
constantly associated on the same thallus; but whereas the spermogonia
were abundant on the younger part of the thallus at the periphery, they
were almost entirely replaced by pycnidia on the older portions near the
centre, only a very few spermogonia (presumably younger pycnidial stages)
being found in that region.

Lindsay[733] has described a great many different lichen pycnidia,
but in many instances he must have been dealing with species of the
“Fungi imperfecti” that were growing in association with the scattered
granules of crustaceous lichens. There are many fungi—Discomycetes
and Pyrenomycetes—parasitic on lichen thalli, and he has, in some
cases, undoubtedly been describing their secondary pycnidial form
of fruit, which indeed may appear far more frequently than the more
perfect ascigerous form, and might easily be mistaken for the pycnidial
fructification of the lichen.


G. GENERAL SURVEY

_a._ SEXUAL OR ASEXUAL. It has been necessary to give the preceding
detailed account of these various structures—pycnidia or spermogonia—in
view of the extreme importance attached to them as the possible male
organs of the lichen plant, and, in giving the results obtained by
different workers, the terminology employed by each one has been adopted
as far as possible: those who consider them to be sexual structures call
them spermogonia; those who refuse to accept that view write of them as
pycnidia.

Tulasne, Nylander and others unhesitatingly accepted them as male
organs without any knowledge of the female cell or of any method of
fertilization. Stahl’s discovery of the trichogyne seemed to settle the
whole question; but though he had evidence of copulation between the
spermatium and the receptive cell or trichogyne he had no real record of
any sexual process.

Many modern lichenologists reject the view that they are sexual; they
regard them as secondary organs of fructification analogous to the
pycnidia so abundant in the related groups of fungi. One would naturally
expect these pycnidia to reappear in lichens, and it might be considered
somewhat arbitrary to classify pycnidia in Sphaeropsideae as asexual
reproductive organs, and then to regard the very similar structures in
lichens as sexual spermogonia. It has also been pointed out that when
undoubted pycnidia do occur on the lichen thallus, as in _Calicium_,
_Strigula_, _Peltigera_, etc., they in no way differ from structures
regarded as spermogonia except in the size of the pycnidiospores—and,
even among these, there are transition forms. The different types of
spermatia can be paralleled among the fungal pycnidiospores and the same
is also true as regards the sporophores generally. Those described as
arthrosterigmata by Nylander—as endosporous by Steiner—were supposed
to be peculiar to lichens; but recently Laubert[734] has described a
fungal pycnidium which grew on the trunk of an apple tree and in which
the spores are not borne on upright sporophores but are budded off from
the cells of the plectenchyma lining the pycnidium. It may be that
future research will discover other such instances, though that type of
sporophore is evidently of very rare occurrence among fungi.

_b._ COMPARISON WITH FUNGI. The most obvious spermogonia among fungi
with which to compare those of lichens occur in the Uredineae where they
are associated with the life-cycle of a large number of rust species.
They are small flask-shaped structures very much like the simpler forms
of pycnidia and they produce innumerable spermatia which are budded off
from the tips of simple spermatiophores. The mature spermatium has a
delicate cell-wall and contains a thin layer of cytoplasm with a dense
nucleus which occupies almost the whole cavity, cytological characters
which, as Blackman[735] has pointed out, are characteristic of male
cells and are not found in any asexual reproductive spores. If we accept
Istvanffi’s[736] description and figures of the lichen spermatia as
correct, their structure is wholly different: there being a very small
nucleus in the centre of the cell comparable in size with those of the
vegetative hyphae (Fig. 115).

[Illustration: Fig. 115. _a_, spermatia; _b_, hypha produced from
spermatium of _Buellia punctiformis_ Th. Fr. × 950 (after Istvanffi).]

Lichen “spermatia” also differ very strikingly from the male cells of any
given group of plants in their very great diversity of form and size;
but the chief argument adduced by the opponents of the sexual theory
is the capacity of germination that has been proved to exist in a fair
number of species. It is true that germination has been induced in the
spermatia of the Uredines by several research workers—by Plowright[737],
Sappin-Trouffy[738] and by Brefeld[739]—who employed artificial nutritive
solutions (sugar or honey), but the results obtained were not much
more than the budding process of yeast cells. Brefeld also succeeded
in germinating the “spermatia” of a pyrenomycetous fungus, _Polystigma
rubrum_, one of the germinating tubes reaching a length four times that
of the spore; but it is now known that all of these fungal spermatia are
non-functional, either sexually or asexually, and degenerate soon after
their expulsion, or even while still in the spermogonium.

_c._ INFLUENCE OF SYMBIOSIS. In any consideration of lichens it is
constantly necessary to hark back to their origin as symbiotic organisms,
and to bear in mind the influence of the composite life on their
development. After germination from the spore, the lichen hypha is so
dependant on its association with the alga, that, in natural conditions,
though it persists without the gonidia for a time, it attains to only a
rather feeble growth of mycelial filaments. In nutritive cultures, as
Möller has proved, the absence of the alga is partly compensated by the
artificial food supply, and a scanty thalline growth is formed up to the
stage of pycnidial fruits. Not only in pycnidia but in all the fruiting
bodies of lichens, symbiosis has entailed a distinct retrogression in the
reproductive importance of the spores, as compared with fungi.

In Ascomycetes, the asci constitute the overwhelming bulk of the
hymenium; in most lichens, there are serried ranks of paraphyses with
comparatively few asci, and the spores are often imperfectly developed.
It would not therefore be surprising if the bodies claimed by Möller and
others as pycnidiospores had also lost even to a considerable extent
their reproductive capacity.

_d._ VALUE IN DIAGNOSIS. Lichen spermogonia have once and again been
found of value in deciding the affinity of related plants, and though
there are a number of lichens in which we have no record of their
occurrence, they are so constant in others, that they cannot be ignored
in any true estimation of species. Nylander laid undue stress on
spermogonial characters, considering them of almost higher diagnostic
value than the much more important ascosporous fruit. They are, after
all, subsidiary organs, and often—especially in crustaceous species—they
are absent, or their relation to the species under examination is
doubtful.




CHAPTER V

PHYSIOLOGY


I. CELLS AND CELL PRODUCTS

Any study of cells or cell-membranes in lichens should naturally include
those of both symbionts, but the algae though modified have not been
profoundly changed, and their response to the influences of the symbiotic
environment has been already described in the discussion of lichen
gonidia. The description of cells and their contents refers therefore
mainly to the fungal tissues which form the framework of the plant; they
have been transformed by symbiosis to lichenoid hyphae in some respects
differing from, in others resembling, the fungal hyphae from which they
are derived.


A. CELL-MEMBRANES

_a._ CHITIN. It was recognized by workers in the early years of the
nineteenth century that the substance forming the cell-walls of fungal
hyphae differed very markedly from the cellulose of the membranes in
other groups of plants, the blue colouration with iodine and sulphuric
acid so characteristic of cellulose being absent in most fungi. Various
explanations were suggested; but it was always held that the doubtful
substance was a cellulose containing something peculiar to fungi, this
view being strengthened by the fact that, after long treatment with
potash, a blue reaction was obtained. It was called fungus-cellulose by
De Bary[740] in order to distinguish it from true cellulose.

It was not till a much later date that any exact work was done on
the fungal cell, and that Gilson[741] by his researches was able to
prove that the membranes of fungi contained probably no cellulose, or,
“if cellulose were present, it was in a different condition from the
cellulose of other plants.” Winterstein[742] followed with the results of
his examination of fungus-cellulose: he found that it contained nitrogen
and therefore differed very considerably from typical plant cellulose.
Gilson[743] published a second paper dealing entirely with fungal tissues
in which he also established the presence of nitrogen, and added that
this nitrogenous compound resembled in various ways the chitin[744] of
animal cells. He further discovered that by heating it with potash a
substance was obtained that took a reddish-violet stain when treated with
iodine and weak sulphuric acid. This substance, called by him mycosin,
was proved later to be similar to chitosan[744], a product of chitin.

Escombe[745] analysed the hyphal membranes of _Cetraria_ and found
that they consisted mainly of a body called by him lichenin and of a
paragalactan. From _Peltigera_ he extracted a substance with physical
properties agreeing fairly well with those of chitosan, though analysis
did not give percentages reconcilable with that substance; the yield
however was very small. No lichenin was detected.

Van Wisselingh[746] examined the hyphae of lichens as well as of
fungi and experimented with a considerable number of both types of
plants. He succeeded in proving the presence of chitin in the higher
fungi (Basidiomycetes and Ascomycetes) and in lichens with one or two
exceptions (_Cladonia_ and _Cetraria_). Though in some the quantity found
was exceedingly small, in others, such as _Peltigera_, the walls of the
hyphae were extremely chitinous. More recently Wester[747] has gone
into the question as regards lichens, and he has practically confirmed
all the results previously obtained by Wisselingh. In some species, as
for instance in _Cladonia rangiferina_, _Cl. squamosa_, _Cl. gracilis_,
_Ramalina calicaris_, _Solorina crocea_ and others, he found that chitin
existed in large quantities, while in _Evernia prunastri_, _Usnea
florida_, _U. articulata_, _Sticta damaecornis_ and _Parmelia saxatilis_
very little was present. The variation in the amount present may be
very great even in the species of one genus: none for instance has been
detected in _Cetraria islandica_ nor in _C. nivalis_ while it is abundant
in other _Cetrariae_. There is also considerable variation in quantity in
different individuals of the same species, and even in different parts
of the thallus of one lichen. Factors such as habitat, age of the plant,
etc., may, however, account to a considerable extent for the differences
in the results obtained.

_b._ LICHENIN AND ALLIED CARBOHYDRATES. It has been proved, as already
stated, that chitin is present in the hyphal cell-walls of all the
lichens examined except in those of _Cetraria islandica_ (Iceland Moss),
_C. nivalis_ and, according to Wester[747], in those of _Bryopogon_
(_Alectoriae_). In these lichens another substance of purely carbohydrate
nature is the chief constituent of the cell-walls which swell up when
soaked in water to a colourless gelatinous substance.

Berzelius[748] first drew attention to the peculiar qualities of this
lichen product, and, recognizing its resemblance in many respects to
ordinary starch, he called it “lichen-starch” or “moss-starch.” More
exact observations were made later by Guérin-Varry[749] who described its
properties and showed by his experiments that it contained no admixture
of either starch or gum. He adopted the name lichenin for this organic
soluble part of Iceland Moss. An analysis of lichenin was made by
Mulder[750] who detected in addition to lichenin, which coloured yellow
with iodine, small quantities of a blue-colouring substance which could
be dissolved out from the lichenin and which he considered to be true
starch. Berg[751] also demonstrated the compound nature of lichenin:
he isolated two isomerous substances with the formula C₆H₁₀O₅. The
name “isolichenin” was given to the second blue-colouring substance by
Beilstein[752] in 1881.

More recently Escombe[753] has chemically analysed the cell-wall of
_Cetraria islandica_: after the elimination of fat, oil, colouring
matter and bitter constituents he found that there remained the compound
lichenin, an anhydride of galactose with the formula C₆H₁₀O₅, which, as
stated above, consists of two substances lichenin and isolichenin[754];
the latter is soluble in cold water and gives a blue reaction with
iodine, lichenin is only soluble in hot water and is not coloured blue.
Both are derivatives of galactose, a sugar found in a great number of
organic tissues and substances, among others in gums.

Lichenin has also been obtained by Lacour[755] from _Lecanora esculenta_,
an edible desert lichen supposed to be the manna of the Israelites.
Wisselingh[756] tested the hymenium of thirteen different lichens for
lichenin. He found it in the walls of the ascus of all those he examined
except _Graphis_. Everniin, a constituent of _Evernia prunastri_, was
isolated and described by Stüde[757]. It is soluble in water and, though
considered by Czapek[758] to be identical with lichenin, it differs,
according to Ulander[759], in being dextro-rotatory to polarized
light; lichenin on the contrary is optically inactive. Escombe[753]
also obtained a substance from _Evernia_ which he considered to be
comparable with chitosan. Usnein which has been extracted[756] from
_Usnea barbata_ may also be identical with lichenin, but that has not yet
been established. Ulander[759] examined chemically the cell-walls of a
fairly large number of lichens. _Cetraria islandica_, _C. aculeata_ and
_Usnea barbata_, designated as the “Cetraria group,” contained soluble
mucilage-forming substances similar to lichenin. A second “Cladonia
group” which included _Cl. rangiferina_ with the variety _alpestris_,
_Stereocaulon paschale_ and _Peltigera aphthosa_ yielded almost none.
After the soluble carbohydrates were removed by hot water, the insoluble
substances were hydrolysed and the “Cetraria group” was found to contain
abundant d-glucose with small quantities of d-mannose and d-galactose;
the “Cladonia group,” abundant d-mannose and d-galactose with but little
d-glucose. Hydrolysis was easier and quicker with the former group than
with the latter.

Besides these, which rank as hexosans, Ulander found small quantities
of pentosans and methyl pentosans. All these substances which are such
important constituents of the hyphal membranes of lichens are classed
by Ulander as hemicelluloses of the same nature as mannan, galactan
and dextran, or as substances between hemicellulose and the glucoses
represented by lichenin, everniin, etc. They are doubtless reserve
stores of food material, and they are chiefly located in the cell-walls
of the medullary hyphae which are often so thick as almost to obliterate
the lumen of the cells. Ulander made no test for chitin in his researches.

Ulander’s results have been confirmed by those obtained by K.
Müller[760]. In _Cladonia rangiferina_, Müller found that the
cell-membranes of the hyphae contained, as hemicelluloses, pentosans in
small quantities and galactan, but no lichenin and very little chitin.
In _Evernia prunastri_ hemicelluloses formed the chief constituents of
the thallus, and from it he was able to isolate galactan soluble in
weak hot acid, and everniin soluble in hot water, the latter with the
formula C₇H₁₅O₆, a result differing from that obtained by Stüde[761] who
has given it as C₉H₁₄O₇; chitin was also present in small quantities.
In _Ramalina fraxinea_, the soluble part of the thallus (in hot water)
differed from everniin and might probably be lichenin. _Cetraria
islandica_ was also analysed and yielded various hemicelluloses, chiefly
dextran and galactan, with less pentosan. No chitin has ever been found
in this lichen. In testing minute quantities of material for chitin,
Wisselingh[762] heated the tissue in potash to 160° C. The potash was
then gradually replaced by glycerine and distilled water; the precipitate
was placed on a slide and the preparation stained under the microscope
by potassium-iodide-iodine and weak sulphuric acid. Chitin, if present,
would have been changed by the potash to mycosin which gives a violet
colour with the staining solution.

It has been stated by Schellenberg[763] that these lichen membranes may
become lignified. He obtained a red reaction with phloroglucine test for
lignin in _Cetraria islandica_ and _Cladonia furcata_. Further research
is required.

_c._ CELLULOSE. Several workers claim to have found true cellulose in
the cell-walls of the hyphal tissues of a few lichens; but the more
careful analyses of Escombe[764], Wisselingh[762] and Wester[765] have
disproved their results. The cell-walls of all the gonidia, however, are
formed of cellulose, or according to Escombe of glauco-cellulose, except
those of _Peltigera_ in which Wester found neither cellulose nor chitin.
Czapek[766] suggests that the blue reaction with iodine characteristic
of the cell-walls in some apothecia, of the asci and of the hyphae in
cortex or medulla in a few instances, may be due to the presence of
carbohydrates of the nature of galactose. Moreau[767] in a recent paper
terms the substance that gives a blue reaction with iodine at the tips of
the asci “amyloid.” In _Peltigera_ the ascus tip is occupied by such a
plug of amyloid which at maturity is projected like a cork from the ascus
and may be found on the surface of the hymenium.


B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS

_a._ CELL-SUBSTANCES. The cells of lichen hyphae contain protoplasm and
nucleus with glucoses. It is doubtful if starch has been found in fungal
hyphae; it is replaced, in some of the tissues at least, by glycogen,
a carbohydrate (C₆H₁₀O₅) very close to, if not identical with, animal
glycogen, a substance which is soluble in water and colours reddish-brown
(wine-red) with iodine. Errera[768] first detected its presence in
Ascomycetes where it is associated with the epiplasm of the cells, more
especially of the asci, and he considered it to be physiologically
homologous with starch. He included lichens, as Ascomycetes, in his
survey of fungi and quotes, in support of his view that lichen hyphae
also contain glycogen, a statement made by Schwendener[769] that “the
contents of the ascogenous hyphae of _Coenogonium Linkii_ stain a
deep-brown with iodine.” Errera also instances the red-brown reaction
with iodine, described by de Bary[770], as characteristic of the large
spores of _Ochrolechia_ (_Lecanora_) _pallescens_, while the germinating
tubes of these spores become yellow with iodine like ordinary protoplasm.
Glycogen has been, so far, found only in the cells of the reproductive
system.

Iodine was found by Gautier[771] in the gonidia of _Parmelia_ and
_Peltigera_, _i.e._ both in bright-green and blue-green algae. The amount
was scarcely calculable.

Herissey[772] claims to have established the presence of emulsin in a
large series of lichens belonging to such widely separated genera as
_Cladonia_, _Cetraria_, _Evernia_, _Peltigera_, _Pertusaria_, _Parmelia_,
_Ramalina_, and _Usnea_. It is a ferment which acts upon amygdalin,
though its presence has been proved in plants such as lichens where no
amygdalin has been found[773]. Diastase was demonstrated in the cells of
_Roccella tinctoria_, _R. Montagnei_ and of _Dendrographa leucophaea_
by Ronceray[774] who states that, in conjunction with air and ammonia,
it forms orchil, the well-known colouring substance of these lichens.
Diastatic ferments have also been determined[775] in _Usnea florida_,
_Physcia parietina_, _Parmelia perlata_ and _Peltigera canina_.

_b._ CALCIUM OXALATE. Oxalic acid (C₂H₂O₄) is an oxidation product of
alcohol and of most carbohydrates and in combination is a frequent
constituent of plant cells. Knop[776] held that it was formed in lichens
by the reduction and splitting of lichen acids, though, as Zopf[777] has
pointed out, these are generally insoluble. Hamlet and Plowright[778]
demonstrated the presence of free oxalic acid in many families of fungi
including _Pezizae_ and _Sphaeriae_. The acid combines with calcium to
form the oxalate (CaC₂0₄), which in the crystalline form is very common
in lichens. In the higher plants the crystals are formed within the
cell, but in lichens they are always deposited on the outer surface of
the hyphal membranes, mainly of the medulla and the cortex.

Calcium oxalate was first detected in lichens by Henri Braconnot[779],
who extracted it by treating the powdered thallus of a number of species
(_Pertusaria communis_, _Diploschistes scruposus_, etc.) with different
reagents. The quantity present varies greatly in lichens: Zopf[780] found
that it was abundant in all the species inhabiting limestone, and states
that in such plants the more purely lichenic acids are relatively scarce.
Errera[781] has calculated the amount of calcium oxalate in _Lecanora
esculenta_, a desert lime-loving lichen, to be about 60 per cent. of the
whole substance of the thallus. Euler[782] gives for the same lichen
even a larger proportion, 66 per cent. of the dry weight. In _Pertusaria
communis_, a corticolous species, the oxalate occurs as irregular
crystalline masses in the medulla (Fig. 116) and has been calculated
as 47 per cent. of the whole substance. Other crustaceous species such
as _Diploschistes scruposus_, _Haematomma coccineum_, _H. ventosum_,
_Lecanora saxicola_, _Lecanora tartarea_, etc., contain large amounts
either in the form of octahedral crystals or as small granules.

[Illustration: Fig. 116. _Pertusaria communis_ DC. Vertical section of
thallus. _a_, cortex; _b_, gonidia; _c_, medulla; _d_, crystal of calcium
oxalate. × ca. 100.]

Rosendahl[783] has recently made observations as to the presence of the
oxalate in the thallus of the brown _Parmeliae_. Of the fourteen species
examined by him, eleven contained calcium oxalate as octahedral crystals
or as small prisms, often piled up in thick irregular masses. Usually the
crystals were located in the medullary part of the thallus, but in two
species, _Parmelia verruculifera_ and _P. papulosa_, they were abundant
on the surface cells of the upper cortex.

_c._ IMPORTANCE OF CALCIUM OXALATE TO THE LICHEN PLANT. It is natural to
conclude that a substance of frequent occurrence in any group of plants
is of some biological significance, and suggestions have not been lacking
as to the value of oxalic acid or of calcium oxalate in the economy of
the lichen thallus. Oxalic acid is known to be one of the most efficient
solvents of argillaceous earth and of iron oxides likely to be in the
soil. These materials are also conveyed to the thallus as air-borne
dust, and would thus, with the aid of the acid, be easily dissolved and
absorbed. As a direct proof of this, Knop[784] has stated that lichen-ash
always contains argillaceous earth. According to Kratzmann[785],
aluminium, a product of clay, is stored up in various lichens. He proved
the amount in the ash of _Umbilicaria pustulata_ to be 4·46 per cent.,
in _Usnea barbata_ 1·79 per cent., in _U. longissima_ considerable
quantities while in _Roccella tinctoria_ it occurred in great abundance.
It was also abundant in _Diploschistes scruposus_, 28·17 per cent.; it
declined in _Variolaria_ (_Pertusaria_) _dealbata_ to 7·77 per cent., in
_Cladonia rangiferina_ to 1·76-2·12 per cent. and in _Ramalina fraxinea_
to 1·8 per cent.

Calcium oxalate is directly advantageous to the thallus by virtue of
the capacity of the crystals to reduce or prevent evaporation, as has
been pointed out by Zukal[786]. A like service afforded by crystals to
the leaves of the higher plants in desert lands has been described by
Kerner[787]. These are frequently encrusted with lime crystals which
allow the copious night dews to soak underneath them to the underlying
cells, while during the day they impede, if they do not altogether check,
evaporation.

Calcium oxalate crystals are insoluble in acetic acid, soluble in
hydrochloric acid without evolution of gas; they deposit gypsum crystals
in a solution of sulphuric acid.


C. OIL-CELLS

_a._ OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens are
able to dissolve the lime of the substratum, and their hyphae penetrate
more or less deeply into the rock. In some forms the entire thallus
may thus be immersed, the fruits alone being visible on the surface of
the stone. In two such species, _Verrucaria calciseda_ and _Petractis_
(_Gyalecta_) _exanthematica_, Steiner[788] detected peculiar sphaeroid
or barrel-shaped cells that differed from the other hyphal cells of the
thallus, not only in their form, but in their greenish-coloured contents.
Similar cells were found by Zukal[789] in another immersed (endolithic)
lichen, _Verrucaria rupestris_ f. _rosea_. He describes them as roundish
organs crowded on the hyphae and filled with a greenish shimmering
protoplasm. He[790] found the same types of sphaeroid and other swollen
cells in the immersed thallus of several calcicolous lichens and he
finally determined the contents as fat in the form of oil. He found
also that these fat-cells, though very frequent, were not constantly
present even in the same species. His observations were confirmed by
Hulth[791] for a number of allied crustaceous lichens that grow not only
on limestone but on volcanic rocks. In them he found a like variety of
fat-cells—intercalary or torulose cells, terminal sphaeroid cells and
hyphae containing scattered oil-drops. Bachmann[792] followed with a
study of the thallus of purely calcicolous lichens. The specialized
oil-cells were fairly constant in the species he examined, and, as a
rule, they were formed either in the tissues immediately below, or at
some distance from, the gonidial zone. Fünfstück[793] has also published
an account of various oil-cells in a large series of calcicolous lichens
(Fig. 117).

[Illustration: Fig. 117. _Lecidea immersa_ Ach. A, sphaeroid fat-cells
from about 8 mm. below the surface × 550. B, oil-hyphae in process
of emptying: _a_, sphaeroid cells containing oil; _b_, cells with
oil-globules × 600 (after Fünfstück).]

The occurrence of oil-(or fat-)cells is not dependent on the presence
of any particular alga as the gonidium of the lichen. Fünfstück[794]
has described the immersed thallus of _Opegrapha saxicola_ as one of
those richest in fat-cells. The gonidia belong to the filamentous alga
_Trentepohlia umbrina_ and form a comparatively thin layer about 160µ
thick near the upper surface; isolated algal branches may grow down to
350µ into the rock, while the fungal elements descend to 11·5 mm., and
though the very lowest hyphae were without oil—as were those immediately
beneath the gonidia—the interlying filaments, he found, were crowded with
oil-cells. Sphaeroid terminal cells were not present.

Fünfstück[794] has re-examined the thallus of _Petractis exanthematica_,
an almost wholly immersed lichen with a gelatinous gonidium, a species of
_Scytonema_. The thallus is homoiomerous: the alga forms no special zone,
it intermingles with the hyphae down to the very base of the thallus; the
hyphae are extremely slender and at the base they measure only about 1µ
in width. Oil-cells are abundant in the form of intercalary cells about
3-5µ in thickness. Nearer the surface sphaeroid cells are formed on short
lateral outgrowths; they measure 14-16µ in diameter and occur in groups
of 15 to 20. The superficial part of the thallus is a mere film; the
hyphae composing it are slightly stouter and more thickly interwoven.

Bachmann[795] and Lang[796] have further described the anatomy of
endolithic thalli especially with reference to oil-cells, and have
supplemented the researches of previous workers. New methods of cutting
the rock in thin slices and of dissolving away the lime enabled them to
see the tissues in their relative positions. In these immersed lichens,
as described by them and by previous writers, and more especially in
calcicolous species, the gonidial zone of Protococcaceous algae lies
near the surface of the rock, and is mingled with delicate, thin-walled
hyphae which usually do not contain oil. The more deeply immersed layer
is formed of a weft of equally thin-walled hyphae, some of the cells
of which are swollen and filled with fat globules. These oil-cells may
occur at intervals along the hyphae or they may form an almost continuous
row. In addition, strands or bundles of hyphae (Fig. 118) containing
few or many oil globules traverse the tissue, and true sphaeroid cells
are generally present. These latter arise in great numbers on short
lateral branchlets, usually near the tip of a filament and the groups
of cells are not unlike bunches of grapes. Sometimes the oil-cells are
massed together into a complex tissue. Hyphae from this layer pierce
still deeper into the rock and constitute the rhizoidal portion of the
thallus. They also produce sphaeroid oil-cells in great abundance (Fig.
119). In the immersed thallus of _Sarcogyne_ (_Biatorella_) _pruinosa_
Lang[797] estimated the gonidial zone as 175-200µ in thickness, while the
colourless hyphae penetrated the rock to a depth of quite 15 mm.

[Illustration: Fig. 118. _Biatorella_ (_Sarcogyne_) _simplex_ Br. and
Rostr. _a_, sphaeroid oil-cells; _b_, strand of oil-hyphae from 10-15 mm.
below the surface. × 585 (after Lang).]

[Illustration: Fig. 119. _Biatorella pruinosa_ Mudd. _a_, complex
of sphaeroid oil-cells from 10 mm. below the surface; _b_, hypha of
sphaeroid cells also from inner part of the thallus. × 585 (after Lang).]

_b._ OIL-CELLS OF EPILITHIC LICHENS. The general arrangement of the
tissues and the occurrence and form of the oil-cells vary in the
different species according to the nature of the substratum. This has
been clearly demonstrated by Bachmann[798] in _Aspicilia_ (_Lecanora_)
_calcarea_, an almost exclusively calcareous lichen as the name implies.
On limestone, he found sphaeroid cells formed in great abundance on the
deeply penetrating rhizoidal hyphae (Fig. 120). On a non-calcareous brick
substratum[799], a specimen had grown which of necessity was epilithic.
The cortex and gonidial zone together were 40µ thick; immediately below
there were hyphae with irregular cells free from oil; lower still there
was formed a compact tissue of globose fat-cells. In this case the
calcareous lichen still retained the capacity to form oil-cells on the
non-calcareous impenetrable substance.

[Illustration: Fig. 120. _Lecanora_ (_Aspicilia_) _calcarea_ Sommerf.
Early stage of sphaeroid cell formation × 175 (after Bachmann).]

Very little oil is formed, as a rule, in the cells of siliceous
crustaceous lichens which are almost wholly epilithic, but Bachmann found
a tissue of oil-cells in the thallus of _Lecanora caesiocinerea_, from
Labrador, on a granite composed of quartz, orthoclase and traces of mica.
A thallus of the same species collected in the Tyrol, though of a thicker
texture, contained no oil. Bachmann[799] suggests no explanation of the
variation.

On granite, rhizoidal hyphae penetrate the rock to a slight extent
between the different crystals, but only in connection with the mica[800]
are typical sphaeroid cells formed.

More or less specialized oil-cells have been demonstrated by
Fünfstück[801] in several superficial (epilithic) lichens which grow
on a calcareous substratum, as for instance _Lecanora_ (_Placodium_)
_decipiens_, _Lecanora crassa_ and other similar species. The oil in
these lichens is usually restricted to more or less swollen or globose
cells; but it may also be present in the ordinary hyphae as globules.
Zukal[802] found that the smooth little round granules sprinkled over the
thallus of the soil-lichens, _Baeomyces roseus_ and _B. rufus_, contained
in the hyphae typical sphaeroid oil-cells and that they were specially
well developed in specimens from Alpine situations. In still another
soil-lichen, _Lecidea granulosa_, shimmering green oil was found in
short-celled torulose hyphae.

Rosendahl’s[803] researches on the brown _Parmeliae_ resulted in
the unexpected discovery of specialized oil-cells situated in the
cortices—upper and lower—of five species out of fourteen which he
examined. In one of the species, _P. papulosa_, they also occurred in
the cortex of the rhizoids. The oil-cells were thinner-walled and larger
than the neighbouring cortical cells; they were clavate or ovate in form
and sometimes formed irregular external processes. They were more or less
completely filled with oil which coloured brown with osmic acid, left a
fat stain on paper and, when extracted, burned with a shining reddish
flame. These oil-cells were never formed in the medulla nor in the
gonidial region.

_c._ SIGNIFICANCE OF OIL-FORMATION. Zukal[804] regarded the oil stored
in these specialized cells as a reserve product of service to the plant
in the strain of fruit-formation, or in times of prolonged drought or
deprivation of light. According to his observations fat was most freely
formed in lichens when periods of luxuriant growth alternated with
periods of starvation. He cites, as proof of his view, the frequent
presence of empty sphaeroid cells, and the varying production of oil
affected by the condition, habitat, etc. of the plant. Fünfstück[805],
on the other hand, considers the oil of the sphaeroid and swollen cells
as an excretion, representing the waste products of metabolism in the
active tissue, but due chiefly to the presence of an excess of carbonic
acid which, being set free by the action of the lichen acids on the
carbonate of lime, forms the basis of fat-formation. He points out that
the development of fat-cells is always greater in endolithic species in
which the gonidial layer—the assimilating tissue—is extremely reduced.
In epilithic lichens with a wide gonidial zone, the formation of oil is
insignificant. He states further that if the oil were a direct product
of assimilation, the cells in which it is stored would be found in
contact with the gonidia, and that is rarely the case, the maximum of fat
production being always at some distance.

Fünfstück tested the correctness of his views by a prolonged series
of growth experiments: he removed the gonidial layer in an endolithic
lichen, and found that fat storage continued for some time afterwards,
its production being apparently independent of assimilative activity.
The correctness of his deductions was further proved by observations on
lichens from glacier stones. In such unfavourable conditions the gonidia
were scanty or absent, having died off, but the hyphae persisted and
formed oil. He[806] also placed in the dark two quick-growing calcicolous
lichens, _Verrucaria calciseda_ and _Opegrapha saxicola_. At the end of
the experiment, he found that they had increased in size without using
up the fat. Lang[807] also is inclined to reject Zukal’s theory, seeing
that the fat is formed at a distance from the tissues—reproductive and
others—in need of food supply. He agrees with Fünfstück that the oil is
an excretion and represents a waste-product of the plant.

Considerable light is thrown on the subject of oil-formation by the
results of recent researches on the nutrition of algae and fungi.
Beijerinck[808] made comparative cultures of diatoms taken from the
soil, and he found that so long as culture conditions were favourable,
any fat that might be formed was at once assimilated. If, however, some
adverse influence checked the growth of the organism while carbonic acid
assimilation was in full vigour, fat was at once accumulated. The adverse
influence in this case was the lack of nitrogen, and Beijerinck considers
it an almost universal rule in plants and animals, that where there is
absence of nitrogen, in a culture otherwise suitable, fat-oils will be
massed in those cells which are capable of forming oil. He observed that
in two of the cultures of diatoms the one which alone was supplied with
nitrogen grew normally, while the other, deprived of nitrogen, formed
quantities of oil-drops. Wehmer[809] records the same experience in his
cultural study of _Aspergillus_. Sphaeroid fat-cells, similar to those
described by Zukal in calcicolous lichens, were formed in the hyphae of
a culture containing an overplus of calcium carbonate, and he judged,
entirely on morphological grounds, that these were not of the nature of
reserve-storage cells.

Stahel[810] has definitely established the same results in cultures
of other filamentous fungi. In an artificial culture medium in which
nitrogen was almost wholly absent, the cells of the mycelium seemed
to be entirely occupied by oil-drops, and this fatty condition he
considered to be a symptom of degeneration due to the lack of nitrogen.
These experiments enable us to understand how the hyphae of calcicolous
lichens, buried deep in the substratum, deprived of nitrogen and
overweighted with carbonic acid, may suffer from fatty degeneration as
shown by the formation of “sphaeroid-cells.” The connection between cause
and effect is more obscure in the case of lichens growing on the surface
of the soil, such as _Baeomyces roseus_, or of tree lichens such as the
brown _Parmeliae_, but the same influence—lack of sufficient nitrogenous
food—may be at work in those as well as in the endolithic species, though
to a less marked extent.

It seems probable that the capacity to form oil- or fat-cells has become
part of the inherited development of certain lichen species and persists
through changes of habitat as exemplified in _Lecanora calcarea_[811].

In considering the question of the formation and the function of fat in
plant cells, it must be remembered that the service rendered to the life
of the organism by this substance is a very variable one. In the higher
plants (in seeds, etc.) fat undoubtedly functions in the same way as
starch and other carbohydrates as a reserve food. It is evidently not
so in lichens, and in one of his early researches, Pfeffer[812] proved
that similarly oil was only an excretion in the cells of hepatics. He
grew various species in which oil-cells occurred in the dark and then
tested the cell contents. He found that after three months of conditions
in which the formation of new carbohydrates was excluded, the oil in
the cells, instead of having served as reserve material, was entirely
unchanged and must in that instance be regarded as an excretion.


D. LICHEN-ACIDS

_a._ HISTORICAL. The most distinctive and most universal of lichen
products are the so-called lichen-acids, peculiar substances found so far
only in lichens. They occur in the form of crystals or minute granules
deposited in greater or less abundance as excretory bodies on the outer
surface of the hyphal cells. Though usually so minute as scarcely to be
recognized as crystals, yet in a fairly large series their form can be
clearly seen with a high magnification. Many of them are colourless;
others are a bright yellow, orange or red, and give the clear pure tone
of colour characteristic of some of our most familiar lichens.

The first definite discovery of a lichen-acid was made towards the
beginning of the nineteenth century and is due to the researches of C.
H. Pfaff[813]. He was engaged in an examination of _Cetraria islandica_,
the Iceland Moss, which in his time was held in high repute, not only as
a food but as a tonic. He wished to determine the chemical properties
of the bitter principle contained in it, which was so much prized by
the Medical Faculty of the period, though the bitterness had to be
removed to render palatable the nutritious substance of the thallus. He
succeeded in isolating an acid which he tested and compared with other
organic acids and found that it was a new substance, nearest in chemical
properties to succinic acid. In a final note, he states that the new
“lichen-acid,” as he named it, approached still nearer to boletic acid,
a constituent of a fungus, though it was distinct from that substance
also in several particulars. The name “cetrarin” was proposed, at a
later date, by Herberger[814] who described it as a “subalkaloidal
substance, slightly soluble in cold water to which it gives a bitter
taste; soluble in hot water, but, on continued boiling, throwing down a
brown powder which is slightly soluble in alcohol and readily soluble in
ether.” Knop and Schnederman[815] found that Herberger’s “cetrarin” was
a compound substance and contained besides other substances “cetraric
acid” and lichesterinic acid. It has now been determined by Hesse[816] as
fumarprotocetraric acid (C₆₂H₅₀O₃₅), a derivative of which is cetraric
acid or triaethylprotocetraric acid with the formula C₅₄H₃₉O₂₄(OC₂H₅)₃
and not C₂₀H₁₈O₉ as had been supposed. Cetraric acid has not yet been
isolated with certainty from any lichen[817].

After this first isolation of a definite chemical substance, further
research was undertaken, and gradually a number of these peculiar acids
were recognized, the lichens examined being chiefly those that were of
real or supposed economic value either in medicine or in the arts. In
late years a wider chemical study of lichen products has been vigorously
carried on, and the results gained have been recently arranged and
published in book form by Zopf[818]. Many of the statements on the
subject included here are taken from that work. Zopf gives a description
of all the acids that had been discovered up to the date of publication,
and the methods employed for extracting each substance. The structural
formulae, the various affinities, derivatives and properties of the
acids, with their crystalline form, are set forth along with a list of
the lichens examined and the acids peculiar to each species. In many
instances outline figures of the crystals obtained by extraction are
given. For a fuller treatment of the subject, the student is referred to
the book itself, as only a general account can be attempted here.

_b._ OCCURRENCE AND EXAMINATION OF LICHEN-ACIDS. Acids have been found,
with few exceptions, in all the lichens examined. They are sometimes
brightly coloured and are then easily visible under the microscope.
Generally their presence can only be determined by reagents. Over 140
different kinds have been recognized and their formulae determined,
though many are still imperfectly known. As a rule related lichen species
contain the same acids, though in not a few cases one species may contain
several different kinds. In growing lichens, they form 1 to 8 per
cent. of the dry weight, and as they are practically, while unchanged,
insoluble in water, they are not liable to be washed out by rain, snow
or floods. Their production seems to depend largely on the presence of
oxygen, as they are always found in greatest abundance on the more freely
aerated parts of the thallus, such as the soredial hyphae, the outer
rind or the loose medullary filaments. They are also often deposited on
the exposed disc of the apothecium, on the tips of the paraphyses, and
on the wall lining the pycnidia. They are absent from the thallus of the
Collemaceae, these being extremely gelatinous lichens in which there can
be little contact of the hyphae with the atmosphere. No free acids, so
far as is known, are contained in _Sticta fuliginosa_, but a compound
substance, trimethylamin, is present in the thallus of that lichen. It
has also been affirmed that acids do not occur in any _Peltigera_ nor
in two species of _Nephromium_, but Zopf[818] has extracted a substance
peltigerin both from species of _Peltigera_ and from the section
_Peltidea_.

For purposes of careful examination freshly gathered lichens are most
serviceable, as the acids alter in herbarium or stored specimens. It is
well, when possible, to use a fairly large bulk of material, as the acids
are often present in small quantities. The lichens should be dried at a
temperature not above 40°C. for fear of changing the character of the
contained substances, and they should then be finely powdered. When only
a small quantity of material is available, it has been recommended that
reagents should be applied and the effect watched under the microscope
with a low power magnification. This method is also of great service in
determining the exact position of the acids in the thallus.

In micro-chemical examination, Senft[819] deprecates the use of
chloroform, ether, etc., seeing that their too rapid evaporation leaves
either an amorphous or crystalline mass of material which does not lend
itself to further examination. He recommends as more serviceable some oil
solution, preferably “bone oil” (neat’s-foot oil), in which a section of
the thallus should be broken up under a cover-glass and subjected to a
process of slow heating; some days must elapse before the extraction is
complete. The surplus oil is then to be drained off, the section further
bruised and the substance examined.

Acids in bulk should be extracted by ether, acetone, chloroform, benzole,
petrol-ether and lignoin or by carbon bisulphide. Such solvents as
alcohols, acetates and alkali solutions should not be used as they tend
to split up or to alter the constitution of the acids. For the same
reason, the use of chloroform is to a certain extent undesirable as it
contains a percentage of alcohol. Ether and acetone, or a mixture of
both, are the most efficient solvents, and all acids can be extracted by
their use, if the material is left to soak a sufficient length of time,
either in the cold or warmed. It is however advisable to follow with a
second solvent in case any other acid should be present in the tissues.
Concentrated sulphuric acid dissolves out all acids but often induces
colour changes in the process.

All known lichen-acids form crystals, though the crystalline form may
alter with the solution used. After filtering and distilling, the residue
will be found to contain a mixture of these crystals along with other
substances, which may be removed by washing, etc.

_c._ CHARACTER OF ACIDS. Many lichen-acids are more or less bitter
to the taste; they are usually of an acid nature though certain of
the substances are neutral, such as zeorin, a constituent of various
Lecanoraceae, Physciaceae and Cladoniaceae, stictaurin, originally
obtained from _Sticta aurata_, leiphemin, from _Haematomma coccineum_,
and others.

A large proportion are esters or alkyl salts formed by the union of an
alcohol and an acid; these are insoluble in alkaline carbonates. It
is considered probable that the fungus generates the acid, while the
alcohol arises in the metabolic processes in the alga. It has indeed
been proved that the alcohol, erythrit, is formed in at least two algae,
_Protococcus vulgaris_ and _Trentepohlia jolithus_; and the lichen-acid,
erythrin (C₂₀H₂₂O₁₀), obtained from species of _Roccella_ in which the
alga is _Trentepohlia_, is, according to Hesse, the erythrit ester of
lecanoric acid (C₁₆H₁₄O₇), a very frequent constituent of lichen thalli.
It is certain that the interaction of both symbionts is necessary for
acid production. This was strikingly demonstrated by Tobler[820] in his
cultural study of the lichen thallus. He succeeded in growing, to a
limited extent, the hyphal part of the thallus of _Xanthoria parietina_
on artificial media; but the filaments remained persistently colourless
until he added green algal cells to the culture. Almost immediately
thereafter the characteristic yellow colour appeared, proving the
presence of parietin, formerly known as chrysophanic acid. Tobler’s
observation may easily be verified in plants from natural habitats.
A depauperate form of _Placodium citrinum_ consisting mainly of a
hypothallus of felted hyphae, with minute scattered granules containing
algae, was tested with potash, and only the hyphae immediately covering
the algal granules took the stain; the hypothallus gave no reaction.

It has been suggested[821] that when a decrease of albumenoids takes
place, the quantity of lichen-acid increases, so that the excreted
substance should be regarded as a sort of waste product of the living
plant, “rather than as a product of deassimilation.” The subject is not
yet wholly understood.

_d._ CAUSES OF VARIATION IN QUANTITY AND QUALITY OF LICHEN-ACIDS. Though
it has been proved that lichen-acids are formed freely all the year round
on any soil or in any region, it happens occasionally that they are
almost or entirely lacking in growing plants. Schwarz[822] found this to
be the case in certain plants of _Lecanora tartarea_, and he suggests
that the gyrophoric acid contained in the outer cortex of that lichen had
been broken up by the ammonia of the atmosphere into carbonic acid and
orcin which is soluble in water, and would thus be washed away by rain.
It has also been shown by Schwendener[823] and others that the outer
layers of the older thallus in many lichens slowly perish, first breaking
up and then peeling off; the denuded areas would therefore have lost, for
some time at least, their particular acids. Fünfstück[824] considers that
the difference in the presence and amount of acid in the same species of
lichen may be due very often to variation in the chemical character of
the substratum, and this view tallies with the results noted by Heber
Howe[825] in his study of American _Ramalinae_. He observed that, though
all showed a pale-yellow reaction with potash, those growing on mineral
substrata gave a more pronouncedly yellow colour.

M. C. Knowles[826] found that in _Ramalina scopulorum_ the colour
reaction to potash varied extremely, being more rapid and more intense,
the more the plants were subject to the influence of the sea-spray.

Lichen-acids are peculiarly abundant in soredia, and as, in some
species, the thallus forms these outgrowths, or even becomes leprose
more freely in damp weather, the amount of acids produced may depend on
the amount of moisture in the atmosphere.

Their formation is also strongly influenced by light, as is well shown
by the varying intensity of colour in some yellow thalli. _Placodium
elegans_, always a brightly coloured lichen, changes from yellow to
sealing-wax red in situations exposed to the full blaze of the sun.
_Haematomma ventosum,_though greenish-yellow in lowland situations
is intensely yellow in the high Alps. The same variation of colour
is characteristic of _Rhizocarpon geographicum_ which is a bright
citron-yellow at high altitudes, and becomes more greenish in hue as it
nears the plains. The familiar foliose lichen _Xanthoria parietina_ is
a brilliant orange-yellow in sunny situations, but grey-green in the
shade, and then yielding only minute quantities of parietin. West[827]
and others have noted its more luxuriant growth and brighter colour when
it grows in positions where nitrogenous food is plentiful, such as the
roofs of farm-buildings, which are supplied with manure-laden dust, and
boulders by the sea-shore frequented by birds.

_e._ DISTRIBUTION OF ACIDS. Some acids, so far as is known, are only
to be found in one or at most in very few lichens, as for instance
cuspidatic acid which is present in _Ramalina cuspidata_, and scopuloric
acid, a constituent of _Ramalina scopulorum_, the acids having been held
to distinguish by their reactions the one plant from the other.

Others of these peculiar products are abundant and widely distributed.
Usninic acid, one of the commonest, has been determined in some 70
species belonging to widely diverse genera, and atranorin, a substance
first discovered in _Lecanora atra_, has been found again many times;
Zopf gives a list of about 73 species or varieties from which it has been
extracted. Another widely distributed acid is salazinic acid which has
been found by Lettau[828] in a very large number of lichens.


E. CHEMICAL GROUPING OF LICHEN-ACIDS

Most of these acids have been provisionally arranged by Zopf in groups
under the two great organic series: I. The Fat series; and II. The
Benzole or Aromatic series.


I. LICHEN-ACIDS OF THE FAT SERIES

=Group 1.= Colourless substances soluble in alkali, the solution not
coloured by iron chloride. Exs. protolichesterinic acid (C₁₉H₃₄O₄)
obtained from species of _Cetraria_, and roccellic acid (C₁₇H₃₂O₄) from
species of _Roccella_, from _Lecanora tartarea_, etc.

=Group 2.= Neutral colourless substances insoluble in alkalies, but
soluble in alcohol, the solution not coloured by iron chloride.
_Exs._ zeorin (C₅₂H₈₈O₄), a product of widely diverse lichens, such
as _Lecanora_ (_Zeora_) _sulphurea_, _Haematomma coccineum_, _Physcia
caesia_, _Cladonia deformis_, etc. and barbatin (C₉H₁₄O), a product of
_Usnea barbata_.

=Group 3.= Brightly coloured acids, yellow, orange or red, all
derivatives of pulvinic acid (C₁₈H₁₂O₅), a laboratory compound which has
not been found in nature. The group includes among others vulpinic acid
(C₁₉H₁₄O₅) from the brilliant yellow _Evernia_ (_Letharia_) _vulpina_,
stictaurin (C₃₆H₂₂O₉) deposited in orange-red crystals on the hyphae of
_Sticta aurata_, and rhizocarpic acid (C₂₆H₂₀O₆) chiefly obtained from
_Rhizocarpon geographicum_: it crystallizes out in slender citron-yellow
prisms.

=Group 4.= Only one acid, usninic (C₁₈H₁₆O₇), a derivative of
acetylacetic acid, is placed in this group. It is of very wide-spread
occurrence, having been found in at least 70 species belonging to very
different genera and families of crustaceous shrubby and leafy lichens.
Zopf himself isolated it from 48 species.

=Group 5.= The thiophaninic acid (C₁₂H₆O₉) group representing only a
small number. They are all sulphur-yellow in colour and soluble in
alcohol, the solution becoming blackish-green or dirty blue on the
addition of iron chloride, with one exception, that of subauriferin
obtained from the yellow-coloured medulla of _Parmelia subaurifera_ which
stains faintly wine-red in an iron solution. Thiophaninic acid, which
gives its name to this group, occurs in _Pertusaria lutescens_ and _P.
Wulfenii_, both of which are yellowish crustaceous lichens growing mostly
on the trunks of trees.


II. LICHEN-ACIDS OF THE BENZOLE SERIES

The larger number of lichen-acids belong to this series, of which 94 at
least are already known. They are divided into two subseries: I. Orcine
derivatives, and II. Anthracene derivatives.


SUBSERIES I. ORCINE DERIVATIVES

Zopf specially insists that the grouping of this series must be regarded
as only a provisional arrangement of the many lichen-acids that are
included therein. All of them are split up into orcine and carbonic acid
by ammonia and other alkalies. On exposure to air, the ammoniacal or
alkaline solution changes gradually into orceine, the colouring principle
and chief constituent of commercial orchil. Orcine is not found free in
nature. The orcine subseries includes five groups:

=Group 1.= The substances in this group form, with hypochlorite of lime
(“CaCl”), red-coloured compounds which yield, on splitting, orsellinic
acid. Zopf enumerates seven acids as belonging to this group, among which
is lecanoric acid (C₁₆H₁₄O₇), found in many different lichens, _e.g._
_Roccella tinctoria_, _Lecanora_ tartarea, etc.: whenever there is a
differentiated pith and cortex it occurs in the pith alone. Erythrin
(C₂₀H₂₂O₁₀), a constituent of the British marine lichen _Roccella
fuciformis_, also belongs to this orsellinic group.

=Group 2.= Substances which also form red products with CaCl, but do not
break up into orsellinic acid. Among the most noteworthy are olivetoric
acid (C₂₁H₂₆O₇), a constituent of _Evernia furfuracea_, perlatic acid
(C₂₈H₃₀O₁₀) and glabratic acid (C₂₄H₂₆O₁₁), which are obtained from
species of _Parmelia_.

=Group 3.= Contains three acids of somewhat restricted occurrence. They
do not form red products with CaCl, and they yield on splitting everninic
acid. They are: evernic acid (C₁₇H₁₆O₇), found in _Evernia prunastri_
var. _vulgaris_, ramalic acid (C₁₇H₁₆O₇) in _Ramalina pollinaria_, and
umbilicaric acid (C₂₅H₂₂O₁₁) in species of _Gyrophora_.

=Group 4.= The numerous acids of this group are not easily soluble and
have a very bitter taste. They are not coloured by CaCl; when extracted
with concentrated sulphuric acid, the solution obtained is reddish-yellow
or deep red. Among the most frequent are fumarprotocetraric acid
(C₆₂H₅₀O₃₅), the bitter principle of _Cetraria islandica_, _Cladonia
rangiferina_, etc., psoromic acid (C₂₀H₁₄O₉), obtained from _Alectoria
implexa_, _Lecanora varia_, _Cladonia pyxidata_ and many other lichens,
and salazinic acid (C₁₉H₁₄O₁₀), recorded by Zopf as occurring in
_Stereocaulon salazinum_ and in several _Parmeliae_, but now found by
Lettau[829] to be very wide-spread. He used micro-chemical methods and
detected its presence in 72 species from twelve different families. The
distribution of the acid in the thallus varies considerably.

=Group 5.= This is called the atranorin group from one of the most
important members. They are colourless substances and, like the preceding
group, are not affected by CaCl, but when split they form bodies that
colour a more or less deep red with that reagent. Atranorin (C₁₉H₁₈O₈)
is one of the most widely spread of all lichen-acids; it occurs in
Lecanoraceae, Parmeliaceae, Physciaceae and Lecideaceae. Barbatinic acid
(C₁₉H₂₀O₇), another member, is found in _Usnea ceratina_, _Alectoria
ochroleuca_ and in a variety of _Rhizocarpon geographicum_. A very large
number of acids more or less fully studied belong to this group.


SUBSERIES II. ANTHRACENE DERIVATIVES

The constituents of this subseries are derived from the carbohydrate
anthracene, and are characterized by their brilliant colours, yellow,
red, brown, red-brown or violet-brown. So far, only ten different kinds
have been isolated and studied. Parietin[830] (C₁₆H₁₂O₅), one of the best
known, has been extracted from _Xanthoria parietina_, _Placodium murorum_
and several other bright-yellow lichens; solorinic acid (C₁₅H₁₄O₅)
occurs in orange-red crystals on the hyphae of the pith and under surface
of _Solorina crocea_; nephromin (C₁₆H₁₂O₆) is found in the yellow medulla
of _Nephromium lusitanicum_; rhodocladonic acid (C₁₂H₈O₆ or C₁₄H₁₀O₇) is
the red substance in the apothecia of the red-fruited _Cladoniae_.

There are, in addition, a short series of coloured substances which
are of uncertain position. They are imperfectly known and are of rare
occurrence. An acid containing nitrogen has been extracted from _Roccella
fuciformis_, and named picroroccellin[831] (C₂₇H₂₉N₃O₅). It crystallizes
in comparatively large prisms, has an exceedingly bitter taste, and is
very sparingly soluble. It is the only lichen-acid in which nitrogen has
been detected.

One acid at least, belonging to the Fat series, vulpinic acid, which
gives the greenish-yellow colour to _Letharia vulpina_, has been prepared
synthetically by Volkard[832].


F. CHEMICAL REAGENTS AS TESTS FOR LICHENS

The employment of chemical reagents as colour tests in the determination
of lichen species was recommended by Nylander[833] in a paper published
by him in 1866. Many acids had already been extracted and examined, and
as they were proved to be constant in the different species where they
occurred, he perceived their systematic importance. As an example of
the new tests, he cited the use of hypochlorite of lime, a solution of
which, applied directly to the thallus of species of _Roccella_, produced
a bright-red “erythrinic” reaction. Caustic potash was also found to
be of service in demonstrating the presence of parietin in lichens by
a beautiful purple stain. Many lichenologists eagerly adopted the new
method, as a sure and ready means of distinguishing doubtful species; but
others have rejected the tests as unnecessary and not always to be relied
on, seeing that the acids are not always produced in sufficient abundance
to give the desired reaction, and that they tend to alter in time.

The reagents most commonly in use are caustic potash, generally indicated
by K; hypochlorite of calcium or bleaching powder by CaCl; and a solution
of iodine by I. The sign + signifies a colour reaction, while- indicates
that no change has followed the application of the test solution. Double
signs ⁺₊ or any similar variation indicate the upper or lower parts of
the thallus affected by the reagent. In some instances the reaction only
follows after the employment of two reagents represented thus: K(CaCl)+.
In such a case the potash breaks up the particular acid and compounds are
formed which become red, orange, etc., on the subsequent application of
hypochlorite of lime.

As an instance of the value of chemical tests, Zopf cites the reaction
of hypochlorite of lime on the thallus of four different species of
_Gyrophora_, the “tripe de roche”:—

  Gyrophora torrefacta CaCl ⁻₊.
     ”      polyrhiza CaCl ⁺₊.
     ”      proboscidea CaCl ⁺₋.
     ”      erosa CaCl ⁻₋.

It must however be borne in mind that these species are well
differentiated and can be recognized, without difficulty, by their
morphological characters. Experienced systematists like Weddell refuse
to accept the tests unless they are supported by true morphological
distinctions, as the reactions are not sufficiently constant.


G. CHEMICAL REACTIONS IN NATURE

Similar colour changes may often be observed in nature. The acids of
the exposed thallus cortex are not unfrequently split up by the gradual
action of the ammonia in the atmosphere, one of the compounds thus set
free being at the same time coloured by the alkali. Thus salazinic acid,
a constituent of several of our native _Parmeliae_, is broken up into
carbonic acid and salazininic acid, the latter taking a red colour.
Fumarprotocetraric acid is acted on somewhat similarly, and the red
colour may be seen in _Cetraria_ at the base of the thallus where contact
with soil containing ammonia has affected the outer cortex of the plant.
The same results are produced still more effectively when the lichen
comes into contact with animal excrement.

Gummy exudations from trees which are more or less ammoniacal may also
act on the thallus and form red-coloured products on contact with the
acids present. _Lecanora_ (_Aspicilia_) _cinerea_ is so easily affected
by alkalies that a thin section left exposed may become red in time owing
to the ammonia in the atmosphere.


II. GENERAL NUTRITION


A. ABSORPTION OF WATER

Lichens are capable of enduring almost complete desiccation, but though
they can exist with little injury through long periods of drought,
water is essential to active metabolism. They possess no special organs
for water conduction, but absorb moisture over their whole surface.
Several interdependent factors must therefore be taken into account in
considering the question of absorption: the type of thallus, whether
gelatinous or non-gelatinous, crustaceous, foliose or fruticose, as
also the nature of the substratum and the prevailing condition of the
atmosphere.

_a._ GELATINOUS LICHENS. The algal constituent of these lichens is some
member of the Myxophyceae and is provided with thick gelatinous walls
which have great power of imbibition and swell up enormously in damp
surroundings, becoming reservoirs of water. Species of _Collema_, for
instance, when thoroughly wet, weigh thirty-five times more than when
dry[834]. There are no interstices in the thallus and frequently no
cortex in these lichens, but the gelatinous substance itself forms on
drying an outer skin that checks evaporation so that water is retained
within the thallus for a longer period than in non-gelatinous forms.
They probably always retain some amount of moisture, as they share with
gelatinous algae the power of revival after long desiccation.

Gelatinous lichens are entirely dependent on a surface supply of water:
their hyphae—or rhizinae when present—rarely penetrate the substratum.

_b._ CRUSTACEOUS NON-GELATINOUS LICHENS. The lichens with this type
of thallus are in intimate contact with the substratum whether it be
soil, rock, tree or dead wood. The hyphae on the under surface of the
thallus function primarily as hold-fasts, but if water be retained in
the substratum, the lichen will undoubtedly benefit, and water, to some
extent, will be absorbed by the walls of the hyphae or will be drawn up
by capillary attraction. In any case, it could only be surface water
that would be available, as lichens have no means of tapping any deeper
sources of supply.

Lichens are, however, largely independent of the substratum for their
supply of water. Sievers[835], who gave attention to the subject, found
that though some few crustaceous lichens took up water from below, most
of them absorbed the necessary moisture on the surface or at the edges of
the thallus or areolae, where the tissue is looser and more permeable.
The swollen gelatinous walls of the hyphae forming the upper layers of
such lichens are admirably adapted for the reception and storage of
water, though, according to Zukal[836], less hygroscopic generally than
in the larger forms. Beckmann[837] proved this power of absorption,
possessed by the upper cortex, by placing a crustaceous lichen,
_Haematomma_ sp., in a damp chamber: he found after a while that water
had been taken up by the cortex and by the gonidial zone, while the lower
medullary hyphae had remained dry.

Herre[838] has recorded an astonishing abundance of lichens from the
desert of Reno, Nevada, and these are mostly crustaceous forms, belonging
to a limited number of species. The yearly rainfall of the region is only
about eight or ten inches, and occurs during the winter months, chiefly
as snow. It is during that period that active vegetation goes on; but
the plants still manage to exist during the long arid summer, when their
only possible water supply is that obtained from the moisture of the
atmosphere during the night, or from the surface deposit of dews.

_c._ FOLIOSE LICHENS. Though many of the leafy lichens are provided with
a tomentum of single hyphae, or with rhizinae on the under surface, the
principal function of these structures is that of attaching the thallus.
Sievers[839] tested the areas of absorption by placing pieces of the
thallus of _Parmeliae_, of _Evernia furfuracea_, and of _Cetraria glauca_
in a staining solution. After washing and cutting sections, it was seen
that the coloured fluid had penetrated by the upper surface and by the
edge of the thallus, as in crustaceous forms, but not through the lower
cortex.

By the same methods of testing, he proved that water penetrates not only
by capillarity between the closely packed hyphae, but also within the
cells. A considerable number of lichens were used for experiment, and
great variations were found to exist in the way in which water was taken
up. It has been proved that in some species of _Gyrophora_ water is
absorbed from below: in those in which rhizinae are abundant, water is
held by them and so gradually drawn up into the thallus; the upper cortex
in this genus is very thick and checks transpiration. Certain other
northern lichens such as _Cetraria islandica_, _Cladonia rangiferina_,
etc., imbibe water very slowly, and they, as well as _Gyrophora_, are
able to endure prolonged wet periods.

That foliose lichens do not normally contain much water was proved by
Jumelle[840] who compared the weight of seven different species when
freshly gathered, and after being dried; he found that the proportion
of fresh weight to dry weight showed least variation in _Parmelia
acetabulum_, as 1·14 to 1; in _Xanthoria parietina_ it was as 1·21 to 1.

_d._ FRUTICOSE LICHENS. There is no water-conducting tissue in the
elongate thallus of the shrubby or filamentous lichens, as can easily
be tested by placing the base in water: it will then be seen that the
submerged parts alone are affected. Many lichens are hygroscopic and
become water-logged when placed simply in damp surroundings. The thallus
of _Usnea_, for instance, can absorb many times its weight of water:
a mass of _Usnea_ filaments that weighed 3·8 grms. when dry increased
to 13·3 grms. after having been soaked in water for twelve hours.
Schrenk[841], who made the experiment, records in a second instance an
increase in weight from 3·97 grms. to 11·18 grms. The _Cladoniae_ retain
large quantities of water in their upright hollow podetia. The Australian
species, _Cladonia retepora_, the podetium of which is a regular network
of holes, competes with the _Sphagnum_ moss in its capacity to take up
water.

To conclude: as a rule, heteromerous, non-gelatinous lichens do not
contain large quantities of water, the weight of fresh plants being
generally about three times only that of the dry weight. Their ordinary
water content is indeed smaller than that of most other plants, though
it varies at once with a change in external conditions. It is noteworthy
that a number of lichens have their habitat on the sea-shore, constantly
subject to spray from the waves, but scarcely any can exist within the
spray of a waterfall, possibly because the latter is never-ceasing.


B. STORAGE OF WATER

The gonidial algae _Gloeocapsa_, _Scytonema_, _Nostoc_, etc. among
Myxophyceae, _Palmella_ and occasionally _Trentepohlia_ among
Chlorophyceae, have more or less gelatinous walls which act as a natural
reservoir of water for the lichens with which they are associated.
In these lichens the hyphae for the most part have thin walls, and
the plectenchyma when formed—as below the apothecium in _Collema
granuliferum_, or as a cortical layer in _Leptogium_—is a thin-walled
tissue. In lichens where, on the contrary, the alga is non-gelatinous—as
generally in Chlorophyceae—or where the gelatinous sheath is not formed
as in the altered _Nostoc_ of the _Peltigera_ thallus, the fungal hyphae
have swollen gelatinous walls both in the pith and the cortex, and not
only imbibe but store up water.

Bonnier[842] had his attention directed to this thickening of the
cell-walls as he followed the development of the lichen thallus. He made
cultures from the ascospore of _Physcia_ (_Xanthoria_) _parietina_ and
obtained a fair amount of hyphal tissue, the cell-walls of which became
thickened, but more slowly and to a much less extent than when associated
with the gonidia.

He noted also that when his cultures were kept in a continuously moist
atmosphere there was much less thickening, scarcely more than in fungi
ordinarily; it was only when they were grown under drier conditions
with necessity for storage, that any considerable swelling of the walls
took place. Further he found that the thallus of forms cultivated in
an abundance of moisture could not resist desiccation as could those
with the thicker membranes. These latter survived drying up and resumed
activity when moisture was supplied.


C. SUPPLY OF INORGANIC FOOD

As in the higher plants, mineral substances can only be taken up when
they are in a state of solution. Lichens are therefore dependent on the
substances that are contained in the water of absorption: they must
receive their inorganic nutriment by the same channels that water is
conveyed to them.

_a._ FOLIOSE AND FRUTICOSE LICHENS. These larger lichens are provided
with rhizinae or with hold-fasts, which are only absorptive to a very
limited extent; the main source of water supply is from the atmosphere
and the salts required in the metabolism of the cell must be obtained
there also—from atmospheric dust dissolved in rain, or from wind-borne
particles deposited on the surface of the thallus which may be
gradually dissolved and absorbed by the cortical and growing hyphae.
That substances received from the atmospheric environment may be all
important is shown by the exclusive habitat of some marine lichens; the
_Roccellae_, _Lichinae_, some species of _Ramalina_ and others which
grow only on rocky shores are almost as dependent on sea-water as are
the submerged algae. Other lichens, such as _Hydrothyria venosa_ and
_Lecanora lacustris_, grow in streams, or on boulders that are subject to
constant inundation, and they obtain their inorganic food mainly, if not
entirely, from an aqueous medium.

Though lichens cannot live in an atmosphere polluted by smoke, they
thrive on trees and walls by the road-side where they are liable to be
almost smothered by soil-dust. West[843] has observed that they flourish
in valleys that are swept by moisture laden winds more especially if
near to a highway, where animal excreta are mingled with the dust. The
favourite habitats of _Xanthoria parietina_ are the walls and roofs
of farm-buildings where the dust must contain a large percentage of
nitrogenous material; or stones by the sea-shore that are the haunts
of sea-birds. Sandstede[844] found on the island of Rügen that while
the perpendicular faces of the cliffs were quite bare, the tops bore
a plentiful crop of _Lecanora saxicola_, _Xanthoria lychnea_ and
_Candellariella vitellina_. He attributed their selection of habitat
to the presence of the excreta of sea-birds. As already stated the
connection of foliose and fruticose lichens with the substratum is
mainly mechanical but occasionally a kind of semiparasitism may arise.
Friedrich[845] gives an instance in a species of _Usnea_ of unusually
vigorous development. It grew on bark and the strands of hyphae,
branching from the root-base of the lichen, had reached down to the
living tissue of the tree-trunk and had penetrated between the cells by
dissolving the middle lamella. It was possible to find holes pierced
in the cell-walls of the host, but it was difficult to decide if the
hyphae had attacked living cells or were merely preying on dead material.
Lindau[846] held very strongly that lichen hyphae were non-parasitic, and
merely split apart the tissues already dead, and the instance recorded by
Friedrich is of rare occurrence[847].

That the substratum does have some indirect influence on these larger
lichens has been proved once and again. Uloth[848], a chemist as well as
a botanist, made analyses of plants of _Evernia prunastri_ taken from
birch bark and from sandstone. Qualitatively the composition of the
lichen substances was the same, but the quantities varied considerably.
Zopf[849] has, more recently, compared the acid content of a form of
_Evernia furfuracea_ on rock with that of the same species growing on
the bark of a tree. In the case of the latter, the thallus produced 4
per cent. of physodic acid and 2·2 per cent. of atranorin. In the rock
specimen, which, he adds, was a more graceful plant than the other, the
quantities were 6 per cent. of physodic acid, and 2·75 per cent. of
atranorin. In both cases there was a slight formation of furfuracinnic
acid. He found also that specimens of _Evernia prunastri_ on dead wood
contained 8·4 per cent. of lichen-acids, while in those from living trees
there was only 4·4 per cent. or even less. Other conditions, however,
might have contributed to this result, as Zopf[850] found later that this
lichen when very sorediate yielded an increased supply of atranoric acid.

Ohlert[851], who made a study of lichens in relation to their habitat,
found that though a certain number grew more or less freely on either
tree, rock or soil, none of them was entirely unaffected. _Usnea
barbata_, _Evernia prunastri_ and _Parmelia physodes_ were the most
indifferent to habitat; normally they are corticolous species, but
_Usnea_ on soil formed more slender filaments, and _Evernia_ on the same
substratum showed a tendency to horizontal growth, and became attached at
various points instead of by the usual single base.

_b._ CRUSTACEOUS LICHENS. The crustaceous forms on rocks are in a more
favourable position for obtaining inorganic salts, the lower medullary
hyphae being in direct contact with mineral substances and able to
act directly on them. Many species are largely or even exclusively
calcicolous, and there must be something in the lime that is especially
conducive to their growth. The hyphae have been traced into the limestone
to a depth of 15 mm.[852] and small depressions are frequently scooped
out of the rock by the action of the lichen, thus giving a lodgement to
the foveolate fruit.

On rocks mainly composed of silica, the lichen has a much harder
substance to deal with, and one less easily affected by acids, though
even silica may be dissolved in time. Uloth[853] concluded from his
observations that the relation of plants to the substratum was chemical
even more than physical, so far as crustaceous species were concerned.
He found that the surface of the area of rock inhabited was distinctly
marked: even such a hard substance as chalcedony was corroded by a
very luxuriant lichen flora, the border of growth being quite clearly
outlined. The corrosive action is due he considered to the carbon dioxide
liberated by the plant, though oxalic acid, so frequent a constituent
of lichens, may also share in the corrosion. Egeling[854] made similar
observations in regard to the effect of lichen growth on granite rocks;
and he further noticed that pieces of glass, over which lichens had
spread, had become clouded, the dulness of the surface being due to a
multitude of small cracks eaten out by the hyphae. Buchet[855] also
gives an instance of glass which had been corroded by the action of
lichen hyphae. It formed part of an old stained window in a chapel
that was obscured by a lichen growth which adhered tenaciously. When
the window was taken down and cleaned, it was found that the surface
of the glass was covered with small, more or less hemispherical pits
which were often confluent. The different colours in the picture were
unequally attacked, some of the figures or draperies being covered with
the minute excavations, while other parts were intact. It happened also,
occasionally, that a colour while slightly corroded in one pane would
be uninjured in another, but the suggestion is made that there might in
that case have been a difference in the length of attack by the lichen.
The selection of colours by the lichens might also be influenced by some
chemical or physical characters.

Bachmann[856] found that on granite there is equally a selection
of material by the hyphae: as a rule they avoid the acid silica
constituents; while they penetrate and traverse the grains of mica which
are dissolved by them exactly as are lime granules.

On another rock consisting mainly of muscovite and quartz he[857] found
that crystals of garnet embedded in the rock were reduced to a powder
by the action of the lichen. He concludes that the destroying action of
the hyphae is accelerated by the presence of carbon dioxide given off
by the lichen, and dissolved in the surrounding moisture. Lang[858] and
Stahlecker[859] have both come to the conclusion that even the quartz
grains are corroded by the lichen hyphae. Stahlecker finds that they
change the quartz into amorphous silicic acid, and thus bring it into
the cycle of organic life. Chalk and magnesia are extracted from the
silicates where no other plant could procure them. Lichens are generally
rare on pure quartz rocks, chiefly, however, for the mechanical reason
that the structure is of too close a grain to afford a foothold.


D. SUPPLY OF ORGANIC FOOD

_a._ FROM THE SUBSTRATUM. The Ascomycetous fungi, from which so many
of the lichens are descended, are mainly saprophytes, obtaining their
carbohydrates from dead plant material, and lichen hyphae have in some
instances undoubtedly retained their saprophytic capacity. It has been
proved that lichen hyphae, which naturally could not exist without the
algal symbiont, may be artificially cultivated on nutrient media without
the presence of gonidia, though the chief and often the only source of
carbon supply is normally through the alga with which the hyphae are
associated in symbiotic union.

A large number of crustaceous lichens grow on the bark of trees, and
their hyphae burrow among the dead cells of the outer bark using up the
material with which they come in contact. Others live on dead wood,
palings, etc. where the supply of disintegrated organic substance is even
greater; or they spread over withered mosses and soil rich in humus.

_b._ FROM OTHER LICHENS. Bitter[860] has recorded several instances
observed by him of lichens growing over other lichens and using up their
substance as food material. Some lichens are naturally more vigorous than
others, and the weaker or more slow growing succumb when an encounter
takes place. _Pertusaria globulifera_ is one of these marauding species;
its habitat is among mosses on the bark of trees, and, being a quick
grower, it easily overspreads its more sluggish neighbours. It can
scarcely be considered a parasite, as the thallus of the victim is first
killed, probably by the action of an enzyme.

_Lecanora subfusca_ and allied species which have a thin thallus are
frequently overgrown by this _Pertusaria_ and a dark line generally
precedes the invading lichen; the hyphae and the gonidia of the
_Lecanorae_ are first killed and changed to a brown structureless mass
which is then split up by the advancing hyphae of the _Pertusaria_ into
small portions. A little way back from the edge of the predatory thallus
the dead particles are no longer visible, having been dissolved and
completely used up. _Pertusaria amara_ also may overgrow _Lecanorae_,
though, generally, its onward course is checked and deflected towards a
lateral direction; if however it is in a young and vigorous condition, it
attacks the thallus in its path, and ahead of it appears the rather broad
blackish line marking the fatal effect of the enzyme, the rest of the
host thallus being unaffected. Neither _Pertusaria_ seems to profit much,
and does not grow either faster or thicker; the thallus appears indeed to
be hindered rather than helped by the encounter. _Biatora_ (_Lecidea_)
_quernea_ with a looser, more furfuraceous thallus is also killed and
dissolved by _Pertusariae_; but if the _Biatora_ is growing near to a
withering or dead lichen it, also, profits by the food material at hand,
grows over it and uses it up. Bitter has also observed lichens overgrown
by _Haematomma_ sp.; the growth of that lichen is indeed so rapid that
few others can withstand its approach.

Another common rock species, _Lecanora sordida_ (_L. glaucoma_), has
a vigorous thallus that easily ousts its neighbours. _Rhizocarpon
geographicum_, a slow-growing species, is especially liable to be
attacked; from the thallus of _L. sordida_ the hyphae in strands push
directly into the other lichen in a horizontal direction and split up the
tissues, the algae persist unharmed for some time, but eventually they
succumb and are used up; the apothecia, though more resistant than the
thallus, are also gradually undermined and hoisted up by the new growth,
till finally no trace of the original lichen is left. _Lecanora sordida_
is however in turn invaded by _Lecidea insularis_ (_L. intumescens_)
which is found forming small orbicular areas on the _Lecanora_ thallus.
It kills its host in patches and the dead material mostly drifts away. On
any strands that are left _Candellariella vitellina_ generally settles
and evidently profits by the dead nutriment. It does not spread to the
living thallus. _Lecanora polytropa_ also forms colonies on these vacant
patches, with advantage to its growth.

Even the larger lichens are attacked by these quick-growing crusts.
_Pertusaria globulifera_ spreads over _Parmelia perlata_ and _P.
physodes_, gradually dissolving and consuming the different thalline
layers; the lower cortex of the victim holds out longest and can be seen
as an undigested black substance within the _Pertusaria_ thallus for some
time. As a rule, however, the lichens with large lobes grow over the
smaller thalli in a purely mechanical fashion.

_c._ FROM OTHER VEGETATION. Zukal[861] has given instances of association
between mosses and lichens in which the latter seemed to play the part of
parasite. The terricolous species _Baeomyces rufus_ (_Sphyridium_) and
_Biatora decolorans_, as well as forms of _Lepraria_ and _Variolaria_,
he found growing over mosses and killing them. Stems and leaves of the
moss _Plagiothecium sylvaticum_ were grown through and through by the
hyphae of a _Pertusaria_, and he observed a leaf of _Polytrichum commune_
pierced by the rhizinae of a minute _Cladonia_ squamule. The cells had
been invaded and the neighbouring tissue was brown and dead.

Perhaps the most voracious consumer of organic remains is _Lecanora
tartarea_, more especially the northern form _frigida_. It is the
well-known cudbear lichen of West Scotland, and is normally a rock
species. It has an extremely vigorous thickly crustaceous and
quick-growing thallus, and spreads over everything that lies in its
path—decaying mosses, dead leaves, other lichens, etc. Kihlman[862] has
furnished a graphic description of the way it covers up the vegetation
on the high altitudes of Russian Lapland. More than any other plant it
is able to withstand the effect of the cold winds that sweep across
these inhospitable plains. Other plant groups at certain seasons or in
certain stages of growth are weakened or killed by the extreme cold of
the wind, and, immediately, a growth of the more hardy grey crust of
_Lecanora tartarea_ begins to spread over and take possession of the area
affected—very frequently a bank of mosses, of which the tips have been
destroyed, is thus covered up. In the same way the moorland _Cladoniae_,
_C. rangiferina_ (the reindeer moss) and some allied species, are
attacked. They have no continuous cortex, the outer covering of the long
branching podetia being a loose felt of hyphae; they are thus sensitive
to cold and liable to be destroyed by a high wind, and their stems,
which are blackened as decay advances, become very soon dotted with the
whitish-grey crust of the more vigorous and resistant _Lecanora_.


III. ASSIMILATION AND RESPIRATION


A. INFLUENCE OF TEMPERATURE

_a._ HIGH TEMPERATURE. It has been proved that plants without chlorophyll
are less affected by great heat than those that contain chlorophyll.
Lichens in which both types are present are more capable of enduring
high temperatures than the higher plants, but with undue heat the alga
succumbs first. In consequence, respiration, by the fungus alone, can go
on after assimilation (photosynthesis) and respiration in the alga have
ceased.

Most Phanerogams cease assimilation and respiration after being subjected
for ten minutes to a temperature of 50° C. Jumelle[863] made a series of
experiments with lichens, chiefly of the larger fruticose or foliaceous
types, with species of _Ramalina_, _Physcia_ and _Parmelia_, also with
_Evernia prunastri_ and _Cladonia rangiferina_. He found that as regards
respiration, plants which had been kept for three days at 45° C., fifteen
hours at 50°, then five hours at 60°, showed an intensity of respiration
almost equal to untreated specimens, gaseous interchange being manifested
by an absorption of oxygen and a giving up of carbon dioxide.

The power of assimilation was more quickly destroyed: as a rule it
failed after the plants had been subjected successively to a temperature
of one day at 45° C., then three hours at 50° and half-an-hour at 60°.
The assimilating green alga, being less able to resist extreme heat, as
already stated, succumbed more quickly than the fungus. Jumelle also
gives the record of an experiment with a crustaceous lichen, _Lecidea_
(_Lecanora_) _sulphurea_, a rock species. It was kept in a chamber heated
to 50° for three hours and when subsequently placed in the sunlight
respiration took place but no assimilation.

Very high temperatures may be endured by lichen plants in quite natural
conditions, when the rock or stone on which they grow becomes heated by
the sun. Zopf[864] tested the thalli of crustaceous lichens in a hot
June, under direct sunlight, and found that the thermometer registered
55° C.

_b._ LOW TEMPERATURE. Lichens support extreme cold even better than
extreme heat. In both cases it is the power of drying up and entering
at any season into a condition of lowered or latent vitality that
enables them to do so. In winter during a spell of severe cold they are
generally in a state of desiccation, though that is not always the case,
and resistance to cold is not due to their dry condition. The water of
imbibition is stored in the cell-walls and it has been found that lichens
when thus charged with moisture are able to resist low temperatures, even
down to -40° C. or -50° as well as when they are dry. Respiration in that
case was proved by Jumelle[865] to continue to -10°, but assimilation
was still possible at a temperature of -40°: _Evernia prunastri_ exposed
to that extreme degree of cold, but in the presence of light, decomposed
carbon dioxide and gave off oxygen.


B. INFLUENCE OF MOISTURE

_a._ ON VITAL FUNCTIONS. Gaseous interchange has been found to vary
according to the degree of humidity present[865]. In lichens growing in
sheltered positions, or on soil, there is less complete desiccation, and
assimilation and respiration may be only enfeebled. Lichens more exposed
to the air—those growing on trees, etc.—dry almost completely and gaseous
interchange may be no longer appreciable. In severe cold any water
present would become frozen and the same effect of desiccation would be
produced. At normal temperatures, on the addition of even a small amount
of moisture the respiratory and assimilative functions at once become
active, and to an increasing degree as the plant is further supplied with
water until a certain optimum is reached, after which the vital processes
begin somewhat to diminish.

Though able to exist with very little moisture, lichens do not endure
desiccation indefinitely, and both assimilation and respiration probably
cease entirely during very dry seasons. A specimen of _Cladonia
rangiferina_ was kept dry for three months, and then moistened:
respiration followed but it was very feeble and assimilation had almost
entirely ceased. Somewhat similar results were obtained with _Ramalina
farinacea_ and _Usnea barbata_.

In normal conditions of moisture, and with normal illumination,
assimilation in lichens predominates over respiration, more carbon
dioxide being decomposed than is given forth; and Jumelle has argued from
that fact, that the alga is well able to secure from the atmosphere all
the carbon required for the nutrition of the whole plant. The intensity
of assimilation, however, varies enormously in different lichens and is
generally more powerful in the larger forms than in the crustaceous: the
latter have often an extremely scanty thallus and they are also more in
contact with the substratum—rock, humus or wood—on which they may be
partly saprophytic, thus obtaining carbohydrates already formed, and
demanding less from the alga.

An interesting comparison might be made with fungi in regard to which
many records have been taken as to their possible duration in a dry
state, more especially on the viability of spores, _i.e._ their
persistent capacity of germination. A striking instance is reported
by Weir[866] of the regeneration of the sporophores of _Polystictus
sanguineus_, a common fungus of warm countries. The plant was collected
in Brazil and sent to Munich. After about two years in the mycological
collection of the University, the branch on which it grew was exposed
in the open among other branches in a wood while snow still lay on the
ground. In a short time the fungus revived and before the end of spring
not only had produced a new hymenium, but enlarged its hymenial surface
to about one-fourth of its original size and had also formed one entirely
new, though small, sporophore.

_b._ ON GENERAL DEVELOPMENT. Lichens are very strongly influenced by
abundance or by lack of moisture. The contour of the large majority
of species is concentric, but they become excentric owing to a more
vigorous development towards the side of damper exposure, hence the
frequent one-sided increase of monophyllous species such as _Umbilicaria
pustulata_. Wainio[867] observed that species of _Cladonia_ growing
in dry places, and exposed to full sunlight, showed a tendency not to
develop scyphi, the dry conditions hindering the full formation of the
secondary thallus. As an instance may be cited _Cl. foliacea_, in which
the primary thallus is much the most abundantly developed, its favourite
habitat being the exposed sandy soil of sea-dunes.

Too great moisture is however harmful: Nienburg[868] has recorded his
observations on _Sphyridium_ (_Baeomyces rufus_): on clay soil the
thallus was pulverulent, while on stones or other dryer substratum it was
granular—warted or even somewhat squamulose.

_Parmelia physodes_ rarely forms fruits, but when growing in an
atmosphere constantly charged with moisture[869], apothecia are more
readily developed, and the same observation has been made in connection
with other usually barren lichens. It has been suggested that, in these
lichens, the abrupt change from moist to dry conditions may have a
harmful effect on the developing ascogonium.

The perithecia of _Pyrenula nitida_ are smaller on smooth bark[870] such
as that of _Corylus_, _Carpinu_s, etc., probably because the even surface
does not retain water.


IV. ILLUMINATION OF LICHENS


A. EFFECT OF LIGHT ON THE THALLUS

As fungi possess no chlorophyll, their vegetative body has little or no
use for light and often develops in partial or total darkness. In lichens
the alga requires more or less direct illumination; the lichen fungus,
therefore, in response to that requirement has come out into the open:
it is an adaptation to the symbiotic life, though some lichens, such as
those immersed in the substratum, grow with very little light. Like other
plants they are sensitive to changes of illumination: some species are
shade plants, while others are as truly sun plants, and others again are
able to adapt themselves to varying degrees of light.

Wiesner[871] made a series of exact observations on what he has
termed the “light-use” of various plants. He took as his standard of
unity for the higher plants the amount of light required to darken
photographic paper in one second. When dealing with lichens he adopted
a more arbitrary standard, calculating as the unit the average amount
of light that lichens would receive in entirely unshaded positions. He
does not take account of the strength or duration of the light, and
the conclusions he draws, though interesting and instructive, are only
comparative.

_a._ SUN LICHENS. The illumination of the Tundra lichens is reckoned by
Wiesner as representing his unit of standard illumination. In the same
category as these are included many of our most familiar lichens, which
grow on rocks subject to the direct incidence of the sun’s rays, such as,
for instance, _Parmelia conspersa_, _P. prolixa_, etc. _Physcia tenella_
(_hispida_) is also extremely dependent on light, and was never found by
Wiesner under 1/8 of full illumination. _Dermatocarpon miniatum_, a rock
lichen with a peltate foliose thallus, is at its best from 1/3 to 1/8 of
illumination, but it grows well in situations where the light varies in
amount from 1 to 1/24. _Psora_ (_Lecidea_) _lurida_, with dark-coloured
crowded squamules, grows on calcareous soil among rocks well exposed
to the sun and has an illumination from 1 to 1/30, but with a poorer
development at the lower figure. Many crustaceous rock lichens are also
by preference sun-plants as, for instance, _Verrucaria calciseda_ which
grows immersed in calcareous rocks but with an illumination of 1 to 1/3;
in more shady situations, where the light had declined to 1/29, it was
found to be less luxuriant and less healthy.

Sun lichens continue to grow in the shade, but the thallus is then
reduced and the plant is sterile. Zukal has made a list of those which
grow best with a light-use of 1 to 1/10, though they are also found not
unfrequently in habitats where the light cannot be more than 1/50. Among
these light-loving plants are the Northern Tundra species of _Cladonia_,
_Stereocaulon_, _Cetraria_, _Parmelia_, _Umbilicaria_, and _Gyrophora_,
as also _Xanthoria parietina_, _Placodium elegans_, _P. murorum_, etc.,
with some crustaceous species such as _Lecanora atra_, _Haematomma
ventosum_, _Diploschistes scruposus_, many species of Lecideaceae, some
Collemaceae and some Pyrenolichens.

Wiesner’s conclusion is that the need of light increases with the
lowering of the temperature, and that full illumination is of still more
importance in the life of the plants when they grow in cold regions and
are deprived of warmth: sun lichens are, therefore, to be looked for in
northern or Alpine regions rather than in the tropics.

_b._ COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight
frequently take on a darker hue. _Cetraria islandica_ for instance in an
open situation is darker than when growing in woods; _C. aculeata_ on
bare sand-dunes is a deeper shade of brown than when growing entangled
among heath plants. _Parmelia saxatilis_ when growing on exposed rocks is
frequently a deep brown colour, while on shaded trees it is normally a
light bluish-grey.

An example of colour-change due directly to light influences is given by
Bitter[872]. He noted that the thallus of _Parmelia obscurata_ on pine
trees, and therefore subject only to diffuse light, grew to a large size
and was of a light greyish-green colour marked by lighter-coloured lines,
the more exposed lobes being always the most deeply tinted. In a less
shaded habitat or in full sunlight the lichen was distinguished by a much
darker colour, and the lobes were seamed and marked by blackish lines and
spots. Bruce Fink[873] noted a similar development of dark lines on the
thallus of certain rock lichens growing in the desert, more especially on
_Parmelia conspersa_, _Acarospora xanthophana_ and _Lecanora muralis_. He
attributes a protective function to the dark colour and observes that it
seemingly spreads from centres of continued exposure, and is thus more
abundant in older parts of the thallus. He contrasts this colouration
with the browning of the tips of the fronds of fruticose lichens by which
the delicate growing hyphae are protected from intense light.

Galløe[874] finds that protection against too strong illumination is
afforded both by white and dark colourations, the latter because the
pigments catch the light rays, the former because it throws them back.
The white colour is also often due to interspaces filled with air which
prevent the penetration of the heat rays.

A deepening of colour due to light effect often visible on exposed
rock lichens such as _Parmelia saxatilis_ is more pronounced still in
Alpine and tropical species: the cortex becomes thicker and more opaque
through the cuticularizing and browning of the hyphal membranes, and the
massing of crystals on the lighted areas. The gonidial layer becomes,
in consequence, more reduced, and may disappear altogether. Zukal[875]
found instances of this in species of _Cladonia_, _Parmelia_, _Roccella_,
etc. The thickened cortex acts also as a check to transpiration and
is characteristic of desert species exposed to strong light and a dry
atmosphere.

Bitter[876] remarked the same difference of development in plants of
_Parmelia physodes_: he found that the better lighted had a thicker
cortex, about 20-30 µ in depth, as compared with 15-22 µ or even only 12
µ in the greener shade-plants, and also that there was a greater deposit
of acids in the more highly illuminated cortices, thus giving rise to the
deeper shades of colour.

Many lichens owe their bright tints to the presence of coloured
lichen-acids, the production of which is strongly influenced by light and
by clear air. _Xanthoria parietina_ becomes a brilliant yellow in the
sunlight: in the shade it assumes a grey-green hue and yields only small
quantities of parietin. _Placodium elegans_, normally a brightly coloured
yellow lichen, becomes, in the strong light of the high Alps, a deep
orange-red. _Rhizocarpon geographicum_ is a vivid citrine-yellow on high
mountains, but is almost green at lesser elevations.

_c._ SHADE LICHENS. Many species grow where the light is abundant though
diffuse. Those on tree-trunks rarely receive direct illumination and may
be generally included among shade-plants. Wiesner found that corticolous
forms of _Parmelia saxatilis_ grew best with an illumination between 1/8
and 1/17 of full light, and _Pertusaria amara_ from 1/12 to 1/21; both of
them could thrive from 1/3 to 1/56, but were never observed on trees in
direct light. _Physcia ciliaris_, which inhabits the trunks of old trees,
is also a plant that prefers diffuse light. In warm tropical regions,
lichens are mostly shade-plants: Wiesner records an instance of a species
found on the aerial roots of a tree with an illumination of only 1/250.

In a study of subterranean plants, Maheu[877] takes note of the lichens
that he found growing in limestone caves, in hollows and clefts of the
rocks, etc. A fair number grew well just within the opening of the
caves; but species such as _Cl. cervicornis_, _Placodium murorum_ and
_Xanthoria parietina_ ceased abruptly where the solar rays failed. Only
a few individuals of one or two species were found to remain normal in
semi-darkness: _Opegrapha hapalea_ and _Verrucaria muralis_ were found at
the bottom of a cave with the thallus only slightly reduced. The nature
of the substratum in these cases must however also be taken into account,
as well as the light influences: limestone for instance is a more
favourable habitat than gypsum; the latter, being more readily soluble,
provides a less permanent support.

Maheu has recorded observations on growth in its relation to light in the
case of a number of lichens growing in caves.

_Physcia obscura_ grew in almost total darkness; _Placodium murorum_
within the cave had lost nearly all colour; _Placodium variabile_
var. deep within the cave, sterile; _Opegrapha endoleuca_ in partial
obscurity; _Verrucaria rupestris_ f. in total obscurity, the thallus much
reduced and sterile; _Verrucaria rupestris_ in partial obscurity, the
asci empty; _Homodium_ (_Collema_) _granuliferum_ in the inmost recess of
the cave, sterile, and the hyphae more spongy than in the open.

Siliceous rocks in darkness were still more barren, but a few odd lichens
were collected from sandstone in various caves: _Cladonia squamosa_,
_Parmelia perlata_ var. _ciliata_, _Diploschistes scruposus_, _Lecidea
grisella_, _Collema nigrescens_ and _Leptogium lacerum_.

_d._ VARYING SHADE CONDITIONS. It has been frequently observed that on
the trees of open park lands lichens are more abundant on the side of
the trunk that faces the prevailing winds. Wiesner[878] remarks that
spores and soredia would more naturally be conveyed to that side; but
there are other factors that would come into play: the tree and the
branches frequently lean away from the wind, giving more light and also
an inclined surface that would retain water for a longer period on the
windward side[879]. Spores and soredia would also develop more readily in
those favourable conditions.

In forests there are other and different conditions: on the outskirts,
whether northern or southern, the plants requiring more light are to be
found on the side of the trunk towards the outside; in the depths of the
forest, light may be reduced from 1/200 to 1/300, and any lichens present
tend to become mere leprose crusts. Krempelhuber[880] has recorded among
his Bavarian lichens those species that he found constantly growing in
the shade: they are in general species of Collemaceae and Caliciaceae,
several species of _Peltigera_ (_P. venosa_, _P. horizontalis_ and _P.
polydactyla_); _Solorina saccata_; _Gyalecta Flotovii_, _G. cupularis_;
_Pannaria microphylla_, _P. triptophylla_, _P. brunnea_; _Icmadophila
aeruginosa_, etc.


B. EFFECT ON REPRODUCTIVE ORGANS

In the higher plants, it is recognized that a certain light-intensity is
necessary for the production of flowers and fruit. In the lower plants,
such as lichens, light is also necessary for reproduction; it is a
common observation that well-lighted individuals are the most abundantly
fruited. In the higher fungi also, the fruiting body is more or less
formed in the light.

_a._ POSITION AND ORIENTATION OF FRUITS WITH REGARD TO LIGHT. There is
an optimum of light for the fruits as well as for the thallus in each
species of lichen: in most cases it is the fullest light that can be
secured.

Zukal[881] finds an exception to that rule in species of _Peltigera_:
when exposed to strong sunlight, the lobes, fertile at the tips, curve
over so that to some extent the back of the apothecium is turned to
the light; with diffuse light, the horizontal position is retained and
the apothecia face upwards. In the closely allied genera _Nephroma_,
_Nephromium_ and _Nephromopsis_, the apothecia are produced on the back
of the lobe at the extreme tip, but as they approach maturity the fertile
lobes turn right back and they become exposed to direct illumination.
In a well-developed specimen the full-grown fruits may thus become so
prominent all over the thallus, that it is difficult to realize they are
on reversed lobes. In one species of _Cetraria_ (_C. cucullata_) the
rarely formed apothecia are adnate to the back of the lobe; but in that
case the margins of the strap-shaped fronds are incurved and connivent,
and the back is more exposed than the front.

In _Ramalina_ the frond frequently turns at a sharp angle at the point
of insertion of the apothecium which is thus well exposed and prominent;
but Zukal[882] sees in this formation an adaptation to enable the frond
to avoid the shade cast by the apothecium which may exceed it in width.
In most lichens, however, and especially in shade or semi-shade species,
the reproductive organs are to be found in the best-lighted positions.

_b._ INFLUENCE OF LIGHT ON COLOUR OF FRUITS. Lichen-acids are secreted
freely in the apothecium from the tips of the paraphyses which give the
colour to the disc, and as acid-formation is furthered by the sun’s rays,
the well-lighted fruits are always deeper in hue. The most familiar
examples are the bright-yellow species that are rich in chrysophanic acid
(parietin). Hedlund[883] has recorded several instances of varying colour
in species of _Micarea_ (_Biatorina_, etc.) in which very dark apothecia
became paler in the shade. He also cites the case of two crustaceous
species, _Lecidea helvola_ and _L. sulphurella_, which have white
apothecia in the shade, but are darker in colour when strongly lighted.


V. COLOUR OF LICHENS

The thalli of many lichens, more especially of those associated with
blue-green gonidia, are hygroscopic, and it frequently happens that
any addition of moisture affects the colour by causing the gelatinous
cell-walls to swell, thus rendering the tissues more transparent and the
green colour of the gonidia more evident. As a general rule it is the dry
state of the plant that is referred to in any discussion of colour.

In the large majority of species the colouring is of a subdued
tone—soft bluish-grey or ash-grey predominating. There are, however,
striking exceptions, and brilliant yellow and white thalli frequently
form a conspicuous feature of vegetation. Black lichens are rare,
but occasionally the very dark brown of foliaceous species such as
_Gyrophora_ or of crustaceous species such as _Verrucaria maura_ or
_Buellia atrata_ deepens to the more sombre hue.


A. ORIGIN OF LICHEN-COLOURING

The colours of lichens may be traced to several different causes.

_a._ COLOUR GIVEN BY THE ALGAL CONSTITUENT. As examples may be cited most
of the gelatinous lichens, Ephebaceae, Collemaceae, etc. which owe, as in
_Collema_, their dark olivaceous-green appearance, when somewhat moist,
to the enclosed dark-green gonidia, and their black colour, when dry, to
the loss of transparency. When the thallus is of a thin texture as in
_Collema nigrescens_, the olivaceous hue may remain constant. _Leptogium
Burgessii_, another thin plant of the same family, is frequently of a
purplish hue owing to the purple colour of the gonidial _Nostoc_ cells.
The dull-grey crustaceous thallus of the Pannariaceae becomes more or
less blue-green when moistened, and the same change has been observed in
the Hymenolichens, _Cora_, etc.

In _Coenogonium_, the alga is some species of _Trentepohlia_, a
filamentous genus mostly yellow, which often gives its colour to the
slender lichen filaments, the covering hyphae being very scanty. Other
filamentous species, such as _Usnea barbata_, etc., are persistently
greenish from the bright-green Protococcaceous cells lying near the
surface of the thalline strands. Many of the furfuraceous lichens are
greenish from the same cause, especially when moist, as are also the
larger lichens, _Physcia ciliaris_, Stereocaulons, Cladonias and others.

_b._ COLOUR DUE TO LICHEN-ACIDS. These substances, so characteristic of
lichens, are excreted from the hyphae, and lie in crystals on the outer
walls; they are generally most plentiful on exposed tissues such as
the cortex of the upper surface or the discs of the apothecia. Many of
these crystals are colourless and are without visible effect, except in
sometimes whitening the surface, strikingly exemplified in _Thamnolia
vermicularis_[884]; but others are very brightly coloured. These
latter belong to two chemical groups and are found in widely separated
lichens[885]:

1. Derivatives of pulvinic acid which are usually of a bright-yellow
colour. They are the colouring substance of _Letharia vulpina_, a
northern species, not found in our islands, of _Cetraria pinastri_ and
_C. juniperina_[886] which inhabit mountainous or hilly regions. The
crustaceous species, _Lecidea lucida_ and _Rhizocarpon geographicum_, owe
their colour to rhizocarpic acid.

The brilliant yellow of the crusts of some species of Caliciaceae is due
to the presence of the substance calycin, while coniocybic acid gives
the greenish sulphur-yellow hue to _Coniocybe furfuracea_. Epanorin
colours the hyphae and soredia of _Lecanora epanora_ a citrine-yellow and
stictaurin is the deep-yellow substance found in the medulla and under
surface of _Sticta aurata_ and _S. crocata_.

2. The second series of yellow acids are derivatives of anthracene. They
include parietin, formerly described as chrysophanic acid, which gives
the conspicuous colour to _Xanthoriae_ and to various wall lichens;
solorinic acid, the crystals of which cover the medullary hyphae and
give a reddish-grey tone to the upper cortex of _Solorina crocea_, and
nephromin which similarly colours the medulla of _Nephromium lusitanicum_
a deep yellow, the colour of the general thallus being, however, scarcely
affected. In this group must also be included the acids that cause
the yellow colouring of the medulla in _Parmelia subaurifera_ and the
yellowish thallus of some _Pertusariae_.

In many cases, changes in the normal colouring[887] are caused by the
breaking up of the acids on contact with atmospheric or soil ammonia.
Alkaline salts are thus formed which may be oxidized by the oxygen in
the air to yellow, red, brown, violet-brown or even to entirely black
humus-like products which are insoluble in water. These latter substances
are frequently to be found at the base of shrubby lichens or on the under
surface of leafy forms that are closely appressed to the substratum.

_c._ COLOUR DUE TO AMORPHOUS SUBSTANCES. These are the various pigments
which are deposited in the cell-walls of the hyphae. The only instance,
so far as is known, of colours within the cell occurs in _Baeomyces
roseus_, in which species the apothecia owe their rose-colour to
oil-drops in the cells of the paraphyses, and in _Lecidea coarctata_
where the spores are rose-coloured when young. In a few instances the
colouring matter is excreted (_Arthonia gregaria_ and _Diploschistes
ocellatus_); but Bachmann[888], who has made an extended study of this
subject and has examined 120 widely diversified lichens, found that with
few exceptions the pigment was in the membranes.

Bachmann was unable to determine whether the pigments were laid down
by the protoplasm or were due to changes in the cell-wall. The middle
layer, he found, was generally more deeply coloured than the inner one,
though that was not universal. In other cases the outer sheath was the
darkest, especially in cortices one to two cells thick such as those
of _Parmelia olivacea_, _P. fuliginosa_ and _P. revoluta_, and in the
brown thick-walled spores of _Physcia stellaris_ and of _Rhizocarpon
geographicum_. Still another variation occurs in _Parmelia tristis_ in
which the dark cortical cells show an outer colourless membrane over the
inner dark wall.

The coloured pigments are mainly to be found in the superficial tissues,
but if the thallus is split by areolation, as in crustaceous lichens, the
internal hyphae may be coloured like those of the outer cortex wherever
they are exposed. The hyphae of the gonidial layer are persistently
colourless, but the lower surface and the rhizoids of many foliose
lichens are frequently very deeply stained, as are the hypothalli of
crustaceous species.

The fruiting bodies in many different families of lichens have dark
coloured discs owing to the abundance of dark-brown pigment in the
paraphyses. In these the walls, as determined by Bachmann, are composed
generally of an inner wall, a second outer wall, and the outermost sheath
which forms the middle lamella between adjacent cells. In some species
the second wall is pigmented, in others the middle lamella is the one
deeply coloured. The hymenium of many apothecia and the hyphae forming
the amphithecium are often deeply impregnated with colour. The wall
hyphae of the pycnidia are also coloured in some forms; more frequently
the cells round the opening pore are more or less brown.

The presence of these coloured substances enables the cell-wall to resist
chemical reactions induced by the harmful influences of the atmosphere
or of the substratum. The darker the cell-wall and the more abundant
the pigment, the less easily is the plant injured either by acids or
alkalies. The coloured tips of the paraphyses thus give much needed
protection to the long lived sporiferous asci, and the dark thalline
tissues prevent premature rotting and decay.

_d._ ENUMERATION OF AMORPHOUS PIGMENTS:

=1. Green.= Bachmann found several different green pigments:
“Lecidea-green,” colouring red with nitric acid, is the dark blue-green
or olive-green (smaragdine) of the paraphyses of many apothecia in
the Lecideaceae, and may vary to a lighter blue; it appears almost
black in thalline cells[889]. “Aspicilia-green” occurs in the thalline
margin and sometimes in the epithecium of the fruits of species of
_Aspicilia_; it becomes a brighter green on the application of nitric
acid. “Bacidia-green,” also a rare pigment, becomes violet with the
same acid; it is found in the epithecium of _Bacidia muscorum_ and
_Bacidia acclinis_ (Lecideaceae). “Thalloidima-green” in the apothecia
of some species of _Biatorina_ is changed to a dirty-red by nitric acid
and to violet by potash. Still another termed “rhizoid-green” gives
the dark greenish colour to the rhizoids of _Physcia pulverulenta_
and _P. aipolia_ and to the spores of some species of _Physcia_ and
_Rhizocarpon_. It becomes more olive-green with potash.

=2. Blue.= A very rare colour in lichens, so far found in only a few
species, _Biatora_ (_Lecidea_) _atrofusca_, _Lecidea sanguinaria_ and
_Aspicilia flavida_ f. _coerulescens_. It forms a layer of amorphous
granules embedded in the outer wall of the paraphyses, becoming more
dense towards the epithecium. A few granules are also present in the
hymenium.

=3. Violet.= “Arthonia-violet” as it is called by Bachmann is a
constituent of the tissues of _Arthonia gregaria_, occurring in minute
masses always near the cortical cells; it is distinct from the bright
cinnabarine granules present in every part of the thallus.

=4. Red.= Several different kinds of red have been distinguished:
“Urceolaria-red,” visible as an interrupted layer on the upper side of
the medulla in the thallus of _Diploschistes ocellatus_, a continental
species with a massive, crustaceous, whitish thallus that shows a faint
rose tinge when wetted. “Phialopsis-red” is confined to the epithecium of
the brightly coloured apothecia of _Phialopsis rubra_. “Lecanora-red,”
by which Bachmann designates the purplish colour of the hymenium, is
an unfailing character of Lecanora atra; the colouring substance is
lodged in the middle lamella of the paraphysis cells; it occurs also in
_Rhizocarpon geographicum_ and in _Rh. viridiatrum_; it becomes more
deeply violet with potash. M. C. Knowles[890] noted the blue colouring of
_Rh. geographicum_ growing in W. Ireland near the sea and she ascribed
it to an alkaline reaction. Two more rare pigments, “Sagedia-red” and
“Verrucaria-red,” are found in species of Verrucariaceae. These tinge the
calcareous rocks in which the lichens are embedded a beautiful rose-pink.
They are scarcely represented in our country.

=5. Brown.= A frequent colouring substance, but also presenting several
different kinds of pigment which may be arranged in two groups:

=(1) Substances with some characteristic chemical reaction.= These are of
somewhat rare occurrence: “Bacidia-brown” in the middle lamella of the
paraphyses of _Bacidia fuscorubella_ stains a clear yellow with acids or
a violet colour with potash; “Sphaeromphale-brown,” which occurs in the
perithecia and in the cortex of _Staurothele clopismoides_, becomes deep
olive-green with potash, changing to yellow-brown on the application of
sulphuric acid; “Segestria-brown” in _Porina lectissima_ changes to a
beautiful violet colour with sulphuric acid, while “Glomellifera-brown,”
which is confined to the outer cortical cells of the upper surface of
_Parmelia glomellifera_, becomes blue with nitric and sulphuric acids,
but gives no reaction with potash. Rosendahl[891] confirmed Bachmann’s
discovery of this colour and further located it in corresponding cells of
_Parmelia prolixa_ and _P. locarensis_.

=(2) Substances with little or no chemical reaction.= There is only one
such to be noted: “Parmelia-brown,” usually a very dark pigment, which is
lodged in the outer membranes of the cells. It becomes a clearer colour
with nitric acid, and if the reagent be sufficiently concentrated, some
of the pigment is dissolved out. Some tissues, such as the lower cortex
of some _Parmeliae_, may be so impregnated and hardened, that nothing
short of boiling acid has any effect on the cells; membranes less deeply
coloured and changed, such as the cortex of the _Gyrophorae_, become
disintegrated with such drastic treatment. With potash the colour becomes
darker, changing from a clear brown to olivaceous-brown or-green, or in
some cases, as in a more faintly coloured epithecium, to a dirty-yellow,
but the lighter colour produced there is largely due to the swelling up
of the underlying tissues to which the potash penetrates readily between
the paraphyses.

“Parmelia-brown” is a colouring substance present in the dark epithecium
and hypothecium of the fruits of many widely diverse lichens, and in the
cortical cells and rhizoids of many thalli. In some plants the thallus is
brown both above and below, in others, as in _Parmelia revoluta_, etc.
only the under surface is dark-coloured.

_e._ COLOUR DUE TO INFILTRATION. There are several crustaceous lichens
that are rusty-red, the colour being due to the presence of iron. These
lichens occur on siliceous rocks of gneiss, granite, etc., and more
especially on rocks rich in iron. Iron as a constituent of lichens
was first demonstrated by John[892] in _Ramalina fraxinea_ and _R.
calicaris_. Grimbel[893] proved that the colour of rust lichens was due
to an iron salt, and Molisch[894] by microscopic examination located
minute granules of ferrous oxide as incrustations on the hyphae of
the upper surface of the thallus. Molisch held that the rhizoids or
penetrating hyphae dissolved the iron from the rocks by acid secretions.
Rust lichens however grow on rocks that are frequently under water in
which the iron is already present.

Among “rusty” lichens are the British forms, _Lecanora lacustris_, the
thallus of which is normally white, though generally more or less tinged
with iron; it inhabits rocks liable to inundation. _L. Dicksonii_ owes
its ferruginous colour to the same influences. _Lecidea contigua_ var.
_flavicunda_ and _L. confluens_ f. _oxydata_ are rusty conditions of
whitish-grey lichens.

Nilson[895] found rusty lichens occurring frequently in the
Sarak-Gebirge, more especially on glacier moraines where they were
liable, even when uncovered by snow, to be flooded by water from the
higher reaches. It is the thallus that is affected by the iron, rarely if
ever are apothecia altered in colour.

BACHMANN’S PIGMENT REACTIONS


  ----------------+---------------+-------------+-------------+----------+
  Name of Pigment |     Colour    |     KOH     |     NH₃     |  Ba(OH)₂ |
     or Lichen    |               |             |             |          |
  ----------------+---------------+-------------+-------------+----------+
  Lecidea-green   |     green     |             |             |          |
                  |               |             |             |          |
  Aspicilia-green |     green     |             |             |          |
                  |               |             |             |          |
  Bacidia-green   |     green     |             |             |          |
                  |               |             |             |          |
  Thalloidima-    |     green     |   violet    |             |          |
            green |               |             |             |          |
                  |               |             |             |          |
  Rhizoid-green   | bluish-green  | olive-green |             |          |
                  |               |  to brown   |             |          |
                  |               |             |             |          |
  _Biatora        |     blue      | dissolves   |             |          |
  atrofusca_      |               |    with     |             |          |
                  |               |greenish-blue|             |          |
                  |               |   colour    |             |          |
                  |               |             |             |          |
  _Phialopsis     |   brick-red   |             |   dirty     |          |
  rubra_          |               |             | purple-red  |          |
                  |               |             |             |          |
  Lecanora-red    |  purple-red   |             | deep violet |          |
                  |               |             |             |          |
  _Sagedia        |  bluish-red   |    blue     |greenish-blue|   blue   |
  declivum_       |               |  (green)    |    then     |          |
                  |               |             | grey-black  |          |
                  |               |             |             |          |
  _Verrucaria     |   rose-red    | dark-green  |             |dark-green|
  Hoffmanni_ f.   |               |             |             |          |
  _purpurascens_  |               |             |             |          |
                  |               |             |             |          |
  _Bacidia        |yellowish-brown|   violet    |   violet    |  violet  |
  fuscorubella_   |               |             |             |          |
                  |               |             |             |          |
  _Sphaeromphale  | leather-brown |    deep     |             |          |
  clopismoides_   |               | olive-green |             |          |
                  |               |             |             |          |
  _Segestria      | yellow-brown  |  rose-red   |             |          |
  lectissima_—    |               |             |             |          |
  perithecia      |               |             |             |          |
                  |               |             |             |          |
  _Segestria      |  brown and    |             |             |          |
  lectissima_—    |  colourless   |             |             |          |
  entire tissue   |               |             |             |          |
                  |               |             |             |          |
  _Parmelia       | leather-brown |             |             |          |
  glomellifera_   |               |             |             |          |
                  |               |             |             |          |
  Parmelia-brown  |  yellow to    |dirty- to    |             |          |
                  |blackish-brown |olive-brown  |             |          |
  ----------------+---------------+-------------+-------------+----------+

  ----------------+---------------+-------------+-------------------------
  Name of Pigment |     HNO₃      |    H₂SO₄    |         Special
     or Lichen    |               |             |        Reactions
  ----------------+---------------+-------------+-------------------------
  Lecidea-green   |  copper or    |             | KOH then HCl: blue
                  |  brick-red    |             |
                  |               |             |
  Aspicilia-green |               |             | HNO₃: brighter green
                  |               |             |
  Bacidia-green   |    violet     |   violet    | HCl: violet
                  |               |             |
  Thalloidima-    | indistinctly  |             | HCl: indistinctly
            green |  purple-red   |             |   purple-red
                  |               |             |
                  |               |             |
  Rhizoid-green   | olive-green   |             |
                  |               |             |
  _Biatora        | violet, then  | dissolves   | H₂O insoluble
  atrofusca_      | yellow, then  |             |
                  | decolourized  |             |
                  |               |             |
  _Phialopsis     |   violet      |             |
  rubra_          |               |             |
                  |               |             |
  Lecanora-red    |               |             |
                  |               |             |
  _Sagedia        |               |             |
  declivum_       |               |             |
                  |               |             |
  _Verrucaria     |               |             | KOH then HNO₃ then
  Hoffmanni_ f.   |               |             | H₂SO₄: violet
  _purpurascens_  |               |             | crystals
                  |               |             |
  _Bacidia        |               |             |
  fuscorubella_   |               |             |
                  |               |             |
  _Sphaeromphale  |               |             | KOH, then H₂SO₄,
  clopismoides_   |               |             | then HNO₃: blackish
                  |               |             |
  _Segestria      | bright yellow |             | dilute H₂SO₄:
  lectissima_—    |               |             | bright yellow
  perithecia      |               |             |
                  |               |             |
  _Segestria      |               |             | Strong H₂SO₄: deep
  lectissima_—    |               |             | violet, then grey
  entire tissue   |               |             |
                  |               |             |
  _Parmelia       |     blue,     |             | CaCl₂O₂: blue, then
  glomellifera_   | then violet,  |             | grey; finally
                  | at last grey  |             | decolourized
                  |               |             |
  Parmelia-brown  |    bright     |             |
                  |   red-brown   |             |
  ----------------+---------------+-------------+-------------------------





CHAPTER VI

BIONOMICS


A. GROWTH AND DURATION

Lichens are perennial plants mostly of slow growth and of long
continuance; there can therefore only be approximate calculations either
as to their rate of increase in dimensions or as to their duration in
time. A series of somewhat disconnected observations have however been
made that bear directly on the question, and they are of considerable
interest.

Meyer[896] was among the first to be attracted by this aspect of lichen
life, and after long study he came to the conclusion that growth varied
in rapidity according to the prevailing conditions of the atmosphere and
the nature of the substratum; but that nearly all species were very slow
growers. He enumerates several,—_Lichen_ (_Xanthoria_) _parietinus_,
_L._ (_Parmelia_) _tiliaceus_, _L._ (_Rhizocarpon_) _geographicus_,
_L._ (_Haematomma_) _ventosus_, and _L._ (_Lecanora_) _saxicolus_,—all
species with a well-defined outline, which, after having attained some
considerable size, remained practically unchanged for six and a half
years, though, in some small specimens of foliose lichens, he noted,
during the same period, an increase of one-fourth to one-third of their
size in diameter. In one of the above crustaceous species, _L. ventosus_,
the specimen had not perceptibly enlarged in sixteen years, though during
that time the centre of the thallus had been broken up by weathering and
had again been regenerated.

Meyer also records the results of culture experiments made in the open,
possibly with soredia or with thalline scraps: he obtained a growth
of _Xanthoria parietina_ (on wrought iron kept well moistened), which
fruited in the second year, and in five years had attained a width of
5-6 lines (about 1 cm.); _Lecanora saxicola_ growing on a moist rock
facing south grew 4-7 lines in six and a half years, and bore very minute
apothecia.

Lindsay[897] quotes a statement that a specimen of _Lobaria pulmonaria_
had been observed to occupy the same area of a tree after the lapse
of half a century. Berkeley[898] records that a plant of _Rhizocarpon
geographicum_ remained in much the same condition of development during
a period of twenty-five years. The latter is a slow grower and, in
ordinary circumstances, it does not fruit till about fifteen years after
the thallus has begun to form. Weddell[899], also commenting on the long
continuance of lichens, says there are crustaceous species occupying on
the rock a space that might be covered by a five-franc piece, that have
taken a century to attain that size.

Phillips[900] on the other hand argues against the very great age of
lichens, and suggests 20 years as a sufficient time for small plants
to establish themselves on hard rocks and attain full development. He
had observed a small vigorous plant of _Xanthoria parietina_ that in the
course of five years had extended outwards to double its original size.
The centre then began to break up and the whole plant finally disappeared.

Exact measurements of growth have been made by several observers.
Scott Elliot[901] found that a _Pertusaria_ had increased about half a
millimetre from the 1st February to the end of September. Vallot[902]
kept under observation at first three then five different plants of
_Parmelia saxatilis_ during a period of eight years: the yearly increase
of the thallus was half a centimetre, so that specimens of twenty
centimetres in breadth must have been growing from forty to fifty years.

Bitter’s[903] observations on _Parmelia physodes_ agree in the main with
those of Vallot: the increase of the upper lobes during the year was 3-4
mm. In a more favourable climate Heere found that _Parmelia caperata_
(Fig. 49) on a trunk of _Aesculus_ in California had grown longitudinally
1·5 cm. and transversely 1 cm. The measurements extended over a period
of seven winter months, five of them being wet and therefore the most
favourable season of growth. In warm regions lichens attain a much
greater size than in temperate or northern countries, and growth must be
more rapid.

A series of measurements was also made by Heere[904] on _Ramalina
reticulata_ (Fig. 64), a rapid growing tree-lichen, and one of
the largest American species. The shorter lobes were selected for
observation, and were tested during a period of seven months from
September to May, five of the months being in the wet season. There was
great variation between the different lobes but the average increase
during that period was 41 per cent.

Krabbe[905] took notes of the colonization of _Cladonia rangiferina_
(Fig. 127) on burnt soil: in ten years the podetia had reached a height
of 3 to 5 cm., giving an annual growth of about 3-5 mm. It is not unusual
to find specimens in northern latitudes 18 inches long (50 cm.), which,
on that computation, must have been 100 to 160 years old; but while
increase goes on at the apex of the podetia, there is constant perishing
at the base of at least as much as half the added length and these plants
would therefore be 200 or 300 years old. Reinke[906] indeed has declared
that apical growth in these _Cladina_ species may go on for centuries,
given the necessary conditions of good light and undisturbed habitat.

Other data as to rate of growth are furnished by Bonnier[907] in the
account of his synthetic cultures which developed apothecia only after
two to three years. The culture experiments of Darbishire[908] and
Tobler[909] with _Cladonia soredia_ are also instructive, the former with
synthetic spore- and alga-cultures having obtained a growth of soredia
in about seven months; the latter, starting with soredia, had a growth of
well-formed squamules in nine months.

It has been frequently observed that abundance of moisture facilitates
growth, and this is nowhere better exemplified than in crustaceous
soil-lichens. Meyer found that on lime-clay soil which had been thrown
up from a ditch in autumn, lichens such as _Gyalecta geoica_ were fully
developed the following summer. He gives an account also of another soil
species, _Verrucaria_ (_Thrombium_) _epigaea_, which attained maturity
during the winter half of the year. Stahl[910] tells us that _Thelidium
minutulum_, a pyrenocarpous soil-lichen, with a primitive and scanty
thallus, was cultivated by him from spore to spore in the space of
three months. Such lichens retain more of the characteristics of fungi
than do those with a better developed thallus. Rapid colonization by a
soil-lichen was also observed in Epping Forest by Paulson[911]. In autumn
an extensive growth of _Lecidea uliginosa_ covered as if with a dark
stain patches of soil that had been worn bare during the previous spring.
The lichen had reached full development and was well fruited.

These facts are quite in harmony with other observations on growth made
on Epping Forest lichens. The writers[912] of the report record the
finding of “fruiting lichens overspreading decaying leaves which can
scarcely have lain on the ground more than two or three years; others
growing on old boots or on dung and fruiting freely; others overspreading
growing mosses.” They also cite a definite instance of a mass of concrete
laid down in 1903 round a surface-water drain which in 1910—seven years
later—was covered with _Lecanora galactina_ in abundant fruit; and of
another case of a Portland stone garden-ornament, new in 1904, and, in
1910, covered with patches of a fruiting _Verrucaria_ (probably _V.
nigrescens_). Both these species, they add, have a scanty thallus and
generally fruit very freely.

A series of observations referring to growth and “ecesis” or the
spreading of lichens have been made by Bruce Fink[913] over a period of
eight years. His aim was mainly to determine the time required for a
lichen to re-establish itself on areas from which it had been previously
removed. Thus a quadrat of limestone was scraped bare of moss and of
_Leptogium lacerum_, except for bits of the moss and particles of the
lichen which adhered to the rock, especially in depressions of the
surface. After four years, the moss was colonizing many small areas on
which grew patches of the lichen 2 to 10 mm. across. Very little change
occurred during the next four years.

Numerous results are also recorded as to the rate of growth, the average
being 1 cm. per year or somewhat under. The greatest rate seems to have
been recorded for a plant of _Peltigera canina_ growing on “a mossy rock
along a brook in a low moist wood, well-shaded.” A plant, measuring 10 by
14 cm., was deprived of several large apothecia. The lobes all pointed
in the same direction, and the plant increased 1·75 cm. in one year.
Two other plants, deprived of their lobes, regenerated and increased
from 2 and 5 cm. respectively to 3·5 and 6 cm. No other measurements are
quite so high as these, though a plant of _Parmelia caperata_ (sterile),
measuring from 1 to 2 cm. across, reached in eight years a dimension of
10 by 13 cm. Other plants of the same species gave much slower rates
of increase. A section of railing was marked bearing minute scattered
squamules of _Cladonia pityrea_. After two years the squamules had
attained normal size and podetia were formed 2 to 4 mm. long.

Several areas of _Verrucaria muralis_ were marked and after ten months
were again measured; the largest plants, measuring 2·12 by 2·4 cm.
across, had somewhat altered in dimensions and gave the measurements 2·2
by 3 cm. Some crustose species became established and produced thalli and
apothecia in two to eight years. Foliose lichens increased in diameter
from 0·3 to 3·5 cm. per year. So far as external appearance goes,
apothecia are produced in one to eight years; it is concluded that they
require four to eight years to attain maturity in their natural habitats.


B. SEASON OF FRUIT FORMATION

The presence of apothecia (or perithecia) in lichens does not always
imply the presence of spores. In many instances they are barren, the
spores having been scattered or not yet matured; the disc in these cases
is composed of paraphyses only, with possible traces of asci. In any
month of the year, however, some lichens may be found in fruit.

Baur[914] found, for instance, that _Parmelia acetabulum_ developed
carpogonia the whole year round, though somewhat more abundantly in
spring and autumn. _Pertusaria communis_ similarly has a maximum period
of fruit-formation at these two seasons. This is probably true of
tree-lichens generally: in summer the shade of the foliage would inhibit
the formation of fruits, as would the extreme cold of winter; but were
these conditions relaxed spore-bearing fruits might be expected at any
season though perhaps not continuously on the same specimen.

An exception has been noted by Baur in _Pyrenula nitida_, a crustaceous
tree Pyrenolichen. He found carpogonia only in February and April, and
the perithecia matured in a few weeks, presumably at a date before the
trees were in full leaf; but even specimens of _Pyrenula_ are not unusual
in full spore-bearing conditions in the autumn of the year.

To arrive at any true knowledge as to the date and duration of spore
production, it would be necessary to keep under observation a series of
one species, examining them microscopically at intervals of a few weeks
or months and noting any conditions that might affect favourably or
unfavourably the reproductive organs. A comparison between corticolous
and saxicolous species would also be of great interest to determine the
influence of the substratum as well as of light and shade. But in any
case it is profitable to collect and examine lichens at all seasons of
the year, as even when the bulk of the spores is shed, there may remain
belated apothecia with a few asci still intact.


C. DISPERSAL AND INCREASE

The natural increase of lichen plants may primarily be sought for in
the dispersal of the spores produced in the fruiting-bodies. These are
ejected, as in fungi, by the pressure of the paraphyses on the mature
ascus. The spores are then carried away by wind, water, insects, etc.
In a few lichens gonidia are enclosed in the hymenium and are ejected
along with the spores, but, in most, the necessary encounter with the
alga is as fortuitous, and generally as certain, as the pollination
of anemophilous flowers. A case of dispersal in _Sagedia microspora_
has been described by Miyoshi[915] in which entire fruits, small round
perithecia, were dislodged and carried away by the wind. The addition
of water caused them to swell enormously and brought about the ejection
of the spores. Areas covered by the thallus are also being continually
enlarged by the spreading growth of the hypothallus.

_a._ DISPERSAL OF CRUSTACEOUS LICHENS. These lichens are distributed
fairly equally on trees or wood (corticolous) and on rocks (saxicolous).
Some species inhabit both substrata. As regards corticolous lichens that
live on smooth bark such as hazel or mountain-ash, the vegetative body
or thallus is generally embedded beneath the epidermis of the host.
Soredia are absent and the thallus is protected from dispersal. In these
lichens there is rather an abundant and constant formation of apothecia
or perithecia.

Other species that affect rugged bark and are more superficial are less
dependent on spore production. The thallus is either loosely granular, or
is broken up into areolae. The areolae are each a centre of growth, and
with an accession of moisture they swell up and exert pressure on each
other. Parts of the thallus thus become loosened and are dislodged and
carried away. If anchored on a suitable substratum they grow again to
a complete lichen plant. Sorediate lichens are dependent almost wholly
on these bud-like portions for increase in number; soredia are easily
separated from the parent plant, and easily scattered. Darbishire[916]
noted frequently that small _Poduridae_ in moving over the surface of
_Pertusaria amara_ became powdered with soredia and very evidently took a
considerable part in the dissemination of the species.

Crustaceous rock lichens are rarely sorediate, but they secure vegetative
propagation[917] by the dispersal of small portions of the thallus. The
thalli most securely attached are cracked into small areolae which, by
unequal growth, become very soon lop-sided, or, by intercalary increase,
form little warts and excrescences on their surface. These irregularities
of development give rise to more or less tension which induces a
loosening of the thallus from the substratum. Weather changes act
similarly and gradually the areolae are broken off. Loosening influence
is also exercised by the developing fruits, the expanding growth of which
pushes aside the neighbouring tissues. Wind or water then carries away
the thalline particles which become new centres of growth if a suitable
substratum is reached.

_b._ DISPERSAL OF FOLIOSE LICHENS. It is a matter of common observation
that, in foliose lichens where fruits are abundant, there are few or
no soredia and _vice versa_. In either case propagation is ensured. In
addition to these obvious methods of increase many lichens form isidia,
outgrowths from the thallus which are easily detached. Bitter[918]
considers for instance that the coralloid branchlets, which occur in
compact tufts on the thallus of _Umbilicaria pustulata_, are of immense
service as organs of propagation. Apothecia and pycnidia are rarely
present in that species, and the plant thus falls back on vegetative
production. Slender crisp thalline outgrowths, easily separable, occur
also on the edges of lobes, as in species of _Peltigera_, _Platysma_, etc.

Owing to the gelatinous character of lichen hyphae, the thallus quickly
becomes soft with moisture and is then easily torn and distributed by
wind, animals, etc. The action of lichens on rocks has been shown to be
of a constantly disintegrating character, and the destruction of the
supporting rock finally entails the scattering of the plant. This cause
of dispersal is common to both crustaceous and foliose species. The older
central parts of a lichen may thus have disappeared while the areolae on
lobes of the circumference are still intact and in full vigour.

As in crustaceous lichens the increase in the area of growth may take
place by means of the lichen mycelium which, originating from the
rhizinae in contact with the substratum, spreads as a hypothallus under
the shelter of the lobes and far beyond them. When algae are encountered
a new lobe begins to form. The process can be seen perhaps most
favourably in lichens on decaying wood which harbours moisture and thus
enables the wandering hyphae to retain life.

_c._ DISPERSAL OF FRUTICOSE LICHENS. Many of these lichens are abundantly
fruited; in others soralia are as constantly developed. Species of
_Usnea_, _Alectoria_, _Ramalina_ and many _Cladoniae_ are mainly
propagated by soredia. They are all peculiarly liable to be broken and
portions of the thallus scattered by the combined action of wind and rain.

Peirce[919] found that _Ramalina reticulata_ (Fig. 65), of which the
fronds are an open network, was mainly distributed by the tearing of the
lichen in high wind. This takes place during the winter rains, when not
only the lichen is wet and soft in texture, but when the deciduous trees
are bare of leaves, at a season, therefore, when the drifting thalline
scraps can again catch on to branch or stem. A series of observations on
the dispersal of forms of long pendulous Usneas was made by Schrenk[920].
In the Middle and North Atlantic States of America these filamentous
species rarely bear apothecia. The high winds break and disperse them
when they are in a wet condition. They generally grow on Spruces and
Firs, because the drifting filaments are more easily caught and entangled
on short needles. The successive wetting and drying causes them to coil
and uncoil, resulting in a tangle impossible to unravel, which holds them
securely anchored to the support.


D. ERRATIC LICHENS

In certain lichens, there is a tendency for the thallus to develop
excrescences of nodular form which easily become free and drift about
in the wind while still living and growing. They are carried sometimes
very long distances, and fall in thick deposits over localities far
from their place of origin. The most famous instance is the “manna
lichen,” _Lecanora esculenta_, which has been scientifically examined
and described by Elenkin[921]. He distinguishes seven different forms
of the species: f. _esculenta_, f. _affinis_, f. _alpina_, and f.
_fruticulosa-foliacea_ which are Alpine lichens, the remainder, f.
_desertoides_, f. _foliacea_ and f. _esculenta-tarquina_, grow on the
steppes or in the desert[922].

Elenkin[921] adds to the list of erratic lichens a variety of _Parmelia
molliuscula_ along with _P. ryssolea_ from S. Russia, from the Asiatic
steppes and from Alpine regions. Mereschkovsky[923] has also recorded
from the Crimea _Parmelia vagans_, probably derived from _P. conspersa_
f. _vaga_ (f. nov.). It drifts about in small rather flattened bits, and,
like other erratics, it never fruits.

Meyer[924] long ago described the development of wandering lichens:
scraps that were torn from the parent thallus continued to grow if there
were sufficient moisture, but at the same time undergoing considerable
change in appearance. The dark colour of the under surface disappears
in the frequently altered position, as the lobes grow out into narrow
intermingling fronds forming a more or less compact spherical mass;
the rhizoids also become modified and, if near the edge, grow out into
thread-like structures which bind the mass together. Meyer says that
“wanderers” have been noted as belonging to _Parmelia acetabulum_,
_Platysma glaucum_ and _Anaptychia ciliaris_.

[Illustration: Fig. 121. _Parmelia revoluta_ var. _concentrica_ Cromb.
_a_, plant on flint with detached fragment; _b_, upper surface of three
specimens; _c_, three specimens as found on chalk downs; _d_, specimens
in section showing central cavity (S. H., _Photo._).]

The most notable instance in Britain of the “erratic” habit is that of
_Parmelia revoluta_ var. _concentrica_ (Fig. 121), first found on Melbury
Hill near Shaftesbury, Dorset, and described as “a spherical unattached
lichen which rolls on the exposed downs.” It has recently been observed
on the downs near Seaford in Sussex, where, however, it seems to be
confined to a small area about eight acres in extent which is exposed to
south-west winds. The lichen is freely distributed over this locality.
To R. Paulson and Somerville Hastings[925] we owe an account of the
occurrence and origin of the _revoluta_ wanderers. The specimens vary
considerably in shape and size, and measure from 1 to 7 cm. in longest
diameter. Very few are truly spherical, some are more or less flattened
and many are quite irregular. The revolute edges of the overlapping
lobes give a rough exterior to the balls, which thereby become entangled
amongst the grass, etc., and movement is impeded or prevented, except
in very high winds. Crombie[926] had suggested that the concentric
plant originated from a corticolous habitat, but no trees are near the
Seaford locality. Eventually specimens were found growing on flints in
the immediate neighbourhood. While still on the stone the lichen tends
to become panniform, a felt of intermingling imbricate lobes is formed,
portions of which, in time, become crowded out and dislodged. When
scattered over the ground, these are liable to be trampled on by sheep or
other animals and so are broken up; each separate piece then forms the
nucleus of new concentric growth.

Crombie[926] observed at Braemar, drifting about on the detritus of
Morrone, an analogous structure in _Parmelia omphalodes_. He concluded
that nodular excrescences of the thallus had become detached from the
rocks on which the lichen grew; while still attached to the substratum
_Parmelia omphalodes_ and the allied species, _P. saxatilis_, form dense
cushion-like masses.


E. PARASITISM

_a._ GENERAL STATEMENT. The parasitism of _Strigula complanata_, an
exotic lichen found on the leaves of evergreen trees, has been already
described[927]; Dufrenoy[928] records an instance of hyphae from a
_Parmelia_ thallus piercing pine-needles through the stomata and causing
considerable injury. Lichen hyphae have attacked and destroyed the
protonemata of mosses. Cases have also been recorded of _Usnea_ and
_Ramalina_ penetrating to the living tissue of the tree on which they
grew, and there may be other similar parasitisms; but these exceptions
serve to emphasize the independent symbiotic growth of lichens.

There are however some lichens belonging to widely diverse genera that
have retained, or reverted to, the saprophytic or parasitic habit of
their fungal ancestors, though the cases that occur are generally of
lichens preying on other lichens. The conditions have been described as
those of “antagonistic symbiosis” when one lichen is hurtful or fatal
in its action on the other, and as “parasymbiosis” when the association
does little or no injury to the host. The parasitism of fungi on lichens,
though falling under a different category, in many instances exhibits
features akin to parasymbiosis.

The parasitism of fungus on fungus is not unusual; there are instances of
its occurrence in all the different classes. In the Phycomycetes there
are genera wholly parasitic on other fungi such as _Woronina_ and other
Chytridiaceae; _Piptocephalus_, one of the Mucorini, is another instance.
_Cicinnobolus_, one of the Sphaeropsideae, preys on _Perisporiae_;
a species of _Cordyceps_ is found on _Elaphomyces_, and _Orbilia
coccinella_ on _Polyporus_; while among Basidiomycetes, _Nyctalis_, an
agaric, grows always on _Russula_.

There are few instances of lichens finding a foothold on fungi, for the
simple reason that the latter are too short lived. On the perennial
_Polyporeae_ a few have been recorded by Arnold[929], but these are
not described as doing damage to the host. They are mostly species of
_Lecidea_ or of allied genera. Kupfer[930] has also listed some 15
different lichens that he found on _Lenzites_ sp.

_b._ ANTAGONISTIC SYMBIOSIS. In discussing the nutrition of lichens[931]
note has been taken of the extent to which some species by means of
enzymes destroy the thallus of other lichens in their vicinity and
then prey on the dead tissues. A constantly cited[932] example is that
of _Lecanora atriseda_ which in its early stages lives on the thallus
of _Rhizocarpon geographicum_ inhabiting mountain rocks. A detailed
examination of the relationship between these two plants was made by
Malme and later by Bitter[933]. Both writers found that the _Lecanora_
thallus as it advanced caused a blackening of the _Rhizocarpon_ areolae,
the tissues of which were killed by the burrowing slender filaments of
the _Lecanora_, easily recognized by their longer cells. The invader
thereafter gradually formed its own medulla, gonidial layer and cortex
right over the surface of the destroyed thallus. _Lecidea insularis_
(_L. intumescens_) similarly takes possession of and destroys the
thallus of Lecanora glaucoma and Malme[932] strongly suspects that
_Buellia verruculosa_ and _B. aethalea_ may be living on the thallus of
_Rhizocarpon distinctum_ with which they are constantly associated.

Other cases of facultative parasitism have been studied by Hofmann[934],
more especially three different species, _Lecanora dispersa_, _Lecanora_
sp. and _Parmelia hyperopta_, which were found growing on the thick
foliose thallus of _Dermatocarpon miniatum_. These grew, at first
independently, on a wall along with many examples of _Endocarpon_ on
to which they spread as opportunity offered. The thallus of the latter
was in all cases distorted, the area occupied by the invaders being
finally killed. The attacking lichens had benefited materially by the
more nutritive substratum: their apothecia were more abundant and their
thallus more luxuriant. The gonidia especially had profited; they were
larger, more brightly coloured, and they increased more freely. Hoffmann
offers the explanation that the strain on the algae of providing organic
food for the hyphal symbiont was relaxed for the time, hence their more
vigorous appearance.

_Arthonia subvarians_ is always parasitic on the apothecia of _Lecanora
galactina_, and Almquist[935] discovered that the hymenium of the host
alone is injured, the hypothecium and excipulum being left intact.

The “parasitism” of _Pertusaria globulifera_ on _Parmelia perlata_ and
_P. physodes_, as described by Bitter[936], may also be included under
antagonistic symbiosis. The hyphae pierce the _Parmelia_ thallus, break
it up and gradually absorb it. Chemical as well as mechanical influences
are concerned in the work of destruction as both the fungus and the alga
of the victim are dissolved. _Lecanora tartarea_ already dealt with as a
marauding lichen[937] over decaying vegetation may spread also to living
lichens. Fruticose soil species, such as _Cetraria aculeata_ and others,
die from the base and the _Lecanora_ gains entrance to their tissues at
the decaying end which is open.

Arnold[938] speaks of these facultative parasites that have merely
changed their substratum as pseudo-parasites, and he gives a list of
instances of such change. In many cases it is rather the older thalli
that are taken possession of, and, in nearly every case, the invader
is some crustaceous species. The plants attacked are generally ground
lichens or more particularly those that inhabit damp localities, such
as _Peltigera_ or _Cladonia_ or certain bark lichens. Drifting soredia
or particles of a lichen would easily take hold of the host thallus and
develop in suitable conditions. To give a few of the instances observed,
there have been found, by Arnold, Crombie and others:

on _Peltigera canina_: _Callopisma cerina_, _Rinodina turfacea_ var.,
_Bilimbia obscurata_ and _Lecanora aurella_;

on _Peltigera aphthosa_: _Lecidea decolorans_;

on _Cladoniae_: _Bilimbia microcarpa_, _Bacidia Beckhausii_ and
_Urceolaria scruposa_, etc.

_Urceolaria_ (_Diploschistes_) has a somewhat bulky crustaceous thallus
which may be almost evanescent in its semi-parasitic condition, the only
gonidia retained being in the margin of the apothecia. Nylander[939]
found isolated apothecia growing vigorously on _Cladonia_ squamules.

Hue[940] describes _Lecanora aspidophora_ f. _errabunda_, an Antarctic
lichen, as not only a wanderer but as a “shameless robber.” It is to be
seen everywhere on and about other lichens, settling small glomeruli of
apothecia here and there on the thallus of _Umbilicariae_ or between
the areolae of _Buelliae_, and always too vigorous to be ousted from its
position.

_Bacidia flavovirescens_ has been regarded by some lichenologists[941] as
a parasite on _Baeomyces_, but recent work by Tobler[942] seems to have
proved that the bright green thallus is that of the _Bacidia_.

_c._ PARASYMBIOSIS. There are certain lichens that are obligative
parasites and pass their whole existence on an alien thallus. They may
possibly have degenerated from the condition of facultative parasitism
as the universal history of parasitism is one of increased dependence
on the host, and of growing atrophy of the parasite, but, in the case
of lichens, there is always the peculiar symbiotic condition to be
considered: the parasite produces its own vigorous hyphae and normal
healthy fruits, it often claims only a share of the carbohydrates
manufactured by the gonidia. The host lichen is not destroyed by this
parasymbiosis though the tissues are very often excited to abnormal
growth by the presence of the invading organism.

Lauder Lindsay[943] was one of the first to study these “microlichens”
as he called them, and he published descriptions of those he had himself
observed on various hosts. He failed however to discriminate between
lichens and parasitic fungi. It is only by careful research in each
case that the affinity to fungi or to lichens can be determined; very
frequently the whole of them, as possessing no visible thallus, have been
classified with fungi, but that view ignores the symbiosis that exists
between the hyphae of the parasite and the gonidia of the host.

Parasitic lichens are rather rare on gelatinous thalli; but even among
these, a few instances have been recorded. Winter[944] has described a
species of _Leptoraphis_, the perithecia of which are immersed in the
thallus of _Physma franconicum_. The host is wholly unaffected by the
presence of the parasite except for a swelling where it is situated.
The foreign hyphae are easily distinguishable; they wander through the
thallus of the host with their free ends in the mucilage of the gonidial
groups from which they evidently extract nourishment. Species of the
lichen genus _Obryzum_ are also parasitic on gelatinous lichens.

The parasitic genus _Abrothallus_[945] has been the subject of frequent
study. There are a number of species which occur as little black discs
on various thalli of the large foliose lichens. They were first of all
described as parasitic fungi, later Tulasne[946] affirmed their lichenoid
nature as proved by the structure, consistence and long duration of the
apothecia. Lindsay[947] wrote a monograph of the genus dealing chiefly
with _Abrothallus Smithii_ (_Buellia Parmeliarum_) and _A. oxysporus_,
with their varieties and forms that occur on several different hosts.
In some instances the thallus is apparently quite unaffected by the
presence of _Abrothallus_, in others, as in _Cetraria glauca_, there
is considerable hypertrophy produced, the portion of the thallus on
which the parasites are situated showing abnormal growth in the form
of swellings or pustules which may be regarded as gall-formations.
Crombie[948] points this out in a note on _C. glauca_ var. _ampullacea_,
figured first by Dillenius, which is merely a swollen condition due to
the presence of _Abrothallus_.

The internal structure and behaviour of _Abrothallus_ has more
recently been followed in detail by Kotte[949]. He recognized a number
of different species growing on various thalli of _Parmelia_ and
_Cetraria_, but _Abrothallus Cetrariae_ was the only one that produced
gall-formation. The mycelium of the parasite in this instance penetrates
to the medulla of the host lichen as a loose weft of hyphae which are
divided into more or less elongate cells. These send out side branches,
which grow towards the algal cells, and by their short-celled filaments
clasp them exactly in the same way as do the normal lichen hyphae. Thus
in the neighbourhood of the parasite an algal cell may be surrounded by
the hyphae not only of the host, but also by those of _Abrothallus_. The
two different hyphae can generally be distinguished by their reaction
to iodine: in some cases _Abrothallus_ hyphae take the stain, in others
the host hyphae. In addition to apothecia, spermogonia or pycnidia are
produced, but in one of the species examined by Kotte, _Abrothallus
Peyritschii_ on _Cetraria caperata_, there was no spermogonial wall
formed. The hyphae also penetrate the host soredia or isidia, so that
on the dispersal of these vegetative bodies the perpetuation of both
organisms is secured in the new growth.

_Abrothallus_ draws its organic food from the gonidia in the same way as
the host species, and possibly the parasitic hyphae obtain also water and
inorganic food along with the host hyphae. They have been traced down
to the rhizinae and may even reach the hypothallus, but no injury to
the host has been detected. It is a case of joint symbiosis and not of
parasitism. Microscopic research has therefore justified the inclusion of
these and other forms among lichens.

_d._ PARASYMBIOSIS OF FUNGI. There occur on lichens, certain parasites
classed as fungi which at an early stage are more or less parasymbionts
of the host; as growth advances they may become parasitic and cause
serious damage, killing the tissues on which they have settled.

Zopf[950] found several instances of such parasymbiosis in his study
of fungal parasites, such as _Rhymbocarpus punctiformis_, a minute
Discomycete which inhabits the thallus of _Rhizocarpon geographicum_.
By means of staining reagents he was able to trace the course of the
parasitic hyphae, and found that they travelled towards the gonidia and
clasped them lichen-wise without damaging them, since these remained
green and capable of division. At no stage was any harm caused to the
host by the alien organism. Another instance he observed was that of
_Conida rubescens_ on the thallus of _Rhizocarpon epipolium_. By means
of fine sections through the apothecia of _Conida_ and the thallus of
the host, he proved the presence of numerous gonidia in the subhymenial
tissue, these being closely surrounded by the hyphae of the parasite,
and entirely undamaged: they retained their green colour, and in size
and form were unchanged. Zopf[951] at first described these parasites as
fungi though later[951] he allows that they may represent lower forms of
lichens.

Tobler[952] has added two more of these parasymbiotic species on the
border line between lichens and fungi, similar to those described
by Zopf. One of these, _Phacopsis vulpina_, belonging to the fungus
family Celidiaceae, is parasitic on _Letharia vulpina_. The fronds of
the host plant are considerably altered in form by its presence, being
more branched and curly. Where the parasite settles a swelling arises
filled with its hyphae, and the host gonidia almost disappear from the
immediate neighbourhood, only a few “nests” being found and these very
mucilaginous. These nests as well as single gonidia are surrounded by
_Phacopsis_ hyphae which have gradually displaced those of the _Letharia_
thallus. The gonidia are excited to division and increase in number on
contact with either lichen or fungus hyphae, but in the latter case the
increase is more abundant owing doubtless to a more powerful chemical
irritant in the fungus. As development advances, the _Phacopsis_ hyphae
multiply to the exclusion of both lichen hyphae and gonidia from the area
of invasion. Finally the host cortex is split, the fungus bursts through,
and the tissue beneath the parasite becomes brown and dead. _Phacopsis_
begins as a “parasymbiont,” then becomes parasitic, and is at last
saprophytic on the dead cells. The hyphae travel down into the medulla of
the host and also into the soredial outgrowths, and are dispersed along
with the host. The effect of _Verrucula_ on the host thallus may also be
cited[953].

Tobler gives the results of his examination of still another fungus,
_Karschia destructans_. It becomes established on the thallus of
_Chaenotheca chrysocephala_ and its hyphae gradually penetrate down to
the underlying bark (larch). The lichen thallus beneath the fungus is
killed, but gonidia in the vicinity are sometimes clasped: _Karschia_
also is thus a parasymbiont, then a parasite, and finally a saprophyte.

Elenkin[954] describes certain fungi which to some extent are
parasymbionts. One of these, _Conidella urceolata_ n. sp., grew on forms
of _Lecanora esculenta_. The other, a stroma-forming species, had invaded
the thallus of _Parmelia molliuscula_, where it caused gall-formation.
As the growth of the gall was due to the co-operation of the lichen
gonidia, the fungus must at first have been a parasymbiont. Only dead
gonidia were present in the stroma; probably they had been digested by
the parasite. Because of the stroma Elenkin placed the fungus in a new
genus, _Trematosphaeriopsis_.

_e._ FUNGI PARASITIC ON LICHENS. A solution or extract of lichen thallus
is a very advantageous medium in which to grow fungi. It is therefore
not surprising that lichens are a favourite habitat for parasitic fungi.
Stahl[955] has noted that the lichens themselves flourish best where
there is frequent moistening by rain or dew with equally frequent drying
which effectively prevents the growth of fungi. Species of _Peltigera_
are however able to live in damp conditions: without being injured, they
have been observed to maintain their vigour when cultivated in a very
moist hothouse while all the other forms experimented with were attacked
and finally destroyed by various fungi.

Lindsay[956] devoted a great deal of attention to the microscopic study
of the minute fruiting bodies so frequently present on lichen thalli and
published descriptions of microlichens, microfungi and spermogonia. He
and others naturally considered these parasitic organisms to be in many
cases either the spermogonia or pycnidia of the lichen itself. It is
often not easy to determine their relationship or their exact systematic
position; many of them are still doubtful forms.

There exists however a very large number of fully recognized parasitic
microfungi belonging to various genera. Lindsay discovered many of
them. Zopf[957] has given exact descriptions of a series of forms, with
special reference to their effect on the host thallus. In an early
paper he described a species, _Pleospora collematum_, that he found on
_Physma compactum_ and other Collemaceae. The hyphae of the parasite
differed from those of the host in being of a yellow colour; they did
not penetrate or spread far, being restricted to rhizoid-like filaments
at the base of their fruiting bodies (perithecia and pycnidia). Their
presence caused a slight protuberance but otherwise did no harm to
the host; the _Nostoc_ cells in their immediate vicinity were even
more brightly coloured than in other parts of the thallus. In another
paper[958] he gives an instance of gall-formation in _Collema pulposum_
induced by the presence of the fungus _Didymosphaeria pulposi_. Small
protuberances were formed on the margins of the apothecia, more rarely
on the lobes of the thallus, each one the seat of a perithecium of the
fungus. No damage was done to either constituent of the thallus.

_Agyrium flavescens_ grows parasitically on the under surface of
_Peltigera polydactyla_. M. and Mme Moreau[959] found that the hyphae
of the fungus spread between the medullary filaments of the lichen;
no haustoria were observed. The mature fruiting body had no distinct
excipulum, but was surrounded by a layer of dead lichen cells.

It is not easy to determine the difference between parasites that are
of fungal nature and those that are lichenoid; but as a general rule
the fungi may be recognized by their more transient character, very
frequently by their effect on the host thallus, which is more harmful
than that produced by lichens, and generally by their affinity to fungi
rather than to lichens. Opinions differ and will continue to differ on
this very difficult question.

The number of such fungi determined and classified has gradually
increased, and now extends to a very long list. Even as far back as
1896 Zopf reckoned up 800 instances of parasitism of 400 species of
fungi on about 350 different lichens and many more have been added. Abbé
Vouaux[960] is the latest writer on the subject, but his work is mostly a
compilation of species already known. He finds representatives of these
parasites in nine families of Pyrenomycetes and six of Discomycetes.
He leaves out of account the much debated Coniocarps, but he includes
with fungi all those that have been proved to be parasymbiotic, such as
_Abrothallus_.

A number of fungus genera, such as _Conida_, etc., are parasitic only on
lichens. Most of them have one host only; others, such as _Tichothecium
pygmaeum_, live on a number of different thalli. Crustaceous species are
often selected by the parasites, and no great damage, if any, is caused
to these hosts, except when the fungus is seated on the disc of the
apothecium, so that the spore-bearing capacity is lessened or destroyed.

In some of the larger lichens, however, harmful effects are more
visible. In _Lobaria pulmonaria_, the fruits of which are attacked by
the Discomycete, _Celidium Stictarum_[961], there is at first induced
an increased and unusual formation of lichen apothecia. These apothecia
are normally seated for the most part on the margins of the lobes or
pustules, but when they are invaded by the fungus, they appear also in
the hollows between the pustules and even on the under surface of the
thallus. In the large majority of cases the fungus is partly or entirely
embedded in the thallus; the gonidia in the vicinity may remain green
and healthy, or all the tissues in the immediate neighbourhood of the
parasite may be killed.

_f._ MYCETOZOA PARASITIC ON LICHENS. Mycetozoa live mostly on decayed
wood, leaves, humus, etc. One minute species, _Listerella paradoxa_,
always inhabits the podetia of _Cladonia rangiferina_. Another species,
_Hymenobolina parasitica_, was first detected and described by Zukal[962]
as a true parasite on the thallus of Physciaceae; it has since been
recorded in the British Islands on _Parmeliae_[963]. This peculiar
organism differs from other mycetozoa in that the spores on germination
produce amoebae. These unite to form a rose-red plasmodium which slowly
burrows into the lichen thallus and feeds on the living hyphae. It is a
minute species, but when abundant the plasmodia can just be detected with
the naked eye as rosy specks scattered over the surface of the lichen.
Later the grey sporangia are produced on the same areas.


F. DISEASES OF LICHENS

_a._ CAUSED BY PARASITISM. Zopf[964] has stated that of all plants,
lichens are the most subject to disease, reckoning as diseases all the
instances of parasitism by fungi or by other lichens. There are however
only rare instances in which total destruction or indeed any permanent
harm to the host is the result of such parasitism. At worst the trouble
is localized and does not affect the organism as a whole. Some of
these cases have been already noted under antagonistic symbiosis or
parasymbiosis. Several instances have however been recorded where real
injury has been caused by the penetration of some undetermined fungus
mycelium. Zukal[965] records two such observed by him in _Parmelia
encausta_ and _Physcia villosa_: the thallus of the former was dwarfed
and deformed by the presence of the alien mycelium, the latter was
excited to abnormal proliferation.

_b._ CAUSED BY CROWDING. Lichens suffer frequently from being overgrown
by other lichens; they may also be crowded out by other plants. My
attention was called by Mr P. Thompson to a burnt plot of ground in
Epping Forest, which, after the fire, had been colonized by _Peltigera
spuria_. In the course of a few years, other vegetation had followed,
depriving the lichen of space and light and gradually driving it out.
When last examined only a few miserable specimens remained, and these
were reduced in vitality by an attack of the lichen parasite _Illosporium
carneum_.

_c._ CAUSED BY ADVERSE CONDITIONS. Zukal considers as pathological, at
least in origin, the cracking of the thallus so frequent in crustaceous
lichens as well as in the more highly developed forms. As the cracks are
beneficial in the aeration of the plant, they can hardly be regarded as
symptoms of a diseased condition. The more evident ringed breaks in the
cortex of _Usneae_, due probably to wind action, have more reason to be
so regarded; they are most pronounced in _Usnea articulata_, where the
portions bounded by the rings are contracted and swollen, and a hollow
space is formed between the cortex and the central axis. The swellings
that are produced on lichen thalli, such as those of _Umbilicaria_ and
some species of _Gyrophora_, due to intercalary growth are normal to
the plant, though occasionally the swollen weaker portions may become
ruptured and the cortex be thrown off. As pathological also must be
regarded the loss of cortex sometimes occasioned by excessive soredial
formation at the margins of the lobes: the upper cortex may be rolled
back and eventually torn away; the gonidial layer is exposed and
transformed into soredia which are swept away by the wind and rain, till
finally only traces of the lower cortex are left.

Zukal[966] has instanced, as a case of diseased condition observed by
him, the undue thickening of the cortex in _Pertusaria communis_ whereby
the formation of the fruiting bodies is inhibited and even vegetative
development is rendered impossible. There arrives finally a stage when
splitting takes place and the whole thallus breaks down and disappears.
As a rule however there need be no limit to the age of the lichen plant.
There is no vital point or area in the thallus; injury of one part leaves
the rest unhurt, and any fragment in growing condition, if it combines
both symbionts, can carry on the life of the plant, the constant renewal
of gonidia preventing either decay or death. Barring accidents many
lichens might exist as long as the world endures.


G. HARMFUL EFFECT OF LICHENS

One lichen only, _Strigula complanata_, a tropical species, has been
proved to be truly and constantly parasitic. It grows on the surface of
thick leathery leaves such as those of _Camellia_[967], etc. and the alga
and fungus both penetrate the epidermis and burrow beneath the cuticle
and outer cells, causing them to become brown. It undoubtedly injures the
leaves.

Friedrich[968] has given an isolated instance of the hold-fast hyphae
of _Usnea_ piercing through the cortex to the living tissue of the
host, and not only destroying the middle lamella by absorption, but
entering the cells. The _Usnea_ plant was characterized by exceptionally
vigorous growth. Practically all corticolous lichens are epiphytic and
the injury they cause is of an accidental nature. Crustaceous species on
the outer bark occupy the dead cortical layers and seem to be entirely
harmless[969]. The larger foliose and fruticose forms are not so
innocuous: by their abundant enveloping growth they hinder the entrance
of air and moisture, and thus impede the life of the higher plant.
Gleditsch[970], one of the earliest writers on Forestry, first indicated
the possibly harmful effect of lichens especially on young trees and
“in addition,” he says, “they serve as cover for large numbers of small
insects which are hurtful in many ways to the trees.” Lindau[971] pointed
out the damage done to pine-needles by _Xanthoria parietina_ which grew
round them like a cuff and probably choked the stomata, the leaves so
clothed being mostly withered. Dufrenoy[972] states that he found the
hyphae of a _Parmelia_ entering a pine-needle by the stomata, and that
the starch disappeared from the neighbouring parenchyma the cells of
which tended to disintegrate.

It is no uncommon sight to see neglected fruit trees with their branches
crowded with various lichens, _Evernia prunastri_, _Ramalina farinacea_,
etc. Such lichens often find the lenticels a convenient opening for
their hold-fasts and exercise a smothering effect on the trees. Lilian
Porter[973] distinctly states that _Ramalinae_ by their penetrating bases
damage the tissues of the trees. The presence of lichens is however
generally due to unhealthy conditions already at work. Friedrich[974]
reported of a forest which he examined, in which the atmospheric moisture
was very high, with the soil water scarce, that those trees that were
best supplied with soil water were free from lichens, while those with
little water at the base bore dead branches which gave foothold to a rich
growth of the epiphytes.

Experiments to free fruit trees from their coating of lichens were made
by Waite[975]. With a whitewash brush he painted over the infested
branches with solutions of Bordeaux mixture of varying strength, and
found that this solution, commonly in use as a fungicide, was entirely
successful. The trees were washed down about the middle of March, and
some three weeks later the lichens were all dead, the fruticose and
foliose forms had changed in colour to a yellowish or brownish tint and
were drooping and shrivelled.

Waite was of opinion that the lichens did considerable damage to the
trees, but it has been held by others that in very cold climates they
may provide protection against severe frost. Instances of damage are
however asserted by Bouly de Lesdain[976]. The bark of willows he found
was a favourite habitat of numerous lichens: certain species, such as
_Xanthoria parietina_, completely surrounded the branches, closing the
stomata; others, such as _Physcia ascendens_, by the mechanical strain of
the rhizoids, first wet and then dry, gradually loosened the outer bark
and gave entry to fungi which completed the work of destruction.


H. GALL-FORMATION

Several instances of gall-formation to a limited extent have been already
noted as caused by parasitic fungi or lichens. Greater abnormality
of development is induced in a few species by the presence of minute
animals, mites, wood-lice, etc. Zopf[977] noted these deformations of
the thallus in specimens of _Ramalina Kullensis_ collected on the coasts
of Sweden. The fronds were frequently swollen in a sausage-like manner,
and branching was hindered or altogether prevented; apothecia were
rarely formed, though pycnidia were abundant. Here and there, on the
swollen portions of the thallus, small holes could be detected and other
larger openings of elliptical outline, about 1-1-1/2 mm. in diameter,
the margins of which had a nibbled appearance. Three types of small
articulated animals were found within the openings: species of mites,
spiders and wood-lice. Mites were the most constant and were more or
less abundant in all the deformations; frequently a minute Diplopodon
belonging to the genus _Polyxenus_ was also met with.

Zopf came to the conclusion that the gall-formation was mainly due to
the mites: they eat out the medulla and possibly through some chemical
irritation excite the algal zone and cortex to more active growth, so
that an extensive tangential development takes place. The small spiders
may exercise the same power; evidently the larger holes were formed by
them.

Later Zopf added to gall-deformed plants _Ramalina scopulorum_ var.
_incrassata_ and _R. cuspidata_ var. _crassa_. He found in the hollow
swollen fronds abundant evidence of mites, but whether identical with
those that attacked _R. Kullensis_ could not be determined. These two
_Ramalinae_ are maritime species; they are morphologically identical,
as are also the deformed varieties, and the presence of mites, excreta,
etc., are plainly visible in our British specimens.

Bouly de Lesdain[978] found evidence of mite action in _Ramalina
farinacea_ collected from _Pinus sylvestris_ on the dunes near Dunkirk.
The cortex had been eaten off either by mites or by a small mollusc
(_Pupa muscorum_) and the fronds had collapsed to a more or less convex
compact mass. Somewhat similar deformations, though less pronounced, were
observed in other _Ramalinae_.

In _Cladonia sylvatica_ and also in _Cl. rangiformis_ Lesdain has
indicated ff. _abortiva_ Harm. as evidently the result of insect attack.
In both cases the tips of the podetia are swollen, brown, bent and
shrivelled.

One of the most curious and constant effects, also worked out by Lesdain,
occurs in _Physcia hispida_ (_Ph. stellaris_ var. _tenella_). In that
lichen the gonidia at the tips of the fronds are scooped out and eaten by
mites, so that the upper cortex becomes separated from the lower part of
the thallus. As the hyphae of the cortex continue to develop, an arched
hood is formed of a whitish shell-like appearance and powdery inside.
Sometimes the mites penetrate at one point only, at other times the
attack is at several places which may ultimately coalesce into one large
cavity. In a crustaceous species, _Caloplaca_ (_Placodium_) _citrina_ he
found constant evidence of the disturbing effect of the small creatures,
which by their action caused the areolae of the thallus to grow into
minute adherent squamules. A pathological variety, which he calls var.
_sorediosa_, is distinguished by the presence of cup-like hollows which
are scooped out by Acarinae and are filled by yellowish soredia. In
another form, var. _maritima_, the margins of the areolae, occasionally
the whole surface, become powdery with a citrine yellow efflorescence as
a result of their nibbling.

Zukal[979] adds to the deformations due to organic agents, the
hypertrophies and abnormalities caused by climatic conditions. He finds
such irregularities of structure more especially developed in countries
with a very limited rainfall, as in certain districts of Chili, Australia
and Africa, where changes in cortex and rhizoids and proliferations of
the thallus testify to the disturbance of normal development.




CHAPTER VII

PHYLOGENY


I. GENERAL STATEMENT


A. ORIGIN OF LICHENS

Though lichens are very old members of the vegetable kingdom, as
symbiotic plants they yet date necessarily from a time subsequent to the
evolution of their component symbionts. Phylogeny of lichens begins with
symbiosis.

The algae, which belong to those families of Chlorophyceae and
Myxophyceae that live on dry land, had become aerial before their
association with fungi to form lichens. They must have been as fully
developed then as now, since it is possible to refer them to the genus
or sometimes even to the species of free-living forms. The fungus hyphae
have combined with a considerable number of different algae, so that,
even as regards the algal symbiont, lichens are truly polyphyletic in
origin.

The fungus is, however, the dominant partner, and the principal line of
development must be traced through it, as it provides the reproductive
organs of the plant. Representatives of two great groups of fungi are
associated with lichens: Basidiomycetes, found in only a few genera,
and Ascomycetes which form with the various algae the great bulk of
lichen families. In respect of their fungal constituents lichens are
also polyphyletic, and more especially in the Ascolichens which can be
traced back to several starting points. But though lichens have no common
origin, the manner of life is common to them all and has influenced them
all in certain directions: they are fitted for a much longer existence
than that of the fungi from which they started; and both the thallus and
the fruiting bodies—at least in the sub-class Ascolichens—can persist
through great climatic changes, and can pass unharmed through prolonged
periods of latent or suspended vitality.

Another striking note of similarity that runs through the members of
this sub-class, with perhaps the exception of the gelatinous lichens, is
the formation of lichen-acids which are excreted by the fungus. These
substances are peculiar to lichens and go far to mark their autonomy. The
production of the acids and the many changes evolved in the vegetative
thallus suggest the great antiquity of lichens.


B. ALGAL ANCESTORS

It is unnecessary to look far for the algae as they have persisted
through the ages in the same form both without and within the lichen
thallus. By many early lichenologists the free-living algae, similar
in type to lichen algae, were even supposed to be lichen gonidia in a
depauperate condition and were, for that reason, termed by Wallroth
“unfortunate brood-cells.” In the condition of symbiosis they may be
considerably modified, but they revert to their normal form, and resume
their normal life-history of spore production, etc., under suitable and
free culture. The different algae taking part in lichen-formation have
been treated in an earlier chapter[980].


C. FUNGAL ANCESTORS

_a._ HYMENOLICHENS. The problem of the fungal origin in this sub-class
is comparatively simple. It contains but three genera of tropical
lichens which are all associated with Myxophyceae, and the fungus in
them, to judge from the form and habit of the plants, is a member of the
Thelephoraceae. It may be that Hymenolichens are of comparatively recent
origin and that the fungi belonging to the Basidiomycetes had, in the
course of time, become less labile and less capable of originating a new
method of existence. Whatever the reason, they lag immeasurably behind
Ascomycetes in the formation of lichens.

_b._ ASCOLICHENS. Lichens are again polyphyletic within this sub-class.
The main groups from which they are derived are evident. Whether there
has been a series of origins within the different groups or a development
from one starting point in each it would be difficult to determine.
In any case great changes have taken place after symbiosis became
established.

The main divisions within the Ascolichens are related to fungi thus:

  Series 1. Pyrenocarpineae } to Pyrenomycetes.
         2. Coniocarpineae  }
         3. Graphidineae      to Hysteriaceae.
         4. Cyclocarpineae    to Discomycetes.


II. THE REPRODUCTIVE ORGANS


A. THEORIES OF DESCENT IN ASCOLICHENS

It has been suggested that ascomycetous fungi, from which Ascolichens are
directly derived, are allied to the Florideae, owing to the appearance
of a trichogyne in the carpogonium of both groups. That organ in the
red seaweeds is a long delicate cell in direct communication with the
egg-cell of the carpogonium. It is a structure adapted to totally
submerged conditions, and fitted to attach the floating spermatia.

In fungi there is also a structure considered as a trichogyne[981],
which, in the Laboulbeniales, is a free, simple or branching organ. There
is no other instance of any similar emergent cell or cells connected with
the ascogonium of the Ascomycetes, though the term has been applied in
these fungi to certain short hyphal branches from the ascogonium which
remain embedded in the tissue. In the Ascomycetes examined all traces of
emergent receptive organs, if they ever existed, have now disappeared;
in some few there are possible internal survivals which never reach the
surface.

In Ascolichens, on the contrary, the “trichogyne,” a septate hyphal
branch extending upwards from the ascogonium, and generally reaching the
open, has been demonstrated in all the different groups except, as yet,
in the Coniocarpineae which have not been investigated. Its presence is
a strong point in the argument of those who believe in the Floridean
ancestry of the Ascomycetes. It should be clearly borne in mind that
Ascolichens are evolved from the Ascomycetes: these latter stand between
them and any more remote ancestry.

In the Ascomycetes, there is a recognized progression of development
in the form of the sporophore from the closed perithecium of the
Pyrenomycetes and possibly through the Hysteriaceae, which are partially
closed, to the open ascocarp of the Discomycetes. If the fungal and
lichenoid “trichogyne” is homologous with the carpogonial organ in
the Florideae, then it must have been retained in all the groups of
Ascomycetes as an emergent structure, and as such passed on from them
to their lichen derivatives. Has that organ then disappeared from fungi
since symbiosis began? There is no trace of it now, except as already
stated in Laboulbeniales with which lichens are unconnected.

Were Ascolichens monophyletic in origin, one could more easily suppose
that both the fungal and lichen series might have started at some
early stage from a common fungal ancestor possessing a well-developed
trichogyne which has persisted in lichens, but has been reduced to
insignificance in fungi, while fruit development proceeded on parallel
lines in both. There is no evidence that such progression has taken place
among lichens; the theory of a polyphyletic origin for the different
series seems to be unassailable. At the same time, there is no evidence
to show in which series symbiosis started first.

It is more reasonable to accept the polyphyletic origin, as outlined
above, from forms that had already lost the trichogyne, if they ever
really possessed it, and to regard the lichen trichogyne as a new organ
developing in lichens in response to some requirement of the deep-seated
ascogonium. Its sexual function still awaits satisfactory proof, and
it is wiser to withhold judgment as to the service it renders to the
developing fruit.


B. RELATION OF LICHENS TO FUNGI

_a._ PYRENOCARPINEAE. In Phycolichens (containing blue-green gonidia)
and especially in the gelatinous forms, fructification is nearly always
a more or less open apothecium. The general absence of the perithecial
type is doubtless due to the gelatinous consistency of the vegetative
structure; it is by the aid of moisture that the hymenial elements become
turgid enough to secure the ejection of the spores through the narrow
ostiole of the perithecium, and this process would be frustrated were
the surrounding and enveloping thallus also gelatinous. There is only
one minutely foliose or fruticose gelatinous family, the Pyrenidiaceae,
in which Pyrenomycetes are established, and the gonidia, even though
blue-green, have lost the gelatinous sheath and do not swell up.

In Archilichens (with bright-green gonidia), perithecial fruits occur
frequently; they are nearly always simple and solitary; in only a few
families with a few representatives, is there any approach to the stroma
formation so marked among fungi. The single perithecium is generally
semi-immersed in the thallus. It may be completely surrounded by a
hyphal “entire” wall, either soft and waxy or dark coloured and somewhat
carbonaceous. In numerous species the outer protective wall covers
only the upper portion that projects beyond the thallus, and such a
perithecium is described as “dimidiate,” a type of fruit occurring in
several genera, though rare among fungi.

As to internal structure, there is a dissolution and disappearance of
the paraphyses in some genera, their protective function not being so
necessary in closed fruits, a character paralleled in fungi. There is a
great variety of spore changes, from being minute, simple and colourless,
to varied septation, general increase in size, and brown colouration. The
different types may be traced to fungal ancestors with somewhat similar
spores, but more generally they have developed within the lichen series.
From the life of the individual it is possible to follow the course of
evolution, and the spores of all species begin as simple, colourless
bodies; in some genera they remain so, in others they undergo more or
less change before reaching the final stage of colour or septation that
marks the mature condition.

As regards direct fungal ancestors, the Pyrenocarpineae, with solitary
perithecia, are nearest in fruit structure to the Mycosphaerellaceae, in
which family are included several fungus genera that are parasitic on
lichens such as _Ticothecium_, _Müllerella_, etc. In that family occurs
also the genus _Stigmatea_, in which the perithecia in form and structure
are very similar to dimidiate _Verrucariae_.

Zahlbruckner[982] has suggested as the starting point for the
Verrucariaceae the fungus genus _Verrucula_. It was established by
Steiner[983] to include two species, _V. cahirensis_ and _V. aegyptica_,
their perithecia being exactly similar to those of _Verrucaria_[984] in
which genus they were originally placed. Both are parasitic on species
of _Caloplaca_ (_Placodium_). The former, on _C. gilvella_, transforms
the host thallus to the appearance of a minutely lobed _Placodium_; the
latter occupies an island-like area in the centre of the thallus of
_Caloplaca interveniens_, and gives it, with its accompanying parasite,
the character of an _Endopyrenium_ (_Dermatocarpon_), while the rest of
the thallus is normal and fertile.

Zahlbruckner may have argued rightly, but it is also possible to regard
these rare desert species as reversions from an originally symbiotic to
a purely parasitic condition. Reinke came to the conclusion that if a
parasitic species were derived directly from a lichen type, then it must
still rank as a lichen, a view that has a direct bearing on the question.
The parallel family of Pyrenulaceae which have _Trentepohlia_ gonidia
is considered by Zahlbruckner to have originated from the fungus genus
_Didymella_.

Compound or stromatoid fructifications occur once and again in
lichen families; but, according to Wainio[985], there is no true
stroma formation, only a pseudostroma resulting from adhesions and
agglomerations of the thalline envelopes or from cohesions of the margins
of developing fruit bodies. These pseudostromata are present in the
genera _Chiodecton_ and _Glyphis_ (Graphidineae) and in _Trypethelium_,
_Mycoporium_, etc. (Pyrenocarpineae). This view of the nature of the
compound fruits is strengthened, as Wainio points out, by the presence in
certain species of single apothecia or perithecia on the same specimen as
the stromatoid fruits.

_b._ CONIOCARPINEAE. This subseries is entirely isolated. Its peculiarity
lies in the character of the mature fruit in which the spores, owing
to the early breaking down of the asci, lie as a loose mass in the
hymenium, while dispersal is delayed for an indefinite time. This type of
fruit, termed a _mazaedium_ by Acharius, is in the form of a stalked or
sessile roundish head—the capitulum—closed at first and only half-open
at maturity rarely, as in _Cyphelium_, an exposed disc. There is a
suggestion, but only a suggestion, of a similar fructification in the
tropical fungus _Camillea_ in which there is sometimes a stalk with one
or more perithecia at the tip, and in some species early disintegration
of the asci, leaving spore masses[986]. But neither in fungi nor in
other lichens is there any obvious connection with Coniocarpineae. In
some of the genera the fungus alone forms the stalk and the wall of the
capitulum; in others the thallus shares in the fruit-formation growing
around it as an amphithecium.

The semi-closed fruits point to their affinity with Pyrenolichens,
though they are more advanced than these judging from the thalline
wall that is present in some genera and also from the half-open disc
at maturity. The latter feature has influenced some systematists to
classify the whole subseries among Cyclocarpineae. The thallus, as in
_Sphaerophorus_, reaches a high degree of fruticose development; in
other genera it is crustaceous without any formation of cortex, while in
several genera or species it is non-existent, the fruits being parasites
on the thalli of other lichens or saprophytes on dead wood, humus, etc.
These latter—both parasites and saprophytes—are included by Rehm[987]
and others among fungi, which has involved the breaking up of this very
distinctive series. Rehm has thus published as Discomycetes the lichen
genera _Sphinctrina_, _Cyphelium_, _Coniocybe_, _Acolium_, _Calicium_ and
_Stenocybe_, since some or all of their species are regarded by him as
fungi.

Reinke[988] in his lichen studies states that it might not be impossible
for a saprophytic fungus to be derived from a crustaceous lichen—a
case of reversion—but that no such instance was then known. More exact
studies[989] of parasymbiosis and antagonistic symbiosis have shown the
wide range of possible life-conditions, and such a reversion does not
seem improbable. We must also bear in mind that in suitable cultures,
lichen hyphae can be grown without gonidia: they develop in that case as
saprophytes.

On Reinke’s[988] view, however, that these saprophytic species, belonging
to different genera in the Coniocarpineae, are true fungi, they would
represent the direct and closely related ancestors of the corresponding
lichen genera, giving a polyphyletic origin within this group. As fungus
genera he has united them in Protocaliciaceae, and the representatives
among fungi he distinguishes, as does Wainio[990], under such names as
_Mycocalicium_ and _Mycoconiocybe_.

If we might consider the saprophytic forms as also retrogressive
lichens, a monophyletic origin from some remote fungal ancestor would
prove a more satisfactory solution of the inheritance problem. This
view is even supported by a comparison Reinke himself has drawn between
the development of the fructification in _Mycocalicium parietinum_, a
saprophyte, and in his view a fungus, and _Chaenotheca chrysocephala_,
a closely allied lichen. Both grow on old timber. In the former (the
fungus), the mycelium pervades the outer weathered wood-cells, and the
fruit stalk rises from a clump of brownish hyphae; there is no trace of
gonidia. _Chaenotheca chrysocephala_ differs in the presence of gonidia
which are associated with the mycelium in scattered granular warts; but
the fruit stalk here also rises directly from the mycelium between the
granules. The presence of a lichen thallus chiefly differentiates between
the two plants, and this thallus is not a casual or recent association;
it is constant and of great antiquity as it is richly provided with
lichen-acids.

Reinke has indicated the course of evolution within the series but that
is on the lines of thalline development and will be considered later.

_c._ GRAPHIDINEAE. This series contains a considerable variety of lichen
forms, but all possess to a more or less marked degree the linear form
of fructification termed a “lirella” which has only a slit-like opening.
There is a tendency to round discoid fruits in the _Roccellae_ and
also in the _Arthoniae_; the apothecia of the latter, called by early
lichenologists “ardellae,” are without margins. In nearly all there is
a formation of carbonaceous black tissue either in the hypothecium or
in the proper margins. In some of them the paraphyses are branched and
dark at the tips, the branches interlocking to form a strong protective
epithecium. There are, however, constant exceptions, in some particular,
to any generalization in genera and in species. Müller-Argau’s[991]
pronouncement might be held to have special reference to Graphidineae:
“that in any genus, species or groups of species are to be found which
outwardly shew something that is peculiar, though of slight importance.”
The most constant type of gonidium is _Trentepohlia_, but _Palmella_ and
_Phycopeltis_ occasionally occur. The spores are various in colour and
form; they are rarely simple.

The genus _Arthonia_ is derived from a member of the Patellariaceae,
from which family many of the Discomycetes have arisen. The course
of development does not follow from a closed to an open fruit; the
apothecium is open from the first, and growth proceeds from the centre
outwards, the fertile cells gradually pushing aside the sterile tissue
of the exterior. The affinity of _Xylographa_ (with _Palmella_ gonidia)
is to be found in _Stictis_ in the fungal family Stictidaceae, the
apothecia of _Stictis_ being at first closed, then open, and with a thick
margin; _Xylographa_ has a more elongate lirella fruit, though otherwise
very similar, and has a very reduced thallus. Rehm[992] has classified
_Xylographa_ as a fungus.

The genera with linear apothecia are closely connected with Hysteriaceae,
and evidently inherit their fruit form severally from that family. There
is thus ample evidence of polyphyletic descent in the series. Stromatoid
fruits occur in Chiodectonaceae, with deeply sunk, almost closed disc,
but they have evidently evolved within the series, possibly from a
dividing up of the lirellae.

In Graphidineae there are also forms, more especially in Arthoniaceae,
on the border line between lichens and fungi: those with gonidia being
classified as lichens, those without gonidia having been placed in
corresponding genera of fungi. These latter athalline species live as
parasites or saprophytes.

The larger number of genera have a poorly developed thallus; in many of
them it is embedded within the outer periderm-cells of trees, and is
known as “hypophloeodal.” But in some families, such as Roccellaceae,
the thallus attains a very advanced form and a very high production of
acids.

The conception of Graphidineae as a whole is puzzling, but one or other
characteristic has brought the various members within the series. It is
in this respect an epitome of the lichen class of which the different
groups, with all their various origins and affinities, yet form a
distinct and well-defined section of the vegetable kingdom.

_d._ CYCLOCARPINEAE. This is by far the largest series of lichens. The
genera are associated with algae belonging both to the Myxophyceae and
the Chlorophyceae, and from the many different combinations are produced
great variations in the form of the vegetative body. The fruit is an
emergent, round or roundish disc or open apothecium in all the members
of the series except Pertusariaceae, where it is partially immersed in
thalline “warts.” In its most primitive form, described as “biatorine” or
“lecideine,” it may be soft and waxy (_Biatora_) or hard and carbonaceous
(_Lecidea_), in the latter the paraphyses being mostly coloured at the
tips; these are either simple or but sparingly branched, so that the
epithecium is a comparatively slight structure. The outer sterile tissue
forms a protective wall or “proper margin” which may be entirely pushed
aside, but generally persists as a distinct rim round the disc.

A great advance within the series arose when the gonidial elements of
the thallus took part in fruit-formation. In that case not only is the
hymenium generally subtended by a layer of algae, but thalline tissue
containing algae grows up around the fruit, and forms a second wall or
thalline margin. This type of apothecium, termed “lecanorine,” is thus
intimately associated with the assimilating tissue and food supply,
and it gains in capacity of ascus renewal and of long duration. This
development from non-marginate to marginate ascomata is necessarily an
accompaniment of symbiosis.

There is no doubt that the Cyclocarpineae derive from some simple form or
forms of Discomycete in the Patellariaceae. The relationship between that
family and the lower _Lecideae_ is very close. Rehm[993] finds the direct
ancestors of _Lecidea_ itself in the fungus genus, _Patinella_, in which
the apothecia are truly lecideine in character—open, flat and slightly
margined, the hypothecium nearly always dark-coloured and the paraphyses
branched, septate, clavate and coloured at the tips, forming a dark
epithecium. More definitely still he describes _Patinella atroviridis_,
a new species he discovered, as in all respects a _Lecidea_, but without
gonidia.

In the crustaceous Lecideaceae, a number of genera have been delimited
on spore characters—colourless or brown, and simple or variously
septate. In Patellariaceae as described by Rehm are included a number
of fungus genera which correspond to these lichen genera. Only two of
them—_Patinella_ and _Patellaria_—are saprophytic; in all the other
genera of the family, the species with very few exceptions are parasitic
on lichens: they are parasymbionts sharing the algal food supply; in any
case, they thrive on a symbiotic thallus.

Rehm unhesitatingly derives the corresponding lichen genera from these
fungi. He takes no account of the difficulty that if these parasitic (or
saprophytic) fungi are primitive, they have yet appeared either later in
time than the lichens on which they exist, or else in the course of ages
they have entirely changed their substratum.

He has traced, for instance, the lichen, _Buellia_, to a saprophytic
fungus species, _Karschia lignyota_, to a genus therefore in which
most of the species are parasitic on lichens and have generally been
classified as parasitic lichens. There is no advance in apothecial
characters from the fungus, _Karschia_, to _Buellia_, merely the change
to symbiosis. It therefore seems more in accordance with facts to regard
_Buellia_ as a genus evolved within the lichen series from _Patinella_
through _Lecidea_, and to accept these species of _Karschia_ on the
border line as parasitic, or even as saprophytic, reversions from
the lichen status. We may add that while these brown-spored lichens
are fairly abundant, the corresponding athalline or fungus forms are
comparatively few in number, which is exactly what might be expected from
plants with a reversionary history.

Occasionally in biatorine or lecideine species with a slight thalline
development all traces of the thallus disappear after the fructification
has reached maturity. The apothecia, if on wood or humus, appear to
be saprophytic and would at first sight be classified as fungi. They
have undoubtedly retained the capacity to live at certain stages, or in
certain conditions, as saprophytes.

The thallus disappears also in some species of the crustaceous genera
that possess apothecia with a thalline margin, and the fruits may be left
stranded and solitary on the normal substratum, or on some neighbouring
lichen thallus where they are more or less parasitic; but as the thalline
margin persists, there has been no question as to their nature and
affinity.

Rehm suggests that many species now included among lichens may be
ultimately proved to be fungi; but it is equally possible that the
reverse may be the case, as for instance _Bacidia flavovirescens_, held
by Rehm and others to be a parasitic fungus species, but since proved by
Tobler[994] to be a true lichen.

A note by Lightfoot[995], one of our old-time botanists who gave lichens
a considerable place in his Flora, foreshadows the theory of evolution
by gradual advance, and his views offer a suggestive commentary on the
subject under discussion. He was debating the systematic position of the
maritime lichen genus _Lichina_, considered then a kind of _Fucus_, and
had observed its similarity with true lichens. “The cavity,” he writes,
“at the top of the fructification (in _Lichina_) is a proof how nearly
this species of _Fucus_ is related to the scutellated lichens. Nature
disdains to be limited to the systematic rules of human invention. She
never makes any sudden starts from one class or genus to another, but is
regularly progressive in all her works, uniting the various links in the
chain of beings by insensible connexions.”


III. THE THALLUS


A. GENERAL OUTLINE OF DEVELOPMENT

_a._ PRELIMINARY CONSIDERATIONS. The evolution of lichens, as such,
has reference mainly to the thallus. Certain developments of the
fructification are evident, but the changes in the reproductive organs
have not kept pace with those of the vegetative structures: the highest
type of fruit, for instance, the apothecium with a thalline margin,
occurs in genera and species with a very primitive vegetative structure
as well as in those that have attained higher development.

Lichens are polyphyletic as regards their algal, as well as their fungal,
ancestors, so that it is impossible to indicate a straight line of
progression, but there is a general process of thalline development which
appears once and again in the different phyla. That process, from simpler
to more complicated forms, follows on two lines: on the one there is the
endeavour to increase the assimilating surface, on the other the tendency
to free the plant from the substratum. In both, the aim has been the
same, to secure more favourable conditions for assimilation and aeration.
Changes in structure have been already described[996], and it is only
needful to indicate here the main lines of evolution.

_b._ COURSE OF EVOLUTION IN HYMENOLICHENS. There is but little trace of
development in these lichens. The fungus has retained more or less the
form of the ancestral _Thelephora_ which has a wide-spreading superficial
basidiosporous hymenium. Three genera have been recognized, the
differences between them being due to the position within the thallus,
and the form of the _Scytonema_ that constitutes the gonidium. The
highest stage of development and of outward form is reached in _Cora_, in
which the gonidial zone is central in the tissue and is bounded above and
below by strata of hyphae.

_c._ COURSE OF EVOLUTION IN ASCOLICHENS. It is in the association with
Ascomycetes that evolution and adaptation have had full scope. In that
sub-class there are four constantly recurring and well-marked stages
of thalline development. (1) The earliest, most primitive stage, is
the crustaceous: at first an accretion of separate granules which may
finally be united into a continuous crust with a protective covering
of thick-walled amorphous hyphae forming a “decomposed” cortex. The
extension of a granule by growth in one direction upwards and outwards
gives detachment from the substratum, and originates (2) the squamule
which is, however, often of primitive structure and attached to the
support, like the granule, by the medullary hyphae. Further growth of
the squamule results in (3) the foliose thallus with all the adaptations
of structure peculiar to that form. In all of these, the principal area
of growth is round the free edges of the thallus. A greater change
takes place in the advance to (4) the fruticose type in which the more
active growing tissue is restricted to the apex, and in which the frond
or filament adheres at one point only to the support, a new series of
strengthening and other structures being evolved at the same time.

The lichen fungi associate, as has been already stated, with two
different types of algae: those combined with the Myxophyceae have been
designated _Phycolichenes_, those with Chlorophyceae as _Archilichenes_.
The latter predominate, not only in the number of lichens, but also in
the more varied advance of the thallus, although, in many instances,
genera and species of both series may be closely related.


B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS

One of the first questions of inheritance concerns the comparative
antiquity of the two gonidial series: with which kind of alga did the
fungus first form the symbiotic relationship? No assistance in solving
the problem is afforded by the type of fructification. The fungus in
Archilichens is frequently one of the more primitive Pyrenomycetes,
though more often a Discomycete, while in Phycolichens Pyrenomycetes are
very rare. There is, as already stated, no correlation of advance between
the fruit and the thallus, as the most highly evolved apothecia with
well-formed thalline margins are constantly combined with thalli of low
type.

Forssell[997] gave considerable attention to the question of
antiquity in his study of gelatinous crustaceous lichens in the
family Pyrenopsidaceae, termed by him Gloeolichens, and he came to
the conclusion that Archilichens represented the older combination,
Phycolichens being comparatively young.

His view is based on a study of the development of certain lichen
fungi that seem able to adapt themselves to either kind of algal
symbiont. He found[997] in _Euopsis_ (_Pyrenopsis_) _granatina_, one
of the Pyrenopsidaceae, that certain portions of the thallus contained
blue-green algae, while others contained _Palmella_, and that these
latter, though retrograde in development, might become fertile. The
granules with blue-green gonidia were stronger, more healthy and capable
of displacing those with _Palmella_, but not of bearing apothecia, though
spermogonia were embedded in them—a first step, according to Forssell,
towards the formation of apothecia. These granules, not having reached
a fruiting stage, were reckoned to be of a more recent type than those
associated with _Palmella_. In other instances, however, the line of
evolution has been undoubtedly from blue-green to more highly evolved
bright-green thalli.

The striking case of similarity between _Psoroma hypnorum_ (bright-green)
and _Pannaria rubiginosa_ (blue-green) may also be adduced. Forssell
considers that _Psoroma_ is the more ancient form, but as the fungus is
adapted to associate with either kind of alga, the type of squamules
forming the thallus may be gradually transformed by the substitution of
blue-green for the earlier bright-green—the _Pannaria_ superseding the
_Psoroma_. There is a close resemblance in the fructification—that is of
the fungus—in these two different lichens.

Hue[998] shares Forssell’s opinion as to the greater antiquity of
the bright-green gonidia and cites the case of _Solorina crocea_. In
that lichen there is a layer of bright-green gonidia in the usual
dorsiventral position, below the upper cortex. Below this zone there is
a second formed entirely of blue-green cells. Hue proved by his study of
development in _Solorina_ that the bright-green were the normal gonidia
of the thallus, and were the only ones present in the growing peripheral
areas; the blue-green were a later addition, and appeared first in small
groups at some distance from the edge of the lobes.

The whole subject of cephalodia-development[999] has a bearing on this
question. These bodies always contain blue-green algae, and are always
associated with Archilichens. Mostly they occur as excrescences, as in
_Stereocaulon_ and in _Peltigera_. The fungus of the host-lichen though
normally adapted to bright-green algae has the added capacity of forming
later a symbiosis with the blue-green. This tendency generally pervades a
whole genus or family, the members of which, as in Peltigeraceae, are too
closely related to allow as a rule of separate classification even when
the algae are totally distinct.


C. EVOLUTION OF PHYCOLICHENS

The association of lichen-forming fungi with blue-green algae may have
taken place later in time, or may have been less successful than with
the bright-green: they are fewer in number, and the blue-green type of
thallus is less highly evolved, though examples of very considerable
development are to be found in such genera as _Peltigera_, _Sticta_ or
_Nephromium_.

_a._ GLOEOLICHENS. Among crustaceous forms the thallus is generally
elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The
algae of that family, _Gloeocapsa_, _Xanthocapsa_ or _Chroococcus_, are
furnished with broad gelatinous sheaths which, in the lichenoid state,
are penetrated and traversed by the fungal filaments, a branch hypha
generally touching with its tip the algal cell-wall. Under the influence
of symbiosis, the algal masses become firmer and more compact, without
much alteration in form; algae entirely free from hyphae are often
intermingled with the others. Even among Gloeolichens there are signs
of advancing development both in the internal structure and in outward
form. Lobes free from the substratum, though very minute, appear in
the genus _Paulia_, the single species of which comes from Polynesia.
Much larger lobes are characteristic of _Thyrea_, a Mediterranean and
American genus. The fruticose type, with upright fronds of minute size,
also appears in our native genus _Synalissa_. It is still more marked in
the coralloid thalli of _Peccania_ and _Phleopeccania_. In most of these
genera there is also a distinct tendency to differentiation of tissues,
with the gonidia congregating towards the better lighted surfaces. The
only cortex formation occurs in the crustaceous genus _Forssellia_ in
which, according to Zahlbruckner[1000], it is plectenchymatous above, the
thallus being attached below by hyphae penetrating the substratum. In
another genus, _Anema_[1001], which is minutely lobate-crustaceous, the
internal hyphae form a cellular network in which the algae are immeshed.
As regards algal symbionts, the members of this family are polyphyletic
in origin.

_b._ EPHEBACEAE AND COLLEMACEAE. In Ephebaceae the algae are tufted and
filamentous, _Scytonema_, _Stigonema_ or _Rivularia_, the trichomes of
which are surrounded by a common gelatinous sheath. The hyphae travel in
the sheath alongside the cell-rows, and the symbiotic plant retains the
tufted form of the alga as in _Lichina_ with _Rivularia_, _Leptogidium_
with _Scytonema_, and _Ephebe_ with _Stigonema_. The last named lichen
forms a tangle of intricate branching filaments about an inch or more in
length. The fruticose habit in these plants is an algal characteristic;
it has not been acquired as a result of symbiosis, and does not signify
any advance in evolution.

A plectenchymatous cortex marks some progress here also in
_Leptodendriscum_, _Leptogidium_ and _Polychidium_, all of which are
associated with _Scytonema_. These genera may well be derived from an
elementary form such as _Thermutis_. They differ from each other in spore
characters, etc., _Polychidium_ being the most highly developed with its
cortex of two cell-rows and with two-celled spores.

_Nostoc_ forms the gonidium of Collemaceae. In its free state it is
extremely gelatinous and transmits that character more or less to the
lichen. In the crustaceous genus _Physma_, which forms the base of the
_Collema_ group or phylum, there is but little difference in form
between the thalline warts of the lichen crust and the original small
_Nostoc_ colonies such as are to be found on damp mosses, etc.

In _Collema_ itself, the less advanced species are scarcely more than
crusts, though the more developed show considerable diversity of lobes,
either short and pulpy, or spreading out in a thin membrane. The _Nostoc_
chains pervade the homoiomerous thallus, but in some species they lie
more towards the upper surface. There is no cortex, though once and again
plectenchyma appears in the apothecial margin, both in this genus and in
_Leprocollema_ which is purely crustaceous.

_Leptogium_ is a higher type than _Collema_, the thallus being
distinguished by its cellular cortex. The tips of the hyphae, lying
close together at the surface, are cut off by one or more septa, giving
a one- or several-celled cortical layer. The species though generally
homoiomerous are of thinner texture and are less gelatinous than those of
_Collema_.

_c._ PYRENIDIACEAE. This small family of pyrenocarpous Phycolichens
may be considered here though its affinity, through the form of the
fruiting body, is with Archilichens. The gonidia are species of _Nostoc_,
_Scytonema_ and _Stigonema_. There are only five genera; one of these,
_Eolichen_, contains three species, the others are monotypic.

The crustaceous genera have a non-corticate thallus, but an advance
to lobate form takes place in _Placothelium_, an African genus. The
two genera that show most development are both British: _Coriscium_
(_Normandina_), which is lobate, heteromerous and corticate—though
always sterile—and _Pyrenidium_ which is fruticose in habit; the latter
is associated with _Nostoc_ and forms a minute sward of upright fronds,
corticate all round; the perithecium is provided with an entire wall and
is immersed in the thallus.

If the thallus alone were under consideration these lichens would rank
with Pannariaceae.

_d._ HEPPIACEAE AND PANNARIACEAE. The next stage in the development of
Phycolichens takes place through the algae, _Scytonema_ and _Nostoc_,
losing not only their gelatinous sheaths, but also, to a large extent,
their characteristic forms. Chains of cells can frequently be observed,
but accurate and certain identification of the algal genus is only
possible by making separate cultures of the gonidia.

_Scytonema_ forms the gonidium of the squamulose Heppiaceae consisting
of the single genus _Heppia_. The ground tissue of the species is either
wholly of plectenchyma with algae in the interstices, or the centre is
occupied by a narrow medulla of loose filaments.

In the allied family Pannariaceae, a number of genera contain _Scytonema_
or _Nostoc_, while two, _Psoroma_ and _Psoromaria_, have bright-green
gonidia. The thallus varies from crustaceous or minutely squamulose, to
lobes of fair dimension in _Parmeliella_ and in _Hydrothyria venosa_,
an aquatic lichen. Plectenchyma appears in the upper cortex of both of
these, and in the proper margin of the apothecia, while the under surface
is frequently provided with rhizoidal filaments.

These two families form a transition between the gelatinous, and mostly
homoiomerous thallus, and the more developed entirely heteromerous
thallus of much more advanced structure. The fructification in all of
them, gelatinous and non-gelatinous, is a more or less open apothecium,
sometimes immarginate, and biatorine or lecideine, but often, even
in species nearly related to these, it is lecanorine with a thalline
amphithecium. Rarely are the sporiferous bodies sunk in the tissue, with
a pseudo-perithecium, as in _Phylliscum_. It would be difficult to trace
advance in all this group on the lines of fruit development. The two
genera with bright-green gonidia, _Psoroma_ and _Psoromaria_, have been
included in Pannariaceae owing to the very close affinity of _Psoroma
hypnorum_ with _Pannaria rubiginosa_; they are alike in every respect
except in their gonidia. _Psoromaria_ is exactly like _Psoroma_, but
with immarginate biatorine apothecia, representing therefore a lower
development in that respect.

These lichens not only mark the transition from gelatinous to
non-gelatinous forms, but in some of them there is an interchange of
gonidia. The progression in the phylum or phyla has evidently been
from blue-green up to some highly evolved forms with bright-green
algae, though there may have been, at the beginning, a substitution of
blue-green in place of earlier bright-green algae, Phycolichens usurping
as it were the Archilichen condition.

_e._ PELTIGERACEAE AND STICTACEAE. The two families just examined
marked a great advance which culminated in the lobate aquatic lichen
_Hydrothyria_. This lichen, as Sturgis pointed out, shows affinity with
other Pannariaceae in the structure of the single large-celled cortical
layer as well as with species of _Nephroma_ (Peltigeraceae). A still
closer affinity may be traced with _Peltigera_ in the presence in both
plants of veins on the under surface. The capacity of _Peltigera_ species
to grow in damp situations may also be inherited from a form like the
submerged _Hydrothyria_. In both families there are transitions from
blue-green to bright-green gonidia, or _vice versa_, in related species.
Thus in Peltigeraceae we find _Peltigera_ containing _Nostoc_ in the
gonidial zone, with _Peltidea_ which may be regarded as a separate genus,
or more naturally as a section of _Peltigera_; it contains bright-green
gonidia, but has cephalodia containing _Nostoc_ associated with its
thallus.

The genus _Nephroma_ is similarly divided into species with a
bright-green gonidial zone, chiefly Arctic or Antarctic in distribution,
and species with _Nostoc_ (subgenus _Nephromium_) more numerous and more
widely distributed.

_Peltigera_ and _Nephroma_ are also closely related in the character of
the fructification. It is a flat non-marginate disc borne on the edge of
the thallus: in _Peltigera_ on the upper surface, in _Nephroma_ on the
under surface. The remaining genus _Solorina_ contains normally a layer
of bright-green algae, but, along with these, there are always present
more or fewer _Nostoc_ cells, either in a thin layer as in _S. crocea_ or
as cephalodia in others, while, in three species the algae are altogether
blue-green.

The members of the Peltigeraceae have a thick upper cortex of
plectenchyma and in some cases strengthening veins, and long rhizinae on
the lower side. Some of the species attain a large size, and, in some,
soredia are formed, an evidence of advance, this being a peculiarly
lichenoid form of reproduction.

The Stictaceae form a parallel but more highly organized family, which
also includes closely related bright-green and blue-green series. They
are all dorsiventral, but they are mostly attached by a single hold-fast
and the lobes in some species suggest the fruticose type in their long
narrow form. A wide cortex of plectenchyma protects both the upper and
the lower surface and a felt of hairs replaces the rhizinae of other
foliose lichens. In the genus _Sticta_ (including the section _Stictina_)
special aeration organs, cyphellae or pseudocyphellae, are provided; in
_Lobaria_ these are replaced by naked areas which serve the same purpose.

Nylander[1002] regarded the Stictaceae as the most highly developed of
all lichens, and they easily take a high place among dorsiventral forms,
but it is generally conceded that the fruticose type is the more highly
organized. In any case they are the highest reach of the phylum or phyla
that started with Pyrenopsidaceae and Collemaceae; the lowly gelatinous
thalli changing to more elaborate structures with the abandonment of the
gelatinous algal sheath, as in the Pannariaceae, and with the replacement
of blue-green by bright-green gonidia. Reinke[1003], considers the
Stictaceae as evolved from the Pannariaceae more directly from the genus
_Massalongia_. Their relationship is certainly with Pannariaceae and
Peltigeraceae rather than with Parmeliaceae; these latter, as we shall
see, belong to a wholly different series.


D. EVOLUTION OF ARCHILICHENS

The study of Archilichens as of Phycolichens is complicated by the many
different kinds of fungi and algae that have entered into combination;
but the two principal types of algae are the single-celled _Protococcus_
group and the filamentous _Trentepohlia_: as before only the broad lines
of thalline development will be traced.

The elementary forms in the different series are of the simplest type—a
somewhat fortuitous association of alga and fungus, which in time bears
the lichen fructification. It has been stated that the greatest advance
of all took place with the formation of a cortex over the primitive
granule, followed by a restricted area of growth outward or upward
which resulted finally in the foliose and fruticose thalli. Guidance
in following the course of evolution is afforded by the character of
the fructification, which generally shows some great similarity of type
throughout the different phyla, and remains fairly constant during the
many changes of thalline evolution. Development starting from one or many
origins advances point by point in a series of parallel lines.

_a._ THALLUS OF PYRENOCARPINEAE. In this series there are two families
of algae that function as gonidia: Protococcaceae, consisting of single
cells, and Trentepohliaceae, filamentous. _Phyllactidium_ (_Cephaleuros_)
appears in a single genus, _Strigula_, a tropical epiphytic lichen.

Associated with these types of algae are a large number of genera and
species of an elementary character, without any differentiation of
tissue. In many instances the thallus is partly or wholly embedded in the
substratum.

Squamulose or foliose forms make their appearance in Dermatocarpaceae:
in _Normandina_ the delicate shell-like squamules are non-corticate, but
in other genera, _Endocarpon_, _Placidiopsis_, etc., the squamules are
corticate and of firmer texture, while in _Dermatocarpon_, foliose fronds
of considerable size are formed. The perithecial fruits are embedded in
the upper surface.

In only one extremely rare lichen, _Pyrenothamnia Spraguei_ (N. America),
is there fruticose development: the thallus, round and stalk-like at the
base, branches above into broader more leaf-like expansions.

_b._ THALLUS OF CONIOCARPINEAE. At the base of this series are
genera and species that are extremely elementary as regards thalline
formation, with others that are saprophytic and parasitic. The
simplest type of thallus occurs in Caliciaceae, a spreading mycelium
with associated algae (Protococcaceae) collected in small scattered
granules, resembling somewhat a collection of loose soredia. The
species grow mostly on old wood, trunks of trees, etc. In _Calicium_
(_Chaenotheca_) _chrysocephalum_ as described by Neubner[1004] the
first thallus formation begins with these scattered minute granules;
gradually they increase in size and number till a thick granular coating
of the substratum arises, but no cortex is formed and there is no
differentiation of tissue.

The genus _Cyphelium_ (Cypheliaceae) is considered by Reinke to be
more highly developed, inasmuch as the thalline granules, though
non-corticate, are more extended horizontally, and, in vertical
section, show a distinct differentiation into gonidial zone and
medulla. The sessile fruit also takes origin from the thallus, and is
surrounded by a thalline amphithecium, or rather it remains embedded
in the thalline granule. A closely allied tropical genus _Pyrgillus_
has reached a somewhat similar stage of development, but with a more
coherent homogeneous thallus, while in _Tylophoron_, also tropical or
subtropical, the fruit is raised above the crustaceous thallus but is
thickly surrounded by a thalline margin. The alga of that genus is
_Trentepohlia_, a rare constituent of Coniocarpineae.

A much more advanced formation appears in the remaining family
Sphaerophoraceae. In _Calycidium_, a monotypic New Zealand genus, the
thallus consists of minute squamules, dorsiventral in structure but with
a tendency to vertical growth, the upper surface is corticate and the
mazaedial apothecia—always open—are situated on the margins. _Tholurna
dissimilis_, (Scandinavian) still more highly developed, has two kinds
of rather small fronds corticate on both surfaces, the one horizontal in
growth, crenulate in outline, and sterile, the other vertical, about 2
mm. in height, hollow and terminating in a papilla in which is seated the
apothecium.

Two other monotypic subtropical genera form a connecting link with the
more highly evolved forms. In the first, _Acroscyphus sphaerophoroides_,
the fronds are somewhat similar to the fertile ones of _Tholurna_, but
they possess a solid central strand and the apical mazaedium is less
enveloped by the thallus. The other, _Pleurocybe madagascarea_, has
narrow flattish branching fronds about 3 cm. in height, hollow in the
centre and corticate with marginal or surface fruits.

The third genus, _Sphaerophorus_, is cosmopolitan; three of the species
are British and are fairly common on moorlands, etc. They are fruticose
in habit, being composed of congregate upright branching stalks, either
round or slightly compressed and varying in height from about 1 to 8
cm. The structure is radiate with a well-developed outer cortex, and a
central strand which gives strength to the somewhat slender stalks. The
fruits are lodged in the swollen tips and are at first enclosed; later,
the covering thallus splits irregularly and exposes the hymenium.

Coniocarpineae comprise only a comparatively small number of genera
and species, but the series is of unusual interest as being extremely
well defined by the fruit-formation and as representing all the various
stages of thalline development from the primitive crustaceous to the
highly evolved fruticose type. With the primitive thallus is associated
a wholly fungal fruit, both stalk and capitulum, which in the higher
forms is surrounded and protected by the thallus. Lichen-acids are freely
produced even in crustaceous forms, and they, along with the high stage
of development reached, testify to the great antiquity of the series.

_c._ THALLUS OF GRAPHIDINEAE. As formerly understood, this series
included only crustaceous forms with an extremely simple development
of thallus, fungi and algae—whether Palmellaceae, etc., or more
frequently Trentepohliaceae—growing side by side either superficially
or embedded in tree or rock, the presence of the vegetative body being
often signalled only by a deeper colouration of the substratum. The
researches of Almquist, and more recently of Reinke and Darbishire, have
enlarged our conception of the series, and the families Dirinaceae and
Roccellaceae are now classified in Graphidineae.

Arthoniaceae, Graphidaceae and Chiodectonaceae are all wholly
crustaceous. The first thalline advance takes place in Dirinaceae with
two allied genera, _Dirina_ and _Dirinastrum_. Though the thallus is
still crustaceous, it is of considerable thickness, with differentiation
of tissues: on the lower side there is a loosely filamentous medulla from
which hyphae pierce the substratum and secure attachment. _Trentepohlia_
gonidia lie in a zone above the medulla, and the upper cortex is formed
of regular palisade hyphae forming a “fastigiate cortex.” It is the
constant presence of _Trentepohlia_ algae as well as the tendency to
ellipsoid or lirellate fruits that have influenced the inclusion of
Dirinaceae and Roccellaceae in the series.

The thallus of Dirinaceae is crustaceous, while the genera of
Roccellaceae are mostly of an advanced fruticose type, though in one,
_Roccellina_, there is a crustaceous thallus with an upright portion
consisting of short swollen podetia-like structures with apothecia
at the tips; and in another, _Roccellographa_, the fronds broaden to
leafy expansions. They are nearly all rock-dwellers, often inhabiting
wind-swept maritime coasts, and a strong basal sheath has been evolved to
strengthen their foothold. In some genera the sheath contains gonidia;
in others the tissue is wholly of hyphae—in nearly every case it is
protected by a cortex.

In the upright fronds the structure is radiate: generally a rather loose
strand of hyphae more or less parallel with the long axis of the plant
forms a central medulla. The gonidia lie outside the medulla and just
within the outer cortex. The latter, in a few genera, is fibrous, the
parallel hyphae being very closely compacted; but in most members of the
family the fastigiate type prevails, as in the allied family Dirinaceae.

_d._ THALLUS OF CYCLOCARPINEAE. This is by far the largest and most
varied series of Archilichens. It is derived, as regards the fungal
constituent, from the Discomycetes, but in these fungi, the vegetative or
mycelial body gives no aid to the classification which depends wholly on
apothecial characters. In the symbiotic condition, on the contrary, the
thallus becomes of extreme importance in the determination of families,
genera and species. There has been within the series a great development
both of apothecial and of thalline characters in parallel lines or phyla.

_AA._ _LECIDEALES._ The type of fruit nearest to fungi in form and origin
occurs in the Lecideales. It is an open disc developed from the fungal
symbiont alone, the alga taking no part. There are several phyla to be
considered.

_aa._ _COENOGONIACEAE._ There are two types of gonidial algae in this
family, and both are filamentous forms, _Trentepohlia_ in _Coenogonium_
and _Cladophora_ in _Racodium_. The resulting lichens retain the slender
thread-like form of the algae, their cells being thinly invested by the
hyphae and both symbionts growing apically. The thalline filaments are
generally very sparingly branched and grow radially side by side in a
loose flat expansion attached at one side by a sheath, or the strands
spread irregularly over the substratum. Plectenchyma appears in the
apothecial margin in _Coenogonium_. Fruiting bodies are unknown in
_Racodium_.

Coenogoniaceae are a group apart and of slight development, only the one
kind of thallus appearing; the form is moulded on that of the gonidium,
and is, as Reinke[1005] remarks, perfectly adapted to receive the maximum
of illumination and aeration.

_bb._ _LECIDEACEAE AND GYROPHORACEAE._ The origin of this thalline phylum
is distinct from that of the previous family, being associated with a
different type of gonidium, the single-celled alga of the Protococcaceae.

The more elementary species are of extremely simple structure as
exemplified in such species as _Lecidea_ (_Biatora_) _uliginosa_
or _Lecidea granulosa_. These lichens grow on humus-soil and the
thallus consists of a spreading mycelium or hypothallus with more or
less scattered thalline granules containing gonidia, but without any
defined structure. The first advance takes place in the aggregation
and consolidation of such thalline granules and the massing of the
gonidia towards the light, thus substituting the heteromerous for the
homoiomerous arrangement of the tissues. The various characters of
thickness, areolation, colour, etc. of the thallus are constant and
are expressed in specific diagnoses. Frequently an amorphous cortex of
swollen hyphae provides a smooth upper surface and forms a protective
covering for such long-lived species as _Rhizocarpon geographicum_, etc.

The squamulose thallus is well represented in this phylum. The squamules
vary in size and texture but are mostly rather thick and stiff. In
_Lecidea ostreata_ they rise from the substratum in serried rows forming
a dense sward; in _L. decipiens_, also a British species, the squamules
are still larger, and more horizontal in direction; they are thick and
firm and the upper cortex is a plectenchyma of cells with swollen walls.
Solitary hyphae from the medulla pass downwards into the support.

Changes in spore characters also arise in these different thalline
series, as for instance in genera such as _Biatorina_ and _Buellia_,
the one with colourless, the other with brown, two-celled spores. These
variations, along with changes in the thallus, are of specific or generic
importance following the significance accorded to the various characters.

In one lichen of the series, the monotypic Brazilian genus
_Sphaerophoropsis stereocauloides_, the thallus is described by
Wainio[1006] as consisting of minute clavate stalks of interwoven
thick-walled hyphae, with gelatinous algae, like _Gloeocapsa_,
interspersed in groups, though with a tendency to congregate towards the
outer surface.

The highest development along this line of advance is to be found in the
Gyrophoraceae, a family of lichens with a varied foliose character and
dark lecideine apothecia. The thallus may be monophyllous and of fairly
large dimensions or polyphyllous; it is mostly anchored by a central
stout hold-fast and both surfaces are thickly corticate with a layer
of plectenchyma; the under surface is mostly bare, but may be densely
covered with rhizina-like strands of dark hyphae. They are all northern
species and rock-dwellers exposed to severe extremes of illumination
and temperature, but well protected by the thick cortex and the dark
colouration common to them all.

_cc._ _CLADONIACEAE._ This last phylum of Lecideales is the most
interesting as it is the most complicated. It possesses a primary,
generally sterile, thallus which is dorsiventral and crustaceous,
squamulose or in some instances almost foliaceous, along with a secondary
thallus of upright radiate structure and of very varied form, known as
the podetium which bears at the summit the fertile organs.

A double thallus has been suggested in the spreading base, containing
gonidia, of some radiate lichens such as _Roccella_, but the upright
portion of such lichens, though analogous, is not homologous with that of
Cladoniaceae.

The algal cells of the family belong to the Protococcaceae.
Blue-green algae are associated in the cephalodia of _Pilophorus_ and
_Stereocaulon_. The primary thallus is a feature of all the members,
though sometimes very slight and very short-lived, as in _Stereocaulon_
or in the section _Cladina_ of the genus _Cladonia_. Where the primary
thallus is most largely developed, the secondary (the podetium) is less
prominent.

This secondary thallus originates in two different ways: (1) the primary
granule may grow upward, the whole of the tissues taking part in the
new development; or (2) the origin may be endogenous and proceed from
the hyphae only of the gonidial zone: these push upwards in a compact
fascicle, as in the apothecial development of _Lecidea_, but instead of
spreading outward on reaching the surface, they continue to grow in a
vertical direction and form the podetium. In origin this is an apothecial
stalk, but generally it is clothed with gonidial tissue. The gonidia may
travel upwards from the base or they may possibly be wind borne from
the open. The podetium thus takes on an assimilative function and is a
secondary thallus.

The same type of apothecium is common to all the genera; the spores are
colourless and mostly simple, but there are also changes in form and
septation not commensurate with thalline advance, as has been already
noted. Thus in _Gomphillus_, with primitive thallus and podetium, the
spores are long and narrow with about 100 divisions.

1. ORIGIN OF CLADONIA. There is no difficulty in deriving Cladoniaceae
from _Lecidea_, or, more exactly, from some crustaceous species of the
section _Biatora_ in which the apothecia—as in Cladoniaceae—are waxy and
more or less light-coloured and without a thalline margin. In only a very
few isolated instances has a thalline margin grown round the _Cladonia_
fruit.

There are ten genera included in the Cladoniaceae, of which five are
British. Considerable study has been devoted to the elucidation of
developmental problems within the family by various workers, more
especially in the large and varied genus _Cladonia_ which is complicated
by the presence of the two thalli. The family is monophyletic in origin,
though many subordinate phyla appear later.

2. EVOLUTION OF THE PRIMARY THALLUS. At the base of the series we find
here also an elementary granular thallus which appears in some species
of most of the genera. In _Gomphillus_, a monospecific British genus,
the granules have coalesced into a continuous mucilaginous membrane.
In _Baeomyces_, though mostly crustaceous, there is an advance to the
squamulose type in _B. placophyllus_, and in two Brazilian species
described by Wainio, one of which, owing to the form of the fronds,
has been placed in a separate genus _Heteromyces_. The primary thallus
becomes almost foliose also in _Gymnoderma coccocarpum_ from the
Himalayas, with dorsiventral stratose arrangement of the tissues, but
without rhizinae. The greatest diversity is however to be found in
_Cladonia_ where granular, squamulose and almost foliose thalli occur.
The various tissue formations have already been described[1007].

3. EVOLUTION OF THE SECONDARY THALLUS. Most of the interest centres round
the development and function of the podetium. In several genera the
primordium is homologous with that of an apothecium; its elongation to an
apothecial stalk is associated with delayed fructification, and though
it has taken on the function of the vegetative thallus, the purpose of
elongation has doubtless been to secure good light conditions for the
fruit, and to facilitate a wide distribution of spores: therefore, not
only in development but in function, its chief importance though now
assimilative was originally reproductive. The vegetative development of
the podetium is correlated with the reduction of the primary thallus
which in many species bears little relation in size or persistence to the
structure produced from it, as, for instance, in _Cladonia rangiferina_
where the ground thallus is of the scantiest and very soon disappears,
while the podetial thallus continues to grow indefinitely and to
considerable size.

4. COURSE OF PODETIAL DEVELOPMENT. In _Baeomyces_ the podetial primordium
is wholly endogenous in some species, but in others the outer cortical
layer of the primary thallus as well as the gonidial hyphae take part
in the formation of the new structure which, in that case, is simply a
vertical extension of the primary granule. This type of podetium—called
by Wainio[1008] a pseudopodetium—also recurs in _Pilophorus_ and in
_Stereocaulon_. To emphasize the distinction of origin it has been
proposed to classify these two latter genera in a separate family, but in
that case it would be necessary to break up the genus _Baeomyces_. We may
assume that the endogenous origin of the “apothecial stalk” is the more
primitive, as it occurs in the most primitive lecideine lichens, whereas
a vertical thallus is always an advanced stage of vegetative development.

Podetia are essentially secondary structures, and they are associated
both with crustaceous and squamulose primary thalli. If monophyletic in
origin their development must have taken place while the primary thallus
was still in the crustaceous stage, and the inherited tendency to form
podetia must then have persisted through the change to the squamulose
type. In species such as _Cl. caespiticia_ the presence of rudimentary
podetia along with large squamules suggests a polyphyletic origin, but
Wainio’s[1008] opinion is that such instances may show retrogression
from an advanced podetial form, and that the evidence inclines to the
monophyletic view of their origin.

The hollow centre of the podetium arises in the course of development
and is common to nearly all advanced stages of growth. There are however
some exceptions: in _Glossodium aversum_, a soil lichen from New Granada,
and the only representative of the genus, a simple or rarely forked
stalk about 2 cm. in height rises from a granular or minutely squamulose
thallus. The apothecium occupies one side of the flattened and somewhat
wider apex. There is no external cortex and the central tissue is of
loose hyphae. In _Thysanothecium Hookeri_, also a monotypic genus from
Australia, the podetia are about the same height, but, though round at
the base, they broaden upwards into a leaf-like expansion. The central
tissue below is of loose hyphae, but compact strands occur above, where
the apothecium spreads over the upper side. The under surface is sterile
and is traversed by nerve-like strands of hyphae.

5. VARIATION IN CLADONIA. It is in this genus that most variation is
to be found. Characters of importance and persistence have arisen by
which secondary phyla may be traced within the genus: these are mainly
(1) the relative development of the horizontal and vertical structures,
(2) formation of the scyphus and branching of the podetium, with (3)
differences in colour both in the vegetative thallus and in the apothecia.

Wainio has indicated the course of evolution on the following lines: (1)
the crustaceous thallus is monophyletic in origin and here as elsewhere
precedes the squamulose. The latter he considers to be also monophyletic,
though at more than one point the more advanced and larger foliose forms
have appeared: (2) the primitive podetium was subulate and unbranched,
and the apex was occupied by the apothecium. Both scyphus and branching
are later developments indicating progress. They are in both cases
associated with fruit-formation—scyphi generally arising from abortive
apothecia[1009], branching from aggregate apothecia. In forms such as
_Cl. fimbriata_, where both scyphiferous and subulate sterile podetia
are frequent, the latter (subspecies _fibula_) are retrogressive, and
reproduce the ancestral pointed podetium. (3) In subgen. _Cenomyce_,
with a squamulose primary thallus, there is a sharp division into two
main phyla characterized by the colour of the apothecia, brown in
_Ochrophaeae_—the colour being due to a pigment—and red in _Cocciferae_
where the colouring substance is a lichen-acid, rhodocladonic acid.
In the brown-fruited _Ochrophaeae_ there are again several secondary
phyla. Two of these are distinguished primarily by the character of the
branching: (_a_) the _Chasmariae_ in which two or several branches arise
from the same level, entailing perforation of the axils (_Cl. furcata_,
_Cl. rangiformis_, _Cl. squamosa_, etc.), the scyphi also are perforated.
They are further characterized by peltate aggregate apothecia, this
grouping of the apothecia according to Wainio being the primary cause
of the complex branching, the several fruit stalks growing out as
branches. The second group (_b_), the _Clausae_, are not perforated and
the apothecia are simple and broad-based on the edge of the scyphus
(_Cl. pyxidata_, _Cl. fimbriata_, etc.), or on the tips of the podetia
(_Cl. cariosa_, _Cl. leptophylla_, etc.). A third very small group also
of _Clausae_ called (_c_) _Foliosae_ has very large primary squamules
and reduced podetia (_Cl. foliacea_, etc.), while finally (_d_) the
_Ochroleucae_, none of which is British, have poorly developed squamules
and variously formed yellowish podetia with pale-coloured apothecia.

The _Cocciferae_ represent a phylum parallel in development with the
_Ochrophaeae_. The species have perhaps most affinity with the _Clausae_,
the vegetative thallus—both the squamules and the podetia—being very
much alike in several species. Wainio distinguishes two groups based on
a difference of colour in the squamules, glaucous green in one case,
yellowish in the other.

6. CAUSES OF VARIATION. External causes of variation in _Cladonia_ are
chiefly humidity and light, excess or lack of either effecting changes
which may have become fixed and hereditary. Minor changes directly
traceable to these influences are also frequent, viz. size of podetia,
proliferation and the production more or less of soredia or of squamules
on the podetia, though only in connection with species in which these
variations are already an acquired character. The squamules on the
podetium more or less repeat the form of the basal squamules.

7. PODETIAL DEVELOPMENT AND SPORE-DISSEMINATION. In a recent paper by
Hans Sättler[1010] the problem of podetial development in _Cladonia_ is
viewed from a different standpoint. He holds that as the podetia are
apothecial stalks, their service to the plant consists in the raising of
the mature fruit in order to secure a wide distribution of the spores,
and that changes in the form of the podetium are therefore but new
adaptations for the more efficient discharge of this function.

Following out this idea he regards as the more primitive forms those
in which both the spermogonia, as male reproductive bodies, and the
carpogonia occur on the primary thallus, ascogonia and trichogynes being
formed before the podetium emerges from the thallus. Fertilization thus
must take place at a very early period, though the ultimate fruiting
stage may be long delayed. Sättler considers that any doubt as to actual
fertilization is without bearing on the question, as sexuality he holds
must have originally existed and must have directed the course of
evolution in the reproductive bodies. In this primitive group, called
by him the “Floerkeana” group, the podetia are always short and simple,
they are terminated by the apothecium and no scyphi are formed (_Cl.
Floerkeana_, _Cl. leptophylla_, _Cl. cariosa_, _Cl. caespiticia_, _Cl.
papillaria_, etc.).

In his second or “pyxidata” group, he places those species in which the
apothecia are borne at the edge of a scyphus. That structure he follows
Wainio in regarding as a morphological reaction on the failure of the
first formed apical apothecium: it is, he adds, a new thallus in the form
of a spreading cup and bears, as did the primary thallus, both the female
primordia and the spermogonia. In some species, such as _Cl. foliacea_,
there may be either scyphous or ascyphous podetia, and spermogonia
normally accompany the carpogonium appearing accordingly along with it
either on the squamule or on the scyphus.

As the pointed podetia are the more primitive, Sättler points out that
they may reappear as retrogressive structures, and have so appeared
in the “pyxidata” group in such species as _Cl. fimbriata_. He refers
to Wainio’s statement that the abortion of the apothecium being a
retrogressive anomaly, while scyphus formation is an evolutionary
advance, the scyphiferous species present the singular case, “that a
progressive transmutation induced by a retrogressive anomaly has become
constant.”

His third group includes those forms that grow in crowded tufts or
swards such as _Cl. rangiferina_, _Cl. furcata_, _Cl. gracilis_, etc.
They originate, as did the pyxidata group, in some _Floerkeana_-like
form, but in the “rangiferina” group instead of cup-formation there is
extensive branching. In the closely packed phalanx of branches water is
retained as in similar growths of mosses, and moist conditions necessary
for fertilization are thus secured as efficiently as by the water-holding
scyphus.

Sättler in his argument has passed over many important points. Above
all he ignores the fact that whatever may have been the original nature
and function of the podetium, it has now become a thalline structure
and provides for the vegetative life of the plant, and that it is in
its thalline condition that the many variations have been formed; the
scyphus is not, as he contends, a new thallus, it is only an extension of
thalline characters already acquired.

8. PILOPHORUS, STEREOCAULON AND ARGOPSIS. These closely related genera
are classified with _Cladonia_ as they share with it the twofold thallus
and the lecideine apothecia. The origin of the podetium being different
they may be held to constitute a phylum apart, which has however taken
origin also from some _Biatora_ form.

The primary thallus is crustaceous or minutely squamulose and the
podetia of _Pilophorus_, which are short and unbranched (or very
sparingly branched), are beset with thalline granules. The podetia of
_Stereocaulon_ and _Argopsis_ are copiously branched and are more or
less thickly covered with minute variously divided leaflets. Cephalodia
containing blue-green algae occur on the podetia of these latter genera;
in _Pilophorus_ they are intermixed with the primary thallus.

The tissue systems are less advanced in these genera than in _Cladonia_:
there is no cortex present either in _Pilophorus_ or in _Argopsis_ or in
some species of _Stereocaulon_, though in others a gelatinous amorphous
layer covers the podetia and also the stalk leaflets. The stalks are
filled with loose hyphae in the centre.

_BB._ _LECANORALES._ This second group of Cyclocarpineae is distinguished
by the marginate apothecium, a thalline layer providing a protecting
amphithecium. The lecanorine apothecium is of a more or less soft
and waxy consistency, and though the disc is sometimes almost black,
neither hypothecium nor parathecium is carbonaceous as in _Lecidea_. The
affinity of _Lecanora_ is with sect. _Biatora_, and development must
have been from a biatorine form with a persistent thallus. The margin or
amphithecium varies in thickness: in some species it is but scanty and
soon excluded by the over-topping growth of the disc, so that a zone of
gonidia underlying the hypothecium is often the only evidence of gonidial
intrusion left in fully formed fruits.

The marginate apothecium has appeared once and again as we have seen.
It is probable however that its first development was in this group of
lichens, and even here there may have been more than one origin as there
is certainly more than one phylum.

_aa._ _COURSE OF DEVELOPMENT._ At the base of the series, the thallus
is of the crustaceous type somewhat similar to that of _Lecidea_, but
there are none of the very simple primitive forms. _Lecanora_ must
have originated when the crustaceous lecideine thallus was already
well established. Its affinity is with _Lecidea_ and not with any
fungus: where the thallus is evanescent or scanty, its lack is due to
retrogressive rather than to primitive characters.

_bb._ _LECANORACEAE._ A number of genera have arisen in this large
family, but they are distinguished mainly if not entirely by spore
characters, and by some systematists have all been included in the
one genus _Lecanora_, since the changes have taken place within the
developing apothecium.

There is one genus, _Harpidium_, which is based on thalline characters,
represented by one species, _H. rutilans_, common enough on the
Continent, but not yet found in our country. It has a thin crustaceous
homoiomerous thallus, the component hyphae of which are divided into
short cells closely packed together and forming a kind of cellular
tissue in which the algae are interspersed. The dorsiventral stratose
arrangement prevails however in the other genera and a more or less
amorphous “decomposed” cortex is frequently present. The medulla rests on
the substratum.

With the stouter thallus, there is slightly more variety of crustaceous
form than in Lecideaceae: there occurs occasionally an outgrowth of the
thalline granules as in _Haematomma ventosum_ which marks the beginning
of fruticulose structure. Of a more advanced structure is the thallus of
_Lecanora esculenta_, a desert lichen which becomes detached and erratic,
and which in some of its forms is almost coralline, owing to the apical
growth of the original granules or branches: a more or less radiate
arrangement of the tissues is thus acquired.

The squamulose type is well represented in _Lecanora_, and the species
with that form of thallus have frequently been placed in a separate
genus, _Squamaria_. These squamules are never very large; they possess an
upper, somewhat amorphous, cortex; the medulla rests on the substratum,
except in such a species as _Lecanora lentigera_, where they are free, a
sort of fibrous cortex being formed of hyphae which grow in a direction
parallel with the surface. In none of them are rhizinae developed.

_cc._ _PARMELIACEAE._ The chief advance, apart from size, of the
squamulose to the foliose type is the acquirement of a lower cortex along
with definite organs of attachment which in Parmeliaceae are invariably
rhizoidal and are composed of compact strands of hyphae extending from
the cells of the lower cortex.

In the genus _Parmelia_ rhizinae are almost a constant character, though
in a few species, such as _Parmelia physodes_, they are scanty or
practically absent. It is not possible, however, to consider that these
species form a lower group, as in other respects they are highly evolved,
and rhizinae may be found at points on the lower surface where there
is irritation by friction. Soredia and isidia occur frequently and, in
several species, almost entirely replace reproduction by spores. In one
or two northern or Alpine species, _P. stygia_ and _P. pubescens_, the
lobes are linear or almost filamentous. They are retained in _Parmelia_
because the apothecia are superficial on the fronds which are partly
dorsiventral, and because rhizinae have occasionally been found. Some of
the _Parmeliae_ attain to a considerable size; growth is centrifugal and
long continued.

Two monotypic genera classified under Parmeliaceae, _Physcidia_ and
_Heterodea_, are of considerable interest as they indicate the bases of
parallel development in _Parmelia_ and _Cetraria_. The former, a small
lichen, is corticate only on the upper surface, and without rhizinae;
and from the description, the cortex is of a fastigiate character. The
solitary species grows on bark in Cuba; it is related to _Parmelia_,
as the apothecia are superficial on the lobes. The second, _Heterodea
Mülleri_, a soil-lichen from Australasia, is more akin to _Cetraria_ in
that the apothecia are terminal. The upper surface is corticate with
marginal cilia, the lower surface naked or only protected by a weft of
brownish hyphae amongst which cyphellae are formed; pseudocyphellae
appear in _Cetraria_.

The genus _Cetraria_ contains very highly developed thalline forms,
either horizontal (subgenus _Platysma_), or upright (_Eucetraria_).
Rhizinae are scanty or absent, but marginal cilia in some upright species
act as haptera. _Cetraria aculeata_ is truly fruticose with a radiate
structure.

An extraordinary development of the under cortex characterizes the genera
_Anzia_[1011] and _Pannoparmelia_: rhizinae-like strands formed from the
cortical cells branch and anastomose with others till a wide mesh of
a spongy nature is formed. They are mostly tropical or subtropical or
Australasian, and possibly the spongy mass may be of service in retaining
moisture. A species of _Anzia_ has been recorded by Darbishire[1012] from
Tierra del Fuego.

_dd._ _USNEACEAE._ As we have seen, the change to fruticose structure
has arisen as an ultimate development in a number of groups; it reaches
however its highest and most varied form in this family. Not only are
there strap-shaped thalli, but a new form, the filamentous and pendulous,
appears; it attains to a great length, and is fitted to withstand severe
strain. The various adaptations of structure in these two types of
thallus have already been described[1013].

In _Parmelia_ itself there are indications of this line of development
in _P. stygia_, with short stiff upright branching fronds, and in _P.
pubescens_, with its tufts of filaments, but these two species are more
or less dorsiventral in structure and do not rise from the substratum.
In _Cetraria_ also there is a tendency towards upright growth and in _C.
aculeata_ even to radiate structure. But advance in these directions has
stopped short, the true line of evolution passing through species like
_Parmelia physodes_ with raised, and in some varieties, tubular fronds,
and the somewhat similar species _P. Kamtschadalis_ with straggling
strap-like lobes, to _Evernia_. That genus is a true link between foliose
and fruticose forms and has been classified now with one series, now with
the other.

In _Evernia furfuracea_, the lobes are free from the substratum except
when friction causes the development of a hold-fast and the branching out
of new lobes from that point. It is however dorsiventral in structure,
the under surface is black and the gonidial zone lies under the upper
cortex. _Evernia prunastri_ is white below and is more fruticose in
habit, the long fronds all rising from one base. They are thin and limp,
no strengthening tissue has been evolved, and they tend to lie over on
one side; both surfaces are corticate and gonidia sometimes travel round
the edge, becoming frequently lodged here and there along the under side.

The extreme of strap-shaped fruticose development is reached in the genus
_Ramalina_. In less advanced species such as _R. evernioides_ there is a
thin flat expansion anchored to the substratum at one point and alike on
both surfaces. In _R. fraxinea_ the fronds may reach considerable width
(var. _ampliata_), but in that and in most species there is a provision
of sclerotic strands to support and strengthen the fronds. One of those
best fitted to resist bending strains is _R. scopulorum_ (_siliquosa_)
which grows by preference on sea-cliffs and safely withstands the maximum
of exposure to wind or weather.

The filamentous structure appears abruptly, unless we consider it as
foreshadowed by _Parmelia pubescens_. The base is secured by strong
sheaths of enduring character; tensile strains are provided for either
by a chondroid axis, as in _Usnea_, or by cortical development, as in
_Alectoria_; the former method of securing strength seems to be the
most advantageous to the plant as a whole, since it leaves the outer
structures more free to develop, and there is therefore in _Usnea_ a
greater variety of branching and greater growth in length, which are less
possible with the thickened cortex of _Alectoria_.

_ee._ _PHYSCIACEAE._ There remains still an important phylum of
Lecanorales well defined by the polarilocular spores[1014]. It also
arises from a _Biatora_ species and forms a parallel development. Even
in this phylum there are two series: one with colourless spores and
mostly yellow or reddish either in thallus or apothecium, and the other
with brown spores and with cinereous-grey or brown thalli. The dark
spores are in many of the species typically polarilocular, though in some
the median septum is not very wide and no canal is visible. Practically
all of the lighter coloured forms contain parietin either in thallus or
apothecia or in both; it is absent in the dark-spored series.

Among the lighter coloured forms it is difficult to decide which of
these two striking characteristics developed first, the acid or the
peculiar spore. Probably the acid has the priority: there is one common
rock lichen in this country, _Placodium rupestre_ (_Lecanora irrubata_),
which gives a strong red acid reaction with potash, but in which the
spores are still simple, and the fruit structure in the biatorine stage.
Another species, _Pl. luteoalbum_, with a purplish reaction in the fruit
only, shows septate spores but with only a rather narrow septum. The
development continues through biatorine forms to lecanorine with a fully
formed thalline margin. Among these latter we encounter _Pl. nivale_
which is well provided with acid but in which the spores have become long
and fusiform with little trace of the polar cells or central canal. We
must allow here also for reversions, and wanderings from the straight
road.

From crustaceous the advance is normal and simple to squamulose forms
which in this phylum maintain a stiff regularity of thalline outline
termed “effigurate”; the squamules, developing from the centre, extend
outwards in a radiate-stellate manner. There are also foliose thalli in
the genus _Xanthoria_ and fruticose in _Teloschistes_. The cortex in the
former horizontal genus is of plectenchyma, and no peculiar structures
have emerged. In _Teloschistes_ the cortex is of compact parallel hyphae
(fibrous) which form the strengthening structure of the narrow compressed
fronds (_T. flavicans_).

In the brown-spored series there is a considerable number of species
that are crustaceous united in the genus _Rinodina_, all of which have
marginate apothecia. One of them, _Rinodina oreina_, approaches in
thalline structure the effigurate forms of _Placodium_; while in _R.
isidioides_, a rare British species, there is an isidioid squamulose
development.

Among foliose genera, the tropical genus _Pyxine_ is peculiar in its
almost lecideine fruit, a few gonidia occurring only in the early stages;
its affinity with _Physcia_ holds, however, through the one-septate brown
spores with very thick walls and the reduced lumen of the cells. The more
simple type of fruit may be merely retrogressive.

_Physcia_, the remaining genus, is mainly foliose and with dorsiventral
thallus. A few species have straggling semi-upright fronds and these
have sometimes been placed in a separate genus _Anaptychia_. Only one
“_Anaptychia_,” _Ph. intricata_, has a radiate structure with fibrous
cortex all round; in the others the upper cortex alone is fibrous—of
long parallel hyphae—but that character appears in nearly every one of
the horizontal species as well, sometimes in the upper, sometimes in the
lower cortex.

In _Physcia_ the horizontal thallus is of smaller dimensions than in
_Parmelia_, and never becomes so free from the substratum: it is attached
by rhizinae and soredia appear frequently. Very often the circular
effigurate type of development prevails.

It is difficult to trace with any certainty the origin of this series of
the phylum. Some workers have associated it with the purely lecideine
genus, _Buellia_, but the brown septate spores of the latter are of
simple structure, though occasionally approaching the _Rinodina_ type.
There are also differences in the thallus, that of _Buellia_, especially
when it is saxicolous, inclining to _Rhizocarpon_ in form. It is more
consistent with the outer and inner structure to derive Rinodina from
some crustaceous _Placodium_ form with a marginate apothecium, therefore
from a form of fairly advanced development. As the parietin content
disappeared—perhaps from the preponderance of other acids—the colouration
changed and the spores became dark-coloured.

Many genera and even families, such as Thelotremaceae, etc., have
necessarily been omitted from this survey of phylogeny in lichens, but
the tracing of the main lines of development has indicated the course of
evolution, and has demonstrated not only the close affinity between the
members of this polyphyletic class of plants, as shown in the constantly
recurring thalline types, but it has proved the extraordinary vigour
gained by both the component organisms through the symbiotic association.

The principal phyla[1015], developing on somewhat parallel lines, are
given in the appended table:

ARCHILICHENS

  ---------------+---------------+----------+------------+----------------
       Phyla     |    Crustose   |Squamulose|  Foliose   |   Fruticose
  ---------------+---------------+----------+------------+----------------
                 |               |          |            |
  Pyrenolichens  | Verrucariaceae|   Dermatocarpaceae    |
                 |               |          |            |
  Coniocarpineae | Caliciaceae   |   Sphaerophoraceae    |Sphaerophoraceae
                 |               |          |            |
                 |{Arthoniaceae  |          |            |
  Graphidineae   |{Graphidaceae  |          |            |Roccellaceae
                 |{Dirinaceae    |          |            |
  Cyclocarpineae |               |          |            |
                 |               |          |            |
                 |{        Lecideaceae   Gyrophoraceae   |
                 |{       Coenogoniaceae    |            |
  Lecideales     |{   (filamentous gonidia) |            |
                 |{        Cladoniaceae     |            |
                 |{  (primary and secondary |            |
                 |{           thalli)       |            |
                 |              |           |            |
  Lecanorales    |         Lecanoraceae     |Parmeliaceae|Usneaceae
                 |              |           |            |
  Polariloculares|              |           |            |
                 |              |           |            |
  {Colourless    |              |           |            |
  {      spores  |           Placodium      |Xanthoria   |Teloschistes
  {Brown spores  |Rinodina,     |           |Physcia     |Physcia
                 |  Pyxine      |           |            |  (Anaptychia)
  ---------------+--------------+-----------+------------+----------------

SCHEME OF SUGGESTED PROGRESSION IN LICHEN STRUCTURE


PYRENOCARPINEAE

        PYRENOCARPEAE

  Pyrenothamniaceae  Phyllopyreniaceae
         |                   |
         |                   |
  Dermatocarpaceae           |
         |                   |
         |                   |
  Dermatocarpaceae           |
         |                   |
         |                   |
         |                   |
  Verrucariaceae        Pyrenulaceae
  (Protococcaceae)  (_Trentepohlia_)


CONIOCARPINEAE

             CONIOCARPEAE

             Sphaerophorus
                   |          Pilophorus
                   |              |
                   +--------------+
   Pleurocybe  Acroscyphus
        |          |
        |          |
  Calycidium   Tholurna
        +-----+----+
         Cyphelium       Tylophoron  Tylophorella
              |              +-----+-----+
         Caliciaceae           Pyrgillus
       (Protococcaceae)    (_Trentepohlia_)


CYCLOCARPINEAE

                                PHYCOLICHENS (CYANOPHILI)

                          --------------------^---------------
                                                   Stictaceae
                                                       |      Peltigeraceae
                                                       |             |
                                                       |        Hydrothyria
                                                       +--+----------+
              Peccania   Phloeopeccania           Pannariaceae
                 |          |                          |
                 |          |    Leptodendriscum and   |
                 |          |         Leptogidium      |
      Synalissa  +----------+              |        Heppiaceae
  Thyrea  |      |                    Polychidium      |
    |     |      |                         |        Leptogium
  Paulia  |      |                         |           |
    |     |      |                         |         Collema
    +---+-+------+                         |           |
    Pyrenopsidaceae                    Thermutis     Physma
    (_Gloeocapsa_)                 (_Scytonema_)   (_Nostoc_)

           LECIDEALES             LECANORALES            POLARILOCULARES
     -----------^-------------
                                             Usneae
                                               |
                    Stereocaulon Eucetraria Ramalina
                             |       |         |
       Ochropheae Cocciferae |       |         |
           |           |     |       |         |
           +--------+--+     |       |      Evernia  Teloschistes   Physcia
                    |        |       |         |         |            sect.
        Sphaero-    |        |   Cetraria   Parmelia Xanthoria   Anaptychia
       phoropsis    |        |  (Platysma)  physodes     |                |
               |    |        |       |         |         |                |
 Gyrophoraceae | Cladonia Pilophoron | Parme-  |         |          Physcia
       |       |    |        |       | liaceae |         |                |
       |       |    +---+----+       |    +----+         |                |
       +-------+     Baeomyces   Heterodea  Physcidia {Euplacodium Rinodina
       |                |            +---------+      {  |                |
 {Sect. Psora      Gonophillus      {Lecanora sect.   {  |     {Callopisma|
 {     |                |           {  Squamaria      {Placodium    |     |
 {Sect. Eulecidea  Sect. Biatora    {      |          {  |     {Blastenia |
       |                |           {  Lecanora    Colourless         Brown
       |                |                  |       spores            spores
       |                |                  |             +------+---------+
       |                |            Sect. Biatora         Sect. Biatora
       |                |                  |                    |
       +----------------+--------+---------+--------------------+
                               Lecidea
                           (Protococcaceae)




CHAPTER VIII

SYSTEMATIC


I. CLASSIFICATION


A. WORK OF SUCCESSIVE SYSTEMATISTS

Since the time when lichens were first recognized as a separate class—as
members of the genus Lichen by Tournefort[1016] or as “Musco-fungi” by
Morison[1017],—many schemes of classification have been outlined, and
the history of the science of lichenology, as we have seen, is a record
of attempts to understand their puzzling structure, and to express that
understanding by relating them to each other and to allied classes of
plants. The great diversity of opinion in regard to their affinities is
directly due to their composite nature.

_a._ DILLENIUS AND LINNAEUS. The first systematists were chiefly
impressed by their likeness to mosses, hepatics or algae. Dillenius[1018]
in the _Historia Muscorum_ grouped them under the moss genera:—IV.
_Usnea_, V. _Coralloides_ and VI. _Lichenoides_. Linnaeus[1019]
classified them among algae under the general name _Lichen_, dividing
them into eight orders based on thalline characters in all but one
instance, the second order being distinguished from the first by bearing
scutellae. The British botanists of the latter part of the eighteenth
century—Hudson, Lightfoot and others—were content to follow Linnaeus and
in general adopted his arrangement.

_b._ ACHARIUS. Early in the nineteenth century Acharius, the Swedish
Lichenologist, worked a revolution in the classification of lichens.
He gave first place to the form of the thallus, but he also noted the
fundamental differences in fruit-formation: his new system appeared in
the _Methodus Lichenum_[1020] with an introduction explaining the terms
he had introduced, many of them in use to this day.

Diagnoses of twenty-three genera are given with their included
species. The work was further extended and emended in _Lichenographia
Universalis_[1021] and in the _Synopsis Lichenum_[1022]. In his final
arrangement the family “Lichenes” is divided into four classes, three of
which are characterized solely by apothecial characters; the fourth class
has no apothecia. They are as follows:

  Class I. Idiothalami with three orders, Homogenei, Heterogenei
  and Hyperogenei: the apothecia differ in texture and colouration
  from the thallus: _Lecidea_, _Opegrapha_, _Gyrophora_, etc.

  Class II. Coenothalami, with three orders, Phymaloidei, Discoidei
  and Cephaloidei. The apothecia are partly formed from the
  thallus: _Lecanora_, _Parmelia_, etc. The Pyrenolichens are also
  included by him in this class, because “the thallus surrounds and
  is concrete with the partly or wholly immersed apothecia.”

  Class III. Homothalami with two orders, Scutellati and Peltati.
  The apothecia are formed from the cortical and medullary tissue
  of the thallus: _Ramalina_, _Usnea_, _Collema_, etc.

  Class IV. Athalami, with but one sterile genus, _Lepraria_.

The orders are thus based on the form of the fruit; the genera in the
_Synopsis_ number 41. Large genera such as _Lecanora_ with 132 species
are divided into sections, many of which have in turn been established as
genera, by S. F. Gray in 1821, and later by other systematists.

The _Synopsis_ was the text-book adopted by succeeding botanists for some
40 years with slight alterations in the arrangement of classes, genera,
etc.

Wallroth[1023] and Meyer[1024] followed with their studies on the lichen
thallus, and Wallroth’s division into “Homoiomerous” and “Heteromerous”
was accepted as a useful guide in the maze of forms, representing as it
did a great natural distinction.

_c._ SCHAERER. This valiant lichenologist worked continuously during
the first half of the nineteenth century, but with very partial use of
the microscope. His last publication in 1850, an _Enumeration of Swiss
Lichens_, was the final declaration of the older school that relied on
field characters. His classification is as follows:

  Class I. Lichenes Discoidei, with ten orders from Usneacei to
  Graphidei; fruits open.

  Class II. Lichenes Capitati, with three orders: Calicioidei,
  Sphaerophorei and Cladoniacei; fruits stalked.

  Class III. Lichenes Verrucarioidei, with three orders:
  Verrucarii, Pertusarii and Endocarpei: fruits closed.

An “Appendix” contains descriptions of Crustacei and Fruticulosi, all
sterile forms, except _Coniocarpon_ and _Arthonia_, which seem out of
place, and finally a “Corollarium” of gelatinous lichens all classified
under one genus _Collema_.

_d._ MASSALONGO AND KOERBER. As a result of their microscopic studies,
these two workers proposed many changes based on fruit and spore
characters, and Koerber in the _Systema Lichenum Germaniae_ (1855)
gave expression to these views in his classification. He also made use
of Wallroth’s distinctions of “homoiomerous” and “heteromerous,” thus
dividing lichens at the outset into those mostly with blue-green and
those with bright-green gonidia.

The following is the main outline of Koerber’s classification:

  Series I. Lichenes Heteromerici.
      Order   I. Lich. Thamnoblasti (fruticose).
      Order  II. Lich. Phylloblasti (foliose).
      Order III. Lich. Kryoblasti (crustaceous).
  Series II. Lichenes Homoeomerici.
      Order IV. Lich. Gelatinosi.
      Order  V. Lich. Byssacei.

With the exception of Order V all are subdivided into two sections,
“gymnocarpi” with open fruits and “angiocarpi” with closed fruits, a
distinction that had long been recognized both in lichens and in fungi.

_e._ NYLANDER. The above writers had been concerned with the
interrelationships of lichens; Nylander, who was now coming forward
as a lichenologist of note, gave a new turn to the study by dwelling
on their relation to other classes of plants. Without for a moment
conceding that they were either algal or fungal, he yet insisted on
their remarkable affinity to algae on the one hand, and to fungi on the
other, and he sought to make evident this double connection by his very
ingenious scheme of classification[1025]. He began with what we may call
“algal lichens,” those associated with blue-green gonidia in the family
“Collemacei”; he continued the series to the most highly evolved foliose
forms and then wound up with those that are most akin to fungi, that
is, those with least apparent thalline formation—according to him—the
“Pyrenocarpei.”

In his scheme, which is the one followed by Leighton and Crombie, the
“family” represents the highest division; series, tribe, genus and
species come next in order. We have thus:

  Fam.   I. Collemacei.
  Fam.  II. Myriangiacei (now reckoned among fungi).
  Fam. III. Lichenacei.

This last family, which includes the great bulk of lichens, is divided
into the following series: I. Epiconiodei; II. Cladoniodei; III.
Ramalodei; IV. Phyllodei; V. Placodei; VI. Pyrenodei. It is an ascending
series up to the Phyllodei, or foliaceous lichens, which he considers
higher in development than the fruticose or filamentous Ramalodei. The
Placodei include four tribes on a descending scale, the Lecanorei,
Lecidinei, Xylographidei and Graphidei. The classification is almost
wholly based on thalline form, except for the Pyrenodei in which are
represented genera with closed fruits, there being one tribe only, the
Pyrenocarpei.

Nylander claims however to have had regard equally to the reproductive
system and was the first to give importance to the spermogonia. The
classification is coherent and easy to follow, though, like all
classifications based on imperfect knowledge, it is not a little
artificial; also while magnifying the significance of spermogonia and
spermatia, he overlooked the much more important characters of the
ascospores.

_f._ MÜLLER(-ARGAU). In preparing his lists of Genevan lichens (1862),
Müller realized that Nylander’s series was unnatural, and he found as he
studied more deeply that lichens must be ranged in parallel or convergent
but detached groups. He recognized three main groups:

  1. Eulichens, divided into Capitularieae, Discocarpeae and
       Verrucaroideae.
  2. Epiconiaceae.
  3. Collemaceae.

He suggested that, in relation to other plants, Eulichens approach
Pezizae, Hysteriaceae and Sphaeriaceae; Epiconiaceae have affinity
with Lycoperdaceae, while Collemaceae are allied to the algal family
Nostocaceae. These three groups of Eulichens, he held, advanced on
somewhat parallel lines, but reached a very varied development, the
Discocarpeae attaining the highest stage of thalline form. Müller
accepted as characters of generic importance the form and structure
of the fruiting body, the presence or absence of paraphyses, and the
septation, colour, etc. of the spores.

A few years later (1867) the composite nature of the lichen thallus was
announced by Schwendener, and, after some time, was acknowledged by most
botanists to be in accordance with the facts of nature. Any system of
classification, therefore, that claims to be a natural one, must, while
following as far as possible the line of plant development, take into
account the double origin of lichens both from algae and fungi, the
essential unity and coherence of the class being however proved by the
recurring similarity between the thalline types of the different phyla.
As Müller had surmised: “they are a series of parallel detached though
convergent groups.”

_g._ REINKE. The arrangement of Ascolichens on these lines was first
seriously studied by Reinke[1026], and his conclusions, which are
embodied[1027] in the _Lichens of Schleswig-Holstein_, have been largely
accepted by succeeding workers. He recognizes three great subclasses: 1.
Coniocarpi; 2. Discocarpi; 3. Pyrenocarpi.

The Coniocarpi are a group apart, but as their fruit is at first entirely
closed—at least in some of the genera—the more natural position for them
is between Discocarpi and Pyrenocarpi. It is in the arrangement of the
Discocarpi that variation occurs. Reinke’s arrangement of orders and
families in that sub-class is as follows:

Subclass 2. Discocarpi.

    Order I. _GRAMMOPHORI_: Fam. _GRAPHIDACEI_ and _XYLOGRAPHACEI_.

   Order II. _LECIDEALES_: Fam. _GYALECTACEI_, _LECIDEACEI_,
               _UMBILICARIACEI_ and _CLADONIACEI_.

  Order III. _PARMELIALES_: Fam. _URCEOLARIACEI_, _PERTUSARIACEI_,
               _PARMELIACEI_, _PHYSCIACEI_, _TELOSCHISTACEI_ and
               _ACAROSPORACEI_.

   Order IV. _CYANOPHILI_: Fam. _LICHINACEI_, _EPHEBACEI_,
               _PANNARIACEI_, _STICTACEI_, _PELTIGERACEI_,
               _COLLEMACEI_ and _OMPHALARIACEI_.

The orders represent generally the principal phyla or groups, the
families subordinate parallel phyla within the orders. The first three
orders are stages of advance as regards fruit development; the Cyanophili
are a group apart.

Wainio[1028] rendered great service to Phylogeny in his elaborate work on
Cladoniaceae, the most complicated of all the lichen phyla. He also drew
up a scheme of arrangement in his work on Brazil Lichens[1029]. There is
in it some divergence from Reinke’s arrangement, as he tends to give more
importance to the thallus than to fruit characters as a guide. He places,
for instance, Gyrophorei beside Parmelei and at a long distance from his
Lecidei. The Cyanophili group of families he has interpolated between
_Buelliae_ (Physciaceae) and _Lecideae_. Many workers approve of Wainio’s
classification but it presents some difficult problems.

_h._ ZAHLBRUCKNER. The systematist of greatest weight in recent times is
A. Zahlbruckner, who is responsible for the systematic account of lichens
in Engler and Prantl’s _Natürlichen Pflanzenfamilien_. It is difficult to
express the very great service he has rendered to Lichenology, in that
and other world-wide studies of lichens. The sketch of lichen phylogeny
as given in the present volume owes a great deal to the sound and clear
guidance of his work, though his conclusions may not always have been
accepted. The classification in the _Pflanzenfamilien_ is the one now
generally followed.

The class _Lichenes_ is divided by Zahlbruckner[1030] into two
subclasses, I. Ascolichens and II. Hymenolichens. He gives a third class,
Gasterolichens[1031], but as it was founded on error[1032], it need not
concern us here. The Ascolichens are by far the more important. These are
subdivided into:

  Series 1. =PYRENOCARPEAE=, with perithecial fruits.
  Series 2. =GYMNOCARPEAE=, with apothecial fruits.

These are again broken up into families, and in the arrangement and
sequence of the families Zahlbruckner indicates his view of development
and relationship. They occur in the following order:


SERIES 1. =PYRENOCARPEAE=

               ALGAL CELLS PROTOCOCCACEAE OR _PALMELLA_.

        I.  _MORIOLACEAE._   }
                             }
       II. _EPIGLOEACEAE._   } Thallus crustaceous, perithecia solitary.
                             }
      III. _VERRUCARIACEAE._ }

       IV. _DERMATOCARPACEAE._ Thallus squamulose or foliose.

        V. _PYRENOTHAMNIACEAE._ Thallus fruticose.
              ALGAL CELLS _PRASIOLA_.

       VI. _MASTOIDIACEAE._
              ALGAL CELLS _TRENTEPOHLIA_.

      VII. _PYRENULACEAE._   }
                             } Thallus crustaceous, perithecia occurring
     VIII. _PARATHELIACEAE._ }   singly.

       IX. _TRYPETHELIACEAE._ }
                              } Thallus crustaceous, perithecia united
        X. _ASTROTHELIACEAE._ }   (stromatoid).

       XI. _MYCOPORACEAE._ Thallus crustaceous, perithecia in compact
                             groups with a common outer wall.

      XII. _PHYLLOPYRENIACEAE._ Thallus minutely foliose.
              ALGAL CELLS _PHYLLACTIDIUM_ OR _MYCOIDEA_.

     XIII. _STRIGULACEAE._ Tropical leaf-lichens.
              ALGAL CELLS _NOSTOC_ OR _SCYTONEMA_.

      XIV. _PYRENIDIACEAE._ Thallus minutely squamulose or fruticose.


SERIES 2. =GYMNOCARPEAE=

  Subseries 1. Coniocarpineae, with subperithecial fruits.
  Subseries 2. Graphidineae, with elongate, narrow fruits.
  Subseries 3. Cyclocarpineae, with round open fruits.


SUBSERIES 1. =CONIOCARPINEAE=

This is a well-defined group, peculiar in the disappearance of the asci
at an early stage so that the spores lie like a powder in the globose
partly closed fruits. Algal cells, bright-green; Protococcaceae. There
are only three families:

       XV.  _CALICIACEAE._ Thallus crustaceous, apothecia stalked.

      XVI. _CYPHELIACEAE._ Thallus crustaceous, apothecia sessile.

     XVII. _SPHAEROPHORACEAE._ Thallus foliose or fruticose, apothecia
                                 sessile.


SUBSERIES 2. =GRAPHIDINEAE=

This subseries comes next in the form of fruit development; generally
the apothecia are elongate, with a narrow slit-like opening, so that a
transverse section shows almost a perithecial outline. Algal cells are
mostly _Trentepohlia_.

    XVIII. _ARTHONIACEAE._ Thallus crustaceous, apothecia oval or linear,
                             flat.

      XIX. _GRAPHIDACEAE._ Thallus crustaceous, apothecia linear, raised.

       XX. _CHIODECTONACEAE._ Thallus crustaceous, apothecia generally
                                immersed in a stroma.

      XXI. _DIRINACEAE._ Thallus crustaceous, corticate above, apothecia
                           round.

     XXII. _ROCCELLACEAE._ Thallus fruticose, apothecia round or elongate.


SUBSERIES 3. =CYCLOCARPINEAE=

A large and very varied group! In most of the families the algal
cells are bright-green (Chlorophyceae), in some they are blue-green
(Cyanophyceae), these latter corresponding to Reinke’s order Cyanophili.
The apothecia, as the name implies, are round and open; the “Cyanophili”
have been placed by Zahlbruckner after those families in which the
apothecium has no thalline margin. They form a phylum distinct from those
that precede and those that follow.

The first family of the Cyclocarpineae, the Lecanactidaceae, is often
placed under Graphidineae; in any case it forms a link between the two
subseries.


1. _Lecideine group_ (apothecia without a thalline margin).

    XXIII. _LECANACTIDACEAE._ Thallus crustaceous. Algal cells
                                _Trentepohlia._ Apothecium with
                                carbonaceous hypothecium or parathecium.

     XXIV. _PILOCARPACEAE._ Thallus crustaceous. Algal cells
                              Protococcaceae. Apothecia with a dense
                              rather dark hypothecium.

      XXV. _CHRYSOTHRICACEAE._ Thallus felted, loose in texture. Algal
                                 cells _Palmella_, Protococcaceae or
                                 _Trentepohlia_. Apothecia with or without
                                 a thalline margin. The affinity of the
                                 “Family” seems to be with Pilocarpaceae.

     XXVI. _THELOTREMACEAE._   } Thallus crustaceous. Algal cells in the
    XXVII. _DIPLOSCHISTACEAE._ }   first _Trentepohlia_; in the second
                               }   Protococcaceae. In both there are
                               }   prominent double margins round the
                               }   apothecium.

   XXVIII. _ECTOLECHIACEAE._ Thallus very primitive in type. Algal cells
                               Protococcaceae. Apothecia with or without
                               a thalline margin. Nearly related to
                               Chrysothricaceae.

     XXIX. _GYALECTACEAE._ Thallus crustaceous. Algal cells _Trentepohlia_,
                             _Phyllactidium_ or rarely _Scytonema_.
                             Apothecia biatorine, _i.e._ of soft
                             consistency and without gonidia.

      XXX. _COENOGONIACEAE._ Thallus confusedly filamentous (byssoid).
                               Algal cells _Trentepohlia_ or _Cladophora_.
                               Apothecia biatorine.

     XXXI. _LECIDEACEAE._ Thallus crustaceous or squamulose. Algal cells
                            Protococcaceae. Apothecia biatorine (soft), or
                            lecideine (carbonaceous).

    XXXII. _PHYLLOPSORACEAE._ Thallus squamulose or foliose. Algal cells
                                Protococcaceae. Apothecia biatorine or
                                lecideine.

   XXXIII. _CLADONIACEAE._ Thallus twofold. Algal cells Protococcaceae.
                             Apothecia biatorine or lecideine.

    XXXIV. _GYROPHORACEAE._ Thallus foliose. Algal cells Protococcaceae.
                              Apothecia lecideine.

     XXXV. _ACAROSPORACEAE._ Thallus primitive crustaceous, squamulose or
                               foliose. Algal cells Protococcaceae.
                               Apothecia with or without a thalline
                               margin; very various, but always with
                               many-spored asci.


2. _Cyanophili group._

In this group the classification depends almost entirely on the nature of
the algal constituents. The apothecia are in most genera provided with a
thalline margin.


_a. More or less gelatinous when moist._

    XXXVI. _EPHEBACEAE._ Algal cells _Scytonema_ or _Stigonema_. Thallus
                           minutely fruticose or filamentous.

   XXXVII. _PYRENOPSIDACEAE._ Algal cells _Gloeocapsa_ (_Gloeocapsa_,
                               _Xanthocapsa_ or _Chroococcus_). Thallus
                               crustaceous, minutely foliose or fruticose.

  XXXVIII. _LICHINACEAE._ Algal cells _Rivularia_. Thallus crustaceous,
                            squamulose or minutely fruticose.

    XXXIX. _COLLEMACEAE._ Algal cells _Nostoc_. Thallus crustaceous,
                            minutely fruticose, or squamulose to foliose.

       XL. _HEPPIACEAE._ Algal cells _Scytonema_. Thallus generally
                           squamulose and formed of plectenchyma.


_b. Not gelatinous when moist._

      XLI. _PANNARIACEAE._ Algal cells _Nostoc_, _Scytonema_ or rarely
                             bright-green, Protococcaceae. Thallus
                             crustaceous, squamulose or foliose.

     XLII. _STICTACEAE._ Algal cells _Nostoc_ or Protococcaceae. Thallus
                           foliose, and very highly developed, corticate
                           on both surfaces.

    XLIII. _PELTIGERACEAE._ Algal cells _Nostoc_ or Protococcaceae.
                              Thallus foliose, corticate above.


3. _Lecanorine group_ (apothecia with a thalline margin).

The remaining families have all bright-green gonidia and nearly always
apothecia with a thalline margin. The group includes several distinct
phyla:

     XLIV. _PERTUSARIACEAE._ Thallus crustaceous. Apothecia, one or
                               several immersed in thalline tubercles;
                               spores mostly very large.

      XLV. _LECANORACEAE._ Thallus crustaceous or squamulose. Apothecia
                             mostly superficial.

     XLVI. _PARMELIACEAE._ Thallus foliose, rarely almost fruticose or
                             filamentous. Apothecia scattered over the
                             surface or marginal, sessile.

    XLVII. _USNEACEAE._ Thallus fruticose or filamentous. Apothecia
                          sessile or shortly stalked.

   XLVIII. _CALOPLACACEAE._ Thallus crustaceous, squamulose or minutely
                              fruticose. Apothecia with polarilocular
                              colourless spores.

     XLIX. _TELOSCHISTACEAE._ Thallus foliose or fruticose. Apothecia with
                                polarilocular colourless spores.

        L. _BUELLIACEAE._ Thallus crustaceous or squamulose. Apothecia
                            (lecideine or lecanorine) with two-celled,
                            thick-walled brown spores (polarilocular in
                            part).

       LI. _PHYSCIACEAE._ Thallus foliose, rarely partly fruticose.
                            Apothecia with two-celled thick-walled brown
                            spores (polarilocular in part).


Subclass 2. Hymenolichens.

There are only three closely related genera of Hymenolichens, _Cora_,
_Corella_ and _Dictyonema_ with _Chroococcus_ or _Scytonema_ algae.

There is reason to dissent from the arrangement in one or two instances
which will be pointed out in the following examination of families and
genera.


B. FAMILIES AND GENERA OF ASCOLICHENS

The necessity for a well-reasoned and well-arranged system of
classification is self-evident: without a working knowledge of the plants
that are the subject of study no progress can be made. The recognition
of plants as isolated individuals is not sufficient, it must be possible
to place them in relation to others; hence the importance of a natural
system. In identifying species artificial aids, such as habitat and
substratum, are also often of great value, and a good working system
should take account of all characteristics.

Lichen development is the result of two organisms mutually affecting
each other, but as the fungus provides the reproductive system, it is
the dominant partner: the main lines of classification are necessarily
determined by fruit characters. The algae occupy a subsidiary position,
but they also are of importance in shaping the form and structure of the
thallus. The different phyla are often determined by the presence of some
particular alga; it is in the delimitation of families that the algal
influence is of most effect.

Zahlbruckner’s system gives due weight to the inheritance from both
fungus and alga with, however, the fungus as the chief factor in
development, and as his work is certain to be generally followed by
modern lichenologists, it is the one of most immediate interest. His
scheme has been accepted in the following more detailed account of
families and genera, and for the benefit of home workers those that have
not so far been recorded from the British Isles have been marked with an
asterisk.

It cannot be affirmed that nomenclature is as yet firmly established in
lichenology. Both on historical grounds and on those of convenience, the
subject is one of extreme importance, and interest in it is one of the
main avenues by which we secure continuity with the past, and by which we
are able to realize not only the difficulty, but the romance of pioneer
work. Besides, there can be no exchange of opinion between students nor
assured knowledge of plants, until the names given to them are beyond
dispute. According to the ruling of the Brussels Botanical Congress in
1910, Linnaeus’s[1033] list of lichens in the _Species Plantarum_ has
been selected as the basis of nomenclature, but since his day many new
families, genera and species have been described and often insufficiently
delimited. It is not easy to decide between priority, which appeals to
the historical sense, and recent use which is the plea of convenience.
Here also it seems there can be no rigid decision; the one aim should be
to arrive at a conclusion satisfactory to all, and accepted by all.

In the following necessarily brief account of families and genera, the
“spermogonia” or “pycnidia” have in most cases been left out of account,
as in many instances they vary within the family and occasionally even
within the genus. Their taxonomic value is not without importance,
but, in the general systematic arrangement, they are only subsidiary
characters. An account of them has already been given, and for more
detailed statements the student is referred to purely systematic works.

There are two main types of spore production in the “pycnidia” which
have been shortly described by Steiner[1034] as “exobasidial” and
“endobasidial.” In the former the sporophores are simple or branched
filaments, at the apices of which a short process grows out and buds off
a pycnidiospore; in the latter the spores are budded directly from cells
lining the walls or filling the cavity of the pycnidium. The exobasidial
type is more simply rendered in the following pages by “acrogenous,”
the endobasidial by “pleurogenous” spore production. In many cases the
“spermogonia” or “pycnidia” are still imperfectly known. In designating
the gonidial algae, the more comprehensive Protococcaceae has been
substituted for _Protococcus_, as in many cases the alga is probably not
_Protococcus_ as now understood, but some other genus of the family[1035].


SUBCLASS I. ASCOLICHENS


SERIES I. PYRENOCARPINEAE

It is on mycological grounds that Pyrenocarpineae are placed at the base
of lichen classification. There is no evidence that the series was first
in time.


I. _MORIOLACEAE_

This family was described by Norman[1036] in 1872 from specimens
collected by himself in Norway or in the Tyrol, on soil or more
frequently on trees. There seems to have been no further record, and
Zahlbruckner, while accepting the family, suggests that an examination or
revision may be necessary.

The thallus is crustaceous. The algal cells, Protococcaceae, occur either
in groups (sometimes stalked) surrounded by a plectenchymatous wall
and called by Norman “goniocysts,” or they form nests in the thallus
termed “nuclei” which are surrounded by a double wall of plectenchyma,
colourless in the interior and brown outside. Norman invented the term
“Allelositismus,” which may be rendered “mutualism,” to indicate this
peculiar form of thallus. The species of _Spheconisca_ are fairly
numerous on poplars, willows and conifers:

  Algae in “goniocysts”                      1. *Moriola Norm.[1037]
  Algae in double-walled “nuclei”            2. *Spheconisca Norm.


II. _EPIGLOEACEAE_

The family consists of but one genus and one species, _Epigloea
bactrospora_, and, according to Zahlbruckner, further examination is
necessary to make certain as to the lichenoid nature of the plant.

Zukal[1038] found the perithecia scattered over the leaves of mosses,
and he alleges that hyphae connected with the perithecium were closely
associated with the alga, _Palmella botryoides_, and were causing it
no harm. Along with the perithecia he also found minute pycnidia. The
“thallus” is of a gelatinous nature and homoiomerous in structure;
the perithecia are soft and clear-coloured with many-spored asci and
colourless one-septate spores.

The small globose pycnidia contain simple sporophores and acrogenous
straight or slightly bent rod-like spores.

  Asci many-spored; spores one-septate       1. *Epigloea Zukal.


III. _VERRUCARIACEAE_

In all the genera of this family the thallus is crustaceous, and, with
very few exceptions, the species are saxicolous or terricolous. The
thallus is variable within the crustaceous limits, and may be superficial
and very conspicuous, almost imperceptible, or wholly immersed in the
substratum. The algal cells are Protococcaceae, and in two of the genera
the green cells penetrate the hymenium and grow in rows alongside of the
asci. The perithecia are small roundish structures scattered over the
thallus, the base immersed, but the upper portion generally projecting.
An outer dark-coloured wall surrounds the whole perithecium (entire) or
only the upper exposed portion (dimidiate); it opens above by a pore or
ostiole more or less prominent.

In some of the genera the paraphyses become dissolved at an early stage,
and somewhat similar filaments near the ostiole, termed periphyses, aid
in the expulsion of the spores. The spores vary in septation, colour
and size, and these variations have served to delimit the genera which
have been formed from the original very large genus _Verrucaria_. The
ascus may be 1-, 2-, 4- or 8-spored. In only one genus is it many-spored
(_Trimmatothele_).

The genera are as follows:

  Perithecia with simple ostioles.
    Paraphyses disappearing early, or wanting.
      Spores simple, ellipsoid               1.  Verrucaria Web.
      Spores simple, elongate vermiform      2.  Sarcopyrenia Nyl.
      Spores simple, numerous in the ascus   3. *Trimmatothele Norm.
      Spores 1-3-septate                     4.  Thelidium Massal.
      Spores muriform (with transverse and
      longitudinal divisions).
        Without hymenial gonidia             5.  Polyblastia Massal.
        With hymenial gonidia                6.  Staurothele Norm.
    Paraphyses present.
      Spores simple.
        Without hymenial gonidia             7.  Thrombium Wallr.
        With hymenial gonidia                8. *Thelenidia Nyl.
      Spores 3-septate, broadly ellipsoid    9. *Geisleria Nitschke.
      Spores acicular, many-septate         10.  Gongylia Koerb.
      Spores muriform                       11.  Microglaena Lönnr.
  Perithecia with a wide ring round the ostiole.
      Spores muriform; paraphyses
        unbranched                          12. *Aspidothelium Wain.
      Spores elongate, many-septate;
      paraphyses branched                   13. *Aspidopyrenium Wain.


IV. _DERMATOCARPACEAE_

In this family there is a much more advanced thalline
development—generally squamulose or with some degree of foliose
structure, though in the genus _Endocarpon_, some of the species
are little more than crustaceous. The gonidia are bright-green
Protococcaceae (according to Chodat, _Coccobotrys_ in _Dermatocarpon_).
In _Endocarpon_ they appear in the hymenium.

The least developed in structure is _Normandina_: the thallus of the
single species consists of delicate shell-like squamules which are
non-corticate above and below. In the other genera there is a cortex of
plectenchyma.

The perithecia are almost wholly immersed, and open above by a straight
ostiole. The fructification of _Dacampia_ is considered by some
lichenologists to be only a parasite on the white thickish squamulose
thallus with which it is associated.

  Hymenial gonidia present.
      Spores muriform                        1.  Endocarpon Hedw.
  Hymenial gonidia absent.
    Thallus non-corticate                    2.  Normandina Wain.
    Thallus corticate.
      Spores simple, colourless              3.  Dermatocarpon Eschw.
      Spores simple, brown                   4. *Anapyrenium Müll.-Arg.
      Spores elongate-septate, colourless    5. *Placidiopsis Beltr.
      Spores elongate-septate, brown         6. *Heterocarpon Müll.-Arg.
      Spores muriform, colourless            7. *Psoroglaena Müll.-Arg.
      Spores muriform, brown                 8.  Dacampia Massal.


V. _PYRENOTHAMNIACEAE_

Thallus more or less fruticose and corticate on both surfaces. Algal
cells Protococcaceae.

Only two genera are included in this family: _Nylanderiella_ with one
species from New Zealand, with a small laciniate thallus up to 15 mm. in
height, partly upright, partly decumbent, and attached to the substratum
by basal rhizinae; the other small genus, _Pyrenothamnia_, belongs to N.
America; the thallus has a short rounded stalk which expands above to an
irregular frond. The perithecia are immersed in the fronds.

  Spores colourless, 1-septate               1. *Nylanderiella Hue[1039].
  Spores brown, muriform                     2. *Pyrenothamnia Tuckerm.


VI. _MASTOIDEACEAE_

A family containing one genus and one species, with a wide distribution,
having been found in Siberia, on the Antarctic continent (Graham’s Land),
as also in Tierra del Fuego, South Georgia, South Shetland Islands and
Kerguelen. The thallus is foliose, of small thin lobes, and without
rhizinae. Algal cells _Prasiola_[1040]. The perithecia are globose and
partly project from the thallus; the asci are 8-spored; the paraphyses
are mucilaginous and partly dissolving.

  Spores elongate-fusiform, simple,
    colourless                               1. *Mastoidea Hook. and Harv.


VII. _PYRENULACEAE_

This family of crustaceous lichens differs from Verrucariaceae chiefly in
the gonidium which is a species of _Trentepohlia_. Genera and species are
largely corticolous and the thallus is inconspicuous, often developing
within the substratum. The perithecia, like those of _Verrucariae_, are
immersed or partly emergent and have an entire or dimidiate outer wall.
They are scattered over the thallus except in _Anthracothecium_ where
they are often coalescent. This genus is tropical or subtropical except
for one species which inhabits S.W. Ireland.

Paraphyses are variable, and in some species tend to disappear, but do
not dissolve in mucilage. The spores are generally colourless, only in
one monotypic genus, _Coccotrema_, are they simple. The cells into which
the spore is divided differ in form according to the genus.

  Paraphyses branched and entangled or wanting.
    Perithecia opening above by stellate
     lobes                                   1. *Asteroporum Müll.-Arg.
    Perithecia opening by a pore.
    Spores variously septate.
      Spore cells cylindrical or cuboid.
      Spores colourless, elongate or
        ovate 1-5-septate                    2.  Arthopyrenia Massal.
      Spores colourless, filiform
        1-multi-septate                      3.  Leptorhaphis Koerb.
      Spores colourless, muriform            4.  Polyblastropsis A. Zahlbr.
      Spores brown, ovoid or elongate
        2-5-septate                          5.  Microthelia Koerb.
      Spore cells globose or lentiform,
        3-multi-septate                      6. *Pseudopyrenula Müll.-Arg.
  Paraphyses unbranched free.
    Spore cells cylindrical or cuboid.
    Perithecia beset with hairs              7.  *Stereochlamys Müll.-Arg.
    Perithecia naked.
      Asci disappearing; spores elongate
          multi-septate, colourless          8. *Belonia Koerb.
      Asci persistent.
        Spores simple, ellipsoid,
          colourless                         9. *Coccotrema Müll.-Arg.
        Spores elongate, 1-multi-septate,
          colourless                        10. Porina Müll.-Arg.
        Spores elongate, 1-multi-septate,
          brown                             11. Blastodesmia Massal.
        Spores muriform, colourless         12. *Clathroporina Müll.-Arg.
        Spores elongate, 2-3-septate,
          colourless                        13. Thelopsis Nyl.
    Spore cells globose or lentiform.
        Spores elongate, 1-5-septate,
          brown                             14. Pyrenula Massal.
        Spores muriform, brown              15. Anthracothecium Massal.


VIII. _PARATHELIACEAE_

This family is peculiar in that the perithecia open by a somewhat
elongate ostiole that slants at an oblique angle. The algal cells are
_Trentepohlia_. Genera and species are endemic in tropical or subtropical
regions of the Western hemisphere, though a species of _Pleurotrema_ has
been found in subantarctic America. They are corticolous and the thallus
is either superficial or embedded. The genera are arranged according to
spore characters:

  Spores elongate, 2- or more-septate.
      Spore cells cylindrical, colourless    1. *Pleurotrema Müll.-Arg.
      Spore cells globose-lentiform.
        Spores colourless                    2. *Plagiotrema Müll.-Arg.
        Spores brown                         3. *Parathelium Müll.-Arg.
  Spores muriform.
        Spores colourless                    4. *Campylothelium Müll.-Arg.
        Spores brown                         5. *Pleurothelium Müll.-Arg.


IX. _TRYPETHELIACEAE_

This and the following two families are distinguished by the pseudostroma
or compound fruit, a character rare among lichens, though the true stroma
is frequent in Pyrenomycetes in such genera as _Dothidea_, _Valsa_, etc.
The genera are crustaceous and corticolous and occur with few exceptions
in tropical or subtropical regions, mostly in the Western Hemisphere.
Several grow on officinal bark (_Cinchona_, etc.). Algal cells are
_Trentepohlia_. As in many tropical lichens, the spores are large. The
genera are based chiefly on spore characters, on septation, and on the
form of the spore cells:

  Spore cells cylindrical or cuboid.
      Spores colourless, elongate,
        multi-septate                        1. *Tomasiella Müll.-Arg.
      Spores colourless, muriform            2. *Laurera Rehb.
      Spores brown, muriform                 3. *Bottaria Massal.
  Spore cells globose-lentiform.
      Spores colourless, elongate,
        multi-septate                        4. *Trypethelium Spreng.
      Spores brown, elongate, multi-septate  5.  Melanotheca Müll.-Arg.


X. _ASTROTHELIACEAE_

The perithecia are either upright or inclined, and occur usually in
radiate groups. They are free or united in a stroma, and the elongate
ostioles open separately or coalesce in a common canal. The genera
are all crustaceous, with _Trentepohlia_ gonidia. They are tropical
or subtropical, mostly in the Western Hemisphere; but species of
_Parmentaria_ and _Astrothelium_ have been recorded also from Australia.

The spores are all many-celled and the form of their cells is a generic
character:

  Spores elongate, multi-septate.
      Spore cells cylindrical                 1. *Lithothelium Müll.-Arg.
      Spore cells globose-lentiform.
        Spores colourless                     2. *Astrothelium Trev.
        Spores brown                          3. *Pyrenastrum Eschw.
  Spores muriform.
        Spores colourless                     4. *Heufleria Trev.
        Spores brown                          5. *Parmentaria Fée.


XI. _MYCOPORACEAE_

A small family with only two genera which are found in both Hemispheres;
species of both occur in Great Britain. They are all corticolous. The
perithecia are united into a partially chambered fruiting body surrounded
by a common wall, but opening by separate ostioles. The thallus is thinly
crustaceous, with _Palmella_ gonidia in _Mycoporum_, and _Trentepohlia_
in _Mycoporellum_. The spores are colourless or brown in both genera:

  Spores muriform                            1. Mycoporum Flot.
  Spores elongate, multi-septate             2. Mycoporellum A. Zahlbr.


XII. _PHYLLOPYRENIACEAE_

Thallus foliose with both surfaces corticate and attached by rhizinae.
Algal cells _Trentepohlia_. There is but one genus, _Lepolichen_, which
has a laciniate somewhat upward growing thallus. Two species, both from
South America, have been described, _L. granulatus_ Müll.-Arg. and _L.
coccophora_ Hue. The latter has been recently examined by Hue[1041]
who finds, on the thalli, cephalodia which are peculiar in containing
bright-green gelatinous algae either _Urococcus_ or _Gloeocystis_, one
of the few instances known of chlorophyllaceous algae forming part
of a cephalodium. _Gloeocystis_ may be the only alga present in the
cephalodium; _Urococcus_ is always accompanied by _Scytonema_.

The perithecia are immersed in thalline tubercles:

  Spores colourless, simple, ovoid or
    ovoid-elongate                           1. *Lepolichen Trevis.


XIII. _STRIGULACEAE_

A family of epiphyllous lichens inhabiting and disfiguring coriaceous
evergreen leaves, or occasionally fern leaves in tropical or subtropical
regions. The algae associated are _Mycoidea_ and _Phycopeltis_
(_Phyllactidium_). The only truly parasitic lichen, _Strigula_, belongs
to this family: the alga precedes the lichen on the leaves and is
gradually invaded by the hyphae of the lichen and altered in character.
The small black perithecia are scattered over the surface. In _Strigula_
the lichen retains the spreading rounded form of the alga. The other
genera are more irregular.

  Thallus orbicular in outline               1. *Strigula Fries.
  Thallus irregular.
    Perithecia without hairs.
      Spores colourless.
        Spores elongate, multi-septate       2. *Phylloporina Müll.-Arg.
        Spores muriform                      3. *Phyllobathelium Müll.-Arg.
      Spores brown.
        Spores simple                        4. *Haplopyrenula Müll.-Arg.
        Spores elongate, 1-3-septate         5. *Microtheliopsis Müll.-Arg.
    Perithecia beset with stiff hairs        6. *Trichothelium Müll.-Arg.


XIV. _PYRENIDIACEAE_

The only family of Pyrenocarpineae associated with blue-green algae. The
genera of Pyrenidiaceae are all monotypic, only one is common and of wide
distribution, _Coriscium_ (_Normandina_ Nyl.). _Pyrenidium_ is the only
member that has a fruticose thallus, and that is of minute dimensions.
_Eolichen Heppii_, found and described by Zukal, is a doubtful
lichen. “_Lophothelium_” Stirton is a case of parasitism of a fungus,
_Ticothecium_, on the squamules of _Stereocaulon condensatum_.

                 Algal cells _Scytonema_ or _Stigonema_.

  Thallus crustaceous[1042]; spores simple,
    colourless                               1. *Rhabdopsora Müll.-Arg.
  Thallus crustaceous; spores 1-septate,
    colourless                               2. *Eolichen Zuk.
  Thallus crustaceous; spores muriform,
    brown                                    3. *Pyrenothrix Riddle[1043].
  Thallus squamulose; spores numerous,
    simple                                   4. *Placothelium Müll.-Arg.

                          Algal cells _Nostoc_.

  Thallus crustaceous; spores filiform,
    simple, colourless                       5. *Hassea A. Zahlbr.
  Thallus fruticose; spores elongate,
    3-septate, brown                         6.  Pyrenidium Nyl.

                Algal cells _Microcystis_ (_Polycoccus_).

  Thallus squamulose; fructification
    unknown                                  7.  Coriscium Wainio.


SERIES II. =GYMNOCARPEAE=


SUBSERIES 1. =Coniocarpineae=

This small subseries is marked by the peculiar “mazaedium” type of
fruit with its disappearing asci. It forms a connecting link between
the families with perithecia and those with apothecia. The thallus is
crustaceous or fruticose, often poorly developed and sometimes absent.
The algal cells are Protococcaceae or rarely _Trentepohlia_.


XV. _CALICEACEAE_

The thallus is thinly crustaceous, sometimes brightly coloured, sometimes
absent, taking no part in the formation of the fruits; these have upright
stalks with a small capitulum, and often look like minute nails. One
genus, _Sphinctrina_, is parasitic on the thallus of other lichens,
mostly _Pertusariae_.

  Fruits with slender stalks.
    Spores simple.
      Spores colourless                      1.  Coniocybe Ach.
      Spores brown                           2.  Chaenotheca Th. Fr.
    Spores septate, brown.
      Spores 1-septate                       3.  Calicium De Not.
      Spores 3-7-septate                     4.  Stenocybe Nyl.
  Fruits with short thick stalks.
      Spores globose, brown (parasitic)      5.  Sphinctrina Fries.
      Spores 1-septate, brown                6. *Pyrgidium Nyl.


XVI. _CYPHELIACEAE_

Thallus crustaceous. Algal cells Protococcaceae or _Trentepohlia_.
Apothecia sessile, more widely open than in the previous family; in
some genera the thallus forms an outer apothecial margin. The genera
_Farriola_ from Norway and _Tylophorella_ from New Granada are monotypic.
The British genus _Cyphelium_ has been known as _Trachylia_.

  Thallus with Protococcaceae.

  Spores colourless, simple                  1. *Farriola Norm.
  Spores brown, 1-3-septate (rarely simple
    or muriform)                             2.  Cyphelium Th. Fr.

  Thallus with _Trentepohlia_.

  Spores simple, many in the ascus           3. *Tylophorella Wainio.
    Spores 8 in the ascus.
      Apothecia with a thalline margin       4. *Tylophoron Nyl.
      Apothecia without a thalline margin    5. *Pyrgillus Nyl.


XVII. _SPHAEROPHORACEAE_

The most highly evolved family of the subseries, as regards the thallus.
Algal cells Protococcaceae. In _Tholurna_, a small lichen endemic in
Scandinavia, there is a double thallus: one of horizontal much-divided
squamules, the other swollen, upright, terminating in the capitulum. The
fruit is lateral in _Calycidium_, a squamulose form from New Zealand, and
in _Pleurocybe_ from Madagascar, with stiff strap-shaped fronds. All the
genera are monotypic except _Sphaerophorus_, of which genus ten species
are recorded, some of them with a world-wide distribution. The spores are
brown and simple or 1-septate.

  Thallus squamulose and upright             1. *Tholurna Norm.
  Thallus wholly squamulose                  2. *Calycidium Stirton.
  Thallus fruticose.
    Fronds hollow in the centre              3. *Pleurocybe Müll.-Arg.
    Fronds not hollow.
      Fruit without a thalline margin        4. *Acroscyphus Lév.
      Fruit inclosed in the tip of the
        fronds                               5.  Sphaerophorus Pers.


SUBSERIES 2. =Graphidineae=

In this subseries are included five families that differ rather widely
from each other both in thallus and apothecia; the latter are more or
less carbonaceous and mostly with a proper margin only. Families and
genera are widely distributed, though most abundant in warm regions.
Algal cells mostly _Trentepohlia_.

A comprehensive study of the apothecia of this series by Bioret[1044]
gives some interesting results in regard to the paraphyses: in _Arthonia_
they are irregular in direction and much-branched; in _Opegrapha_,
the paraphyses are vertical and parallel with more regular branching;
_Stigmatidium_ (_Enterographa_) resembles _Opegrapha_ in this respect as
does also _Platygrapha_, a genus of Lecanactidaceae, while in _Graphis_
the paraphyses are vertical, unbranched and free; _Melaspilea_ paraphyses
are somewhat similar to those of _Graphis_.


XVIII. _ARTHONIACEAE_

The thallus of Arthoniaceae is corticolous with few exceptions and
is very inconspicuous, being largely embedded in the substratum. The
apothecia (ardellae) are round, irregular or stellate, without any
margin, the hymenium being protected by the dense branching of the
paraphyses at the tips.

_Arthonia_ is abundant everywhere. The species of the other genera
belong mostly to tropical or subtropical countries. _Arthoniopsis_ is
similar to _Arthonia_ in the character of the fruits, but the gonidium
is a _Phycopeltis_, and it is only found on leaves. _Synarthonia_ with
peculiar stromatoid fructification is monotypic; it occurs in Costa Rica.

                  Thallus with _Trentepohlia_ gonidia.

  Apothecia scattered.
    Spores elongate 1- or pluri-septate      1.  Arthonia Ach.
    Spores muriform                          2.  Arthothelium Massal.
  Apothecia stromatoid.
    Spores elongate, multi-septate           3. *Synarthonia Müll.-Arg.

                    Thallus with _Palmella_ gonidia.

    Spores 1- or more-septate                4.  Allarthonia Nyl.
    Spores muriform                          5. *Allarthothelium Wain.

                   Thallus with _Phycopeltis_ gonidia.

    Spores elongate 1- or more-septate       6. *Arthoniopsis Müll.-Arg.


XIX. _GRAPHIDACEAE_

Thallus crustaceous, inconspicuous, partly immersed, mainly growing
on bark but occasionally on dead wood or stone. Algal cells chiefly
_Trentepohlia_, very rarely _Palmella_ or _Phycopeltis_ (epiphyllous).
Apothecia (lirellae) carbonaceous more or less linear, opening by a
narrow slit with a well-developed proper margin except in _Gymnographa_,
a monotypic Australian genus. In two genera, the fruit is of a compound
nature, several parallel discs occurring in one lirella: these are
_Ptychographa_ (on bark in Scotland) and _Diplogramma_ (Australia), both
are monotypic. They must not be confused with _Graphis elegans_ and
allied species in which the sterile carbonaceous margin is furrowed. Two
tropical genera associated with _Phycopeltis_ are epiphyllous.

Graphidaceae are among the oldest recorded lichens, attention having been
drawn to them since early times by the resemblance of the lirellae on the
bark of trees to hieroglyphic writing.

                    Thallus with _Palmella_ gonidia.

  Apothecia single.
    Hypothecium dark-brown.
      Spores simple                          1.  Lithographa Nyl.
  Hypothecium colourless or brownish.
    Spores colourless.
      Spores simple                          2.  Xylographa Fries.
      Spores elongate 3-8-septate            3. *Aulaxina Fée.
    Spores brown.
      Spores 1-septate                       4.  Encephalographa Massal.
      Spores pluri-septate, then muriform    5. *Xyloschistes Wain.
  Apothecia compound.
      Spores simple, colourless              6.  Ptychographa Nyl.
      Spores pluri-septate, colourless       7. *Diplogramma Müll.-Arg.

                  Thallus with _Trentepohlia_ gonidia.

  Spores elongate 1-multi-septate, the cells longer than wide.
    Spores brown.
      Spores 1-(rarely more)-septate         8.  Melaspilea Nyl.
      Spores 3-septate (apothecia
        rudimentary)                         9. *Gymnographa Müll.-Arg.
    Spores colourless.
      Spores acicular, coiled (many in the
        ascus)                              10. *Spirographa A. Zahlbr.
      Spores fusiform, straight             11.  Opegrapha Humb.
  Spores muriform.
      Spores elongate, central cells
        finally muriform                    12. *Dictyographa Müll.-Arg.

  Spores elongate, septate, cells wider than long.
    Paraphyses unbranched, filiform.
      Spores multi-septate, colourless      13.  Graphis Adans.
      Spores multi-septate, brown           14.  Phaeographis Müll.-Arg.
      Spores muriform, colourless           15.  Graphina Müll.-Arg.
      Spores muriform, brown                16.  Phaeographina Müll.-Arg.
    Paraphyses clavate, warted at tips      17. *Acanthothecium Wain.
    Paraphyses branched, interwoven above   18. *Helminthocarpon Fée.

            Thallus with _Phycopeltis_ gonidia (epiphyllous).

    Spores elongate, 3-9-septate,
      colourless                            19. *Opegraphella Müll.-Arg.
    Spores elongate, 1-septate, brown       20. *Micrographa Müll.-Arg.


XX. _CHIODECTONACEAE_

Specially distinguished in this subseries by the grouping of the somewhat
rudimentary apothecia in pseudostromata in which they are almost wholly
immersed. In form they are roundish or linear; the spores are septate or
muriform. The thallus is thinly crustaceous and continuous: in _Glyphis_,
_Sarcographa_ and _Sarcographina_ there is an amorphous upper cortex, the
other genera are non-corticate. Algal cells are _Trentepohlia_ with the
exception of two epiphyllous genera associated with _Phycopeltis_.

Genera and species are mostly tropical. _Sclerophyton_ with five
species is represented in Europe by a single British specimen, _S.
circumscriptum_.

The form of the paraphyses is a distinguishing character of the genera.

                  Thallus with _Trentepohlia_ gonidia.

  Paraphyses free, unbranched.
    Spore cells short or almost globose.
      Spores elongate, multi-septate,
        colourless                           1.  Glyphis Fée.
      Spores elongate, multi-septate, brown  2. *Sarcographa Fée.
      Spores muriform, brown                 3. *Sarcographina Müll.-Arg.
    Spore cells longer and cuboid.
      Spores muriform, colourless            4. *Enterodictyon Müll.-Arg.
  Paraphyses branched, interwoven above.
      Spores elongate, multi-septate,
        colourless                           5.  Chiodecton Ach.
      Spores elongate, multi-septate, brown  6.  Sclerophyton Eschw.
      Spores muriform, colourless            7. *Minksia Müll.-Arg.
      Spores muriform, brown                 8. *Enterostigma Müll.-Arg.

            Thallus with _Phycopeltis_ gonidia (epiphyllous).

  Paraphyses free.
      Spores unequally 2-celled,
        colourless                           9. *Pycnographa Müll.-Arg.
  Paraphyses branched, interwoven above.
      Spores elongate, multi-septate,
        colourless                          10. *Mazosia Massal.


XXI. _DIRINACEAE_

A small family, which is associated with and often included under
Graphidaceae. The thallus is crustaceous and corticate on the upper
surface, the cortex being formed of palisade hyphae. Algal cells
_Trentepohlia_. Apothecia are rounded or with a tendency to elongation,
and, in addition to a thin proper margin, possess a stout thalline
margin; the hypothecium is thick and carbonaceous. There are two genera:
_Dirina_ with twelve species has a wide distribution; _Dirinastrum_ is
monotypic and occurs on maritime rocks in Australia. In both the spores
are elongate-septate, differing only in colour:

  Spores colourless                          1.  Dirina Fr.
  Spores brown                               2. *Dirinastrum Müll.-Arg.


XXII. _ROCCELLACEAE_

The Roccellaceae differ from the preceding Dirinaceae chiefly in the
fruticose thallus which is more or less characteristic of all the genera,
though in _Roccellographa_ it expands into foliose dimensions and in
_Roccellina_ is reduced to short podetia-like processes from a crustose
base. The fronds—mostly long and strap-shaped—are protected in most of
the genera by a cortex of compact palisade hyphae; in a few the outer
hyphae are parallel with the long axis. The medulla is of parallel
hyphae, either loose or compact. The algal cells are _Trentepohlia_.

The apothecia are lateral except in _Roccellina_ where they occur at the
tips of the short upright fronds, and only in _Roccellaria_ is there
no thalline margin. They are superficial in all of the genera except
_Roccellographa_, in which they are immersed and almost closed, recalling
the perithecia-like fruits of _Chiodecton_ (sect. _Enterographa_). The
spores are elongate, narrow, pluri-septate, and colourless or brownish,
except in _Darbishirella_ in which they are ovoid, 2-septate and brown.

The affinity of Dirinaceae and Roccellaceae with Graphidaceae was first
indicated by Reinke[1045] and elaborated later by Darbishire[1046] in
his monograph of Roccellaceae. The apothecia in some species of _Dirina_
are ellipsoid rather than round; in several genera of Roccellaceae they
are distinctly lirellate, and in _Roccella_ itself some species have
ellipsoid fruits. The fruticose thallus is predominant in Roccellaceae,
but its evolution from the crustaceous type may be traced through
_Roccellina_ which is partly crustaceous and only imperfectly fruticose.

In most of the genera only one species is recorded. _Roccella_,
represented by twelve species, is well known for its dyeing properties,
and has a wide distribution. Like other Graphidineae they are mainly
plants of warm regions, many of them exclusively maritime rock-dwellers.

The following synopsis of the genera is the one given by Darbishire in
his monograph.

  Cortex fastigate, of palisade hyphae.
    Spores colourless.
      Hypothecium black-carbonaceous.
        Apothecia round.
          Thallus fruticose                  1.  Roccella DC.
          Thallus crustaceous-fruticose      2. *Roccellina Darbish.
        Apothecia lirellate                  3. *Reinkella Darbish.
      Hypothecium colourless.
        Gonidia present under the
          hypothecium                        4. *Pentagenella Darbish.
        Gonidia absent from hypothecium      5. *Combea De Not.
    Spores brown or brownish.
        Medulla of parallel somewhat loose
          hyphae                             6. *Schizopelte Th. Fr.
        Medulla solid, black                 7. *Simonyella Steiner.
  Cortex fibrous, of parallel hyphae.
    Apothecia round.
      Hypothecium black-carbonaceous.
        Apothecia with thalline margin       8. *Dendrographa Darbish.
        Apothecia with proper margin         9. *Roccellaria Darbish.
      Hypothecium colourless                10. *Darbishirella A. Zahlbr.
    Apothecia lirellate                     11. *Ingaderia Darbish.


SUBSERIES 3. =Cyclocarpineae=

This last subseries includes the remaining twenty-nine families of
Ascolichens. They are very varied both in the fungal and the algal
symbionts. The fruit is more or less a discoid open apothecium. The
gonidia belong to different genera of Myxophyceae and Chlorophyceae,
but the most frequent are Protococcaceae. Families are based largely on
thalline structure.


XXIII. _LECANACTIDACEAE_

By many systematists this family is included under Graphidineae
on account of the fruit structure which in some of the forms is
carbonaceous and almost lirellate, and also because the algal symbiont is
_Trentepohlia_. The thallus is primitive, being thinly crustaceous and
non-corticate; the apothecium has a black carbonaceous hypothecium in two
of the genera, _Lecanactis_ and _Schismatomma_ (_Platygrapha_); in the
third genus, _Melampydium_, it is colourless. The latter is monotypic,
and the spores become muriform. In the other genera they are elongate and
multi-septate.

  Apothecia with prominent proper margin     1.  Lecanactis Eschw.
  Apothecia with thin proper margin          2. *Melampydium Müll.-Arg.
  Apothecia with thalline margin             3.  Schismatomma Flot.


XXIV. _PILOCARPACEAE_

A small family with but one genus, _Pilocarpon_. It is distinguished
as one of the few epiphyllous genera of lichens associated with
Protococcaceous gonidia and with a distribution extending far beyond
the tropics. The best known species, _P. leucoblepharum_, encircles the
base of pine-needles with a white felted crust, or inhabits coriaceous
evergreen leaves. Another species lives on fern leaves. The fruit is
a discoid apothecium with a dark carbonaceous hypothecium and proper
margin, and with a second thalline margin. The paraphyses are branched
and interwoven above.

  Spores elongate, 3-septate, colourless     1.  Pilocarpon Wain.


XXV. _CHRYSOTRICHACEAE_

This family now, according to Hue[1047], includes two genera, _Crocynia_
and _Chrysothrix_. In both there is a thallus of interlaced hyphae with
Protococcaceous algae scattered through it or in groups. The structure
is thus homoiomerous, and Hue has suggested for it a new series,
“Intertextae.” The only British species, _Crocynia lanuginosa_, first
placed by Nylander[1048] in _Amphiloma_ and later transferred by him to
_Leproloma_[1049], has a soft crustaceous lobate thallus, furfuraceous on
the surface; no fructification has been found. A West Indian species, _C.
gossypina_, has discoid apothecia with a thalline margin. There is only
one species of _Chrysothrix_, _Ch. nolitangere_, which forms small clumps
or tufts on the spines of Cactus in Chili. The structure is somewhat
similar to that of _Crocynia_.

  Spores colourless, simple                  1.  Crocynia Nyl.
  Spores colourless, 2-3-septate             2. *Chrysothrix Mont.


XXVI. _THELOTREMACEAE_

A tropical or subtropical family of which the leading characteristic is
the deeply sunk disc of the apothecium: it has a proper hyphal margin,
and, round that, an overarching thalline margin. The apothecia occur
singly, or they are united in a kind of pseudostroma: in _Tremotylium_
several grow together, while in _Polystroma_ each new apothecium develops
as an outgrowth from the thalline margin of the one already formed,
so that an upright, branching succession of fruits is built up. It is
a very unusual type of lichen fructification, with one species, _P.
Ferdinandezii_, found in Spain and in Guiana.

The thallus in all the genera is crustaceous with an amorphous
(decomposed) cortex; or it is non-corticate. The algal cells are
_Trentepohlia_ except in _Phyllophthalmaria_, an epiphyllous genus
associated with the alga _Phycopeltis_. In _Polystroma_ the alga is
unknown.

Only one genus is represented in the British Isles.

  Apothecia growing singly.
    Thallus with _Trentepohlia_ gonidia.
      Paraphyses numerous, unbranched, free.
        Spores colourless.
          Spores elongate, 2- or
            multi-septate                    1. *Ocellularia Spreng.
          Spores muriform                    2.  Thelotrema Ach.
        Spores brown.
          Spores elongate, septate           3. *Phaeotrema Müll.-Arg.
          Spores muriform                    4. *Leptotrema Mont.
      Paraphyses scanty, branched.
        Spores muriform, brown               5. *Gyrostomum Fr.
    Thallus with _Phycopeltis_ gonidia       6. *Phyllophthalmaria A.
                                                   Zahlbr.
  Apothecia in pseudostromata.
      Apothecia united in tubercles          7. *Tremotylium Nyl.
      Apothecia united by the margins        8. *Polystroma Clem.


XXVII. _DIPLOSCHISTACEAE_

Scarcely differing from the preceding family except in the gonidia which
are Protococcaceous algae. The thallus is crustaceous and non-corticate.
The apothecia have a double margin but the outer thalline margin is less
overarching than in Thelotremaceae. The spores in the two genera are
somewhat peculiar: in _Conotrema_ they are exceedingly long and divided
by parallel septa into thirty to forty small cells; in _Diploschistes_
(_Urceolaria_) they are large, muriform and brown. _Conotrema_ contains
two corticolous species; _Diploschistes_ about thirty species mostly
saxicolous. Both genera are represented in the British Isles.

  Spores elongate, multi-septate,
    colourless                               1.  Conotrema Tuck.
  Spores muriform, brown                     2.  Diploschistes Norm.


XXVIII. _ECTOLECHIACEAE_

A family of tropical epiphyllous lichens that are associated with
Protococcaceous gonidia. The thallus is primitive in character, mostly a
weft of hyphae with intermingled algal cells, described as homoiomerous.

The apothecia are without a thalline margin, and with a scarcely
developed proper margin: their affinity is with the Lecideaceae, though
in two genera, _Lecaniella_ and _Arthotheliopsis_, there are gonidia
below the hypothecium, a character of Lecanoraceae. The genera are nearly
all monotypic; in _Sporopodium_ has been included _Lecidea phyllocharis_
Wainio (Sect. _Gonothecium_), which is distinguished by hymenial gonidia.

  Apothecia at first covered by a “veil.”
        Spores elongate, colourless,
          septate                            1. *Asterothyrium Müll.-Arg.
  Apothecia uncovered from the first.
    Gonidia not present below the hypothecium.
      Paraphyses unbranched, free.
        Spores muriform                      2. *Lopadiopsis Wain.
      Paraphyses branched.
        Spores 1-septate                     3. *Actinoplaca Müll.-Arg.
        Spores elongate, multi-septate       4. *Tapellaria Müll.-Arg.
        Spores muriform                      5. *Sporopodium Mont.
    Gonidia present below the hypothecium.
        Spores elongate, 2-septate           6. *Lecaniella Wain.
        Spores muriform                      7. *Arthotheliopsis Wain.


XXIX. _GYALECTACEAE_

The algal cells in this family are filamentous; either Myxophyceae
(_Scytonema_) or Chlorophyceae (_Trentepohlia_ or _Phyllactidium_).
The thallus is crustaceous, and in some cases homoiomerous, as in
_Petractis_, where the alga, _Scytonema_, penetrates the substratum as
deeply as the hyphae. _Monophiale_, a tropical genus, possesses two
kinds of gonidia: the species that grow on bark or mosses are associated
with _Trentepohlia_; others that have invaded the surface of leathery
evergreen leaves resemble most epiphyllous lichens in being associated
with the leaf alga _Phyllactidium_ (_Phycopeltis_). Some species of
_Trentepohlia_ exhale when moist an odour of violets. This scent is
retained in at least one genus, _Jonaspis_.

The apothecia are superficial, and are soft, waxy and bright-coloured,
with prominent margins which are however entirely hyphal: the affinity
is therefore with Lecideaceae. In one genus, _Sagiolechia_, the fruit
is carbonaceous and dark coloured. The spores of all the genera are
colourless.

  Apothecia waxy, bright-coloured.
    Thallus with _Scytonema_ gonidia.
      Spores elongate, 3-septate             1.  Petractis Fr.
    Thallus with _Trentepholia_ gonidia.
      Asci 6-8-spored.
        Spores simple                        2.  Jonaspis Th. Fr.
        Spores 1-septate                     3. *Microphiale A. Zahlbr.
        Spores septate or muriform           4.  Gyalecta Ach.
      Asci 12-many-spored.
        Spores 1-septate                     5. *Ramonia Stizenb.
        Spores fusiform or acicular,
          many-septate                       6.  Pachyphiale Lönnr.
  Apothecia carbonaceous.
        Spores elongate, 2-3-septate         7. *Sagiolechia Massal.


XXX. _COENOGONIACEAE_

There are only two genera in this small family, _Coenogonium_ with
_Trentepohlia_ gonidia, and _Racodium_ with _Cladophora_. Both genera
follow the algal form and are filamentous. In _Coenogonium_ the filaments
are sometimes matted into a loose felted expansion. The genus is mainly
tropical or subtropical and mostly rather light-coloured. There is only
one British species, _C. ebeneum_[1050], a sterile form, in which the
hyphae are very dark-brown; it often covers large areas of stone or rock
with its sooty-like creeping filaments.

_Racodium_ includes 2 (?) species. One of these, _R. rupestre_, is
sterile and resembles _C. ebeneum_ in form and colour.

The apothecia of _Coenogonium_ are waxy and light-coloured; they are
borne laterally on the filaments; the spores are simple or 1-septate.

  Thallus with _Trentepohlia_ gonidia        1. Coenogonium Ehrenb.
  Thallus with _Cladophora_ gonidia          2. Racodium Fr.


XXXI. _LECIDEACEAE_

One of the largest lichen families as regards both genera and species,
and of world-wide distribution. The algal cells are Protococcaceae. The
thallus is mostly crustaceous but it becomes squamulose in _Psora_, a
section of _Lecidea_; and in _Sphaerophoropsis_, a Brazilian genus, there
are small upright fronds or stalks with lateral apothecia. The prevailing
colour of the thallus is some shade of grey, but it ranges from white or
yellow to dark-brown or almost black. Cephalodia appear in some of the
species.

The apothecia have a proper margin only, no gonidia taking part in the
fruit-formation. They may be soft and waxy (biatorine) or hard and
carbonaceous (lecideine). The genera are mainly based on spore characters
which are very varied.

The arrangement of genera given below follows that of Zahlbruckner;
in several instances, both as to the limitations of genera and to the
nomenclature, it differs from that of British text-books, though the
general principle of classification is the same.

  Thallus crustaceous non-corticate.
    Spores simple.
      Spores small, thin-walled.
        Spores colourless                    1.  Lecidea Ach.
        Spores brown                         2. *Orphniospora Koerb.
      Spores large, thick-walled             3.  Mycoblastus Norm.
    Spores 1-septate.
      Spores small, thin-walled              4.  Catillaria Th. Fr.
      Spores large, thick-walled             5.  Megalospora Mey. and Flot.
    Spores elongate, 3-multi-septate.
      Spores elongate, narrow, thin-walled   6.  Bacidia A. Zahlbr.
      Spores elongate, large and
        thick-walled                         7.  Bombyliospora De Not.
    Spores muriform.
      Spores colourless; on trees            8.  Lopadium Koerb.
      Spores colourless to brown; on rocks   9.  Rhizocarpon Th. Kr.
  Thallus warted or squamulose, corticate.
      Spores elongate, 1-7-septate,
        thin-walled                         10.  Toninia Th. Fr.
  Thallus of upright podetia-like small fronds.
      Spores ellipsoid, becoming 1-septate  11. *Sphaerophoropsis Wain.


XXXII. _PHYLLOPSORACEAE_

A small family of exotic lichens with a somewhat more developed thallus
than that of the Lecideaceae, being in both of the genera squamulose or
almost foliose.

The apothecia are without a thalline margin; they are biatorine or
lecideine; the hypothecium is formed of plectenchyma and is purple-red
in one species, _Phyllopsora furfuracca_. The two genera differ only
in spore characters. There are fifteen species, mostly corticolous,
belonging to _Phyllopsora_; only one, from New Zealand, is recorded for
_Psorella_.

  Spores simple                              1. *Phyllopsora Müll.-Arg.
  Spores elongate, septate                   2. *Psorella Müll.-Arg.


XXXIII. _CLADONIACEAE_

Associated with Lecideaceae in the type of apothecium, but differing
widely in thallus formation. The latter is of a twofold type: the
primary thallus is crustaceous, squamulose, or very rarely foliose; the
secondary thallus or podetium, upright, simple or branched, is terminated
by the apothecia, or broadens upwards to cup-like scyphi. Algal cells,
Protococcaceae, according to Chodat, _Cystococcus_.

Much attention has been given to the origin and development of the
podetia in this family. They are superficial on granule or squamule
except in the monotypic Himalayan genus _Gymnoderma_ where they are
marginal on the large leaf-like lobes. Though in origin the podetia are
doubtless fruit stalks, they have become in most cases vegetative in
function.

The fruits are coloured yellowish, brown or red (or dark and carbonaceous
in _Pilophorus_), and are borne on the tips of the branches or on the
margins of the scyphi. In _Glossodium_ and _Thysanothecium_—the former
from New Granada, the latter from Australia—the apothecia occupy one side
of the widened surface at the tips.

Cephalodia are developed on the primary thallus of _Pilophorus_, and on
the podetia of _Stereocaulon_ and _Argopsis_.

  Podetia simple, short, not widening upwards.
    Podetial stalks naked.
      Primary thallus thin, continuous       1.  Gomphillus Nyl.
      Primary thallus granular or
        squamulose                           2.  Baeomyces Pers.
      Primary thallus foliose.
        Podetia superficial                  3. *Heteromyces Müll.-Arg.
        Podetia marginal                     4. *Gymnoderma[1051] Nyl.
    Podetial stalks granular, squamulose     5.  Pilophorus Th. Fr.
  Podetia short, widening upwards.
        Podetia simple above, rarely
          divided                            6. *Glossodium Nyl.
        Podetia lobed, leaf-like             7. *Thysanothecium Berk. &
                                                 Mont.
  Podetia elongate, variously branched, or
    scyphous and hollow                      8.  Cladonia Hill.
  Podetia elongate, not scyphous, the stalks solid.
        Spores elongate, septate             9.  Stereocaulon Schreb.
        Spores muriform                     10. *Argopsis Th. Fr.


XXXIV. _GYROPHORACEAE_

A small family of foliose lichens allied to Lecideaceae by the character
of the fruit—a superficial apothecium in the formation of which the
gonidia take no share. There are only three genera, distinguished
by differences in spore and other characters. _Dermatiscum_ has
light-coloured thallus and fruits; of the two species, one occurs in
Central Europe, the other in North America. _Umbilicaria_ and _Gyrophora_
are British; they are dark-coloured rock-lichens and are extremely
abundant in Northern regions where they are known as “tripe de roche.”
Algal cells Protococcaceae.

_Umbilicaria_, _Dermatiscum_, and some species of _Gyrophora_ are
attached to the substratum by a central point. Other species of
_Gyrophora_ are rhizinose. In all there is a cortex of plectenchyma
above and below. In _Gyrophora_ the thallus may be monophyllous as in
_Umbilicaria_, or polyphyllous and with or without rhizinae. New lobes
frequently arise from protuberances or warts on the older parts of the
thallus. At the periphery, in most species, growth is equal along the
margins, in _G. erosa_[1052] the edge is formed of numerous anastomosing
lobes with lateral branching, the whole forming a broadly meshed open
network. Further back the tissues become continuous owing to the active
growth of the lower tissue or hypothallus, which grows out from all
sides and meets across the opening. The overlying layers, with gonidia,
follow more slowly, but they also in time become continuous, so that the
“erose” character persists only near the periphery. This forward growth
of the lower thallus occurs in other species, though to a much less
marked degree.

There is abundant detritus formation in this family; the outer layers of
the cortex are continually being sloughed, the dead tissues lying on the
upper surface as a dark gelatinous layer, continuous or in small patches.
On the under surface the cast-off cortex gathers into a loose confused
mass of dead tissues.

  Asci 8-spored.
      Spores mostly simple (disc gyrose)     1.  Gyrophora Ach.
      Spores 1-septate                       2. *Dermatiscum Nyl.
  Asci 1-2-spored.
      Spores muriform                        3.  Umbilicaria Hoffm.


XXXV. _ACAROSPORACEAE_

Thallus foliose, squamulose or crustaceous, sometimes scarcely developed.
Algal cells Protococcaceae.

Into this family Zahlbruckner has gathered the genera in which the asci
are many-spored, as he considers that a character of great importance in
determining relationship, but he has in doing so overlooked other very
great differences. The fruit-bodies are round and completely enclosed
in a thalline wall in _Thelocarpon_, which has however no perithecial
wall. They have a proper margin only (lecideine) in _Biatorella_, and a
thalline margin (lecanorine) in the remaining genera. In _Acarospora_ the
apothecia are sunk in the thallus. Stirton’s genus _Cryptothecia_[1053]
is allied to _Thelocarpon_ in the fruit-formation, but the basal thallus
is well developed and the spores are few in number and variously divided.

  Thallus none.
    Apothecia (or perithecia) in thalline
      warts                                  1.  Thelocarpon Nyl.
  Thallus crustaceous.
    Apothecia lecideine; spores simple       2.  Biatorella Th. Fr.
    Apothecia lecanorine; spores septate     3. *Maronea Massal.
  Thallus of small squamules                 4.  Acarospora Massal.
  Thallus almost foliose, attached
    centrally                                5. *Glypholecia Nyl.


XXXVI. _EPHEBACEAE_

A family of very simple structure either filamentous, foliose or
crustaceous. The algal cells which give a dark colour to the thallus
are _Stigonema_ or _Scytonema_, members of the blue-green Myxophyceae,
and consist of minute simple or branched filaments—single cell-rows in
_Scytonema_, compound in _Stigonema_.

In some of the genera the lichen hyphae travel within the gelatinous
sheath of the filaments, both algae and hyphae increasing by apical
growth so that filaments many times the length of the alga are formed as
in _Ephebe_. In others the filaments scarcely increase beyond the normal
size of the alga as in _Thermutis (Gonionema)_; or the gelatinous algal
cells may be distributed in a stratum of hyphae.

The apothecia are minute and almost closed; they may be embedded in
swellings of the thallus, or are more or less superficial. The spores are
rather small, colourless and simple or 1-septate.

The lichens of this family are rock-dwellers and are mostly to be found
in hilly or Alpine regions. A tropical species, _Leptogidium dendriscum_,
occurs in sterile condition in south-west Ireland. There are few species
in any of the genera.

  Algal cells _Scytonema_.
    Thallus minutely fruticose,
      non-corticate                          1.  Thermutis Fr.
    Thallus minute, of felted filaments,
      cortex one cell thick                  2. *Leptodendriscum Wain.
    Thallus of elongate filaments, cortex
      of several cells                       3.  Leptogidium Nyl.
    Thallus foliose or fruticose, cellular
      throughout                             4.  Polychidium Ach.
    Thallus crustaceous, non-corticate       5.  Porocyphus Koerb.
  Algal cells _Stigonema_.
    Thallus minutely fruticose,
      non-corticate                          6.  Spilonema Born.
    Thallus of long branching filaments.
      Spores septate; paraphyses wanting     7.  Ephebe Fr.
      Spores simple; paraphyses present      8.  Ephebeia Nyl.
    Thallus crustaceous; upper surface
      non-corticate, lower surface
      corticate                              9. *Pterygiopsis Wain.


XXXVII. _PYRENOPSIDACEAE_

In this family are included gelatinous lichens of which the gonidium
is a blue-green alga with a thick gelatinous coat, either _Gloeocapsa_
(including _Xanthocapsa_) or _Chroococcus_. In _Gloeocapsa_ and
_Chroococcus_ the gelatinous envelope is often red, in _Xanthocapsa_
it is yellow, and these colours persist more or less in the lichens,
especially in the outer layers.

The thallus is in many cases a formless gelatinous crust of hyphal
filaments mingling with colonies of algal cells as in _Pyrenopsis_;
but small fruticose tufts are characteristic of _Synalissa_, and
larger foliose and fruticose thalli appear in some exotic genera. A
plectenchymatous cortex is formed on the thallus of _Forssellia_, a
crustaceous genus from Central Europe, with two species only; the whole
thallus is built up of a kind of plectenchyma in some others, but in most
of the genera there is no tissue formed.

The apothecia, as in Ephebaceae, are generally half-closed.

  Thallus with _Gloeocapsa_ gonidia.
    Thallus crustaceous.
      Spores simple                          1.  Pyrenopsis Nyl.
      Spores 1-septate                       2. *Cryptothele Forss.
    Thallus shortly fruticose                3.  Synalissa Fr.
    Thallus lobate, centrally attached       4. *Phylliscidium Forss.
  Thallus with _Chroococcus_ gonidia.
    Thallus crustaceous                      5.  Pyrenopsidium Forss.
    Thallus lobate, centrally attached       6. *Phylliscum Nyl.
  Thallus with _Xanthocapsa_ gonidia.
    Thallus crustaceous.
      Thallus non-corticate.
        Spores simple.
          Apothecia open, asci 8-spored      7.  Psorotichia Forss.
          Apothecia covered, asci
            many-spored                      8. *Gonohymenia Stein.
        Spores 1-septate.
          Apothecia closed                   9. *Collemopsidium Nyl.
      Thallus with plectenchymatous cortex  10. *Forssellia A. Zahlbr.
    Thallus lobate, centrally attached.
      Spores simple.
        Thallus plectenchymatous
          throughout                        11. *Anema Nyl.
        Thalline tissue of loose hyphae     12. *Thyrea Massal.
        Cortex of upright parallel hyphae   13. *Jenmania Wächt.
      Spores 1-septate.
        Thalline tissue of loose hyphae     14. *Paulia Fée.
    Thallus fruticose.
      Thallus without a cortex              15. *Peccania Forss.
      Thallus with cortex of parallel
        hyphae                              16. *Phloeopeccania Stein.


XXXVIII. _LICHINACEAE_

The only family of lichens associated with _Rivularia_ gonidia, the
trichomes of which retain their filamentous form to some extent in the
more highly developed genera; they lie parallel to the long axis of the
squamule or of the frond except in _Lichinella_ in which genus they are
vertical to the surface. The thallus may be crustaceous, or minutely
foliose, or fruticose; in all cases it is dark-brown in colour, and the
gelatinous character is evident in the moist condition. The best known
British genus is _Lichina_ which grows on rocks by the sea.

The apothecia are more or less immersed in the tissue; in _Pterygium_
and _Steinera_ they are open and superficial (the latter monotypic
genus confined to Kerguelen). They are also open in _Lichinella_ and
_Homopsella_, both very rare genera. The spores are colourless and
simple except in _Pterygium_ and _Steinera_ where they are elongate, and
1-3-septate.

  Thallus crustaceous squamulose.
    Apothecia immersed in thalline warts     1. *Calothricopsis Wain.
    Apothecia superficial, with thalline
      margin                                 2. *Steinera A. Zahlbr.
    Apothecia superficial, without a
      thalline margin                        3.  Pterygium Nyl.
  Thallus of small fruticose fronds.
    Gonidia occupying the central strand     4. *Lichinodium Nyl.
    Gonidia not in the centre.
      Apothecia immersed                     5.  Lichina Ag.
      Apothecia superficial.
        Paraphyses present                   6. *Lichinella Nyl.
        Paraphyses absent                    7. *Homopsella Nyl.


XXXIX. _COLLEMACEAE_

The most important family of the gelatinous lichens and the most
numerous. _Collema_ is historically interesting as having first suggested
the composite thallus. Algal cells, _Nostoc_, which retain the chain-like
form except in _Leprocollema_, a doubtful member of the family. The
thallus varies from indeterminate crusts to lobes of considerable size;
occasionally the lobes are narrow and erect, forming minute fruticose
structures. In the more primitive genera the thallus is non-corticate,
but in the more evolved, the apical cells of the hyphae coalesce to form
a continuous cellular cortex, one or more cells thick, well marked in
some species, in others rudimentary; the formation of plectenchyma also
occurs occasionally in the apothecial tissues of some non-corticate
species.

The apothecia are superficial except in _Pyrenocollema_, a monotypic
genus of unknown locality. They are generally lecanorine, with gonidia
entering into the formation of the apothecium: in some genera they are
lecideine or biatorine, being formed of hyphae alone. The spores are
colourless and vary in form, size and septation.

  Apothecia immersed; spores fusiform,
    1-septate                                1. *Pyrenocollema Reinke.
  Apothecia superficial.
    Thallus without a cortex.
      Spores simple, globose or ellipsoid.
        Thallus crustaceous                  2. *Leprocollema Wain.
        Thallus largely squamulose-fruticose.
          Apothecia lecideine
            (dark-coloured)                  3. *Leciophysma Th. Fr.
          Apothecia lecanorine               4.  Physma Massal.
      Spores variously septate or muriform.
          Apothecia biatorine
            (light-coloured)                 5. *Homothecium Mont.
          Apothecia lecanorine               6.  Collema Wigg.
    Thallus with cortex of plectenchyma.
      Spores simple.
        Spores globose                       7.  Lemmopsis A. Zahlbr.
        Spores ellipsoid, with thick
          subverrucose wall                  8. *Dichodium Nyl.
        Spores vermiform, spirally curved    9. *Koerberia Massal.
      Spores variously septate or muriform.
        Apothecia biatorine
          (light-coloured)                  10. *Arctomia Th. Fr.
        Apothecia lecanorine                11.  Leptogium S. F. Gray.


XL. _HEPPIACEAE_

A family belonging to the “blue-green” series as it is associated with
a gelatinous alga, _Scytonema_, but is of almost entirely cellular
structure and is non-gelatinous. The thallus is squamulose or minutely
foliose, or is formed of narrow almost fruticose lobes; the apothecia are
semi-immersed; the asci are 4-many-spored.

_Heppia_ is a wide-spread genus both in northern and tropical regions
with about forty species that live on soil or rock. So far, no
representative has been recorded in our Islands.

  Spores simple, colourless, globose or
    ellipsoid                                1. *Heppia Naeg.
  Spores muriform, colourless, ellipsoid     2. *Amphidium[1054] Nyl.


XLI. _PANNARIACEAE_

The members of this family are also non-gelatinous, though for the
most part associated with blue-green gelatinous algae, _Nostoc_ or
_Scytonema_. The gonidia are bright-green in the genera _Psoroma_ and
_Psoromaria_, the former often included under _Lecanora_, but too closely
resembling _Pannaria_ to be dissociated from that genus.

The thallus varies from being crustaceous to squamulose or foliose, and
has a cortex of plectenchyma on the upper and sometimes also on the lower
surface. The apothecia are superficial or lateral and with or without a
thalline margin (lecanorine or biatorine), the spores are colourless.

Zahlbruckner has included _Hydrothyria_ in this family. It is a
monotypic aquatic genus found in North America and very closely allied
to _Peltigera_. The British species of the genus, familiarly known as
_Coccocarpia_, have been placed under _Parmeliella_, the former name
being restricted to the tropical or subtropical species first assigned
to _Coccocarpia_ and distinguished by the cortex, the hyphae forming it
lying parallel with the surface though forming a regular plectenchyma.

An Antarctic lichen _Thelidea corrugata_ with _Palmella_ gonidia is
doubtfully included: the thallus is foliose, the apothecia biatorine with
colourless 1-septate spores.

  Thallus with bright-green gonidia.
    With _Palmella_                          1. *Thelidea Hue.
    With Protococcaceae.
      Apothecia non-marginate (biatorine)    2. *Psoromaria Nyl.
      Apothecia marginate                    3.  Psoroma Nyl.
  Thallus with _Scytonema_ gonidia.
      Apothecia marginate, spores 1-septate  4.  Massalongia Koerb.
      Apothecia non-marginate; spores simple.
        Upper surface smooth                 5. *Coccocarpia Pers.
        Upper surface felted                 6. *Erioderma Fée.
  Thallus with _Nostoc_ gonidia.
      Apothecia marginate; spores simple     7.  Pannaria Del.
      Apothecia non-marginate; spores various.
        Thallus crustaceous or minutely
          squamulose                         8.  Placynthium Ach.
        Thallus squamulose, cortex
          indistinct                         9. *Lepidocollema Wain.
        Thallus squamulose or foliose,
          cortex cellular                   10.  Parmeliella Müll.-Arg.
        Thallus foliose, thin veined
          below                             11. *Hydrothyria Russ.


XLII. _STICTACEAE_

Thallus foliose, mostly horizontal, with a plectenchymatous cortex
on both surfaces, a tomentum of hair-like hyphae taking the place of
rhizinae on the lower surface. Algal cells Protococcaceae or _Nostoc_.
Cephalodia and cyphellae or pseudocyphellae often present. Apothecia
superficial or lateral; spores colourless or brown, variously septate.

The highly organized cortex and the presence of aeration organs—cyphellae
or pseudocyphellae—which are almost solely confined to the genus _Sticta_
give this family a high position as regards vegetative development. The
two genera are of wide distribution, but _Sticta_ is more abundant in the
Southern Hemisphere. _Lobaria pulmonaria_ is one of our largest lichens.

  Under surface dotted with cyphellae
    or pseudocyphellae                       1.  Sticta Schreb.
  Under surface without these organs         2.  Lobaria Schreb.


XLIII. _PELTIGERACEAE_

A family of heteromerous foliose lichens containing in some instances
blue-green (_Nostoc_), in others bright-green (Protococcaceae) gonidia,
and thus representing a transition between these two series. They have
large or small lobes and grow on the ground or on trees.

Cephalodia, either ectotrophic (Peltidea) or endotrophic (Solorina),
occur in the family and further exemplify the capacity of the fungus
hyphae to combine with different types of algae.

The upper surface is a wide cortex of plectenchyma, which in some forms
(_Nephromium_) is continued below. In the non-corticate under surface
of _Peltigera_, the lower hyphae grow out in hairs or rhizinae, very
frequently brown in colour. Intercalary growth of the upper tissues
stretches the thallus and tears apart the lower under surface so that
the hair-bearing areas become a network of veins, with the white
exposed medulla between. In _Peltigera canina_ there is further growth
and branching of the hyphae in the veins, adding to the bulk of the
interlacing ridges.

From all other foliose lichens Peltigeraceae are distinguished by the
flat wholly appressed or peltate apothecia without a thalline margin
which arise mostly on the upper surface, but in _Nephromium_ on the
extreme margin of the under surface, the tip of the fertile lobe in
that case is turned back as the apothecium matures, so that the fruit
eventually faces the light. In _Nephroma_ has been included _Eunephroma_
with bright-green gonidia and _Nephromium_ with blue-green.

Bitter[1055] has recorded the finding of apothecia on the under surface
of _Peltigera malacea_ and not at the margin, as in _Nephromium_. The
plant was otherwise normal and healthy. _Solorinella_, from Central
Europe and _Asteristion_ from Ceylon are monotypic genera with poorly
developed thalli.

  Thallus poorly developed.
      Asci 6-8-spored; spores 3-5-septate    1. *Asteristion Leight.
      Asci many-spored; spores 1-septate     2. *Solorinella Anzi.
  Thallus generally well developed.
      Apothecia superficial, sunk in the
        thallus                              3.  Solorina Ach.
      Apothecia terminal on upper surface
        of lobes                             4.  Peltigera Willd.
      Apothecia terminal on lower surface
        of lobes                             5.  Nephroma Ach.


XLIV. _PERTUSARIACEAE_

Thallus crustaceous, often rather thick and with an amorphous cortex on
the upper surface. Algal cells Protococcaceae. Apothecia solitary or
several immersed in thalline warts, generally with a narrow opening which
barely exposes the disc, and which in one genus, _Perforaria_, is so
small as almost to constitute a perithecium; spores are often very large
and with thick walls; some if not all are multinucleate and germinate at
many points.

In the form of the fruit, this family stands between Pyrenocarpeae and
Gymnocarpeae, though more akin to the latter. _Perforaria_, with two
species, belongs to New Zealand and Japan. _Pertusaria_ has a world-wide
distribution, and _Varicellaria_, a monotypic genus, with a very large
two-celled spore, is an Alpine plant, recorded from Europe and from
Antarctic America.

  Spores simple.
      Apothecia with pore-like opening       1. *Perforaria Müll.-Arg.
      Apothecia with a wider opening         2.  Pertusaria DC.
  Spores 1-septate                           3.  Varicellaria Nyl.


XLV. _LECANORACEAE_

Thallus mostly crustaceous, occasionally squamulose or very rarely
minutely fruticulose. The squamulose thallus is corticate above,
the under surface appressed and attached to the substratum by
penetrating hyphae, often effigurate at the circumference. Algal cells
Protococcaceae. Apothecia well distinguished by the thalline margin;
spores colourless, simple or variously septate or muriform.

_Lecanora_, _Ochrolechia_, _Lecania_, _Haematomma_ and _Phlyctis_ are
cosmopolitan genera, some of them with a very large number of species;
the other genera are more restricted in distribution and generally with
few species.

The genus _Candelariella_ is of uncertain position; the spores are 8
or many in the ascus and are simple or 1-septate, and not unfrequently
become polarilocular as in Caloplacaceae, but there is no parietin
present.

  Algae distributed through the thallus.
    Spores simple                             1. *Harpidium Koerb.
  Algae restricted to a definite zone.
    Spores simple.
      Thallus grey, white or yellowish.
        Spores rather small                   2.  Lecanora Ach.
        Spores large                          3.  Ochrolechia Massal.
      Thallus bright yellow.
        Spores simple or 1-septate            4.  Candelariella Müll.-Arg.
    Spores 1-septate (rarely pluri-septate).
      Paraphyses free.
        Thallus squamulose, effigurate        5.  Placolecania Zahlbr.
        Thallus crustaceous.
          Apothecial disc brownish            6.  Lecania Zahlbr.
          Apothecial disc flesh-coloured      7.  Icmadophila Trevis.
      Paraphyses branched, intricate          8. *Calenia Müll.-Arg.
    Spores elongate, pluri-septate.
      Apothecia superficial                   9.  Haematomma Massal.
      Apothecia immersed.
        Paraphyses free                      10. *Phlyctella Müll.-Arg.
        Paraphyses branched, intricate       11. *Phlyctidia Müll.-Arg.
    Spores muriform.
      Apothecia superficial                  12. *Myxodictyon Massal.
      Apothecia immersed                     13.  Phlyctis Wallr.


XLVI. PARMELIACEAE

A very familiar family of foliose lichens. Genera and species are
dorsiventral and stratose in structure, though some _Cetrariae_ are
fruticose in habit. Algal cells are Protococcaceae; in _Physcidia_
they are _Palmellae_. In every case the upper surface of the thallus
is corticate and generally of plectenchyma, the lower being somewhat
similar, but in _Heterodea_ and _Physcidia_, monotypic Australasian
genera, the upper cortex is of branching hyphae parallel with the
surface, the lower surface being non-corticate.

The _Parmeliae_ are mostly provided with abundant rhizinae; in
_Cetrariae_ and _Nephromopsis_ these are very sparingly present, while
in _Anzia_ (including _Pannoparmelia_) the medulla passes into a wide
net-like structure of anastomosing hyphae.

In _Heterodea_, cyphellae occur on the under surface as in Stictaceae;
and in _Cetraria islandica_ bare patches have been described as
pseudocyphellae. The latter lichen is one of the few that are of value as
human food. Special aeration structures are present on the upper cortex
of _Parmelia aspidota_.

  Thallus non-corticate below.
    Apothecia terminal                       1. *Heterodea Nyl.
    Apothecia superficial                    2. *Physcidia Tuck.
  Thallus spongy below                       3. *Anzia Stizenb.
  Thallus corticate below.
      Asci poly-spored                       4.  Candelaria Massal.
      Asci 8-spored.
        Spermatia acrogenous                 5.  Parmeliopsis Nyl.
        Spermatia pleurogenous.
          Apothecia superficial              6.  Parmelia Ach.
          Apothecia lateral.
            Apothecia on upper surface       7.  Cetraria Ach.
            Apothecia on lower surface       8. *Nephromopsis Müll.-Arg.


XLVII. _USNEACEAE_

This also is a familiar family of lichens, _Usnea barbata_ the “bearded
moss” being one of the first lichens noted and chronicled. Algal cells
Protococcaceae. Structure radiate, the upright or pendulous habit
characteristic of the family securing all-round illumination. Special
adaptations of the cortex or of the internal tissues have been evolved to
strengthen the thallus against the strains incidental to their habit of
growth as they are attached in nearly all cases by one point only, by a
special sheath, or by penetrating hold-fasts.

Apothecia are superficial or marginal and sometimes shortly stalked;
spores are simple or variously septate.

_Ramalina_ and _Usnea_, the most numerous, are cosmopolitan genera;
_Alectoria_ inhabits northern or hilly regions.

The genus _Evernia_, also cosmopolitan, represents a transition between
foliose and fruticose types; the fronds of the two species, though
strap-shaped and generally upright, are dorsiventral and stratose, the
gonidia for the most part lying beneath one surface; the other (lower)
surface is either white or very dark-coloured. _Everniopsis_, formed of
thin branching strap-shaped fronds, is also dorsiventral.

A number of genera, _Thamnolia_, _Siphula_, etc. are of podetia-like
structure, generally growing in swards. Several of them have been
classified with _Cladoniae_, but they lack the double thallus.
One of these, _Endocena_, a sterile monotypic Patagonian lichen,
with stiff hollow coralloid fronds, was classified by Hue[1056]
along with _Siphula_; recently he has transferred it to his family
Polycaulionaceae[1057] based on _Polycauliona regale_ (_Placodium
frustulosum_ Darbish.), and allied to _Placodium_ Sect. _Thamnoma_[1058].
In recent studies Hue has laid most stress on thalline characters.
He places the new family between “Ramalinaceae” and “Alectoriaceae.”
_Dactylina arctica_ is a common Arctic soil-lichen.

  Thallus strap-shaped.
    Structure dorsiventral.
      Greyish-green above                    1.  Evernia Ach.
      Whitish-yellow above                   2. *Everniopsis Nyl.
    Structure radiate alike on both surfaces.
      Fronds grey; medulla of loose hyphae   3.  Ramalina Ach.
      Fronds yellow; medulla traversed by
        strands                              4. *Letharia A. Zahlbr.
  Thallus filamentous.
    Medulla a strong “chondroid” strand      5.  Usnea Dill.
    Medulla of loose hyphae.
      Spores simple                          6.  Alectoria Ach.
      Spores muriform, brown                 7. *Oropogon Fr.
  Thallus of upright podetia-like fronds.
    Fronds rather long (about two inches),
      tapering, white                        8.  Thamnolia Ach. (Cerania
                                                 S. F. Gray).
    Fronds shorter, blunt.
      Medulla solid                          9. *Siphula Fr.
      Medulla partly or entirely hollow.
        Fronds swollen and tall (about two
          inches)                           10. *Dactylina Nyl.
        Fronds coralloid, entangled         11. *Endocena Cromb.
        Fronds short, upright               12. *Dufourea Nyl.


XLVIII. _CALOPLACACEAE_

In this family Zahlbruckner has included the squamulose or crustaceous
lichens with colourless polarilocular spores, relegating those with more
highly developed thallus or with brown spores to other families. He has
also substituted the name _Caloplaca_ for the older _Placodium_, the
latter being, as he considers, less well defined.

Algal cells are Protococcaceae. The thallus is mostly light-coloured,
generally some shade of yellow, and, with few exceptions, contains
parietin, which gives a purple colour on the application of potash. The
squamulose forms are closely appressed to the substratum, and have often
a definite rounded outline (effigurate). The spores have a thick median
septum with a loculus at each end and a connecting canal[1059].

In _Blastenia_ the outer thalline margin is obscure or absent—though
gonidia are frequently present below the hymenium. Caloplacaceae occur
all over the globe; they are among the most brilliantly coloured of all
lichens. _Polycauliona_ Hue[1060] possibly belongs here: though based
on thalline rather than on spore characters, one species at least has
polarilocular spores.

  Apothecia with a distinct thalline margin  1.  Caloplaca Th. Fr.
  Apothecia without a thalline margin        2.  Blastenia Th. Fr.


XLIX. _TELOSCHISTACEAE_

Polarilocular colourless spores are the distinguishing feature of this
family as of the Caloplacaceae. Algal cells Protococcaceae. The thallus
of Teloschistaceae is more highly developed, being either foliose or
fruticose, though never attaining to very large dimensions. The cortex
of _Xanthoria_ (foliose) is plectenchymatous, that of _Teloschistes_
(fruticose) is fibrous. The species of both genera are yellow or
greenish-yellow due to the presence of the lichen-acid parietin.

Both genera have a wide distribution over the globe, more especially in
maritime regions.

  Thallus foliose                            1.  Xanthoria Th. Fr.
  Thallus fruticose                          2.  Teloschistes Norm.


L. _BUELLIACEAE_

A family of crustaceous lichens distinguished by the brown two-celled
spores. Algal cells Protococcaceae. Zahlbruckner has included here
_Buellia_ and _Rinodina_; the former with a distinctly lecideine fruit
and with thinly septate spores; the latter lecanorine and with spores
of the polarilocular type, with a very wide central septum pierced in
most of the species by a canal which may or may not traverse the middle
lamella of the wall. _Rinodina_ is closely allied to Physciaceae, while
_Buellia_ has more affinity with Lecideaceae and is near to _Rhizocarpon_.

Both genera are of world-wide distribution.

  Apothecia lecideine, without a thalline
    margin                                   1.  Buellia De Not.
  Apothecia lecanorine, with a thalline
    margin                                   2.  Rinodina Massal.


LI. _PHYSCIACEAE_

Thallus foliose or partly fruticose, and generally attached by rhizinae.
Algal cells Protococcaceae. The spores resemble those of _Rinodina_,
dark-coloured with a thick septum and reduced cell-lumina. As in that
species there may be a second septum in each cell, giving a 3-septate
spore; but that is rare.

_Pyxine_, a tropical or subtropical genus, is lecanorine only in the
very early stages; it soon loses the thalline margin. _Anaptychia_ is
differentiated from _Physcia_ by the subfruticose habit, though the
species are nearly all dorsiventral in structure, only a few of them
being truly radiate and corticate on both surfaces. The upper cortex
of _Anaptychia_ is fibrous, but that character appears also in most
species of _Physcia_ either on the upper or the lower side. _Physcia_ and
_Anaptychia_ are widely distributed.

  Thalline margin absent in apothecia        1. *Pyxine Nyl.
  Thalline margin present in apothecia.
    Thallus foliose                          2.  Physcia Schreb.
    Thallus fruticose                        3.  Anaptychia Koerb.


C. *HYMENOLICHENS

Fungus a Basidiomycete, akin to _Thelephora_. Algal cells _Scytonema_
or _Chroococcus_. Thallus crustaceous, squamulose or foliose. Spores
colourless, produced on basidia, on the under surface of the free thallus.

The Hymenolichens[1061] are few in number and are endemic in tropical or
warm countries. They inhabit soil or trees.

  Thallus of extended lobes.
    Gonidia near the upper surface           1. *Dictyonema Zahlbr.
    Gonidia in centre of tissue              2. *Cora Fr.
  Thallus squamulose, irregular              3. *Corella Wain.


II. NUMBER AND DISTRIBUTION OF LICHENS


1. ESTIMATES OF NUMBER

Calculations have been made and published, once and again, as to the
number of lichen species occurring over the globe or in definite areas.
In 1898 Fünfstück stated that about 20,000 different species had been
described, but as many of them had been proved to be synonyms, and since
many must rank as forms or varieties, the number of well-authenticated
species did not then, according to his estimate, exceed 4000. Many
additional genera and species have, however, been discovered since then.
In Engler and Prantl’s _Pflanzenfamilien_, over 50 families and nearly
300 genera find a place, but even in these larger groupings opinions
differ as to the limits both of genera and families, and lichenologists
would not all accept the arrangement given in that volume.

Fünfstück has reckoned that of his estimated 4000, about 1500 are
European and of these at least 1200 occur in Germany. Probably this is
too low an estimate for that large country. Leighton in 1879 listed, in
his _British Lichen Flora_, 1710 in all, and, as the compilation includes
varieties, it cannot be considered as very far astray. On comparing it
with Olivier’s[1062] recent statistics of lichens, we find that of the
larger fruticose and foliose species, 310 are recognized by him for the
whole of Europe, 206 of these occurring in the British Isles. Leighton’s
estimate of similar species is about 145, without including varieties now
reckoned as good species. In a more circumscribed area, Th. Fries[1063]
described for Spitzbergen about 210 different lichens, a number that
closely approximates to the 206 recent records by Darbishire[1064] for
the same area.

A general idea of the comparative numbers of the different types of
lichens may be gathered from Hue’s compilation of exotic lichens[1065],
examined or described by Nylander, and now in the Paris herbarium.
There are 135 genera with 3686 species. Of these, about 829 belong to
the larger foliose and fruticose lichens (including _Cladoniae_); the
remaining 2857 belong to the smaller kinds, most of them crustaceous.


2. GEOGRAPHICAL DISTRIBUTION


A. GENERAL SURVEY

The larger foliose and fruticose lichens are now fairly well known and
described for Europe, and the knowledge of lichens in other continents
is gradually increasing. It is the smaller crustaceous forms that baffle
the investigator. The distribution of all lichens over the surface of the
earth is controlled by two principal factors, climate and substratum; for
although lichens as a rule require only support, they are most of them
restricted to one or another particular substratum, either organic or
inorganic. As organisms which develop slowly, they require an unchanging
substratum, and as sun-plants they avoid deeply shaded woodlands: their
occurrence thus depends to a large extent on the configuration and
general vegetation of the country.

Though so numerous and so widely distributed, lichens have not evolved
that great variety of families and genera characteristic of the allied
fungi and algae. They conform to a few leading types of structure,
and thus the Orders and Families are comparatively few, and more or
less universal. They are most of them undoubtedly very old plants and
were probably wide-spread before continents and climates had attained
their present stability. Arnold[1066] indeed considers that a large
part of the present-day lichens were almost certainly already evolved
at the end of the Tertiary period, and that they originated in a warm
or probably subtropical climate. As proof of this he cites such genera
as _Graphis_, _Thelotrema_ and _Arthonia_[1067] which are numerous in
the tropics though rare in the colder European countries; and he sees
further proof in the fact that many fruticose and gelatinous lichens do
not occur further north than the forest belt, though they are adapted
to cold conditions. Several genera that are abundant in the tropics are
represented outside these regions by only one or few species, as for
instance _Conotrema urceolatum_ and _Bombyliospora incana_.

During the Ice age of the Quaternary period, not many new species can
have arisen, and such forms as were not killed off must have been driven
towards the south. As the ice retreated the valleys were again stocked
with southern forms, and northern species were left behind on mountain
tops all over the globe.

In examining therefore the distribution of lichens, it will be found
that the distinction between different countries is relative, certain
families being more or less abundant in some regions than others, but,
in general, nearly all being represented. Certain species are universal,
where similar conditions prevail. This is especially true of those
species adapted to extreme cold, as that condition, normal in polar
regions, recurs even on the equator if the mountains reach the limit of
perpetual snow; the vertical distribution thus follows on the lines of
the horizontal.

In all the temperate countries we find practically the same families,
with some few exceptions; there is naturally more diversity of genera
and species. Genera that are limited in locality consist, as a rule,
of one or few species. In this category, however, are not included the
tropical families or genera which may be very rich in species: these are
adapted to extreme conditions of heat and often of moisture, and cannot
exist outside tropical or subtropical regions, extreme heat being more
restricted as to geographical position than extreme cold.

In the study of distribution the question which arises as to the place
of origin of such widely distributed plants is one that is difficult to
solve. Wainio[1068] has attempted the task in regard to _Cladonia_, one
of the most unstable genera, the variations of form, which are dependent
on external circumstances, being numerous and often bewildering. In his
fine monograph of the genus, 132 species are described and 25 of these
are cosmopolitan.

The distribution of Phanerogams is connected, as Wainio points out, with
causes anterior to the present geological era, but this cannot be the
case in a genus so labile and probably so recent as _Cladonia_, though
some of the species have existed long enough to spread and establish
themselves from pole to pole. Endemic species, or those that are confined
to a comparatively limited area, are easily traced to their place of
origin, that being generally the locality where they are found in most
abundance, and as a general rule in the centre of that area, though there
may be exceptions: a plant for instance that originated on a mountain
would migrate only in one direction—towards the regions of greater cold.

The difficulty of determining the primitive stations of cosmopolitan,
or of widely spread, species is much greater, but generally they also
may be referred to their area of greatest abundance. Thus a species
may occur frequently in one continent and but rarely in another, even
where the conditions of climate, etc., are largely comparable. It may
therefore be inferred that the plant has not yet reached the full extent
of possible distribution in the less frequented area. As examples of
this, Wainio cites, among other instances, _Cladonia papillaria_, which
has a very wide distribution in Europe, but, as yet, has been found only
in the eastern parts of North America; and _Cl. pycnoclada_, a plant
which braves the climate of Cape Horn and the Falkland Islands, but has
not travelled northward beyond temperate North America: the southern
origin of that species is thus plainly indicated. Wainio also finds that
evidence of the primitive locality of a very widely spread species may be
obtained by observing the locality of species derived from it, which are
as yet of limited distribution; presumably these arose in the ancestral
place of origin, though this indication is not always to be relied on.
If, however, the ancestral plant has given rise to several of these
rarer related species, those of them that are most closely allied to the
primitive plant would be found near to it in the original locality.

A detailed account of species distribution according to these indications
is given by Wainio and is full of interest. No such attempt has been
made to deal with any other group, and the distribution of genera and
species can only be suggested. An exhaustive comparison of the lichens
of different regions is beyond the purpose of our study and is indeed
impossible as, except in some limited areas, or for certain species,
the occurrence and distribution are not fully known. It is in any case
only tentatively that genera or species can be described as local or
rare, until diligent search has been made for them over a wider field.
The study of lichens from a floristic point of view lags behind that of
most other groups of plants. The larger lichen forms have received more
attention, as they are more evident and more easily collected; but the
more minute species are not easily detected, and, as they are largely
inseparable from their substratum of rocks, or trees, etc., on which
they grow, they are often difficult to collect. They are also in many
instances so indefinite, or so alike in outward form, that they are
liable to be overlooked, only a microscopic examination revealing the
differences in fruit and vegetative structure.

Though much remains to be done, still enough is known to make the
geographical distribution of lichens a subject of extreme interest. It
will be found most instructive to follow the usual lines of treatment,
which give the three great divisions: the Polar, the Temperate and the
Tropical regions of the globe.


B. LICHENS OF POLAR REGIONS

Strictly speaking, this section should include only lichens growing
within the Polar Circles; but in practice the lichens of the whole of
Greenland and those of Iceland are included in the Arctic series, as
are those of Alaska: the latitudinal line of demarcation is not closely
adhered to. With the northern lichens may also be considered those of the
Antarctic continent, as well as those of the islands just outside the
Antarctic Circle, the South Shetlands, South Orkneys, Tierra del Fuego,
South Georgia and the Falkland Islands. During the Glacial period, the
polar forms must have spread with the advancing cold; as the snow and ice
retreated, these forms have been left, as already stated, on the higher
colder grounds, and representatives of polar species are thus to be found
very far from their original haunts. There are few exclusively boreal
genera: the same types occur at the Poles as in the higher subtemperate
zones. One of the most definitely polar species, for instance, _Usnea_
(_Neuropogon_) _melaxantha_ grows in the whole Arctic zone, and, in the
Antarctic, is more luxuriant than any other lichen, but it has also been
recorded from the Andes in Chili, Bolivia and Peru, and from New Zealand
(South Island).

Cold winds are a great feature of both poles, and the lichens that by
structure or habit can withstand these are the most numerous; those that
have a stout cortical layer are able to resist the low temperatures, or
those that grow in tufts and thus secure mutual protection. In Arctic
and Subarctic regions, 495 lichens have been recorded, most of them
crustaceous. Among the larger forms the most frequently met are certain
species of _Peltigera_, _Parmelia_, _Gyrophora_, _Cetraria_, _Cladonia_,
_Stereocaulon_ and _Alectoria_. Among smaller species _Lecanora tartarea_
spreads everywhere, especially over other vegetation, _Lecanora varia_
reaches the farthest limits to which wood, on which it grows, has
drifted, and several species of _Placodium_ occur constantly, though not
in such great abundance. Over the rocks spread also many crustaceous
Lecideaceae too numerous to mention, one of the most striking being the
cosmopolitan _Rhizocarpon geographicum_.

Wainio[1069] has described the lichens collected by Almquist at Pitlekai
in N.E. Siberia just on the borders of the Arctic Circle, and he gives a
vivid account of the general topography. The snow lies on the ground till
June and falls again in September, but many lichens succeed in growing
and fruiting. It is a region of tundra and sand, strewn more or less with
stones. Most of the sand is bare of all vegetation; but where mosses,
etc., have gained a footing, there are also a fair number of lichens:
_Lecanora tartarea_, _Psoroma hypnorum_, with _Lecideae_, _Parmeliae_,
_Cladoniae_, _Stereocaulon alpinum_, _Solorina crocea_, _Sphaerophorus
globosus_, _Alectoria nigricans_ and _Gyrophora proboscidea_. Some
granite rocks in that neighbourhood rise to a height of 200 ft., and
though bare of vegetation on the north side, yet, in sheltered nooks,
several species are to be found. Stunted bushes of willow grow here
and there, and on these occur always the same species: _Placodium
ferrugineum_, _Rinodina archaea_, _Buellia myriocarpa_ and _Arthopyrenia
punctiformis_. Some species such as _Sphaerophorus globosus_, _Dactylina
arctica_ (a purely Arctic genus and species) and _Thamnolia vermicularis_
are so abundant that they bulk as largely as other better represented
genera such as _Cladoniae_, _Lecanorae_ or _Lecideae_. On the soil,
_Lecanorae_ cover the largest areas.

Wainio determined a large number of lichens with many new species, but
the region is colder than that of Lappland, and trees with tree-lichens
are absent, with the exception of those given above. In Arctic Siberia,
Elenkin[1070] discovered a new lichen _Placodium subfruticulosum_
which scarcely differs from Darbishire’s[1071] Antarctic species _Pl.
fruticulosum_ (or _P. regale_); both are distinguished by the fruticose
growth of the thallus, for which reason Hue[1072] placed them in a new
genus, _Polycauliona_.

The Antarctic Zone and the neighbouring lands are less hospitable to
plant life than the northern regions, and there is practically no
accumulation of detritus. Collections have been made by explorers, and
several lists have been published which include a marvellous number of
species common to both Poles, if the subantarctic lands are included in
the survey. An analytic study of the various lists has been published
by Darbishire[1073]. He recognizes 106 true Antarctic lichens half of
which are Arctic as well. The greater number are crustaceous and are
plants common also to other lands though a certain number are endemic.
The most abundant genera in species as well as individuals are _Lecidea_
and _Lecanora_. Several bright yellow species of _Placodium_—_Pl.
elegans_, _Pl. murorum_, etc., are there as at the North Pole. Among
the larger forms, _Parmeliae_, _Cetrariae_, and _Cladoniae_ are fairly
numerous; _Usneae_ and _Ramalinae_ rather uncommon, while members of the
Stictaceae are much more abundant than in the North. The common species
of _Peltigera_ also occur in Antarctica, though _P. aphthosa_ and _P.
venosa_ are wanting; both of these latter are boreal species. Darbishire
adds that lichens have so great a capacity to withstand cold, that they
are only checked by the snow covering, and were bare rocks to be found
at the South Pole, he is sure lichens would take possession of them.
The most southerly point at which any plant has been found is 78° South
latitude and 162° East longitude, in which locality the lichen _Lecanora
subfusca_ was collected by members of Scott’s Antarctic expedition
(1901-1904) at a height of 5000 ft.

A somewhat different view of the Antarctic lichen flora is indicated by
Hue[1072] in his account of the plants brought back by the second French
Antarctic Expedition. The collection was an extremely favourable and
important one: great blocks of stone with their communities of lichens
were secured, and these blocks were entirely covered, the crustaceous
species, especially, spreading over every inch of space.

Hue determined 126 species, but as 15 of these came from the Magellan
regions only 111 were truly Antarctic. Of these 90 are new species, 29
of them belonging to the genus _Buellia_. Hue considers, therefore, that
in Antarctica there is a flora that, with the exception of cosmopolitan
species, is different from every other, and is special to these southern
regions. Darbishire himself described 34 new Antarctic species, but only
10 of these are from true Antarctica; the others were collected in South
Georgia, the Falkland Islands or Tierra del Fuego. Even though many
species are endemic in the south, the fact remains that a remarkable
number of lichens which occur intermediately on mountain summits are
common to both Polar areas.


C. LICHENS OF THE TEMPERATE ZONES

Regions outside the Polar Circles which enjoy, on the whole, cool
moist climates, are specially favourable to lichen growth, and the
recorded numbers are very large. The European countries are naturally
those in which the lichen flora is best known. Whereas polar and high
Alpine species are stunted in growth and often sterile, those in milder
localities grow and fruit well, and the more highly developed species are
more frequent. _Parmeliae_, _Nephromae_, _Usneae_ and _Ramalinae_ become
prominent, especially in the more northern districts. Many Arctic plants
are represented on the higher altitudes. A comparison has been made
between the lichens of Greenland and those of Germany: of 286 species
recorded for the former country, 213 have been found in Germany, the
largest number of species common to both countries being crustaceous.
Lindsay[1074] considered that Greenland lichens were even more akin to
those of Scandinavia.

There is an astonishing similarity of lichens in the Temperate Zone
all round the world. Commenting on a list of Chicago lichens by
Calkins[1075], Hue[1076] pointed out that with the exception of a
few endemic species they resemble those of Normandy. The same result
appears in Bruce Fink’s[1077] careful compilation of Minnesota lichens,
which may be accepted as typical of the Eastern and Middle States of
North Temperate America. The genera from that region number nearly 70,
and only two of these, _Omphalaria_ and _Heppia_, are absent from our
British Flora. The species naturally present much greater diversity.
Very few Graphideae are reported. In other States of North America there
occurs the singular aquatic lichen, _Hydrothyria venosa_, nearly akin to
_Peltigera_.

If we contrast American lichens with these collected in South Siberia
near Lake Baikal[1078], we recognize there also the influence of
temperate conditions. Several species of _Usnea_ are listed, _U.
barbata_, _U. florida_, _U. hirta_ and _U. longissima_, all of them
also American forms, _U. longissima_ having been found in Wisconsin.
_Xanthoria parietina_, an almost cosmopolitan lichen, is absent from
this district, and is not recorded from Minnesota. The opinion[1079] in
America is that it is a maritime species: Tuckerman gives its habitat
as “the neighbourhood of a great water,” and reports it from near Lake
Superior. In our country it grows at a good distance from the sea, in
Yorkshire dales, etc., but all our counties would rank as maritime in the
American sense. _Lecanora tartarea_ which is rare in Minnesota is also
absent from the Lake Baikal region. It occurs frequently both in Arctic
and in Antarctic regions, and is probably also somewhat maritime in
habitat. Many of the _Parmeliae_, _Nephromiae_ and _Peltigerae_, common
to all northern temperate climes, are Siberian as are also _Cladoniae_
and many crustaceous species. There is only one _Sticta_, _St. Wrightii_,
a Japanese lichen, recorded by Wainio from this Siberian locality.

A marked difference as regards species is noted between the Flora of
Minnesota and that of California. Herre[1080] has directed attention to
the great similarity between the lichens of the latter state and those
of Europe: many European species occur along the coast and nowhere else
in America so far as is yet known; as examples he cites, among others,
_Calicium hyperellum_, _Lecidea quernea_, _L. aromatica_, _Gyrophora
polyrhiza_, _Pertusaria amara_, _Roccella fuciformis_, _R. fucoides_
and _R. tinctoria_. The Scandinavian lichen, _Letharia vulpina_, grows
abundantly there and fruits freely; it is very rare in other parts of
America. Herre found, however, no specimens of _Cladonia rangiferina_,
_Cl. alpestris_ or _Cl. sylvatica_, nor any species of _Graphis_; he
is unable to explain these anomalies in distribution, but he considers
that the cool equable climate is largely responsible: it is so much more
like that of the milder countries of Europe than of the states east
of the Sierra Nevada. His contention is supported by a consideration
of Japanese lichens. With a somewhat similar climate there is a great
preponderance of European forms. Out of 382 species determined by
Nylander[1081], 209 were European. There were 17 _Graphideae_, 31
_Parmeliae_, and 23 _Cladoniae_, all of the last named being European.
These results of Nylander’s accord well with a short list of 30 species
from Japan compiled by Müller[1082] at an earlier date. They were chiefly
crustaceous tree-lichens; but the _Cladoniae_ recorded are the familiar
British species _Cl. fimbriata_, _Cl. pyxidata_ and _Cl. verticillata_.

With the Japanese Flora may be compared a list[1083] of Maingay’s lichens
from China, 35 in all. _Collema limosum_, the only representative
of Collemaceae in the list, is European, as are the two species of
_Ramalina_, _R. gracilenta_ and _R. pollinaria_; four species of
_Physcia_ are European, the remaining _Ph. picta_ being a common
tropical or subtropical plant. _Lecanora saxicola_, _L. cinerea_,
_Placodium callopismum_ and _Pl. citrinum_ are cosmopolitan, other
_Lecanorae_ and most of the _Lecideae_ are new. _Graphis scripta_,
_Opegrapha subsiderella_ and _Arthonia cinnabarina_—the few Graphideae
collected—are more or less familiar home plants. Among the Pyrenocarpei,
_Verrucaria_ (_Pyrenula_) _nitida_ occurs; it is a widely distributed
tree-lichen.

It is unnecessary to describe in detail the British lichens. Some
districts have been thoroughly worked, others have barely been touched.
The flora as a whole is of a western European type showing the influence
of the Gulf Stream, though there is also a representative boreal growth
on the moorlands and higher hills, especially in Scotland. Such species
as _Parmelia pubescens_, _P. stygia_ and _P. alpicola_ recall the Arctic
Circle while _Alectoriae_, _Cetrariae_ and _Gyrophorae_ represent
affinity with the colder temperate zone.

In the southern counties such species as _Sticta aurata_, _S.
damaecornis_, _Phaeographis Lyellii_ and _Lecanora_ (_Lecania_)
_holophaea_ belong to the flora of the Atlantic seaboard, while in
S.W. Ireland the tropical genera _Leptogidium_ and _Anthracothecium_
are each represented by a single species. The tropical or subtropical
genus _Coenogonium_ occurs in Great Britain and in Germany, with one
sterile species, _C. ebeneum_. _Enterographa crassa_ is another of our
common western lichens which however has travelled eastwards as far as
Wiesbaden. _Roccella_ is essentially a maritime genus of warm climates:
two species, _R. fuciformis_ and _R. fucoides_, grow on our south and
west coasts. The famous _R. tinctoria_ is a Mediterranean plant, though
it is recorded also from a number of localities outside that region and
has been collected in Australia.

In the temperate zones of the southern hemisphere are situated the great
narrowing projections of South Africa and South America with Australia
and New Zealand. As we have seen, the Antarctic flora prevails more or
less in the extreme southern part of America, and the similarity between
the lichens of that country and those of New Zealand is very striking,
especially in the fruticulose forms. There is a very abundant flora in
the New Zealand islands with their cool moist climate and high mountains.
Churchill Babington[1084] described the collections made by Hooker.
Stirton[1085] added many species, among others _Calycidium cuneatum_,
evidently endemic. Later, Nylander[1086] published the species already
known, and Hellbom[1087] followed with an account of New Zealand lichens
based on Berggsen’s collections; many more must be still undiscovered.
Especially noticeable as compared with the north, are the numbers
of Stictaceae which reach their highest development of species and
individuals in Australasia. They are as numerous and as prominent as are
Gyrophoraceae in the north. A genus of Parmeliaceae, _Hetorodea_, which,
like the _Stictae_, bears cyphellae on the lower surface, is peculiar to
Australia.

A warm current from the tropical Pacific Ocean passes southwards along
the East Coast of Australia, and Wilson[1088] has traced its influence on
the lichens of Australia and Tasmania to which countries a few tropical
species of _Graphis_, _Chiodecton_ and _Trypethelium_ have migrated.
Various unusual types are to be found there also: the beautiful _Cladonia
retepora_ (Fig. 71), which spreads over the ground in cushion-like
growths, with the genera _Thysanothecium_ and _Neophyllis_, genera of
Cladoniaceae endemic in these regions.

The continent of Africa on the north and east is in so close connection
with Europe and Asia that little peculiarity in the flora could be
expected. In comparing small representative collections of lichens, 37
species from Egypt and 20 from Palestine, Müller[1089] found that there
was a great affinity between these two countries. Of the Palestine
species, eight were cosmopolitan; among the crustaceous genera,
_Lecanorae_ were the most numerous. There was no record of new genera.

The vast African continent—more especially the central region—has
been but little explored in a lichenological sense; but in 1895
Stizenberger[1090] listed all of the species known, amounting to
1593, and new plants and new records have been added since that day.
The familiar genera are well represented, _Nephromium_, _Xanthoria_,
_Physcia_, _Parmelia_, _Ramalina_ and _Roccella_, some of them by
large and handsome species. In the Sahara Steiner[1091] found that
genera with blue-green algae such as the Gloeolichens were particularly
abundant; _Heppia_ and _Endocarpon_ were also frequent. Algeria has a
Mediterranean Flora rather than tropical or subtropical. Flagey[1092]
records no species of _Graphis_ for the province of Constantine, and only
22 species of other Graphideae. Most of the 519 lichens listed by him
there are crustaceous species. South America stretches from the Tropics
in the north to Antarctica in the south. Tropical conditions prevail
over the central countries and tropical tree-lichens, Graphidaceae,
Thelotremaceae, etc. are frequent; further West, on the Pacific slopes,
_Usneae_ and _Ramalinae_ hang in great festoons from the branches, while
the foliose _Parmeliae_ and _Stictae_ grow to a large size on the trunks
of the trees.

Wainio’s[1093] _Lichens du Brésil_ is one of the classic systematic books
and embodies the writer’s views on lichen classification. There are no
new families recorded though a number of genera and many species are
new, and, so far as is yet known, these are endemic. Many of our common
forms are absent; thus _Peltigera_ is represented by three species only,
_P. leptoderma_, _P. spuriella_ and _P. Americana_, the two latter being
new species. _Sticta_ (including _Stictina_) includes only five species,
and _Coenogonium_ three. There are 39 species of _Parmelia_ with 33 of
_Lecanora_ and 68 of _Lecidea_, many of them new species.


D. LICHENS OF TROPICAL REGIONS

In the tropics lichens come under the influence of many climates: on the
high mountains there is a region of perpetual snow, lower down a gradual
change to temperate and finally to tropical conditions of extreme heat,
and, in some instances, extreme moisture. There is thus a bewildering
variety of forms. By “tropical” however the warmer climate is always
implied. Several families and genera seem to flourish best in these warm
moist conditions and our familiar species grow there to a large size.
Among crustaceous families Thelotremaceae and Graphidaceae are especially
abundant, and probably originated there. In the old comprehensive genus
_Graphis_, 300 species were recorded from the tropics. It should be
borne in mind that _Trentepohlia_, the alga that forms the gonidia of
these lichens, is very abundant in the tropics. _Coenogonium_, a genus
containing about twelve species and also associated with _Trentepohlia_,
is scarcely found in Europe, except one sterile species, _C. ebeneum_.
Other species of the genus have been recorded as far north as Algeria in
the Eastern Hemisphere and Louisiana in the Western, while one species,
_C. implexum_, occurs in the southern temperate zone in Australia and New
Zealand.

Of exclusively tropical lichens, the Hymenolichens are the most
noteworthy. They include three genera, _Cora_, _Corella_ and
_Dictyonema_, the few species of which grow on trees or on the ground
both in eastern and western tropical countries.

Other tropical or subtropical forms are _Oropogon loxensis_, similar to
_Alectoria_ in form and habit, but with one brown muriform spore in the
ascus; it is only found in tropical or subtropical lands. _Physcidia
Wrightii_ (Parmeliaceae) is exclusively a Cuban lichen. Several small
genera of Pyrenopsidaceae such as _Jenmania_ (British Guiana), _Paulia_
(Polynesia) and _Phloeopeccania_ (South Arabia) seem to be confined to
very hot localities. On the other hand Collemaceae are rare: Wainio
records from Brazil only four species of _Collema_, with nine of
_Leptogium_.

Among Pyrenolichens, Paratheliaceae, Mycoporaceae and Astrotheliaceae are
almost exclusively of tropical distribution, and finally the leaf lichens
with very few exceptions. These follow the leaf algae, _Mycoidea_,
_Phycopeltis_, etc., which are so abundant on the coriaceous long-lived
green leaves of a number of tropical Phanerogams. All the Strigulaceae
are epiphytic lichens. _Phyllophthalmaria_ (Thelotremaceae) is also a
leaf genus; one of the species, _Ph. coccinea_, has beautiful carmine-red
apothecia. The genera of the tropical family Ectolechiaceae also inhabit
leaves, but they are associated with Protococcaceae; one of the genera
_Sporopodium_[1094] is remarkable as having hymenial gonidia. Though
tropical in the main, epiphyllous lichens may spread to the regions
beyond: _Sporopodium Caucasium_ and a sterile _Strigula_ were found by
Elenkin and Woronichin[1095] on leaves of _Buxus sempervirens_ in the
Caucasus, well outside the tropics.

_Pilocarpon_, an epiphytic genus, is associated with Protococcaceae; one
of the species, _P. leucoblepharum_, spreads from the bark to the leaves
of pine-trees; it is widely distributed and has also been reported in the
Caucasus[1096]. _Chrysothrix_, in which the gonidia belong to the algal
genus _Palmella_, grows on Cactus spines in Chili, and may also rank as a
subtropical epiphyllous lichen.

A series of lichens from the warm temperate region of Transcaucasia
investigated by Steiner[1097] were found to be very similar to those
of Central Europe. Lecanoraceae were, however, more abundant than
Lecideaceae and Verrucariaceae were comparatively rare.

Much of Asia lies within tropical or subtropical influences. Several
regions have received some amount of attention from collectors. From
Persia there has been published a list of 59 species determined by
Müller[1098]; several of them are Egyptian or Arabian plants, 15 are new
species, but the greater number are European.

A small collection of 53 species from India, near to Calcutta, published
by Nylander[1099], included a new genus of Caliciaceae, _Pyrgidium_
(_P. bengalense_), allied to _Sphinctrina_. He also recorded _Ramalina
angulosa_ in African species, along with _R. calicaris_, _R. farinacea_
and _Parmelia perlata_, f. _isidiophora_, which are British. Other
foliose forms, _Physcia picta_, _Pyxine Cocoës_ and _P. Meissnerii_ are
tropical or subtropical; along with these were collected crustaceous
tropical species belonging to _Lecanorae_, _Lecideae_, Graphideae, etc.

Leighton[1100] published a collection of Ceylon lichens and found that
Graphideae predominated. Nylander[1101] came to the same conclusion
with regard to lichens referred to him: out of 159 species investigated
from Ceylon, there were 36 species of Graphideae. In another list[1102]
of Labuan, Singapore and Malacca lichens, 164 in all, he found that 56
belonged to the Graphidei, 36 to Pyrenocarpei, 14 to Thelotremei and 11
to Parmelei; only 15 species were European.

On the whole it is safe to conclude from the above and other publications
that the exceptional conditions of the tropics have produced many
distinctive lichens, but that a greater abundance both of species and
individuals is now to be found in temperate and cold climates.


III. FOSSIL LICHENS

In pronouncing on the great antiquity of lichens, proof has been adduced
from physiological rather than from phytogeological evidence. It would
have been of surpassing interest to trace back these plants through the
ages, even if it were never possible to assign to any definite period
the first symbiosis of the fungus and alga; but among fossil plants there
are only scanty records of lichens and even these few are of doubtful
determination.

The reason for this is fairly obvious: not only are the primitive
thalline forms too indistinct for recognizable preservation, but all
lichens are characterized by the gelatinous nature of the hyphal or of
the algal membranes which readily imbibe water. They thus become soft and
flaccid and unfit to leave any impress on sedimentary rocks. It has also
been pointed out by Schimper[1103] that while deciduous leaves with fungi
on them are abundant in fossil beds, lichens are entirely wanting. These
latter are so firmly attached to the rocks or trees on which they grow
that they are rarely dislodged, and form no part of wind- or autumn-fall.
Trunks and branches of trees lose their bark by decay long before they
become fossilized and thus all trace of their lichen covering disappears.

The few records that have been made are here tabulated in chronological
order:

1. PALAEOZOIC. Schimper decides that there are no records of lichens in
the earlier epochs. Any allusions[1104] to their occurrence are held to
be extremely vague and speculative.

2. MESOZOIC. Braun[1105] has recorded a _Ramalinites lacerus_ from the
Keuper sandstone at Eckersdorf, though later[1106] he seemed to be
doubtful as to his determination. One other lichen, an _Opegrapha_, has
been described[1107] from the chalk at Aix.

3. CAINOZOIC. In the brown-coal formations of Saxony Engelhardt[1108]
finds two lichens: _Ramalina tertiaria_, a much branched plant, the
fronds being flat and not channelled “and of further interest that it is
attached to a carbonized stem.” The second form, _Lichen dichotomus_,
has a dichotomously branching strap-shaped frond. “There is sufficient
evidence that these fronds were cylindrical and that the width is due to
pressure. In one place a channel is visible, filled with an ochraceous
yellow substance.”

Other records on brown coal or lignite are: _Verrucarites
geanthricis_[1109] Goepp., somewhat similar to _Pyrenula nitida_, found
at Muskau in Silesia; _Opegrapha Thomasiana_[1110] Goepp., near to
_Opegrapha varia_, and _Graphis scripta succinea_ Goepp.[1111] on a piece
of lignite in amber beds, all of them doubtful.

Schimper has questioned, as he well might, Ludwig’s[1112] records from
lignite from the Rhein-Wetterau Tertiary formations; these are: _Cladonia
rosea_, _Lichen albineus_, _L. diffissus_ and _L. orbiculatus_; he thinks
they are probably fungus mycelia. Another lichen, a _Parmelia_ with
apothecia, which recalls somewhat _P. saxatilis_ or _P. conspersa_,
collected by Geyler also in the brown coal of Wetterau is accepted by
Schimper[1113] as more trustworthy.

More authentic also are the lichens from the amber beds of Königsberg
and elsewhere collected by Goeppert and others. These deposits are
Cainozoic and have been described by Goeppert and Menge[1114] as middle
Miocene. Schimper gives the list as: _Parmelia lacunosa_ Meng. and
Goepp., fragments of thallus near to _P. saxatilis_; _Sphaerophorus
coralloides_; _Cladonia divaricata_ Meng. and Goepp.; _Cl. furcata_;
_Ramalina calicaris_ vars. _fraxinea_ and _canaliculata_; _Cornicularia
aculeata_, _C. subpubescens_ Goepp., _C. ochroleuca_, _C. succinea_
Goepp., and _Usnea barbata_ var. _hirta_. Schimper rather deprecates
specific determinations when dealing with such imperfect fragments.

In a later work Goeppert and Menge[1114] state that they have found
twelve different amber lichens and that among these are _Physcia
ciliaris_, _Parmelia physodes_ and _Graphis_ (probably _G. scripta
succinea_) along with _Peziza resinae_ which is more generally classified
among lichens as _Lecidea_ (_Biatorella_) _resinae_.

Another series of lichens found in recent deposits in North Europe
has been described by Sernander[1115] as “subfossil.” While engaged
on the investigations undertaken by the Swedish Turf-Moor Commission,
he noted the alternation of slightly raised _Sphagnum_ beds with
lower-lying stretches of _Calluna_ and lichen moor—in some instances
dense communities of _Cladonia rangiferina_. In time the turf-forming
_Sphagnum_ overtopped and invaded the drier moorland, covering it with a
new formation of turf. Beneath these layers of “regenerated turf” were
found local accumulations of blackened remains of the _Cladonia_ still
recognizable by the form and branching. Some specimens of _Cetraria
islandica_ were also determined.

Of especial lichenological interest in these northern regions was the
Calcareous Tufa or Calc-sinter in which Sernander also found subfossil
lichens—distinct impressions of _Peltigera_ spp. and the foveolae of
endolithic calcicolous species.

In another category he has placed _Ramalina fraxinea_, _Graphis_ sp. and
_Opegrapha_ sp., traces of which were embedded with drift in the Tufa. In
the two Graphideae the walls of apothecia and pycnidia were preserved.
Sernander considers their presence of interest as testifying to warmer
conditions than now prevail in these latitudes.




CHAPTER IX

ECOLOGY


A. GENERAL INTRODUCTION

Ecology is the science that deals with the habitats of plants and their
response to the environment of climate or of substratum. Ecology in
the lichen kingdom is habitat “writ large,” and though it will not be
possible in so wide a field to enter into much detail, even a short
examination of lichens in this aspect should yield interesting results,
especially as lichens have never, at any time, been described without
reference to their habitat. In very early days, medicinal Usneas were
supposed to possess peculiar virtues according to the trees on which they
grew and which are therefore carefully recorded, and all down the pages
of lichen literature, no diagnosis has been drawn up without definite
reference to the nature of the substratum. Not only rocks and trees are
recorded, but the kind of rock and the kind of tree are often specified.
The important part played by rock lichens in preparing soil for other
plants has also received much attention[1116].

Several comprehensive works on Ecology have been published in recent
times and though they deal mainly with the higher vegetation, the general
plan of study of land plants is well adapted to lichens. A series of
definitions and explanations of the terms used will be of service:

Thus in a work by Moss[1117] we read “The flora is composed of the
individual species: the vegetation comprises the groupings of these
species into ensembles termed vegetation units or plant communities.” And
again:

1. “A _plant formation_ is the whole of the vegetation which occurs on
a definite and essentially uniform habitat.”—All kinds of plants are
included in the formation, so that strictly speaking a _lichen formation_
is one in which lichens are the dominant plants. Cf. p. 394. The term
however is very loosely used in the literature. A uniform habitat, as
regards lichens, would be that of the different kinds of soil, of rock,
of tree, etc.

2. “A _plant association_ is of lower rank than a formation, and
is characterized by minor differences within the generally uniform
habitat.”—It represents a more limited community within the formation.

3. “A _plant society_ is of lower rank than an association, and is marked
by still less fundamental differences of the habitat.”—The last-named
term represents chiefly aggregations of single species. Moss adds that:
“_plant community_ is a convenient and general term used for a vegetation
unit of any rank.”

Climatic conditions and geographical position are included in any
consideration of habitat, as lichens like other plants are susceptible to
external influences.

Ecological plant-geography has been well defined by Macmillan[1118]
as “the science which treats of the reciprocal relation between
physiographic conditions and life requirements of organisms in so far as
such relations manifest themselves in choice of habitats and method of
establishment upon them ... resulting in the origin and development of
plant formations.”


B. EXTERNAL INFLUENCES

The climatic factors most favourable to lichen development are direct
light (already discussed)[1119], a moderate or cold temperature, constant
moisture and a clear pure atmosphere. Wind also affects their growth.

_a._ _TEMPERATURE._ Lichens, as we have seen, can endure the heat of
direct sunlight owing to the protection afforded by thickened cortices,
colour pigments, etc. Where such heat is so intense as to be injurious
the gonidia succumb first[1120].

Lichens endure low temperatures better than other plants, their
xerophytic structure rendering them proof against extreme conditions:
the hyphae have thick walls with reduced cell lumen and extremely meagre
contents. Freezing for prolonged periods does them little injury; they
revive again when conditions become more favourable. Efficient protection
is also afforded by the thickened cortex of such lichens as exist in
Polar areas, or at high altitudes. Thus various species of _Cetrariae_
with a stout “decomposed” amorphous cortex can withstand very low
temperatures and grow freely on the tundra, while _Cladonia rangiferina_,
also a northern lichen, but without a continuous cortex, cannot exist in
such cold conditions, unless in localities where it is protected by a
covering of snow during the most inclement seasons.

_b._ _HUMIDITY._ A high degree of humidity is distinctly of advantage to
the growth of the lichen thallus, though when the moist conditions are
excessive the plants become turgid and soredial states are developed.

The great abundance of lichens in the western districts of the British
Isles, where the rainfall is heaviest, is proof enough of the advantage
of moisture, and on trees it is the side exposed to wind and rain that
is most plentifully covered. A series of observations on lichens and
rainfall were made by West[1121] and have been published since his death.
He has remarked in more than one of his papers that a most favourable
situation for lichen growth is one that is subject to a drive of wind
with much rain. In localities with an average of 216 days of rain in
the year, he found abundant and luxuriant growths of the larger foliose
species. In West Ireland there were specimens of _Ricasolia laetevirens_
measuring 165 by 60 cm. In West Scotland with an “average of total days
of rain, 225,” he found plants of _Ricasolia amplissima_ 150 × 90 cm. in
size, of _R. laetevirens_ 120 × 90 cm., while _Pertusaria globulifera_
formed a continuous crust on the trees as much as 120 × 90 cm. _Lecanora
tartarea_ seemed to thrive exceptionally well when subject to driving
mists and rains from mountain or moorland, and was in these circumstances
frequently the dominant epiphyte. Bruce Fink[1122] also observed in his
ecological excursions that the number of species and individuals was
greater near lakes or rivers.

Though a fair number of lichens are adapted to life wholly or partly
under water, land forms are mostly xerophytic in structure, and die off
if submerged for any length of time. The _Peltigerae_ are perhaps the
most hydrophilous of purely land species. Many Alpine or Polar forms
are covered with snow for long periods. In the extreme north it affords
more or less protection; and Kihlman[1123] and others have remarked on
the scarcity of lichens in localities denuded of the snow mantle and
exposed to severe winter cold. On the other hand lichens on the high
Alpine summits that are covered with snow the greater part of the year
suffer, according to Nilson[1124], from the excessive moisture and the
deprivation of light. Foliose and fruticose forms were, he found, dwarfed
in size; the crustaceous species had a very thin thallus and in all of
them the colour was impure. _Gyrophorae_ seemed to be most affected:
folds and outgrowths of the thallus were formed and the internal tissues
were partly disintegrated. Lichens on the blocks of the glacier moraines
which are subject to inundations of ice-cold water after the snow has
melted, were unhealthy looking, poorly developed and often sterile,
though able to persist in a barren state. Lindsay[1125] noted as a result
of such conditions on _Cladoniae_ not only sterility but also deformity
both of vegetative and reproductive organs; discolouration and mottling
of the thallus and an increased development of squamules of the primary
thallus and on the podetia.

_c._ _WIND._ Horizontal crustaceous or foliose lichens are not liable
to direct injury by wind as their close adherence to the substratum
sufficiently shelters them. It is only when the wind carries with it any
considerable quantity of sand that the tree or rock surfaces are swept
bare and prevented from ever harbouring any vegetation, and also, as
has been already noted, the terrible winds round the poles are fatal to
lichens exposed to the blasts unless they are provided with a special
protective cortex. After crustaceous forms, species of _Cetraria_,
_Stereocaulon_ and _Cladonia_ are best fitted for weathering wind storms:
the tufted[1126] cushion-like growth adopted by these lichens gives them
mutual protection, not only against wind, but against superincumbent
masses of snow. Kihlman[1123] has given us a vivid account of wind action
in the Tundra region. He noted numerous hollows completely scooped out
down to the sand: in these sheltered nooks he observed the gradual
colonization of the depressions, first by a growth of hepatics and mosses
and by such ground lichens as _Peltigera canina_, _P. aphthosa_ and
_Nephromium arcticum_; they cover the soil and in time the hollow becomes
filled with a mass of vegetation consisting of Cladonias, mosses, etc.
On reaching a certain more exposed level these begin to wither and die
off at the tips, killed by the high cold winds. Then arrives _Lecanora
tartarea_, one of the commonest Arctic lichens, and one which is readily
a saprophyte on decayed vegetation. It covers completely the mound of
weakened plants which are thus smothered and finally killed. The collapse
of the substratum entails in turn the breaking of the _Lecanora_ crust,
and the next high wind sweeps away the whole crumbling mass. How long
recolonization takes, it was impossible to find out.

Upright fruticose lichens are necessarily more liable to damage by
wind, but maritime _Ramalinae_ and _Roccellae_ do not seem to suffer
in temperate climates, though in regions of extreme cold fruticose
forms are dwarfed and stunted. The highest development of filamentous
lichens is to be found in more or less sheltered woods, but the effect
of wind on these lichens is not wholly unfavourable. Observations have
been made by Peirce[1127] on two American pendulous lichens which are
dependent on wind for dissemination. On the Californian coasts a very
large and very frequent species, _Ramalina reticulata_ (Fig. 64), is
seldom found undamaged by wind. In Northern California the deciduous oaks
_Quercus alba_ and _Q. Douglasii_ are festooned with the lichen, while
the evergreen “live oak,” _Q. chrysolepis_, with persistent foliage,
only bears scraps that have been blown on to it. Nearer the coast and
southward the lichen grows on all kinds of trees and shrubs. The fronds
of this _Ramalina_ form a delicate reticulation and when moist are easily
torn. In the winter season, when the leaves are off the trees, wind- and
rain-storms are frequent; the lichen is then exposed to the full force of
the elements and fragments and shreds are blown to other trees, becoming
coiled and entangled round the naked branches and barky excrescences, on
which they continue to grow and fruit perfectly well. A succeeding storm
may loosen them and carry them still further. Peirce noted that only
plants developed from the spore formed hold-fasts and they were always
small, the largest formed measuring seven inches in length. Both the
hold-fast and the primary stalk were too slight to resist the tearing
action of the wind.

Schrenk[1128] made a series of observations and experiments with the
lichens _Usnea plicata_ and _U. dasypoga_, long hanging forms common on
short-leaved conifers such as spruce and juniper. The branches of these
trees are often covered with tangled masses of the lichens not due to
local growth, but to wind-borne strands and to coiling and intertwining
of the filaments owing to successive wetting and drying. Tests were
made as to the force of wind required to tear the lichens and it was
found that velocities of 77 miles per hour were not sufficient to cause
any pieces of the lichen to fly off when it was dry; but after soaking
in water, the first pieces were torn off at 50 miles an hour. These
figures are, however, considered by Schrenk to be too high as it was
found impossible in artificially created wind to keep up the condition of
saturation. It is the combination of wind and rain that is so effective
in ensuring the dispersal of both these lichens.

_d._ _HUMAN AGENCY._ Though lichens are generally associated with
undisturbed areas and undisturbed conditions, yet accidents or
convulsions of nature, as well as changes effected by man, may at times
prove favourable to their development. The opening up of forests by
thinning or clearing will be followed in time by a growth of tree and
ground forms; newly planted trees may furnish a new lichen flora, and
the building of houses and walls with their intermixture of calcareous
mortar will attract a particular series of siliceous or of lime-loving
lichens. A few lichens are partial to the trees of cultivated areas, such
as park-lands, avenues or road-sides. Among these are several species of
_Physcia_: _Ph. pulverulenta_, _Ph. ciliaris_ and _Ph. stellaris_, some
species of _Placodium_, and those lichens such as Lecanora varia that
frequently grow on old palings.

On the other hand lichens are driven away from areas of dense population,
or from regions affected by the contaminated air of industrial centres.
In our older British Floras there are records of lichens collected in
London during the eighteenth century—in Hyde Park and on Hampstead
Heath—but these have long disappeared. A variety of _Lecanora galactina_
seems to be the only lichen left within the London district: it has been
found at Camden Town, Notting Hill and South Kensington.

So recently as 1866, Nylander[1129] made a list of the lichens growing
in the Luxembourg gardens in Paris; the chestnuts in the alley of the
Observatory were the most thickly covered, and the list includes about
35 different species or varieties, some of them poorly developed and
occurring but rarely, others always sterile, but quite a number in
healthy fruiting condition. All of them were crustaceous or squamulose
forms except _Parmelia acetabulum_, which was very rare and sterile;
_Physcia obscura_ var. and _Ph. pulverulenta_ var., also sterile;
_Physcia stellaris_ with occasional abortive apothecia and _Xanthoria
parietina_, abundant and fertile. In 1898, Hue[1130] tells us, there were
no lichens to be found on the trees and only traces of lichen growth on
the stone balustrades.

The question of atmospheric pollution in manufacturing districts and its
effect on vegetation, more especially on lichen vegetation, has received
special attention from Wheldon and Wilson[1131] in their account of the
lichens of South Lancashire, a district peculiarly suitable for such an
inquiry, as nowhere, according to the observations, are the evil effects
of impure air so evident or so wide-spread. The unfavourable conditions
have prevailed for a long time and the lichens have consequently become
very rare, those that still survive leading but a meagre existence. The
chief impurity is coal smoke which is produced not only from factories
but from private dwellings, and its harmful effect goes far beyond
the limits of the towns or suburbs, lichens being seen to deteriorate
as soon as there is the slightest deposition of coal combustion
products—especially sulphur compounds—either on the plants or on the
surfaces on which they grow. The larger foliose and fruticose forms have
evidently been the most severely affected. “While genera of bark-loving
lichens such as _Calicium_, _Usnea_, _Ramalina_, _Graphis_, _Opegrapha_,
_Arthonia_ etc. are either wholly absent or are poorly represented in
the district,” corticolous species now represent about 15 per cent.
of those that are left; those that seem best to resist the pernicious
influences of the smoky atmosphere are, principally, _Lecanora varia,
Parmelia saxatilis, P. physodes_ and to a less degree _P. sulcata_, _P.
fuliginosa_ var. _laetevirens_ and _Pertusaria amara_.

Saxicolous lichens have also suffered severely in South Lancashire; not
only the number of species, but the number of individuals is enormously
reduced and the specimens that have persisted are usually poorly
developed. The smoke-producing towns are situated in the valley-bottoms,
and the smoke rises and drifts on to the surrounding hills and moorlands.
The authors noted that crustaceous rock-lichens were in better condition
on horizontal surfaces such as the copings of walls, or half-buried
stones, etc. than on the perpendicular or sloping faces of rocks or
walls. This was probably due to what they observed as to the effect
of water trickling down the inclined substrata and becoming charged
with acid from the rock surfaces. They also observed further that a
calcareous substratum seemed to counteract the effect of the smoke, the
sulphuric acid combining with the lime to form calcium sulphate, and
the surface-washings thus being neutralized, the lichens there are more
favourably situated. They found in good fruiting condition, on mortar,
cement or concrete, the species _Lecanora urbana_, _L. campestris_, _L.
crenulata_, _Verrucaria muralis_, _V. rupestris_, _Thelidium microcarpum_
and _Staurothele hymenogonia_. Some of these occurred on the mortar
of sandstone walls close to the town, “whilst on the surface of the
sandstone itself no lichens were present.”

Soil-lichens were also strongly affected, the Cladoniae of the moorlands
being in a very depauperate condition, and there was no trace of
_Stereocaulon_ or of _Sphaerophorus_ species, which, according to older
records, previously occurred on the high uplands.

The influence of human agency is well exemplified in one of the London
districts. In 1883 Crombie published a list of the lichens recorded from
Epping Forest during the nineteenth century. They numbered 171 species,
varieties or forms, but, at the date of publication, many had died out
owing to the destruction of the older trees; the undue crowding of the
trees that were left and the ever increasing population on the outskirts
of the Forest. Crombie himself made a systematic search for those that
remained, and could only find some 85 different kinds, many of them in a
fragmentary or sterile condition.

R. Paulson and P. Thompson[1132] commenced a lichen exploration of the
Forest 27 years after Crombie’s report was published, and they have found
that though the houses and the population have continued to increase
round the area, the lichens have not suffered. “Species considered by
Crombie as rare or sterile are now fairly abundant, and produce numerous
apothecia. Such are _Baeomyces rufus_, _B. roseus_, _Cladonia pyxidata_,
_Cl. macilenta_ var. _coronata_, _Cl. Floerkeana_ f. _trachypoda_,
_Lecanora varia_, _Lecidea decolorans_ and _Lecidea tricolor_.” They
conclude that “some at least of the Forest lichens are in a far more
healthy and fertile condition than they were 27 years ago.” They
attribute the improvement mainly to the thinning of trees and the opening
up of glades through the Forest, letting in light and air not only to the
tree trunks but to the soil. In 1912[1133] the authors in a second paper
reported that 109 different kinds had been determined, and these, though
still falling far short of the older lichen flora, considerably exceed
the list of 85 recorded in 1883.


C. LICHEN COMMUNITIES

Lichen communities fall into a few definite groups, though, as we shall
see, not a few species may be found to occur in several groups—species
that have been designated by some workers as “wanderers.” The leading
communities are:

1. ARBOREAL, including those that grow on leaves, bark or wood.

2. TERRICOLOUS, ground-lichens.

3. SAXICOLOUS, rock-lichens.

4. OMNICOLOUS, lichens that can exist on the most varied substrata, such
as bones, leather, iron, etc.

5. LOCALIZED COMMUNITIES in which owing to special conditions the lichens
may become permanent and dominant.

In all the groups lichens are more or less abundant. In arboreal and
terricolous formations they may be associated with other plants; in
saxicolous and omnicolous formations they are the dominant vegetation. It
will be desirable to select only a few of the typical communities that
have been observed and recorded by workers in various lands.


1. ARBOREAL

Arboreal communities may be held to comprise those lichens that grow
on wood, bark or leaves. They are usually the dominant and often the
sole vegetation, but in some localities there may be a considerable
development of mosses, etc., or a mantle of protococcaceous algae may
cover the bark. Certain lichens that are normally corticolous may also
be found on dead wood or may be erratic on neighbouring rocks: _Usnea
florida_ for instance is a true corticolous species, but it grows
occasionally on rocks or boulders generally in crowded association with
other foliose or fruticose lichens.

Most of the larger lichens are arboreal, though there are many
exceptions: _Parmelia perlata_ develops to a large size on boulders as
well as on trees; some species of _Ramalinae_ are constantly saxicolous
while there are only rare instances of _Roccellae_ that grow on trees.
The purely tropical or subtropical genera are corticolous rather than
saxicolous, but species that have appeared in colder regions may have
acquired the saxicolous habit: thus _Coenogonium_ in the tropics grows on
trees, but the European species, _C. ebeneum_, grows on stone.

_a._ EPIPHYLLOUS. These grow on Ferns or on the coriaceous leaves
of evergreens in the tropics. Many of them are associated with
_Phycopeltis_, _Phyllactidium_ or _Mycoidea_, and follow in the wake
of these algae. Observations are lacking as to the associations or
societies of these lichens whether they grow singly or in companies. The
best known are the Strigulaceae: there are six genera in that family,
and some of the species have a wide distribution. The most frequent
genus is _Strigula_ associated with _Phycopeltis_ which forms round grey
spots on leaves, and is almost entirely confined to tropical regions.
Chodat[1134] records a sterile species, _S. Buxi_, on box leaves from the
neighbourhood of Geneva.

Other genera, such as those of Ectolechiaceae, which inhabit fern
scales and evergreen leaves, are associated with Protococcaceae.
_Pilocarpon leucoblepharum_ with similar gonidia grows round the
base of pine-needles. It is found in the Caucasus. In our own woods,
along the outer edges, the lower spreading branches of the fir-trees
are often decked with numerous plants of _Parmelia physodes_, a true
“plant society,” but that lichen is a confirmed “wanderer.” _Biatorina
Bouteillei_, on box leaves, is a British and Continental lichen.

_b._ CORTICOLOUS. In this series are to be found many varying groups, the
type of lichen depending more on the physical nature of the bark than
on the kind of trees. Those with a smooth bark such as hazel, beech,
lime, etc., and younger trees in general, bear only crustaceous species,
many of them with a very thin thallus, often partly immersed below the
surface. As the trees become older and the bark takes on a more ragged
character, other types of lichens gain a foothold, such as the thicker
crustaceous forms like _Pertusaria_, or the larger foliose and fruticose
species. The moisture that is collected and retained by the rough bark is
probably the important factor in the establishment of the thicker crusts,
and, as regards the larger lichens, both rhizinae and hold-fasts are able
to gain a secure grip of the broken-up unequal surface, such as would be
quite impossible on trees with smooth bark.

Among the first to pay attention to the ecological grouping of
corticolous lichens was A.L. Fée[1135], a Professor of Natural Science
and an Army doctor, who wrote on many literary and botanical subjects.
In his account of the Cryptogams that grow on “officinal bark,” he
states that the most lichenized of all the _Cinchonae_ was the one known
as “Loxa,” the bark of which was covered with species of _Parmelia_,
_Sticta_ and _Usnea_ along with crustaceous forms of _Lecanora_,
_Lecidea_, _Graphis_ and _Verrucaria_. Another species, _Cinchona
cordifolia_, was completely covered, but with crustaceous forms only:
species of Graphidaceae, _Lecanora_ and _Lecidea_ were abundant, but
_Trypethelium_, _Chiodecton_, _Pyrenula_ and _Verrucaria_ were also
represented. On each species of tree some particular lichen was generally
dominant:

  A species of _Thelotrema_ on _Cinchona oblongifolia_.
  A species of _Chiodecton_ on _C. cordifolia_.
  A species of _Sarcographa_ on _C. condaminea_.

Fries[1136], in his geography of lichens, distinguished as arboreal and
“hypophloeodal” species of Verrucariaceae, while the Graphideae, which
also grew on bark, were erumpent. _Usnea barbata_, _Evernia prunastri_,
etc., though growing normally on trees might, he says, be associated with
rock species.

More extensive studies of habitat were made by Krempelhuber[1137] in his
_Bavarian Lichens_. In summing up the various “formations” of lichens, he
gives lists of those that grow, in that district, exclusively on either
coniferous or deciduous trees, with added lists of those that grow on
either type of tree indifferently. Among those found always on conifers
or on coniferous wood are: _Letharia vulpina_, _Cetraria Laureri_,
_Parmelia aleurites_ and a number of crustaceous species. Those that are
restricted to the trunks and branches of leafy trees are crustaceous
with the exception of some foliose Collemaceae such as _Leptogium
Hildenbrandii_, _Collema nigrescens_, etc.

Arnold[1138] carried to its furthest limit the method of arranging
lichens ecologically, in his account of those plants from the
neighbourhood of Munich. He gives “formation” lists, not only for
particular substrata and in special situations, but he recapitulates the
species that he found on the several different trees. It is not possible
to reproduce such a detailed survey, which indeed only emphasizes the
fact that the physical characters of the bark are the most important
factors in lichen ecology: that on smooth bark, whether of young trees,
or on bark that never becomes really rugged, there is a preponderance
of species with a semi-immersed thallus, and very generally of those
that are associated with _Trentepohlia_ gonidia, such as Graphidaceae or
Pyrenulaceae, though certain species of _Lecidea_, _Lecanora_ and others
also prefer the smooth substratum.

Bruce Fink[1139] has published a series of important papers on lichen
communities in America, some of them similar to what we should find in
the British Isles.

On trees with smooth bark he records in the Minnesota district:

  _Xanthoria polycarpa._
  _Candelaria concolor._
  _Parmelia olivacea_, _P. adglutinata_.
  _Placodium cerinum._
  _Lecanora subfusca._
  _Bacidia fusca-rubella._
  _Lecidea enteroleuca._
  _Graphis scripta._
  _Arthonia lecideella_, _A. dispersa_.
  _Arthopyrenia punctiformis_, _A. fallax_.
  _Pyrenula nitida_, _P. thelena_, _P. cinerella_, _P. leucoplaca_.

On rough bark he records:

  _Ramalina calicaris_, _R. fraxinea_, _R. fastigiata_.
  _Teloschistes chrysophthalmus._
  _Xanthoria polycarpa_, _X. lychnea_.
  _Candelaria concolor._
  _Parmelia perforata_, _P. crinita_, _P. Borreri_, _P. tiliacea_,
      _P. saxatilis_, _P. caperata_.
  _Physcia granulifera_, _Ph. pulverulenta_, _Ph. stellaris_, _Ph.
      tribacia_, _Ph. obscura_.
  _Collema pycnocarpum_, _C. flaccidum_.
  _Leptogium mycochroum._
  _Placodium aurantiacum_, _Pl. cerinum_.
  _Lecanora subfusca._
  _Pertusaria leioplaca_, _P. velata_.
  _Bacidia rubella_, _B. fuscorubella_.
  _Lecidea enteroleuca._
  _Rhizocarpon alboatrum_, _Buellia parasema_.
  _Opegrapha varia._
  _Graphis scripta._
  _Arthonia lecideella_, _A. radiata_.
  _Arthopyrenia quinqueseptata_, _A. macrospora_.
  _Pyrenula nitida_, _P. leucoplaca_.

Finally, as generally representative of the commonest lichens in our
woods of deciduous trees, including both smooth- and rough-barked, the
community of oak-hazel woods as observed by Watson[1140] in Somerset may
be quoted:

  _Collema flaccidum._
  _Calicium hyperellum._
  _Ramalina calicaris_, _R. fraxinea_ with var. _ampliata_, _R.
      fastigiata_, _R. farinacea_ and _R. pollinaria_.
  _Parmelia saxatilis_ and f. _furfuracea_, _P. caperata_, _P. physodes_.
  _Physcia pulverulenta_, _Ph. tenella_ (_hispida_).
  _Lecanora subfusca_, _L. rugosa_.
  _Pertusaria amara_, _P. globulifera_, _P. communis_, _P. Wulfenii_.
  _Lecidea_ (_Buellia_) _canescens_.
  _Graphis scripta._

And on the soil of these woods:

  _Cladonia pyxidata_, _Cl. pungens_, _Cl. macilenta_, _Cl. pityrea_,
      _Cl. squamosa_ and _Cl. sylvatica_.

Paulson[1141], from his observations of lichens in Hertfordshire, has
concluded that the presence or absence of lichens on trees is influenced
to a considerable degree by the nature of the soil. They were more
abundant in woods on light well-drained soils than on similar communities
of trees on heavier soils, though the shade in the former was slightly
more dense and therefore less favourable to their development; the cause
of this connection is not known.

_c._ LIGNICOLOUS. Lichens frequenting the branches of trees do not long
continue when these have fallen to the ground. This may be due to the
lack of light and air, but Bouly de Lesdain[1142] has suggested that
the chemical reactions produced by the decomposition of the bast fibres
are fatal to them, _Lecidea parasema_ alone continuing to grow and even
existing for some time on the detached shreds of bark.

On worked wood, such as old doors or old palings, light and air are
well provided and there is often an abundant growth of lichens, many of
which seem to prefer that substratum: the fibres of the wood loosened by
weathering retain moisture and yield some nutriment to the lichen hyphae
which burrow among them. Though a number of lichens grow willingly on
dead wood, there are probably none that are wholly restricted to such a
habitat. A few, such as the species of _Coniocybe_, are generally to be
found on dead roots of trees or creeping loosely over dead twigs. They
are shade lichens and fond of moisture.

The species on palings—or “dead wood communities”—most familiar to us in
our country are:

  _Usnea hirta._
  _Cetraria diffusa._
  _Evernia furfuracea._
  _Parmelia scortia_, _P. physodes_.
  _Xanthoria parietina._
  _Placodium cerinum._
  _Rinodina exigua._
  _Lecanora Hageni_, _L. varia_ and its allies.
  _Lecidea ostreata_, _L. parasema_.
  _Buellia myriocarpa._
  Cladoniaceae and Caliciaceae (several species).

These may be found in very varying association. It has indeed been
remarked that the dominant plant may be simply the one that has first
gained a footing, though the larger and more vigorous lichens tend to
crowd out the others. Bruce Fink[1143] has recorded associations in
Minnesota:

On wood:

  _Teloschistes chrysophthalmus._
  _Placodium cerinum._
  _Lecanora Hageni_, _L. varia_.
  _Rinodina sophodes_, _R. exigua_.
  _Buellia parasema_ (_disciformis_), _B. turgescens_.
  _Calicium parietinum._
  _Thelocarpon prasinellum._

On rotten stumps and prostrate logs: _Peltigera canina_, _Cladonia
fimbriata_ var. _tubaeformis_, _Cl. gracilis_, _Cl. verticillata_, _Cl.
symphicarpia_, _Cl. macilenta_, _Cl. cristatella_.

Except for one or two species such as _Buellia turgescens_, _Cladonia
symphicarpia_, etc., the associations could be easy paralleled in our own
country, though with us _Peltigera canina_, _Cladonia gracilis_ and _Cl.
verticillata_ are ground forms.


2. TERRICOLOUS

In this community other vegetation is dominant, lichens are subsidiary.
In certain conditions, as on heaths, they gain a permanent footing,
in others they are temporary denizens and are easily crowded out. As
they are generally in close contact with the ground they are peculiarly
dependent on the nature of the soil and the water content. There are
several distinct substrata to be considered each with its characteristic
flora. Cultivated soil and grass lands need scarcely be included, as in
the former the processes of cultivation are too harassing for lichen
growth, and only on the more permament somewhat damp mossy meadows do
we get such a species as _Peltigera canina_ in abundance. Some of the
earth-lichens are among the quickest growers: the apothecia of _Baeomyces
roseus_ appear and disappear within a year. _Thrombium epigaeum_ develops
in half a year; _Thelidium minutulum_ in cultures grew from spore to
spore, according to Stahl[1144], in three months.

There are three principal types of soil composition: (1) that in which
there is more or less of lime; (2) soils in which silica in some form or
other predominates, and (3) soils which contain an appreciable amount of
humus.

Communities restricted to certain soils such as sand-dunes, etc., are
treated separately.

_a._ ON CALCAREOUS SOIL. Any admixture of lime in the soil, either as
chalk, limy clay or shell sand is at once reflected in the character
of the lichen flora. On calcareous soil we may look for any of the
squamulose _Lecanorae_ or _Lecideae_ that are terricolous species, such
as _Lecanora crassa_, _L. lentigera_, _Placodium fulgens_, _Lecidea
lurida_ and _L. decipiens_. There are also the many lichens that grow on
mortar or on the accumulated debris mixed with lime in the crevices of
walls, such as _Biatorina coeruleonigricans_, species of _Placodium_,
several species of _Collema_ and of Verrucariaceae.

Bruce Fink[1145] found in N.W. Minnesota an association on exposed
calcareous earth as follows:

  _Heppia Despreauxii._
  _Urceolaria scruposa._
  _Biatora_ (_Lecidea_) _decipiens_.
  _Biatora_ (_Bacidia_) _muscorum_.
  _Dermatocarpon hepaticum._

This particular association occupied the slope of a hill that was washed
by lime-impregnated water. It was normally a dry habitat and the lichens
were distinguished by small closely adnate thalli.

There are more lichens confined to limy than to sandy soil. Arnold[1146]
gives a list of those he observed near Munich on the former habitat:

  _Cladonia sylvatica_ f. _alpestris_.
  _Cladonia squamosa_ f. _subsquamosa_.
  _Cladonia rangiformis_ f. _foliosa_.
  _Cladonia cariosa_ and f. _symphicarpa_.
  _Peltigera canina_ f. _soreumatica_.
  _Solorina spongiosa._
  _Heppia virescens._
  _Lecanora crassa._
  _Urceolaria scruposa_ f. _argillacea_.
  _Verrucaria_ (_Thrombium_) _epigaea_.
  _Lecidea decipiens._
  _Dermatocarpon cinereum._
  _Collema granulatum._
  _Collema tenax._
  _Leptogium byssinum._

It is interesting to note how many of these lichens specialized as to
habitat are forms of species that grow in other situations.

_b._ ON SILICEOUS SOIL. Lichens are not generally denizens of cultivated
soil; a few settle on clay or on sandbanks. _Cladonia fimbriata_ and
_Cl. pyxidata_ grow frequently in such situations; others more or less
confined to sandy or gravelly soil are, in the British Isles:

  _Baeomyces roseus._
  _Baeomyces rufus._
  _Baeomyces placophyllus._
  _Endocarpon_ spp.
  _Gongylia viridis._
  _Dermatocarpon lachneum_
  _Dermatocarpon hepaticum._
  _Dermatocarpon cinereum._

These very generally grow in extended societies of one species only.

In his enumeration of soil-lichens Arnold[1146] gives 40 species that
grow on siliceous soil, as against 57 on calcareous. Many of them
occurred on both. Those around Munich on siliceous soil only were:

  _Cladonia coccifera._
  _Cladonia agariciformis._
  _Secoliga_ (_Gyalecta_) _bryophaga_.
  _Baeomyces rufus._
  _Lecidea gelatinosa._
  _Psorotichia lutophila._

Mayfield[1147] in his account of the Boulder Clay lichen flora of Suffolk
found only four species that attained to full development on banks and
hedgerows. These were: _Collema pulposum_, _Cladonia pyxidata_, _Cl.
furcata_ var. _corymbosa_ and _Peltigera polydactyla_.

On bare heaths of gravelly soil in Epping Forest Paulson and
Thompson[1148] describe an association of such lichens as:

  _Baeomyces roseus._
  _Baeomyces rufus._
  _Pycnothelia papillaria._
  _Cladonia coccifera._
  _Lecidea granulosa._
  _Cladonia macilenta._
  _Cladonia furcata._
  _Cetraria aculeata._
  _Peltigera spuria._

And on flints in the soil: _Lecidea crustulata_ and _Rhizocarpon
confervoides_. They found that _Peltigera spuria_ colonized very quickly
the burnt patches of earth which are of frequent occurrence in Epping
Forest, while on wet sandy heaths amongst heather they found associated
_Cladonia sylvatica_ f. _tenuis_ and _Cl. fimbriata_ subsp. _fibula_.

_c._ ON BRICKS, ETC. Closely allied with siliceous soil-lichens are those
that form communities on bricks. As these when built into walls are
more or less smeared with mortar, a mixture of lime-loving species also
arrives. Roof tiles are more free from calcareous matter. Lesdain[1149]
noted that on the dunes, though stray bricks were covered by algae,
lichens rarely or never seemed to gain a footing.

There are many references in literature to lichens that live on tiles.
A fairly representative list is given by Lettau[1150] of “tegulicolous”
species.

  _Verrucaria nigrescens._
  _Lecidea coarctata._
  _Candelariella vitellina._
  _Lecanora dispersa._
  _Lecanora galactina._
  _Lecanora Hageni._
  _Lecanora saxicola._
  _Parmelia conspersa._
  _Placodium teicholytum._
  _Placodium pyraceum._
  _Placodium decipiens._
  _Placodium elegans._
  _Placodium murorum._
  _Xanthoria parietina._
  _Rhizocarpon alboatrum_ var.
  _Buellia myriocarpa._
  _Lecidea demissa._
  _Physcia ascendens._
  _Physcia caesia._
  _Physcia obscura._
  _Physcia sciastrella._

Several of these are more or less calcicolous and others are wanderers,
indifferent to the substratum. Though certain species form communities
on bricks, tiles, etc., none of them is restricted to such artificial
substrata.

_d._ ON HUMUS. Lichens are never found on loose humus, but rocks or
stumps of trees covered with a thin layer of earth and humus are a
favourite habitat, especially of _Cladoniae_. One such “formation” is
given by Bruce Fink[1151] from N. Minnesota; with the exception of
_Cladonia cristatella_, the species are British as well as American:

  _Cladonia furcata._
  _Cladonia cristatella._
  _Cladonia gracilis._
  _Cladonia verticillata._
  _Cladonia rangiferina._
  _Cladonia uncialis._
  _Cladonia alpestris._
  _Cladonia turgida._
  _Cladonia coccifera._
  _Cladonia pyxidata._
  _Cladonia fimbriata._
  _Peltigera malacea._
  _Peltigera canina._
  _Peltigera aphthosa._

_e._ ON PEATY SOIL. Peat is generally found in most abundance in northern
and upland regions, and is characteristic of mountain and moorland,
though there are great moss-lands, barely above sea-level, even in our
own country. Such soil is of an acid nature and attracts a special type
of plant life. The lichens form no inconsiderable part of the flora, the
most frequent species being members of the Cladoniaceae.

The principal crustaceous species on bare peaty soil in the British Isles
are _Lecidea uliginosa_ and _L. granulosa_. The former is not easily
distinguishable from the soil as both thallus and apothecia are brownish
black. The latter, which is often associated with it, has a lighter
coloured thallus and apothecia that change from brick-red to dark brown
or black; Wheldon and Wilson[1152] remarked that after the burning of the
heath it was the first vegetation to appear and covered large spaces with
its grey thallus. Another peat species is _Icmadophila ericetorum_, but
it prefers damper localities than the two Lecideae.

To quote again from Arnold[1153]: 24 species were found on turf around
Munich, 13 of which were _Cladoniae_, but only four species could be
considered as exclusively peat-lichens. These were:

  _Cladonia Floerkeana._
  _Biatora terricola._
  _Thelocarpon turficolum._
  _Geisleria sychnogonioides._

The last is a very rare lichen in Central Europe and is generally found
on sandy soil. Arnold considered that near Munich, for various reasons,
there was a very poor representation of turf-lichens.

_f._ ON MOSSES. Very many lichens grow along with or over mosses,
either on the ground, on rocks or on the bark of trees, doubtless owing
to the moisture accumulated and retained by these plants. Besides
_Cladoniae_ the commonest “moss” species in the British Isles are
_Bilimbia sabulosa_, _Bacidia muscorum_, _Rinodina Conradi_, _Lecidea
sanguineoatra_, _Pannaria brunnea_, _Psoroma hypnorum_ and _Lecanora
tartarea_, with species of _Collema_ and _Leptogium_ and _Diploschistes
bryophilus_.

Wheldon and Wilson[1154] have listed the lichens that they found in
Perthshire on subalpine heath lands, on the ground, or on banks amongst
mosses:

  _Leptogrum_ spp.
  _Peltigera_ spp.
  _Cetraria_ spp.
  _Parmelia physodes._
  _Psoroma hypnorum._
  _Lecanora epibryon._
  _Lecanora tartarea._
  _Lecidea coarctata._
  _Lecidea granulosa._
  _Lecidea uliginosa._
  _Lecidea neglecta._
  _Bilimbia sabulosa._
  _Bilimbia liguiaria._
  _Bilimbia melaena._
  _Baeomyces_ spp.
  _Cladonia_ spp.

As already described _Lecanora tartarea_[1155] spreads freely over the
mosses of the tundra. Aigret[1156] in a study of _Cladoniae_ notes that
_Cl. pyxidata_, var. neglecta chooses little cushions of acrocarpous
mosses, which are particularly well adapted to retain water. _Cl.
digitata_, _Cl. flabelliformis_ and some others grow on the mosses which
cover old logs or the bases of trees.

_g._ ON FUNGI. Some of the fungi, such as Polyporei, are long lived, and
of hard texture. On species of _Lenzites_ in Lorraine, Kieffer[1157] has
recorded 15 different forms, but they are such as naturally grow on wood
and can scarcely rank as a separate association.


3. SAXICOLOUS

Lichens are the dominant plants of this and the following formations,
they alone being able to live on bare rock; only when there has been
formed a nidus of soil can other plants become established.

_a._ CHARACTERS OF MINERAL SUBSTRATA. It has been often observed that
lichens are influenced not only by the chemical composition of the
rocks on which they grow but also by the physical structure. Rocks that
weather quickly are almost entirely bare of lichens: the breaking up of
the surface giving no time for the formation either of thallus or fruit.
Close-grained rocks such as quartzite have also a poor lichen flora, the
rooting hyphae being unable to penetrate and catch hold. Other factors,
such as incidence of light, and proximity of water, are of importance in
determining the nature of the flora, even where the rocks are of similar
formation.

_b._ COLONIZATION ON ROCKS. When a rock surface is laid bare it becomes
covered in time with lichens, and quite fresh surfaces are taken
possession of preferably to weathered surfaces[1158]. The number of
species is largest at first and the kind of lichen depends on the flora
existing in the near neighbourhood. Link[1159], for instance, has stated
that _Lichen candelarius_ was the first lichen to appear on the rocks he
observed, and, if trees were growing near, then _Lichen parietinus_ and
_Lichen tenellus_ followed soon after. After a time the lichens change,
the more slow-growing being crowded out by the more vigorous. Crustaceous
species, according to Malinowski[1160], are most subject to this struggle
for existence, and certain types from the nature of their thallus are
more easily displaced than others. Those with a deeply cracked areolated
thallus become disintegrated in the older central areas by repeated
swelling and contracting of the areolae as they change from wet to dry
conditions. Particles of the thallus are thus easily dislodged, and bare
places are left, which in time are colonized again by the same lichen
or by some invading species. There may result a bewildering mosaic
of different thalli and fruits mingling together. Some forms such as
_Rhizocarpum geographicum_ which have a very close firm thallus do not
break away. In the course of time lichen communities come and go, and
the plants of one locality may be different from those of another for no
apparent reason.

The question of colonization[1161] was studied by Bruce Fink[1162] on
a “riprap” wall of quartz, 30 years old, built to protect and brace
a railway in Iowa. Near by was a grass swamp which supplied moisture
especially to the lower end of the wall. A few boulders were present in
the vicinity, but the nearest lichen “society” was on trees about 150
metres away and these bore corticolous Parmelias, Physcias, Ramalinas,
Placodiums, Lecanoras and Rinodines which were only very sparingly
represented on the riprap. Moisture-loving species never gained a
footing; the extreme xerophytic conditions were evidenced by the
character of the lichens, _Biatora myriocarpoides_ (_Lecidea sylvicola_)
occupying the driest parts of the wall. Lower down where more moisture
prevailed _Bacidia inundata_ and _Stereocaulon paschale_ were the
dominant species. Some 30 species or forms were listed of which 11 were
Cladonias that grew mainly on debris from the disintegration of the wall.
With the exception of two or three species the number of individuals was
very small.

Some of these lichens had doubtless come from the boulders, others from
the trees; the Cladonias were all known to occur within a few miles,
but most of the species had been wind-borne from some distance. The
_Stereocaulon_ present did not exist elsewhere in Iowa; it had evidently
been brought by the railroad cars, possibly on telegraph poles.

A similar wall on the south side of the railway, subject to even more
xerophytic conditions but with less disintegration of the surface, had a
larger number of individuals though fewer species. Only one _Cladonia_
and one _Parmelia_ had gained a footing, the rest were crustaceous,
_Buellia myriocarpa_ being one of the most frequent.

There are two types of rock of extreme importance in lichen ecology:
those mainly composed of lime (calcareous), and those in which silica
or silicates preponderate (siliceous). They give foothold to two
corresponding groups of lichen communities, calcicolous and silicicolous.

_c._ CALCICOLOUS. The pioneer in this section of lichen ecology is H.
F. Link, who was a Professor of Natural Science and Botany at Rostock,
then at Breslau, and finally in Berlin. He[1163] published in 1789, while
still at Rostock, an account of limestone plants in his neighbourhood,
most of them being lichens. In a later work he continues his Botanical
Geography or “Geology” and gives more precise details as to the plants,
some of which are essentially calcicolous though many of them he records
also on siliceous rocks.

Most calcicolous lichens are almost completely dependent on the lime
substratum which evidently supplies some constituent that has become
necessary to their healthy growth. Calcareous rocks are usually of softer
texture than those mainly composed of silica, and not only the rhizoidal
hyphae but the whole thallus—both hyphae and gonidia—may be deeply
embedded. Only the fruits are visible and they are, in some species,
lodged in tiny depressions (foveolae) scooped out of the surface by the
lichen-acids acting on the easily dissolved lime.

Those obligate lime species may be found in associations on almost
any calcareous rock. Watson[1164] has given us a list of species that
inhabit carboniferous limestone in Britain. Wheldon and Wilson[1165]
have described in West Lancashire the “grey calcareous rocks blotched
with black patches of Pannarias (_Placynthium nigrum_) and Verrucarias,
or dark gelatinous rosettes of Collemas. White and grey _Lecanorae_
and _Verrucariae_ spread extensively, some of them deeply pitting the
surface. These more sombre or colourless species are enlivened by an
intermixture of orange-yellow _Physciae_ (_Xanthoriae_) and _Placodii_
by the ochrey films of _Lecanora ochracea_ and lemon-yellow of _Lecanora
xantholyta_. Amongst the greenish scaly crusts of _Lecanora crassa_ may
be seen the bluish cushions of _Lecidea coeruleonigricans_, the whole
forming an exquisite blend of tints.”

The flora recorded by Flagey[1166] on the cretaceous rocks of Algeria in
the Province of Constantine does not greatly differ, some of the species
being identical with those of our own country. Placodiums and Rinodinas
were abundant, as also _Lecanora calcarea_, _Acarospora percaenoides_
and _Urceolaria actinostoma_ var. _calcarea_. Also a few _Lecideae_
along with _Verrucaria lecideoides_, _V. fuscella_, _V. calciseda_ and
_Endocarpon monstrosum_. The rocks of that region are sometimes so
covered with lichens that the stone is no longer visible.

Bruce Fink[1167] gives a typical community on limestone bluffs in
Minnesota:

  _Pannaria_ (_Placynthium_) _nigra_.
  _Crocynia lanuginosa._
  _Omphalaria pulvinata._
  _Collema plicatile._
  _Collema pustulatum._
  _Leptogium lacerum._
  _Placodium citrinum._
  _Bacidia inundata._
  _Rhizocarpon alboatrum_ var.
  _Dermatocarpon miniatum._
  _Staurothele umbrinum._

Forssell[1168] pointed out an interesting selective quality in
the Gloeolichens which are associated with the gelatinous algae,
_Chroococcus_, _Gloeocapsa_ and _Xanthocapsa_. The genera containing the
two former grow on siliceous rocks with the exception of _Synalissa_. The
genera _Omphalaria_, _Peccania_, _Anema_, _Psorotichia_ and _Enchylium_,
in which _Xanthocapsa_ is the gonidium, grow on calcareous rocks.
_Collemopsidium_ is the only _Xanthocapsa_ associate that is silicicolous.

_d._ SILICICOLOUS. There is greater variety in the mineral composition
and in the nature of the surface in siliceous than in calcareous rocks;
they are also more durable and give support to a large number of
slow-growing forms.

Silicon enters into the composition of many different types, from
the oldest volcanic to the most recent of sedimentary rocks. Some of
these are of hard unyielding surface on which only a few lichens are
able to attach themselves. Such a rock is instanced by Servit[1169] as
occurring in Bohemia, and is known as Lydite or Lydian stone, a black
flinty jasper. The association of lichens on this smooth rock was
almost entirely _Acarospora chlorophana_ and _Rinodina oreina_, which
as we shall see occur again as a “desert” association in Nevada; these
two lichens grow equally well in sun or shade, and either sheltered or
exposed as regards wind and rain. _Acarospora chlorophana_, according
to Malinowski[1170], arrives among the first on rocks newly laid bare,
and forms large societies, though in time it gives place to _Lecanora
glaucoma_ (_L. sordida_), a common silicicolous lichen.

A difference has been pointed out by Bachmann[1171] between the lichens
of acid and of basic rocks. The acid series, such as quartz- and
granite-porphyry, contain 70 per cent. and more of oxide of silica;
the basic—diabase and basalt—not nearly 50 per cent. He observed that
_Rhizocarpon geographicum_ was the most frequent lichen of the acid
porphyry, while on basalt there were only small scattered patches.
_Pertusaria corallina_ was abundant only on granitic rocks. On the other
hand _Pertusaria lactea_ f. _cinerascens_, _Diploschistes scruposus_,
_D. bryophilus_ and _Buellia leptocline_ preferred the basic substratum
of diabase and basalt. In this case it is the chemical rather than
the physical character of the rocks that affects the lichen flora, as
porphyry and basalt are both close-grained, and are outwardly alike
except in colouration.

Other rocks, such as granite, in which the different crystals, quartz,
mica and felspar are of varying hardness, are favourite habitats as
affording not only durability but a certain openness to the rhizoidal
hyphae, though in Shetland, West[1172] found the granitic rocks bare
owing to their too rapid weathering. In these rocks the softer basic
constituents such as the mica are colonized first; the quartz remains
a long time naked, though in time it also is covered. Wheldon and
Wilson[1173] point out that the sandstone near to intrusive igneous
rocks has become close-grained and indurated and bears _Lecanora
squamulosa_, _L. picea_, _Lecidea rivulosa_ and _Rhizocarpon petraeum_,
which were not seen on the unaltered sandstone. It was also observed by
Stahlecker[1174], that, in layered rocks, the lichen chose the surface at
right angles to the layering as the hyphae thus gain an easier entrance.

It will only be possible to give a few typical associations from the many
that have been published. Crustaceous forms are the most abundant.

On granite and on quartzite not disintegrated Malinowski[1175] listed:

  _Acarospora chlorophana._
  _Lecanora glaucoma._
  _Rhizocarpon viridiatrum._
  _Lecidea tumida._
  _Biatorella sporostatia._
  _Biatorella testudinea._

On granite and quartzite disintegrated:

  _Aspicilia cinerea._
  _Aspicilia gibbosa._
  _Aspicilia tenebrosa._
  _Buellia coracina._
  _Catillaria_ (_Biatorina_) _Hochstetteri_.
  _Rhizocarpon petraeum._
  _Rhizocarpon geographicum_ vars.
  _Biatorella cinerea._
  _Lecanora badia._
  _Lecanora cenisia._
  _Lecidea confluens._
  _Lecidea fuscoatra._
  _Lecidea platycarpa._
  _Lecidea lapicida._
  _Haematomma ventosum._

On these disintegrated rocks there is a constant struggle for existence
between the various species; the victorious association finally consists
of _Lecanora badia_, _L. cenisia_ and _Lecidea confluens_ with occasional
growths of the following species:

  _Aspicilia cinerea._
  _Haematomma ventosum._
  _Rhizocarpon geographicum_ vars.
  _Biatorella cinerea._
  _Lecidea platycarpa._

A number of rock associations have been tabulated by Wheldon and
Wilson[1176] for Perthshire. Among others they give some of the most
typical lichens on granitic and eruptive rocks:

  _Sphaerophorus coralloides._
  _Sphaerophorus fragilis._
  _Platysma Fahlunense._
  _Platysma commixtum._
  _Platysma glaucum._
  _Platysma lacunosum._
  _Parmelia saxatilis._
  _Parmelia omphalodes._
  _Parmelia Mougeotii._
  _Parmelia stygia._
  _Parmelia tristis._
  _Parmelia lanata._
  _Gyrophora proboscidea._
  _Gyrophora cylindrica._
  _Gyrophora torrefacta._
  _Gyrophora polyphylla._
  _Gyrophora flocculosa._
  _Lecanora gelida._
  _Lecanora atra._
  _Lecanora badia._
  _Lecanora tartarea._
  _Lecanora parella._
  _Lecanora ventosa._
  _Lecanora Dicksonii._
  _Lecanora cinerea._
  _Lecanora peliocypha._
  _Pertusaria dealbata._
  _Stereocaulon Delisei._
  _Stereocaulon evolutum._
  _Stereocaulon coralloides._
  _Stereocaulon denudatum._
  _Psorotichia lugubris._
  _Lecidea inserena._
  _Lecidea panaeola._
  _Lecidea contigua._
  _Lecidea confluens._
  _Lecidea lapicida._
  _Lecidea plana._
  _Lecidea mesotropa._
  _Lecidea auriculata._
  _Lecidea diducens._
  _Lecidea aglaea._
  _Lecidea rivulosa._
  _Lecidea Kochiana._
  _Lecidea pycnocarpa._
  _Buellia atrata._
  _Rhizocarpon Oederi._

On siliceous rocks in West Lancashire the same authors[1177] depict
the lichen flora as follows: “There are many grey _Parmeliae_ and
_Cladoniae_ with coral-like _Sphaerophorei_ on the rocks, and on the
walls smoky-looking patches of _Parmelia fuliginosa_ and ragged fringes
of _Platysma glaucum_ and _Evernia furfuracea_. On the higher scars, flat
topped tabular blocks exhibit black scaly _Gyrophoreae_, dingy green
_Lecidea_ (_Rhizocarpon_) _viridiatra_ and mouse-coloured _L. rivulosa_.
Suborbicular (whitish) patches of _Pertusaria lactea_ and _P. dealbata_
enliven the general sadness of tone, and everywhere loose rocks and
stones are covered with the greyish-black spotted thallus of _Lecidea
contigua_.”

On the Silurian series of rocks in the same district they describe a
somewhat brighter coloured flora: “First Stereocaulons invite attention,
and greenish or yellowish shades are introduced by an abundance of
_Lecanora sulphurea_, _L. polytropa_, _Rhizocarpon geographicum_ and
_Parmelia conspersa_, often beautifully commingled with grey species
such as _Lecidea contigua_ and _L. stellulata_, and reddish angular
patches of _Lecanora Dicksonii_. Also an abundance of orbicular patches
of _Haematomma ventosum_ with its reddish-brown apothecia.” A brightly
coloured association on the cretaceous sand-rocks of Saxon Switzerland
has been described as “Sulphur lichens.” These have recently[1178] been
determined as chiefly _Lepraria chlorina_, in less abundance _Lecidea
lucida_ and _Calicium arenarium_, with occasional growths of _Coniocybe
furfuracea_ and _Calicium corynellum_.


4. OMNICOLOUS LICHENS

Some account must be taken in any ecological survey of those lichens that
are indifferent to substrata. Certain species have become so adapted
to some special habitat that they never or rarely wander; others, on
the contrary, are true vagabonds in the lichen kingdom and settle
on any substance that affords a foothold: on leather, bones, iron,
pottery, etc. There can be no sustenance drawn from these supports,
or at most extremely little, and it is interesting to note in this
connection that while some rock-lichens are changed to a rusty-red
colour by the infiltration of iron—often from a water medium containing
iron-salts—those that live directly on iron are unaffected.

The “wanderers” are more or less the same in every locality and they
pass easily from one support to another. Bouly de Lesdain[1179] made a
tabulation of such as he found growing on varied substances on the dunes
round Dunkirk and they well represent these omnicolous communities. It is
in such a no man’s land that one would expect to find an accumulation of
derelict materials, not only favourably exposed to light and moisture,
but undisturbed for long periods and bordering on normal lichen
associations of soil, tree and stones. Arnold[1180] also noted many of
these peculiar habitats.

The following were noted by Lesdain and other workers:

=On iron=—_Xanthoria parietina_, _Physcia obscura_ and var. _virella_,
_Ph. ascendens_, _Placodium_ (_flavescens_) _sympageum_, _Pl. pyraceum_,
_Pl. citrinum_, _Candelariella vitellinum_, _Rinodina exigua_, _Lecanora
campestris_, _L. umbrina_, _L. galactina_, _Lecania erysibe_, _Bacidia
inundata_. _Xanthoria parietina_ is one of the commonest wandering
species; it was found by Richard[1181] on an old cannon lying near water,
that was exfoliated by rust.

=On tar=—_Lecanora umbrina._

=On charcoal=—_Rinodina exigua_, _Lecanora umbrina_.

=On bones=—_Xanthoria parietina_, _Physcia ascendens_, _Ph. tenella_,
_Placodium citrinum_, _Pl. lacteum_, _Rinodina exigua_, _Lecanora
galactina_, _L. dispersa_, _L. umbrina_, _Lecania erysibe_, _L.
cyrtella_, _Acarospora pruinosa_, _A. Heppii_, _Bacidia inundata_, _B.
muscorum_, _Verrucaria anceps_, _V. papillosa_.

In Arctic regions in Ellesmere Land and King Oscar Land, Darbishire[1182]
found on bones: _Lecanora varia_, _L. Hageni_, _Rinodina turfacea_ and
_Buellia parasema_ (_disciformis_). He could not trace any effect of the
lichens on the substratum.

=On charcoal=—_Rinodina exigua_, _Lecanora umbrina_.

=On dross or clinkers=—_Parmelia dubia_, _Physcia obscura_, _Ph.
ascendens_ f. _tenella_, _Ph. pulverulenta_, _Xanthoria parietina_,
_Placodium pyraceum_, _Pl. citrinum_, _Rinodina exigua_, _Lecanora
dispersa_, _L. umbrina_, _Lecania erysibe_.

=On glass=[1183]—_Physcia ascendens_ f. _tenella_, _Buellia canescens_.
Richard has recorded the same lichens on the broken glass of walls and
in addition: _Xanthoria parietina_, _Lecanora crenulata_, _L. dispersa_,
_Lecania erysibe_, _Rinodina exigua_, and _Buellia canescens_.

=On earthenware, china, etc.=—Physcia ascendens f. tenella, Lecanora
umbrina, _L. dispersa_, _Lecania_ (_? Biatorina_) _cyrtella_, _Verrucaria
papillosa_, _Bacidia inundata_.

=On leather=—Nearly fifty species or varieties were found by Lesdain on
old leather on the dunes. Cladonias, Parmelias and Physcias were well
represented with one Evernia and a large series of crustaceous forms.
He adds a note that leather is an excellent substratum: lichens covered
most of the pieces astray on the dunes. Similar records have been made
in Epping Forest by Paulson and Thompson[1184] who found _Cladonia
fimbriata_ var. _tubaeformis_ and _Lecidea granulosa_ growing on an old
boot. These authors connect the sodden condition of the leather with its
attraction for lichens.

=On pasteboard=—Even on such a transient substance as this Lesdain found
a number of forms, most of them, however, but poorly developed: _Cladonia
furcata_ (thallus), _Parmelia subaurifera_ (beginning), _Xanthoria
parietina_ (beginning), _Physcia obscura_, _Placodium citrinum_
(thallus), _Pl. pyraceum_, _Lecanora umbrina_, _Bacidia inundata_ and
_Polyblastia Vouauxi_ var. _charticola_.

=On linoleum=—_Xanthoria parietina_, _Physcia ascendens f. tenella_,
_Rinodina exigua_, _Lecanora umbrina_.

=On indiarubber=—_Physcia ascendens f. tenella_.

=On tarred cloth=—_Xanthoria parietina_, _Placodium citrinum_, _Pl.
pyraceum_, _Rinodina exigua_, _Lecanora umbrina_, _Lecania erysibe_,
_Bacidia inundata_.

=On felt=—_Bacidia inundata_, _B. muscorum_.

=On cloth= (cotton, etc.)—_Bacidia inundata_.

=On silk=—_Physcia ascendens_, _Ph. obscura_, _Placodium citrinum_
(thallus), _Lecanora umbrina_, _Bacidia inundata_.

=On cord=—_Physcia ascendens f. tenella_, _Placodium citrinum_ (thallus).

=On excreta=—One would scarcely expect to find lichens on animal
droppings, but as some of these harden and lie exposed for a considerable
time, some quick-growing species attain to more or less development
on what is, in any case, an extremely favourable habitat for fungi
and for many minute organisms. Paulson and Thompson found tiny
fruiting individuals of _Cladonia macilenta_ and _Cl. fimbriata_ var.
_tubaeformis_ growing on the dry dung of rabbits in Epping Forest. On the
same type of pellets Lesdain records _Physcia ascendens_ f. _leptalea_,
_Cladonia pyxidata_, _Bacidia inundata_ and _B. muscorum_; and on sheep
pellets: _Physcia ascendens_ f. _leptalea_ and _Placodium citrinum_;
while on droppings of musk-ox in Ellesmere Land Darbishire found
_Biatorina globulosa_, _Placodium pyraceum_, _Gyalolechia subsimilis_,
_Lecanora epibryon_, _L. verrucosa_, _Rinodina turfacea_ and even, firmly
attached, _Thamnolia vermicularis_.

It would be difficult to estimate the age of these lichens, but it seems
evident that the “wanderers” are all more or less quick growers, and
the lists also prove conclusively their complete indifference to the
substratum, as the same species occur again and again on the very varied
substances.


5. LOCALIZED COMMUNITIES

Lichens may be grouped ecologically under other conditions than those
of substratum. They respond very readily to special environments, and
associations arise either of species also met with elsewhere, or of
species restricted to one type of surroundings. Such associations or
communities might be multiplied indefinitely, but only a few of the
outstanding ones will be touched on.

_a._ MARITIME LICHENS. This community is the most specialized of any,
many of the lichens having become exclusively adapted to salt-water
surroundings. They are mainly saxicolous, but the presence of sea-water
is the factor of greatest influence on their growth and distribution,
and they occur indifferently on any kind of shore rock either siliceous
or calcareous. Wheldon and Wilson[1185] noted this indifference to
substratum on the Arran shores, where a few calcicolous species such as
_Verrucaria nigrescens_, _V. maculiformis_, _Placodium tegularis_ and
_Pl. lobulatum_, grow by the sea on siliceous rocks. They suggest that
the spray-washed habitat affords the conditions, which, in other places,
are furnished by limestone.

The greater or less proximity of the salt water induces in lichens, as in
other maritime plants, a distribution into belts or zones which recede
gradually or abruptly according to the slope of the shore and the reach
of the tide. Weddell[1186] on the Isle d’Yeu delimited three such zones:
(1) marine, those nearest the sea and immersed for a longer or shorter
period at each tide; (2) semi-marine, not immersed but subject to the
direct action of the waves, and (3) maritime or littoral, the area beyond
the reach of the waves but within the influence of sea-spray. In the
course of his work he indicates the lichens of each zone.

[Illustration: Fig. 122. _Ramalina siliquosa_ A. L. Sm. Upper zone of
barren plants (after M. C. Knowles, R. Welch. _Photo._).]

In Ireland, a thorough examination has been made of a rocky coast at
Howth near Dublin by M. C. Knowles[1187]. She recognizes five distinct
belts beginning with those furthest from the shore though within the
influence of the salt water:

  1. =The Ramalina belt.=
  2. =The Orange belt.=
  3. =Lichina Vegetation.=
  4. =Verrucaria maura belt.=
  5. =The belt of Marine Verrucarias.=

(1) =The Ramalina belt.= In this belt there are two zones of lichen
vegetation: those in the upper zone consist mainly of barren plants
of _Ramalina siliquosa_[1188], rather dark or glaucous in colour with
much branched fronds which are incurved at the tips (Fig. 122). They
are beyond the direct action of the waves. The lower zone consists
also mainly of the same _Ramalina_, the plants bearing straight,
stiff, simple, or slightly branched fertile fronds of a pale-green or
straw colour (Fig. 123). The pale colour may be partly due to frequent
splashings by sea-spray.

_Ramalina siliquosum_ in both zones takes several distinct forms,
according to exposure to light, wind or spray, the effects of which are
most marked in the upper zone. The plants growing above the ordinary
spray zone generally form sward-like growths (Fig. 124); at the higher
levels the sward growth is replaced by isolated tufts with a smaller
more amorphous thallus which passes into a very small stunted condition.
The latter form alone has gained and retained a footing on the steep
faces of the hard and close-grained quartzite rocks. “On the western
faces, indeed, it is the only visible vegetation.” The dwarfed tufts
with lacerated fronds measuring from 1/4 to 1/2 an inch in height are
dotted all over the quartzites. On the sea faces the plants are larger,
but everywhere they are closely appressed to the rock surface. At lower
levels the fronds lengthen to more normal dimensions. “On these steep
rock-faces there is a complete absence of any of the crustaceous species.
The problem, therefore, as to how the _Ramalina_ has obtained a foothold
on these very hard precipitous rocks, which are too inhospitable even for
crustaceous species is an interesting and puzzling one.”

In the _Ramalina_ zone along with the dominant species there occur
occasional tufts of _R. Curnowii_ and _R. subfarinacea_, the latter
more especially in shady and rather moist situations. There are also
numerous foliaceous and crustaceous lichens mingling with the _Ramalina_
vegetation (Fig. 125), several Parmelias, _Physcia aquila_, _Xanthoria
parietina_, _Buellia canescens_, _B. ryssolea_, _Lecanora atra_, _L.
sordida_, _Rhizocarpon geographicum_ and others. In the main these are
arranged in the following order descending towards the sea:

  1. _Parmeliae._
  2. _Physcia aquila._
  3. _Xanthoria parietina._
  4. Crustaceous species.

[Illustration: Fig. 123. _Ramalina siliquosa_ A. L. Sm. Lower zone of
fertile plants (after M. C. Knowles, R. Welch, _Photo._).]

[Illustration: Fig. 124. Sward of young _Ramalinae_ (after M. C. Knowles,
R. Welch, _Photo._).]

_Parmelia prolixa_ is the most abundant of the Parmelias: it covers large
spaces of the rocks and frequently competes for room with the Ramalinas,
or in other areas with _Physcia aquila_ and _Lecanora parella_.

A number of crustaceous species which form the sub-vegetation of the
_Ramalina_ belt, and also on the same level, clothe the steeper rock
faces where shelter and moisture are insufficient to support the foliose
forms. “In general the sub-vegetation of the eastern and northern coasts
is largely composed of species that are common in Alpine and upland
regions. This is due to the steepness of the rocks and also to the
colder and drier conditions prevailing on these coasts.” An association
of _Rhizocarpon geographicum_, _Lecanora_ (_sordida_) _glaucoma_ and
_Pertusaria concreta_ f. _Westringii_ forms an almost continuous covering
in some places, descending nearly to sea-level.

[Illustration: Fig. 125. Crustaceous communities in the _Ramalina_ belt.
_Lecanora atra_ Ach. (grey patches) and _Buellia ryssolea_ A. L. Sm.
(dark patches). (After M. C. Knowles, R. Welch, _Photo._)]

On sunnier and moister rocks with a south and south-west aspect the
association is of more lowland forms such as _Buellia colludens_, _B.
stellulata_, _Lecanora smaragdula_ and _L. simplex_ f. _strepsodina_.

(2) =The Orange belt.= “Below the Ramalinas, and between them and the
sea, several deep yellow or orange-coloured lichens form a belt of
varying width all round the coast. In summer, the colour of these
lichens is so brilliant that the belt is easily recognized from a
considerable distance.” The most abundant species occur mainly in the
following order descending towards the sea:

  1. _Xanthoria parietina._
  2. _Placodium murorum._
  3. _Placodium tegularis._
  4. _Placodium decipiens._
  5. _Placodium lobulatum._

“On the stones and low shore rocks that lie just above the ordinary
high-tide level _Placodium lobulatum_ grows abundantly, covering the
rocks with a continuous sheet of brilliant colour.” With these brightly
coloured lichens are associated several with greyish thalli such as:

  _Lecanora prosechoides._
  _Lecanora umbrina._
  _Lecanora Hageni._
  _Rhizocarpon alboatrum._
  _Biatorina lenticularis._
  _Rinodina exigua var. demissa._
  _Opegrapha calcarea f. heteromorpha._

(3) =The Lichina vegetation=, and (4) =The Verrucaria maura belt=. These
two communities are intermingled, and it will therefore be better to
consider them together. There are only two species of _Lichina_ on this
or any other shore, _L. pygmaea_ and _L. confinis_; the latter grows
above the tide-level, and sometimes high up on the cliffs, where it is
subject to only occasional showers of spray: it forms on the Howth coast
a band of vegetation four to five inches wide above the =Verrucaria
belt=. _Lichina pygmaea_ occurs nearer the water, and therefore mixed
with and below _Verrucaria maura_. Those three zones were first pointed
out by Nylander[1189] at Pornic, where however they were all submerged at
high tide.

_Verrucaria maura_ is one of the most abundant lichens of our rocky
coasts, and is reported from Spitzbergen in the North to Graham Land in
the Antarctic. It grows well within the range of sea-spray, covering
great stretches of boulders and rocks with its dull-black crustaceous
thallus. At Howth it is submerged only by the highest spring tides.
Though it is the dominant lichen on that beach, other species such as
_V. memnonia_, _V. prominula_, and _V. aquatilis_ form part of the
association, and more rarely _V. scotina_ along with _Arthopyrenia
halodytes_, _A. leptotera_ and _A. halizoa_.

(5) =The belt of marine Verrucarias.= This association includes the
species that are submerged by the tide for a longer or shorter period
each day. The dominant species are _Verrucaria microspora_, _V.
striatula_ and _V. mucosa_. _Arthopyrenia halodytes_ is also abundant;
_A. halizoa_ and _A. marina_ are more rarely represented. Among the
plants of _Fucus spiralis_, _Verrucaria mucosa_, the most wide-spreading
of these marine forms, is “very conspicuous as a dark-green, almost
black, band of greasy appearance stretching along the shore.” When
growing in the shade, the thallus is of a brighter green colour.

An examination[1190] of the west coast of Ireland yielded much the same
results, but with a still higher “white belt” formed mainly of _Lecanora
parella_ and _L. atra_ which covered the rocks lying above high-water
mark, “giving them the appearance of having been whitewashed.” A more
general association for the same position as regards the tide is given by
Wheldon and Wilson[1191] on the coasts of Arran as:

  _Physcia aquila._
  _Xanthoria parietina._
  _Lecanora parella._
  _Lecanora atra._
  _Lecanora campestris._
  _Placodium ferrugineum_ var. _festivum._
  _Placodium tegularis._
  _Ramalina cuspidata._
  _Physcia stellaris._
  _Physcia tenella._
  _Verrucaria maura._

A somewhat similar series of “formations” was determined by
Sandstede[1192] on the coast of Rügen. On erratic granite boulders washed
by the tide he found:

  _Verrucaria maura._
  _Lichina confinis._
  _Lecanora prosechoides._
  _Placodium lobulatum._

While in a higher position on similar boulders:

  _Lecanora exigua._
  _Lecanora dispersa._
  _Lecanora galactina._
  _Lecanora sulphurea._
  _Lecanora saxicola._
  _Lecanora caesiocinerea._
  _Lecanora gibbosa._
  _Lecanora atra._
  _Lecanora parella._
  _Lecidea colludens._
  _Lecidea lavata._
  _Lecidea nigroclavata_ f. _lenticularis._
  _Xanthoria parietina_ and f. _aureola._
  _Physcia subobscura._
  _Physcia caesia._

And more rarely a few species of _Lecidea_.

_b._ LICHENS OF SAND-DUNES. These lichens might be included with those
of the terricolous communities, but they really represent a maritime
community of xerophytic type, subject to the influence of salt spray
but not within reach of the tide. They are sun-lichens and react to the
strong light in the deeper colour of the thallus. In such a sun-baked
area at Findhorn a luxuriant association of lichens was observed growing
among short grass and plant debris. It consisted chiefly of:

  _Parmelia physodes._
  _Evernia prunastri._
  _Cetraria aculeata._
  _Cladonia cervicornis._
  _Cladonia endiviaefolia._
  _Peltigera_ spp.

On very arid situations the species of _Cladonia_ are those that have
a well-developed rather thick primary thallus, probably because such
a thallus is able to retain moisture for a prolonged period[1193].
On shifting sand, as in the desert, there are no lichens; it is only
on surfaces more or less fixed by marram grass that lichens begin
to develop, though in the cool damp weather of autumn and winter, as
observed by Wheldon and Wilson[1194], certain species associated with
Myxophyceae, such as Collemaceae, may make their appearance, among others
_Leptogium scotinum_, _Collemodium turgidum_ and _Collema ceranoides_.
Watson[1195] makes the same observation in his study of sand-dunes.

When the loose sand on the dunes of South Lancashire becomes cemented
by algae and mosses several rare _Lecideae_ are to be found on the
decaying vegetation, and with further accumulation of humus _Cladoniae_
appear and spread rapidly along with several species of _Peltigera_ and
the ubiquitous _Parmelia physodes_. The latter starts on dead twigs of
_Salix repens_ and spreads on to the surrounding soil where it forms
patches some inches in diameter. The association also includes _Lecidea
uliginosa_ and _Bilimbia sphaeroides_.

On the more inland portions of the dunes numerous rather poorly developed
_Cladoniae_ and _Cetraria aculeata_ were associated, while on the sides
of “slacks” or “dune-pans” _Collema pulposum_, _Cladonia sylvatica_ and
several crustaceous lichens covered the soil. The wetter parts of the
dunes were not found to be favourable to lichen growth.

Sandstede[1196] found on the sandy shores of Rügen, from the shore
upwards: first a stretch of bare sand, then a few dune grasses with
scattered scraps of _Cladoniae_, _Peltigerae_ and _Cetraria aculeata_.
Next in order sandbanks with _Parmelia physodes_, _Cladonia sylvatica_,
_Cl. alcicornis_ and _Stereocaulon paschale_. All these are species
that occur on similar shores in the British Islands. Sandstede adds an
extensive list of maritime species observed by him in Rügen.

A very careful tabulation of lichens at Blakeney Point in Norfolk was
made by McLean[1197] and the table on p. 386 is reproduced from his
paper. Sand, he writes, is present in all the associations and the
presence or absence of stones marks the great difference between the two
formations determined by dune and shingle.

(1) =Bare sand=, which is the first association listed, is an area
practically without phanerogams; the few lichen plants, _Cladonia
furcata_ and _Cetraria aculeata_ f. _acanthella_, are attached by slight
embedding in the soil.

(2) =Grey dune.= The sand-loving lichens of the association grow
in company with _Hypnum cupressiforme_ and attain their greatest
development. Other species which also occur there are _Parmelia physodes_
and _Evernia prunastri_ var. _stictocera_.

(3) =Derelict dune.= This part of the dune formation occurs here and
there on the seaward margin where the grey dune has been worn down by
the wind. It is more shingly, hence the presence of stone lichens; dune
phanerogams are interspersed and with them a few fruticose lichens, such
as _Cladonia furcata_.

(4) =High shingle.= The term indicates shingle aggregated into banks
lying well above all except the highest tides. A large percentage of sand
may be mixed with the stones and if no humus is present and the stones
of small size, lichens may be absent altogether. Those occurring in the
“loose shingle” are saxicolous. In the “bound shingle” where there is no
grass the stones, fixed in a mixture of sand and humus, are well covered
with lichens. With the presence of grass, a thin layer of humus covers
the stones and a dense lichen vegetation is developed both of shingle and
of dune species.

(5) =Low shingle.= This last association lies in the hollows among plants
of _Suacda fruticosa_. Stability is high and tidal immersions regular and
frequent. The dominant factor of the association is the quantity of humus
and mud deposited around and over the stones. The lichens cover almost
every available spot on the firmly embedded pebbles. The characteristic
species of such areas are _Lecanora badia_ and _L._ (_Placodium_)
_citrina_ which effect the primary colonization. To these succeed
_Lecanora atra_ and _Xanthoria parietina_. In time the mud overwhelms and
partly destroys the lichens, so that the phase of luxuriant growth is
only temporary.

_Lecanora badia_ is conspicuously abundant at the sand end of this
formation. _Lecanora_ (_Placodium_) _citrina_ disappears as the mud is
left behind. _Collema_ spp. also occur frequently on the mixture of mud
and sand round the stones. The species on “low shingle” are those most
tolerant of submersion: _Verrucaria maura_ is confined to this area,
where it is covered by the tide several hours each day.

  FORMATION  ASSOCIATION            PRINCIPAL SPECIES

         {1. Bare Sand            _Cetraria aculeata_ f. _acanthella_
         {                        _Cladonia furcata_
  Dune   {2. Grey Dune            _Cladonia rangiferina_,
         {                        _Peltigera rufescens_
         {                        _Cladonia furcata_, _Cl. alcicornis_
         {3. Derelict Dune        _Cladonia furcata_, _Parmelia fuliginosa_
         {                        _Rhizocarpon confervoides_


         {4. High Shingle
         {                       {_Lecanora atra_, _L. galactina_
         {         {With sand    {_Rhizocarpon confervoides_
         {         {             {_Lecanora citrina_
         {  Loose
         {         {             {_Physcia tenella_, _Lecanora citrina_,
         {         {Without sand {_Xanthoria parietina_
         {         {             {_Squamaria saxicola_
         {                       {_Parmelia saxatilis_, _P. fuliginosa_
  Shingle{
         {                       {_Cladonia rangiferina_, _Cl. furcata_,
         {         {With grasses {_Cl. pungens_
         {         {             {_Cetraria aculeata_
         {   Bound
         {         {             {_Xanthoria parietina_,
         {         {             {_Biatorina chalybeia_,
         {         {Without      {_Lecanora atra_
         {         {  grasses    {_Aspicilia gibbosa_, _Buellia colludens_,
         {         {             {_Verrucaria microspora_
         {                       {_Physcia tenella_, _Lecanora atroflava_
         {
         {                        _Rhizocarpon confervoides_,
         {                        _Lecanora citrina_ var. _incrustans_
         {5. Low Shingle          _L. badia_, _L. atra_, _Xanthoria
                                  parietina_
                                  _Verrucaria maura_

McLean adds that _Xanthoria parietina_ in its virescent form on _Suaeda
fruticosa_ also endures constant immersion; _Lecanora badia_ does not
occur above the tidal line and _Lecanora galactina_ does not descend
below tidal limits; the latter is an arenicolous species and colonizes
some of the loosest and sandiest areas of shingle. _Rhizocarpon
confervoides_ is ubiquitous.

_c._ MOUNTAIN LICHENS. On the mountain summits of our own and other
lands are to be found lichens very similar to those of the far North
the climatic conditions being the chief factors of importance in
determining the formations. These regions are occupied by what Wheldon
and Wilson[1198] describe as “a zone of Arctic-Alpine vegetation,” and
they have recorded a series of lichen associations belonging to that zone
on the schistose summits of the Perthshire mountains. The following is
one of the most typical:

  _Euopsis granatina._
  _Sphaerophorus coralloides._
  _Sphaerophorus fragilis._
  _Gyrophora polyphylla._
  _Cetraria tristis._
  _Cetraria nivalis._
  _Lecanora tartarea_ var. _frigida_.
  _Lecanora upsaliensis._
  _Aspicilia oculata._
  _Pertusaria dactylina._
  _Pertusaria glomerata._
  _Stereocaulon denudatum._
  _Parmelia saxatilis._
  _Parmelia omphalodes._
  _Parmelia lanata._
  _Parmelia stygia._
  _Stereocaulon tomentosum._
  _Stereocaulon alpinum._
  _Cladonia coccinca._
  _Cladonia gracilis._
  _Cladonia uncialis._
  _Cladonia destricta._
  _Cladonia racemosa._
  _Lecidea arctica._
  _Parmelia alpicola._
  _Cetraria aculeata._
  _Cetraria crispa._
  _Cetraria islandica._
  _Lecidea limosa._
  _Lecidea alpestris._
  _Lecidea demissa._
  _Lecidea uliginosa._
  _Lecidea cuprea._
  _Lecidea Berengeriana._
  _Lecidea cupreiformis._
  _Lecidea atrofusca._

Again on the summit of Ben-y-Gloe the same authors[1199] have recorded
_Gyrophora erosa_, _G. torrefacta_ and _G. cylindrica_, _Parmelia
alpicola_, _Lecanora tartarea_ var. _frigida_, _Lecidea limosa_ and _L.
arctica_, the last two lichens thriving in the most bleak and exposed
situations. _Cladonia cervicornis_ grew in reduced squamulose cushions;
_Stereocaulon_ and _Sphaerophorus_ in very compact forms, the outer
stalks prostrate, the next inclined, the central ones erect so that
points only are exposed and no lateral stress is caused by wind storms.
Erect fruticose lichens are absent in this region, being represented
only by _Parmelia lanata_, a semi-decumbent plant, and by _Thamnolia
vermicularis_ which is prostrate on the ground except where the points
of the stalks turn up to catch the dew. Many of the _Lecideae_ were
observed to have large fruits and very little thallus: “the hyphae ramify
in the minute interstices of the stone and the gonidia cluster under the
lea of the apothecia: this is especially the case on loose stones where
conditions are extremely dry.”

On the Continent an interesting study of the lichens of high altitudes
was made by Maheu[1200] in the Savoyard Oberland. On the Great Casse at
a height of 3861 m. he collected four mosses and sixteen lichens. These
were:

  _Stereocaulon condensatum._
  _Gyrophora cylindrica._
  _Gyrophora spodochroa._
  _Solorina crocea._
  _Solorina saccata._
  _Parmelia encausta._
  _Candelaria concolor._
  _Caloplaca pyracea_ var. _nivalis_.
  _Haematomma ventosum._
  _Acarospora smaragdula._
  _Psora decipiens._
  _Buellia discolor._
  _Buellia stellulata._
  _Lecidea contigua_ var. _steriza_.
  _Lecidea confluens._
  _Dermatocarpon hepaticum._

He found that as he climbed higher and higher foliaceous species became
rarer and crustaceous more abundant. The colour of the lichens on the
high summits was slightly weakened and the thallus often reduced, but
all were fertile and the apothecia normal and sporiferous. Lichens at
less high altitudes where they emerge from the snow covering for longer
periods and enjoy light and sunshine are, as already observed, often very
brightly coloured and of luxuriant growth.

_d._ TUNDRA LICHENS. In phyto-geography the term “tundra” is given to
great stretches of country practically treeless and unsheltered within
the Polar climate; the tundra extends from the zone of dwarfed trees
on to the permanent ice or snow fields. The vegetation includes a few
dwarfed trees, shrubs, etc., but is mainly composed of mosses and
lichens; the latter being the most abundant. These are true climatic
lichen formations.

Leighton[1201], in describing lichens from Arctic America brought home
by the traveller, Sir John Richardson, quotes from the latter that: “the
terrestrial lichens were gathered on Great Bear, and Great Slave Lakes
before starting on our summer voyages after the snow had melted....
The barren grounds are densely covered for many hundreds of miles with
_Corniculariae_ and _Cetrariae_, and where the ground is moist with
_Cladoniae_, while the boulders thickly scattered over the surface are
clothed with _Gyrophorae_.... The smaller stones on the gravelly ridges
of the Barren Grounds are covered with lichens.”

The accounts of tundra lichens that have been given by various travellers
deal chiefly with the more prominent terricolous forms. They have been
classified as “Cladina tundra,” including _Cladonia rangiferina_ and
_Sphaerophorus coralloides_, “Cetraria tundra,” and “Alectoria heath,”
the latter the hardiest of all. Great swards of these lichens often
alternate with naked stony soil.

Kihlman[1202] has noted, as characteristic of tundra formations, the
compact cushion-like growth of the mosses which are thus enabled to store
up water and to conduct it by capillarity throughout the mass to the
highest stalks. Certain tundra lichens take on the same growth character
as adaptations to the strenuous life conditions. _Cetraria glauca_ f.
_spadicea_ with f. _congesta_ and _C. crispa_ are examples of this
compact growth: they form a soft thick carpet of a yellowish-grey colour.
_Cladoniae_ also grow in crowded tufts, but are generally to be found in
the more sheltered positions, in valleys between the tundra hills and in
the clefts of the rocks, or between great boulders and stones where there
is also more moisture.

The same kinds of lichens occur all over these northern regions. Birger
Nilson[1203] gives as the principal earth-lichens in Swedish Lappland,
_Alectoria ochroleuca_, _A. nigricans_, _Cetraria nivalis_, _C.
cucullata_, _Cladonia uncialis_, _Thamnolia_ (_Cerania_) _vermicularis_
and _Sphaerophorus coralloides_.

Darbishire[1204] speaks of the extensive beds of various species of
_Cetraria_ in Ellesmere Land and King Oscar Land. _Alectoria nigricans_
and _A. ochrolenca_ were often found in pure communities, but even more
frequently in close company with mosses. Though these fruticose lichens
are not represented by many species in Arctic regions, they cover a very
extensive area and form a very important feature in the vegetation.

Crustaceous lichens are not wanting: _Lecanora tartarea_ f. _frigida_,
_L. epibryon_ and others are to be found in great sheets covering the
mosses or the soil, or spreading over the stones and boulders. Cold has
no deterrent effect, and their advance is only checked by the presence of
perpetual snow.

_e._ DESERT LICHENS. The reduced rainfall of desert countries is
unfavourable to general lichen growth and only the more xerophytic
species—those with a stout cortex—can flourish in the adverse conditions
of excessive light and dryness. Lichens, however, there are, in great
numbers as far as individuals are concerned, though the variety is not
great. The abundance of the crustaceous _Lecanora esculenta_ in the
deserts of Asia has already been noted. Flagey[1205] found it one of the
dominant species at Biskra in the Sahara where it grows on the rocks.
Patouillard[1206] in describing the flora of Tunis speaks of the great
patches (societies) of _Lecanora crassa_ f. _deserti_ which at a distance
look like milk spilled on the ground, or if growing on unequal surfaces
take the aspect of plaster that has been passed over by some wheeled
vehicle. At Biskra species of _Heppia_ grow on the sand. Steiner[1207]
also records the frequency of _Heppia_ and of _Endocarpon_ in the Sahara
as well as of Gloeolichens which, as they are associated with gelatinous
blue-green algae, can endure extreme and long-continued desiccation.
These lichens, however, only form communities in clefts among the rocks
where these abut on the desert. In the great plains the sand is too
mobile and too often shifted by the sirocco to enable them to settle.

Bruce Fink[1208] discusses desert lichens and their adaptive characters:
crustaceous species with a stout cortex are best able to withstand the
long dry periods; conspicuously lobed thalli are lacking, as are lichens
with fruticose structure though he thinks the latter are prevented
from developing by the exposure to high winds and driving sand storms.
Herre’s[1209] study of the desert lichen flora at Reno, Nevada, is
full of interest. The district is situated at an altitude of 4500 feet
east of the Sierra Nevada Mountains. The annual rainfall averages 8·21
inches, and a large part falls as snow during the winter months or as
early spring rain. The summer is hot and dry and the diurnal changes of
temperature are very great. Strong drying winds from the west or north
are frequent.

At 5000 feet and upwards lichens are, in general, exceedingly abundant
on all rock substrata and represent 57 species or subspecies, only three
of these being arboreal: _Buellia triphragmia_ occurs rarely, _Xanthoria
polycarpa_ is frequent on sage brush, while _Candelariella cerinella_
though a rock-lichen grows occasionally on the same substratum.
_Caloplaca_ (_Placodium_) _elegans_ is one of the most successful and
abundant species and along with _Lecanora_ (nine forms), _Acarospora_
(seven forms) and _Lecidia_ (five forms) comprises three-fourths of the
rock surface occupied by lichens. The addition of _Rinodina_ with two
species and _Gyrophora_ with four brings the computation of individuals
in these desert rock formations up to nine-tenths of the whole. As the
desert rocks pass to the Alpine, _Gyrophora_ becomes easily the dominant
genus followed by _Acarospora_, _Caloplaca_ and _Lecidea_.

“The colouring characteristic of the rock ledges of the desert and
cañon walls is often entirely due to lichens, and in a general way they
form the only brilliant plant formations in a landscape notable for
its subdued pale monotonous tones. Most conspicuous are _Acarospora
chlorophana_ and _Caloplaca elegans_, which form striking landmarks when
covering great crags and rock walls. The next most conspicuous lichens
are _Rinodina oreina_ and _Lecanora rubina_ and its allies, which often
entirely cover immense boulders and northerly sloping rock walls.” Herre
concludes that though desert conditions are unfavourable to most species
of lichens, yet some are perfectly at home there and the rocks are just
as thickly covered as in regions of greater humidity and less sunshine.

_f._ AQUATIC LICHENS. There is only one of the larger lichens that has
acquired a purely aquatic habit, _Hydrothyria venosa_, a North American
plant. It grows on rocks[1210] in the beds of streams, covering them
often with a thick felt; it is attached at the base and the rather narrow
fronds float freely in the current. The gonidium is _Nostoc_ sp., and
the thallus is of a bluish-grey colour; the fruits are small discoid
reddish apothecia with an evanescent margin. It is closely allied to
_Peltigerae_, some of which are moisture-loving though not truly aquatic.

The nearest approach to aquatic habit among the foliose forms in our
country is _Dermatocarpon aquaticum_, with thick coriaceous rather
contorted lobes; it inhabits rocks and stones in streams and lakes.
Somewhat less continuously aquatic is _D. miniatum_ var. _complicatum_
which grows on damp rocks exposed to spray or occasionally to inundation.
Lindsay[1211] has described it “on boulders by the side of the Tay,
frequently covered by the river when flooded, and of a deep olive colour
when under water”: both these lichens have a wide distribution in Europe,
Africa, America and New Zealand.

In a discussion of lake shore plants Conway Macmillan[1212] describes on
the flat shores a _Dermatocarpon_ zone on the wet area nearest the lake,
behind that a _Biatora_ zone and further landward a _Cladonia_ zone. On
rounded rocky shores the same zones followed each other but were less
broad: they were so close together that the _Cladoniae_, which with
_Stereocaulon paschale_ grow in profusion on all such shores, occurred
within a couple of feet of the high-water mark.

M. C. Knowles[1213] reports concerning the lichen flora of some mountain
lakes in Waterford, that a band of _Dermatocarpon miniatum_ var.
_complicatum_ six feet wide grew all the way round the lakes between the
winter and summer level of the water. Below that zone _D. aquaticum_
formed another belt mingled with the moss _Fontinalis_ and several
species of crustaceous lichens _Staurotheleae_, _Polyblastiae_, etc.

Bruce Fink[1214] gives as a typical “amphibious angiocarpous lichen
formation” of wet rocks in Minnesota: _Dermatocarpon aquaticum_, _D.
miniatum_ var. _complicatum_, _Staurothele clopima_ and _Verrucaria
viridula_. These “formations,” he says, “may be seen complete in places
along the shores of Vermillion Lake and less well represented at other
portions of the lake shore.” Macmillan found that on the rocky shores of
Lake Superior the _Dermatocarpon_ zone also occurred nearest the water.

Species with closed fruits such as Pyrenolichens, or with apothecia
deeply sunk in the thallus and thus also well protected, seem to
be best adapted to the aquatic life. Such in our own country are
_Lecanora lacustris_, _Bacidia inundata_ and others, with a number of
_Verrucariae_: _V. aethiobola_, _V. hydrela_, _V. margacea_, etc.

Lettau[1215] gives as “formations” on rocks or boulders in the beds of
streams in Thuringia:

  _Verrucaria aethiobola._
  _Verrucaria hydrela._
  _Dermatocarpon aquaticum._
  _Bacidia inundata._
  _Lecanora aquatica._

In their ecological study of Perthshire lichens Wheldon and Wilson[1216]
give two “formations.” The first is on rocks submerged for long periods,
though in dry weather the lichens may be exposed, and can withstand
desiccation for a considerable time:

  _Pterygium Kenmorensis._
  _Collema fluviatile._
  _Lecanora lacustris._
  _Lecanora epulotica._
  _Bacidia inundata._
  _Rhizocarpum obscuration._
  _Rhizocarpum petraeum._
  _Lecidea contigua._
  _Lecidea albocoerulescens._
  _Dermatocarpon miniatum var. complicatum._
  _Dermatocarpon aquaticum._
  _Verrucaria laevata._
  _Verrucaria aethiobola._
  _Verrucaria margacea._

The second group of species usually inhabits damp, shaded rocks of
ravines or large boulders by streams or near waterfalls. It includes
species of _Collema_, _Sticta_, _Peltigera_, _Solorina_, _Pannaria_,
etc., with _Opegrapha zonata_, _Porina lectissima_ and _Verrucaria
nigrescens_.

The last-mentioned lichen grows by preference on limestone, but in
excessive moisture[1217], as by the sea-side, the substratum seems to be
of minor importance.


D. LICHENS AS PIONEERS

_a._ SOIL-FORMERS. The part played by lichens in the “Economy of Nature”
is of very real importance: to them is allotted the pioneer work of
breaking down the hard rock surfaces and preparing a soil on which more
highly developed plants can grow. This was pointed out by Linnaeus[1218]
who thus describes the succession of plants: “Crustaceous lichens,” he
writes, “are the first foundation of vegetation. Though hitherto we have
considered theirs a trifling place among plants, nevertheless they are
of great importance at that first stage in the economy of nature. When
the rocks emerge from the seas, they are so polished by the force of
the waves, that scarcely any kind of plant could settle on them, seen
more especially near the sea. But very soon, in truth, the smallest
crustaceous lichens begin to cover those arid rocks, and are sustained by
minute quantities of soil and by imperceptible particles brought to them
by rain and by the atmosphere. These lichens in time become converted by
decay into a thin layer of humus, so that at length imbricate lichens
are able to thrust their rhizoids into it. As these in turn change to
humus by natural decay, various mosses such as _Hypnum_, _Bryum_ and
_Polytrichum_ follow, and find suitable place and nourishment. In time
there is produced by the dying down of the mosses such a quantity of soil
that herbs and shrubs are able to establish themselves and maintain their
existence.”

Similar observations have been made since Linnaeus’s day, among others
by Guembel[1219] in his account of _Lecanora ventosa_. Either by the
excretion of carbon dioxide which acidifies the surrounding moisture, or
by the mechanical action of hyphae and rhizinae, the component particles
of rocks such as granite are gradually dissolved and broken up. Rocks
exposed to weather alone are unchanged, while those covered with lichens
have their surface disintegrated and destroyed.

The decaying parts of the lichen thallus add to the soil material as
observed by Linnaeus, and in time mosses follow, and, later, phanerogams.
Goeppert[1220] has pointed out the succession observed on roofs of
houses as: “first some lichen such as _Lecanora saxicola_, then the moss
_Grimmia pulvinata_, which forms compact cushions on which later grow
_Poa compressa_, small crucifers, etc.”

Goeppert[1220] has noted as special rock-destroyers some foliaceous
species, _Parmelia saxatilis_, _P. stygia_ and _P. encausta_, the
underlying rock being roughened and broken up by their rhizoids. Species
of _Gyrophora_ and _Sphaerophorus_ have the same disintegrating effect,
so that the surface of the rock may in time lose its coherence to a depth
of 2 to 4 inches. Crustaceous species such as _Lecanora polytropa_,
_Candelariella vitellina_, etc., exercise an equally powerful solvent
action, while underneath closely appressed growers like _Lecanora atra_
and _Acarospora smaragdula_ the stone is converted to a friable substance
that can be sliced away with a knife.

Salter[1221] concluded that oxalic acid was the principal agent in
disintegration. He found that it acted more or less rapidly on minerals
and almost any class of saline compounds; it even attacked glass finely
powdered, though silica remained unchanged.

Bachmann[1222] found that granite was reduced by lichens to a clay-like
granular yellow mass in a comparatively short time, the lichen seizing on
the particles of mica first; but the spread of the lichen over the rock,
he observes, is largely directed by the amount of humidity and by the
chance of gaining a foothold. In the case of calcareous rocks he[1223]
tested the relative dampness of those containing lichens and those that
were lichen-free. In the former case water was absorbed more freely and
retained much longer than in the barren rock, thus encouraging further
vegetation.

Lucy E. Braun[1224] has described the successive colonization of
limestone conglomerate in Cincinnati. The rock is somewhat resistant
to erosion and stands out in irregular outcrops on the hillsides of
the region. The first plants to gain a footing are certain crustaceous
lichens, _Lecidea_ sp., _Pertusaria communis_, _Staurothele umbrina_,
_Verrucaria muralis_ and _Placodium citrinum_ which occur as patches on
the smoother and more exposed rock faces. With these were associated
small quantities of a moss, _Grimmia apocarpa_. In the second stage of
growth _Dermatocarpon miniatum_, and, to a lesser degree, a gelatinous
_Omphalaria_ sp. were the most prominent plants, but mosses were more in
evidence, and the next stage consisted almost exclusively of mosses and
hepatics with _Peltigera canina_. A thick layer of humus was gradually
built up by these plants on which Phanerogamous plants were able to
flourish.

In tropical countries the first vegetation to settle on bare rocks would
seem to be blue-green gelatinous algae. Three years after the eruption of
Krakatoa, dark-green layers of these plants were found by Treub[1225] on
the surface of the pumice and ash, and on the loose stones in the ravines
of the mountain. It was only at a later stage that lichens appeared.

_b._ OUTPOSTS OF VEGETATION. Lichens are the only plants that can survive
extreme conditions of cold or of heat. They grow in Polar regions where
no other vegetation could obtain sustenance; they are to be found at
great heights on mountains all over the globe; and, on arid desert rocks
they persist through long dry seasons, depending almost entirely on night
dews for the supply of moisture. Here we have true lichen formations in
the sense of modern ecology.




CHAPTER X

ECONOMIC AND TECHNICAL


A. LICHENS AS FOOD.

_a._ FOOD FOR INSECTS, ETC. Some of the earlier botanists made careful
observations on the important place occupied by lichens in nature as
affording food to many small animals. In 1791 Jacques Brez[1226] wrote
his _Flore des Insectophyles_, and in the list of food-plants he includes
seven species of lichens. The “insects” that frequented these lichens
were species of the genera _Acarus_ (mites) and _Phalena_ (moths). A
few years later Persoon[1227] noted that lichens formed the main food
supply of many insects, slugs, etc. Zukal[1228], quoting from Otto Wilde
(_Die Pflanzen und Raupen Deutschlands_, Berlin, 1860), gives a list of
caterpillars that are known to feed on and destroy lichens.

A very considerable number of small creatures feed eagerly on lichens,
and traces of their depredations are constantly to be seen in the empty
fruit discs, and in the cortices eaten away in patches so as to expose
the white medulla. It has been argued by Zukal[1229] that the great
formation of acid substances in lichens is for shielding them against
the attacks of animals; Zopf[1230] on the contrary insists that these
substances afford the plants no real protection. He made a series of
experiments with snails, feeding them with slices of potato smeared with
pure lichen acids. Many snails ate the slices with great readiness even
when covered with bitter acids such as cetraric, or with those which
are poisonous for other animals such as rhizocarpic and pinastrinic.
The only acid they refused was vulpinic, which is said to be poisonous
for vertebrates. The crystals of the acids passed unchanged through the
alimentary canal of the snails, and were found in masses in the excreta.
They were undissolved, but, enclosed in slime, their sharp edges did no
damage to the digestive tract.

Stahl[1231] however upholds Zukal’s theory of the protective function
of lichen acids against the attacks of small animals. Some few snails,
caterpillars, etc., that are omnivorous feeders consume most lichens with
impunity, and the bitter taste seems to attract rather than repel them;
but many others he contends are certainly prevented from eating lichens
by the presence of the acids. He proved this by soaking portions of the
thalli of certain bitter species for about twenty-four hours in a one per
cent. soda solution, which was sufficiently strong to extract the acids.
He found that these treated specimens were in most cases preferred to
fresh portions that had been simply moistened with water.

Even the omnivorous snail, _Helix hortensis_, was several times observed
to touch the fresh thallus and then creep away, while it ate continuously
the soda-washed portion as soon as it came into contact with it. Calcium
Oxalate, on the other hand, formed no protection; omnivorous feeders
ate indifferently calcicolous lichens such as _Aspicilia calcarea_ and
_Lecanora saxicola_, whether treated with soda or not, but would only
accept lichens with acid contents, such as _Parmelia caperata_, _Evernia
prunastri_, etc., after they had been duly soaked.

Experiments were also made with wood-lice (_Oniscus murarius_), and with
earwigs (_Forficula auricularia_), and the result was the same: they
would only eat bitter lichens after the acids had been extracted by the
soda method. Stahl therefore concludes that acids must be regarded as
eminently adapted to protect lichens which otherwise, owing to their
slowness of growth, would scarcely escape extinction.

The gelatinous Collemaceae, as also _Nostoc_, the alga with which these
are associated, are unharmed by snails, etc., on account of their
slippery consistency when moist, which prevents the creatures from
getting a foothold on the thallus. These lichens however do not contain
acids, and if, when dry, they are reduced to powder and then moistened,
they are eagerly eaten both by snails and by wood-lice. _Peltigera
canina_, on account of a disagreeable odour it acquires on being chewed,
is avoided to a certain extent, but even so it is frequently found with
much of the thallus eaten away.

Hue[1232] in his study of Antarctic lichens, comments on the abundance
and perfect development of the lichens, especially the crustaceous
species, which cover every inch of rock surface. He ascribes this to
the absence of snails and insects which in other regions so seriously
interfere with the normal and continuous growth of these plants.

Snails do not eat lichens when they are dry and hard, but on damp or
dewy nights, and on rainy days, all kinds, both large and small, come
out of their shells and devour the lichen thalli softened by moisture.
Large slugs (_Limax_) have been seen devouring with great satisfaction
_Pertusaria faginea_, a bitter crustaceous lichen. The same _Limax_
species eats many different lichens, some of them containing very bitter
substances. Zopf[1233] observed that _Helix cingulata_ ate ten different
lichens, containing as many different kinds of acid.

Other creatures such as mites, wood-lice, and the caterpillars of
many butterflies live on lichens, though, with the exception of the
caterpillars, they eat them only when moist. Very frequently the
apothecial discs and the soredia are taken first as being evidently the
choicest portions. All lichens are, however, not equally palatable.
Bitter[1234] observed that the insect _Psocus_ (_Orthoptera_) had a
distinct preference for certain species, and restricted its attention to
them probably because of their chemical constitution. He noted that in
a large spreading thallus of _Graphis elegans_ on holly, irregular bare
spots appeared, due to the ravages of insects—probably _Psocus_. In other
places, the thallus alone had been consumed, leaving the rather hard
black fruits (lirellae) untouched. In time the thallus of _Thelotrema
lepadinum_, also a crustaceous lichen, invaded the naked areas, and
surrounded the _Graphis_ lirellae. The new comer was not to the taste of
the insects and was left untouched.

Petch[1235] says that lichens form the staple food of _Termes monoceros_,
the black termite of Ceylon. These ants really prefer algae, but as the
supply is limited they fall back on lichens, though they only consume
those of a particular type, or at a particular stage of development.
Those with a tough smooth cortex are avoided, preference being given
to thalli with a loose powdery surface. At the feeding ground the ants
congregate on the suitable lichens. With their mandibles they scrape
off small fragments of the thallus which they form into balls, varying
in size from 1·5 mm. to 2·5 mm. in diameter. The workers then convey
these to the nests in their mandibles. It would seem that they carry
about these balls of food, and allow the ants busy in the nest to nibble
off portions. Lichen balls are not used by termites as fungi are, for
“gardens.”

Other observations have been made by Paulson and Thompson[1236] in their
study of Epping Forest lichens: “Mites of the family Oribatidae must
be reckoned among the chief foes of these plants upon which they feed,
seeming to have a special predilection for the ripe fruits. We have had
excellent specimens of _Physcia parietina_ spoiled by hidden mites of
this family, which have eaten out the contents of the mature apothecia
after the lichens have been gathered. One can sometimes see small flocks
of the mites browsing upon the thallus of tree-dwelling lichens, like
cattle in a meadow.” The Oribatidae, sometimes called beetle-mites, a
family of Acarinae, are minute creatures familiar to microscopists. They
live chiefly on or about mosses, but Michael[1237] is of opinion that a
large number frequent these plants for the fungi and lichens which grow
in and about the mosses. In Michael’s _Monograph of British Oribatidae_,
four species are mentioned as true lichen-lovers, _Leiosoma palmicinetum_
found on _Peltigera canina_ and allied species; _Cepteus ocellatus_ and
_Oribata parmeliae_ which live on _Physciae_, the latter exclusively
on _Physcia_ (_Xanthoria_) _parietina_; and _Scutovertes maculatus_
which confines itself to lichens by the sea-shore. Another species,
_Notaspis lucorum_, frequents maritime lichens, but it is also found on
other substrata; while _Tegeocranus labyrinthicus_, though usually a
lichen-eating species, lives either on mosses or on lichens on walls.
Zopf[1238] reckoned twenty-nine species of lichens, mostly the larger
foliose and fruticose kinds, that were eaten by mites. Lesdain[1239] in
his observations on mite action notes that frequently the thallus round
the base of the perithecia of _Verrucaria_ sp. was eaten clean away,
leaving the perithecia solitary and extremely difficult to determine.

[Illustration: Fig. 126. 1, _Tetranychus lapidus_, enlarged; 2,
_Verrucaria calciseda_ with eggs _in situ_, slightly enlarged; 3 and 4,
eggs attached to lichen fruits, much magnified (after Wheldon).]

J. A. Wheldon[1240] found the eggs of a species of mite, _Tetranychus
lapidus_, attached to the fruits of _Verrucaria calciseda_, _Lecidea
immersa_ and _L. Metzleri_, calcicolous lichens of which the thallus not
only burrows deep down into the limestone, but the fruits form in shallow
excavated pits (Fig. 126). The eggs of this stone mite are found fairly
frequently on exposed limestone rocks, bare of vegetation, except for a
few crustaceous lichens. “There is usually a single egg, rarely two, in
each pit apparently attached to the old lichen apothecium. The eggs are
very attractive objects under a lens; they measure ·5 mm. in diameter,
and are disc-like with a central circular depression from which numerous
ridges radiate to the circumference, like the spokes of a wheel. When
fresh, they have a white pearly lustre, becoming chalk-white when dry and
old.” Wheldon’s observations were made in the Carnforth and Silverdale
district of West Lancashire.

A minute organism, _Hymenobolina parasitica_[1241], first described
by Zukal and doubtfully grouped among the mycetozoa, feeds, in the
plasmodium stage, on living lichens. The parasitic habit is unlike that
of true mycetozoa. It has recently been recorded from Aberdeenshire.

_b._ INSECT MIMICRY OF LICHENS. Paulson and Thompson[1242] give instances
of moth caterpillars, which not only feed on lichens, but which take on
the coloration of the lichens they affect, either in the larval or in
the perfect moth stage. “One of the most remarkable examples of this
protective resemblance to lichens is that of the larva of the geometrid
moth, _Cleora lichenaria_, which feeds upon foliose lichens growing upon
tree-trunks and palings, and being of a green-grey hue, and possessed of
two little humps on many of their body-segments, they so exactly resemble
the lichens in colour and appearance as to be extremely difficult of
detection.” Several instances are recorded of moths that resemble the
lichens on which they settle: perfect examples of such similarity
are exhibited at the Natural History Museum, South Kensington, where
_Teras literana_, _Moma orion_, and other moths are shown at rest on
lichen-covered bark from which they can hardly be distinguished.

Another curious instance of suggested mimicry is recorded by G.E.
Stone[1243]. He spotted a number of bodies on the bark of some sickly
elms in Massachusetts. They were about 1/8 of an inch in diameter “with a
dark centre and a drab foliaceous margin.” They were principally lodged
in the crevices of the bark and Stone collected them under the impression
that they were the apothecia of a lichen most nearly resembling those of
_Physcia hypoleuca_. Some of the bodies were even attached to the thallus
of a species of _Physcia_; others were on the naked bark and had every
appearance of lichen fruits. Only closer examination proved their insect
nature, and they were identified as belonging to a species _Gossypina
Ulmi_, an elm-leaf beetle common in Europe where it causes a disease of
the tree. It had been imported into the United States and had attacked
American elms.

It is stated by Tutt[1244] that the larvae of many of the Psychides
(_Lepidoptera_) live on the lichens of trees and walls, such as
_Candelaria concolor_, _Xanthoria parietina_, _Physcia pulverulenta_
and _Buellia canescens_, and that their larvae pupate on their feeding
grounds. Each species makes a “case” peculiar to itself, but those of the
lower families are usually covered externally with grains of sand, scraps
of lichens, etc. The “case” of _Narcyria monilifera_, for instance, is
somewhat raised on a flat base and is obscured with particles of sand
and yellow lichen, giving the whole a yellow appearance. That of _Luffia
lapidella_ is roughly conical and is held up at an angle of 30° to 45°
when the larva moves. The “cases” of _Bacotia sepium_ are always upright;
they measure about 5·5 mm. in height and 2·75 mm. in width and present a
hoary appearance from the minute particles of lichen with which they are
covered, so that the structure is not unlike the podetium of a _Cladonia_.

_c._ FOOD FOR THE HIGHER ANIMALS. It has been affirmed, especially by
Henneguy, that many lichens, if deprived of the bitter principle they
contain, by soaking in water, or with the addition of sodium or potassium
carbonate, might be used with advantage as fodder for animals. He cites
as examples of such, _Lobaria pulmonaria_, _Evernia prunastri_, _Ramalina
fraxinea_, _R. farinacea_, and _R. fastigiata_, all of which grow
abundantly on trees, and owe their nutritive quality to the presence of
lichenin, a carbohydrate allied to starch.

[Illustration: Fig. 127. _Cladonia rangiferina_ Web. (S. H., _Photo._).]

_Cladonia rangiferina_ (Fig. 127), the well-known “reindeer moss,” is,
however, the lichen of most economic importance, as food for reindeer,
cattle, etc. It is a social plant and forms dense tufts and swards of
slender, much branched, hollow stalks of a greenish-grey colour which may
reach a height of twelve inches or even more; the stalks decay slowly
at the base as they increase at the apex, so that very great length is
never attained. In normal conditions they neither wither nor die, and
growth continues indefinitely. It is comparatively rare in the northern
or hilly regions of the British Isles, and is frequently confused with
the somewhat smaller species _Cl. sylvatica_ which is very common on our
moorlands, a species which Zopf[1245] tells us reindeer absolutely refuse
to eat.

The true reindeer moss is abundant in northern countries, more especially
in forest regions[1246] and in valleys between the tundra hills which
are more or less sheltered from the high winds; it is independent of the
substratum and flourishes equally on barren sand and on wet turf; but
grows especially well on soil devastated by fire. For long periods it
may be covered with snow without injury and the reindeer are accustomed
to dig down with horns and hoofs in order to reach their favourite food.
Though always considered as peculiarly “reindeer” moss, deer, roebuck
and other wild animals, such as Lemming rats[1247], feed on it largely
during the winter. In some northern districts it is collected and stored
as fodder for domestic cattle; hot water is poured over it and it is
then mixed with straw and sprinkled with a little salt. Johnson[1248]
has reported that the richness of the milk yielded by the small cows of
Northern Scandinavia is attributed by some to their feeding in great
measure on the “reindeer moss.”

[Illustration: Fig. 128. _Cetraria islandica_ Ach. (S. H., _Photo._).]

When _Cladonia rangiferina_ is scarce, a few other lichens[1249] are made
use of, _Alectoria jubata_, a brownish-black filamentous tree-lichen
being one of the most frequent substitutes. _Stereocaulon paschale_,
which grows in large dense tufts on the ground in mountainous regions,
is also eaten by reindeer and other animals; and Iceland moss, _Cetraria
islandica_, is stored up in large quantities by the Icelanders and used
as fodder. Willemet[1250] reports it as good for horses, oxen, cows and
pigs.

It is interesting to recall a discovery of prehistoric remains at the
Abbey of Schussenried on the Lake of Constance and described by F.
Keller[1251]: under successive beds of peat and crumbly tufa, there was
found a layer, 3 feet thick, containing flints, horns of reindeer and
bones of various animals, and, along with these, masses of reindeer moss;
a sufficient proof of its antiquity as a fodder-plant.

_d._ FOOD FOR MAN. Lichens contain no true starch nor cellulose, but the
lichenin present in the cell-walls of the hyphae has long been utilized
as a food substance. It is peculiarly abundant in _Cetraria islandica_
(Fig. 128), which grows in northern countries, covering great stretches
of ground with its upright strap-shaped branching fronds of varying
shades of brown. In more southern lands it is to be found on high hills
or on upland moors, but in much smaller quantities. Commercial “Iceland
moss” is supplied from Sweden, Norway or Iceland. In the last-named
country the inhabitants harvest the lichen preferably from bare stony
soil where there is no admixture of other vegetation. They revisit the
locality at intervals of three years, the time required for the lichen
to grow to a profitable size; and they select the wet season for the
ingathering of the plants as they are more easily detached when they are
wet. If the weather should be dry, they collect it during the night.
When gathered it is cleansed from foreign matter and washed in water to
remove as much as possible of the bitter principle. It is then dried and
reduced to powder. When required, the powder is put to macerate in water
for 24 hours, or it is soaked in a weak solution of soda or of carbonate
of potassium, by which means the bitter cetraric acid is nearly all
eliminated. When boiled[1252] it yields a jelly which forms the basis of
various light and easily digested soups or of other delicacies prepared
by boiling in milk, which have been proved to be valuable for dyspeptics
or sufferers from chest diseases. The northern nations also make the
powder into bread, porridge or gruel. Johnson[1253] states in his account
of “Useful Plants” that considerable quantities of Iceland moss were
formerly employed in the manufacture of sea biscuit, and that ship’s
bread mixed with it was said to be less liable to the attacks of weevil
than when made from wheat flour only.

An examination of the real food value of the mucilaginous extract from
“Iceland moss” has been made by several workers. Church[1254] states
that for one part of flesh formers, there are eight parts of heat-givers
reckoned as starch. Brown[1255] isolated the two carbohydrates, lichenin
and isolichenin. The former, a jelly which yields on hydrolysis a large
quantity of a reducing sugar, dextrose, ferments with yeast and gives no
phloroglucin reaction; it is unaffected by digestion and probably does
not form glycogen. Iso-lichenin is much less abundant and resembles
soluble starch, but on digestion yields only dextrins—no sugar. It may
be concluded, judging from the chemical nature of the mucilage, from the
resistance of its constituents to digestion and from the small amount
present in the jelly, that its nutritive value is practically nil[1256].

It has been stated that “reindeer moss” in times of food scarcity is
powdered and mixed with “Iceland moss” and rye to make bread in North
Finland. Johnson confirms this and cites the evidence of a Dr Clarke
that: “to our surprise we found we might eat of it with as much ease as
of the heart of a fine lettuce. It tasted like wheat-bran, but after
swallowing it, there remained in the throat and upon the palate a gentle
heat, or sense of burning, as if a small quantity of pepper had been
mixed with the lichen.”

The Egyptians[1257] have used _Evernia prunastri_, more rarely _E.
furfuracea_, in baking. In the eighteenth century fermentative agents
such as yeast were unknown to them, and these lichens, which were
imported from more northern lands, were soaked in water for two hours and
the solution then mixed with the flour to give a much appreciated flavour
to the unleavened bread.

In India[1258] a species of _Parmelia_ (near to _P. perlata_) known in
the Telegu language as “rathapu” or rock-flower has been used as a food,
generally prepared as a curry, by the natives in the Bellary district
(Madras Presidency), and is esteemed as a delicacy. It is also used
medicinally. The collecting of rathapu is carried on during the hot
weather in April and May, and forms a profitable business.

A note has been published by Calkins[1259], on the authority of
a correspondent in Japan, that large quantities of _Endocarpon_
(_Dermatocarpon_) _miniatum_ (Fig. 56) are collected in the mountains of
that country for culinary purposes, and largely exported to China as an
article of luxury. The local name is “iwataka,” meaning stone-mushroom.
Properly prepared it resembles tripe. It is possibly the same lichen
under a different name, _Gyrophora esculenta_, which is described by
Manabu Miyoshi[1260] as of great food value in Japan where it is known
as “iwatake.” It is a greyish-brown leathery “monophyllous” plant of
somewhat circular outline and fairly large size, measuring 3 to 13 cm.
across. Fertile specimens are rare, and are smaller than the sterile.
It grows generally on the steep declivities of damp granitic rocks and
is common in various districts of Japan, being especially abundant on
such mountains as Kiso, Nikko, Kimano, etc. The face of the precipices
is often thickly covered with the lichen growth. The inhabitants collect
the plants in large quantities. They dry them and send them to the towns,
where they are sold in all vegetable stores; some are even exported to
other countries. These lichens are not bitter to the taste, nor are they
irritating as are other species of the genus. They are on the contrary
quite harmless and are much relished by the Japanese on account of their
agreeable flavour, in spite of their being somewhat indigestible. Though
only determined scientifically in recent times, this edible lichen has
long been known, and the risks attending its collection have frequently
been described in Old Chinese and Japanese writings.

[Illustration: Fig. 129. _Gyrophora polyrhiza_ Koerb. (S. H., _Photo._,
reduced).]

Other species of _Gyrophora_ including _G. polyrhiza_ (Fig. 129) and
_Umbilicaria_, black leathery lichens which grow on rocks in northern
regions, have also been used as food. They are the “Tripe de Roche” or
Rock Tripe of Arctic regions, a name given to the plants by Canadian
fur-hunters. They have been eaten by travellers and others in desperate
straits for food; but though to a certain extent nutritious, they are
bitter and nauseous, and cause severe internal irritation if the bitter
acids are not first extracted by boiling or soaking.

Of more historical interest is the desert lichen _Lecanora esculenta_,
supposed to be the manna[1261] of the Israelites, and still called “bread
from heaven.” Eversmann[1262] wrote an account of its occurrence and
qualities, and fuller information was given by Berkeley[1263]: when mixed
with meal to a third of its weight it is made into bread and eaten by the
desert tribes. It grows abundantly in North Africa and in many parts of
Western Asia, on the rocks or on soil. It is easily broken off and driven
into heaps by the wind; and has been reported as covering the soil to a
depth of 15 cm. to 20 cm. with irregular contorted lumps varying in size
from a pea to a small nut (Fig. 130). Externally these are clear brown
or whitish; the interior is white, and consists of branching interlaced
hyphae, with masses of calcium oxalate crystals, averaging about 60 per
cent. or more of the whole substance.

[Illustration: Fig. 130. _Lecanora esculenta_ Eversm. Loose nodules of
the sterile thallus.]

A still more exhaustive account is given by Visiani[1264], who quotes
the experience of a certain General Jussuf, who had tested its value in
the Sahara as food for his soldiers. When bread was made from the lichen
alone it was friable and without consistency; when mixed with a tenth
portion of meal it was similar to the soldiers’ ordinary bread, and had
something of the same taste. The General also gave it as fodder to the
horses, some of them being nourished with the lichen and a mixture of
barley for three weeks without showing any ill effects. It is also said
that camels, gazelles and other quadrupeds eat it with advantage, though
it is in any case a very defective food.

A remarkable deposit of the lichen occurred in recent times in
Mesopotamia during a violent storm of hail. After the hail had
melted, the ground was seen to be covered, and specimens were sent to
Errera[1265] for examination. He identified it as _Lecanora esculenta_.
In his opinion two kinds of manna are alluded to in the Bible: in one
case (Exodus xvi.) it is the sweet gum exuded from the tamarisk that is
described; the other kind (Numbers xi.), he thinks, plainly refers to the
lichen. He considers that its nutritive value must be very low, and it
can only be valued as food in times of famine.


B. LICHENS AS MEDICINE

_a._ ANCIENT REMEDIES. An interesting note has been published by
Müller-Argau[1266] which seems to trace back the medicinal use of lichens
to a very remote age. He tells us that Dr Schweinfurth, the distinguished
traveller, who made a journey through the valley of the Nile in 1864,
sent to him from Cairo a piece of lichen thallus found in a vase along
with berries of _Juniperus excelsa_ and of _Sapindus_, with some other
undetermined seeds. The vase dated from the 18th Dynasty (1700 to 1600
B.C.), and the plants contained in it must thus have lain undisturbed
over 3000 years. The broken pieces of the lichen thallus were fairly
well preserved; they were extremely soft and yellowish-white and almost
entirely decorticate, but on the under surfaces there remained a few
black patches, which, on microscopical examination, enabled Müller
to identify them as scraps of _Evernia furfuracea_. This lichen does
not grow in Egypt, but it is still sold there along with _Cetraria
islandica_ and some other lichens as foreign drugs. Dr Schweinfurth
considered his discovery important as proving the use of foreign remedies
by the ancient Egyptians.

_b._ DOCTRINE OF “SIGNATURES.” In the fifteenth century A.D. there was
in the study and treatment of disease a constant attempt to follow the
guidance of nature. It was believed that Providence had scattered here
and there on plants “signatures,” or resemblances more or less vague to
parts of the human body, or to the diseases to which man is subject, thus
indicating the appropriate specific.

[Illustration: Fig. 131. _Parmelia saxatilis_ Ach. (S. H., _Photo._).]

Lichens among other plants in which any “signature” could be detected
or imagined were therefore constantly prescribed: the long filaments of
_Usnea barbata_ were used to strengthen the hair; _Lobaria pulmonaria_,
the true lung-wort, with its pitted reticulate surface (Fig. 72), was
marked as a suitable remedy for lung troubles; _Xanthoria parietina_
being a yellow lichen was supposed to cure jaundice, and _Peltigera
aphthosa_, the thallus of which is dotted with small wart-like
tubercles[1267], was recommended for children who suffered from the
“thrush” eruption.

The doctrine reached the height of absurdity in the extravagant value
set on a lichen found growing on human skulls, “Muscus cranii humani”
or “Muscus ex cranio humano.” There are a number of lichens that grow
indifferently on a variety of substances, and not infrequently on bones
lying in the open. This skull lichen[1268], _Parmelia saxatilis_ (Fig.
131) or some other, was supposed to be worth its weight in gold as a cure
for epilepsy. Parkinson[1269] tells us in all confidence “it groweth
upon the bare scalps of men and women that have lyen long ... in former
times much accounted of because it is rare and hardly gotten, but in our
own times much more set by, to make the ‘Unguentum Sympatheticum’ which
cureth wounds without the local application of salves ... but as Crollius
hath it, it should be taken from the sculls of those that have been
hanged or executed for offences.” Ray[1270] says that the same gruesome
plant “is celebrated by several authors as useful in haemorrhages and
is said to be an ingredient of the famous ‘Unguentum Armarium[1271],’
reported to have been invented by Paracelsus.” Another lost ointment!

_c._ CURE FOR HYDROPHOBIA. Still another lichen to which extraordinary
virtue was ascribed, was the very common ground species _Peltigera
canina_ (Fig. 54), a preparation of which was used in the cure of rabies.
Dillenius[1272] has published in full the prescription as “A certain Cure
for the Bite of a Mad Dog” which was given to him by a very celebrated
physician of that day, Dr Richard Mead, who had found it effective:

“Let the patient be blooded at the arm, nine or ten ounces. Take of
the herb called in Latin _Lichen cinereus terrestris_, in English
Ash-coloured ground liverwort, clean’d, dry’d and powder’d half an ounce.
Of black pepper powder’d two drachms.

“Mix these well together and divide the Powder into four Doses, one
of which must be taken every Morning, fasting, for four Mornings
successively in half a Pint of Cow’s Milk warm. After these four Doses
are taken, the Patient must go into the cold bath, or a cold Spring or
River, every Morning fasting, for a Month. He must be dipt all over but
not stay in (with his head above water) longer than half a minute, if the
Water be very cold. After this he must go in three Times a Week for a
Fortnight longer.”

Lightfoot[1273], some forty years later, refers to this medicine as “the
once celebrated ‘Pulvis antilyssus,’ much recommended by the great Dr
Mead.” He adds that “it is much to be lamented that the success of this
medicine has not always answered the expectation. There are instances
where the application has not prevented the Hydrophobia, and it is very
uncertain whether it has been at all instrumental in keeping off that
disorder.” Belief in the efficacy of the powder died out before the end
of the century but the echo of the famous remedy remains in the name
_Peltigera canina_, the dog lichen.

_d._ POPULAR REMEDIES. Lichens with very few exceptions are non-poisonous
plants. They owed their repute as curative herbs to the presence in
the thallus of lichenin and of some bitter or astringent substances,
which, in various ailments, proved of real service to the patient,
though they have now been discarded in favour of more effective drugs.
Some of them, on account of their bitter taste, were frequently used
as tonics to replace quinine in attacks of fever. Several species of
_Pertusaria_, such as the bitter _P. amara_ (Fig. 132), and of _Cladonia_
as well as _Cetraria islandica_ (Fig. 128), were recommended in cases
of intermittent fever; species of _Usnea_ and others, as for instance
_Evernia furfuracea_, were used as astringents in haemorrhages; others
were given for coughs, _Cladonia pyxidata_ (Fig. 69) being supposed to be
specially valuable in whooping cough.

[Illustration: Fig. 132. _Pertusaria amara_ Nyl. on bark (S. H.,
_Photo._).]

One of the most frequently prescribed lichens was the tree lung-wort
(Lobaria pulmonaria) (Fig. 72). It was first included among medical
plants by Dorstenius[1274], a Professor at Marburg; he gives a good
figure and supplies directions for its preparation as a cure for chest
complaints. The doctrine of “signatures” influenced practitioners in its
favour, but it contains lichenin which acts as an emollient. In England,
it was taken up by the famous Dr Culpepper[1275], who, however, believed
in astrology even more than in signatures. He says: “it is of great use
with many physicians to help the diseases of the lungs and for coughs,
wheesings and shortness of breath which it cureth both in man and beast.”
He adds that “Jupiter seems to own the herb.” A century later we find Dr
John Hill[1276], who was a physician as well as a naturalist, stating
that the great tree lung-wort has been at all times famous in diseases
of the breast and lungs, but by that time “it was not much used owing to
change in fashions.”

The only lichen that has stood the test of time and experience as a real
remedy is _Cetraria islandica_, and even the “Iceland moss” is now rarely
prescribed. The first mention in literature of this famous plant occurs
in Cordus[1277] as the _Muscus_ with crisp leaves. Some years later it
figures among the medicinal plants in Sibbald’s[1278] _Chronicle of the
Scottish Flora_, and Ray[1279] wrote of it about the same time as being
known for its curative and alimentary properties. It was Linnaeus[1280],
and later Scopoli[1281], who gave it the important place it held so long
in medicine. It has been used with advantage in many chronic affections
as an emollient and tonic. Cramer[1282] in a lengthy dissertation
gathered together the facts pertaining to its use as a food, a medicine
and for dyeing, and he gives recipes he had himself prescribed with
marked success in many different maladies. It has been said that if
“Iceland moss” accomplished all the good it was alleged to do, it was
indeed a “Divine gift to man.”

The physiological action of cetrarin (acid principle of the lichen) on
living creatures has been studied by Kobert[1283] and his pupils. It
has not any poisonous effect when injected into the blood, nor does
it work any harm when taken into the stomach even of small animals,
so that it may be safely given to the most delicate patients. Nearly
always after small doses peristaltic movements in the intestines are
induced which indicate that as a drug it might be of service in the case
of enfeebled organs. In larger doses it may cause collapse in animals,
but if administered as free cetraric acid it passes through the stomach
unchanged to become slowly and completely dissolved in the intestine. The
mucous membrane of the intestine of animals that had been treated with an
overdose, was found to be richer in blood so that it seems as if cetrarin
might be of service in chlorosis and in assisting digestion.

Cetrarin has also been proved to be a nerve excitant which might be used
with advantage in mental maladies.


C. LICHENS AS POISONS

Though the acid substances of lichens are most of them extremely
irritating when taken internally, very few lichens are poisonous.
Keegan[1284] writing on this subject considers this quality of
comparative innocuousness as a distinctive difference between fungi and
lichens and he decides that it proves the latter to be higher organisms
from a physiological point of view: “the colouring matters being true
products of deassimilation, whereas those of fungi are decomposition or
degradation waste products of the albuminoids akin to alkaloids.”

The two outstanding exceptions to this general statement are the two
Alpine species _Letharia vulpina_ and _Cetraria pinastri_. The former
contains vulpinic acid in the cortical cells, the crystals of which are
lemon-yellow in the mass. _Cetraria pinastri_ produces pinastrinic acid
in the hyphae of the medulla and the crystals are a beautiful orange or
golden yellow.

These lichens, more especially _Letharia vulpina_, have been used by
Northern peoples to poison wolves. Dead carcasses are stuffed with a
mixture of lichen and powdered glass and exposed in the haunts of wolves
in time of frost. Henneguy[1285], who insists on the non-poisonous
character of all lichens, asserts that the broken glass is the fatal
ingredient in the mixture, but Kobert[1286], who has proved the poisonous
nature of vulpinic acid, says that the wounds caused by the glass render
the internal organs extremely sensitive to the action of the lichen.

Kobert, Neubert[1287] and others have recorded the results of experiments
on living animals with these poisons. They find that _Letharia vulpina_
either powdered or in solution has an exciting effect on the mucous
membrane. Elementary organisms treated with a solution of the lichen
succumbed more quickly than in a solution of the acid as a salt. Kobert
concluded that vulpinic acid is a poison of protoplasm.

He further tested the effect of the poison on both cold- and warm-blooded
animals. Administered as a sodium salt, 4 mg. proved fatal to frogs.
The effect on warm-blooded animals was similar. A sodium salt, whether
swallowed or administered as subcutaneous or intravenous injections, was
poisonous. Cats were the most sensitive—hedgehogs the least—of all the
animals that were subjected to the experiments. Volkard’s[1288] synthetic
preparation of vulpinic acid gave the same results as the solution
directly extracted from the lichens.


D. LICHENS USED IN TANNING, BREWING AND DISTILLING

The astringent property in _Cetraria islandica_ and in _Lobaria
pulmonaria_ has been made use of in tanning leather. The latter lichen
grows commonly on oak and could hardly be gathered in sufficient quantity
to be of commercial importance. Like many other lichens it develops very
slowly. _Lobaria pulmonaria_ has also been used to replace hops in the
brewing of beer. Gmelin[1289] in his journey through Siberia visited a
monastery at Ussolka where the monks employed it for this purpose. The
beer tasted exactly like that made with hops, but was more intoxicating.
The lichen in that country grew on pine-trees.

Lichens have in more modern times been used in the preparation of
alcohol. The process of manufacture was discovered by Roy of Tonnerre,
early in the nineteenth century, and was described by Léorier[1290].
It was further improved by Stenberg[1291], a Professor of Chemistry in
Stockholm. Roy had worked with _Physcia ciliaris_, _Ramalina fraxinea_,
_R. fastigiata_, _R. farinacea_ and _Usnea florida_, but Stenberg and
distillers after his time[1292] made more use of _Cladonia rangiferina_
(Fig. 127), _Cetraria islandica_ (Fig. 128) and _Alectoria jubata_.

By treatment with weak sulphuric or nitric acid the lichenin of the
thallus is transformed into glucose which on fermentation forms alcohol.
Stenberg found that 68 per cent. of the weight in _Cladonia rangiferina_
was a “sugar” from which a good brandy could be prepared: a kilogramme of
the lichens furnished half a litre of alcohol. The Professor followed up
his researches by establishing a distillery near Stockholm. His papers
contain full instructions as to collecting and preparing the plants.
Henneguy[1293], writing in 1883, stated that the fabrication of alcohol
from lichens was then a large and increasing industry in Sweden. The
whole industry seems, however, to have fallen into disuse very soon:
Wainio[1294], quoting Hellbom[1295], states that the various distilleries
were already closed in 1884, because of the exhaustion of the lichen in
the neighbourhood, and the impossibility of obtaining sufficient supplies
of such slow-growing plants.


E. DYEING PROPERTIES OF LICHENS

_a._ LICHENS AS DYE-PLANTS. Knowledge as to the dyeing properties
of lichens dates back to a remote antiquity. It has been generally
accepted that lichen-colours are indicated by the prophet Ezekiel in his
denunciation of Tyre: “blue and purple from the Isles of Elishah was that
which covered thee.” Theophrastus describes certain plants as growing
in Crete, and being used to dye wool, etc., and Pliny in his _Phycos
Thalassion_ is also understood as referring to the lichen _Roccella_,
“with crisp leaves, used in Crete for dyeing garments.”

Information as to the dyeing properties of certain lichens is given in
most of the books or papers dealing with these plants from the herbals
onwards. Hoffmann[1296] devoted a large part of his _Commentatio de
vario Lichenum usu_ to the dye-lichens, and, illustrating his work, are
a series of small rectangular coloured blocks representing samples of
woollen cloth dyed with different lichens. There are seventy-seven of
these samples with the colour names used by French dyers.

An important treatise on the subject translated into French was also
contributed by Westring[1297]. He desired to draw attention to the
tinctorial properties of lichens other than the _Roccellae_ which do
not grow in Sweden. The Swedes, he states, already used four to six
lichens as dye-plants, but only for one colour. He demonstrated by his
improved methods that other colours and of finer tint could be obtained.
He describes the best methods both of extraction and of dyeing, and
then follows with an account of the different lichens likely to be of
service. The treatise was subsequently published at greater length in
Swedish[1298] with twenty-four very fine coloured illustrations of the
lichens used, and with sample blocks of the colours to be obtained.

_b._ THE ORCHIL LICHEN, ROCCELLA. The value of _Roccella_ as a dye-plant
had been lost sight of until it was accidentally rediscovered, early in
the fourteenth century, by a Florentine merchant called Federigo. He
introduced its use into Florence, and as he retained the industry in
his own hands he made a large fortune, and founded the family of the
Orcellarii, called later the Rucellarii or Rucellai, hence the botanical
name, _Roccella_. The product was called _orseille_ for which the
English name is orchil or archil. Another origin suggested for orchil
is the Spanish name of the plant, _Orcigilia_. There are a number of
different species that vary in the amount of dye-product. Most of them
grow on rocks by the sea-side in crowded bluish-grey or whitish tufts of
strap-shaped or rounded stiff narrow fronds varying in length up to about
six inches or more. The main supply of “weeds” came from the Levant until
the fifteenth century when supplies were obtained from the Canaries (long
considered to produce the best varieties), Cape Verd and the African
coasts. The geographical distribution of the _Roccellae_ is very wide:
they grow on warm sea-coasts all over the globe, more particularly in
Angola, the Cape, Mozambique, Madagascar, in Asia, in Australia, and in
Chili and Peru.

Zopf[1299] has proved the existence of two different colouring substances
among the Roccellas: in _R. fuciformis_ (Fig. 57) and _R. fucoides_
(both British species), in _R. Montagnei_ and _R. peruensis_ the
acid present is erythrin; in _R. tinctoria_, _R. portentosa_ and _R.
sinuensis_ it is lecanoric acid. In _R. tinctoria_ (Fig. 133), according
to Ronceray[1300], the acid is located chiefly in the gonidial layer and
the soredia but is absent from the cortex and centre. In _R. portentosa_
it is abundant in the cortex and central layer, while scarcely to be
detected in the gonidial layer, and it is wanting altogether in the
soredia. In _R. Montagnei_ it is chiefly found in the cortex and the
gonidial layer, and is absent from the soredia and from the medulla.

_c._ PURPLE DYES: ORCHIL, CUDBEAR AND LITMUS. Orseille or orchil is
formed not only from erythrin and lecanoric acid (orseillic acid), but
also from erythrinic, gyrophoric, evernic and ramalic acids[1301] and may
be obtained from any lichen containing these substances. By the action
of ammonia the acids are split up into orcin and carbonic acid. In time,
under the influence of ammonia and the oxygen of the air[1302], orcin
becomes orcein which is the colouring principle of orchil; the perfecting
of the process may take a month. The dye is used for animal fibres such
as wool and silk; it has no effect on cotton.

There are several different preparations on the market, chiefly obtained
from France and Holland; orchil or orseille in the form of a solution,
cudbear (persio of Germany) almost the same, but manufactured into a
violet-reddish powder, and litmus (tournesol of France) which is prepared
in a slightly different manner. At one time the lichen, broken into small
pieces, was soaked in urine; a fermentation process was set up, then lime
and potash with an admixture of alum were added. The mass of material
when ready was pressed into cubes and dried in the air. Commercial litmus
contains three substances, erythrolein, erythrolitmin and azolitmin; the
last named, which is the true litmus, is a dark brown amorphous powder
soluble in water, and forming a blue solution with alkalies.

[Illustration: Fig. 133. _Roccella tintoria_ Ach. From the Cape of Good
Hope.]

An aqueous solution of litmus when exactly neutralized by an acid
is violet coloured; it becomes red with the smallest trace of free
acid, or blue with free alkali. Litmus paper is prepared by steeping
specially prepared unsized paper in the dye solution. It is as a ready
and sensitive indicator of acidity or alkalinity that litmus is of so
much value. According to Zopf[1303] it is also used as a blueing agent
in washing and as a colouring of wine. Litmus is chiefly manufactured
in Holland. Still another substance somewhat differently prepared from
the same lichens is sold as French purple, a more brilliant and durable
colour than orchil.

[Illustration: Fig. 134. _Lecanora tartarea_ Ach. (S. H., _Photo._).]

_d._ OTHER ORCHIL LICHENS. Though species of _Roccella_ rank first
in importance as dye-plants, purple and blue colours are obtained,
as indicated above, from other very different lichens. Lindsay[1304]
extracted orchil from about twenty species. Those most in use in northern
countries are on the whole less rich in colouring substances; they
are: _Umbilicaria pustulata_, species of _Gyrophora_, _Parmelia_ and
_Pertusaria_, and above all _Lecanora tartarea_ (Fig. 134). The last
named, one of the hardiest and most abundant of rock- or soil-lichens,
is chiefly used in Scotland and Sweden (hence the name “Swedish moss”)
to furnish a red or crimson dye. In Scotland all dye-lichens are called
“crottles,” but the term “cudbear” was given to _Lecanora tartarea_
(either the lichen or the dye-product); it was acquired from a corrupt
pronunciation of the Christian name of Dr Cuthbert Gordon, a chemist,
who, according to Bohler[1305], obtained a patent for his process
of producing the dye, or who first employed it on a great scale in
Glasgow. Johnson[1306] remarks that the colour yielded by cudbear, if
well prepared, is a fine, clear, but not very bright purple. It is, he
alleges, not permanent. Like other orchil substances it is without effect
on cotton or linen.

_e._ PREPARATION OF ORCHIL. A general mode of treatment of dye-lichens
recommended by Lauder Lindsay[1307] for home production of orchil,
cudbear and litmus is as follows:

  1. Careful washing, drying and cleansing to separate earthy and
  other impurities.

  2. Pulverization into a coarse or fine pulp with water.

  3. Repeated addition of ammoniacal liquor of a certain strength,
  obtainable from several sources (_e.g._ putrid urine, gas liquor,
  etc.).

  4. Frequent stirring of the fermenting mass so as to ensure full
  exposure of every part thereof to the action of atmospheric
  oxygen.

  5. Addition of alkalies in some cases (_e.g._ potash or soda),
  to heighten or modify colour; and of chalk, gypsum and other
  substances to impart consistence.

_f._ BROWN AND YELLOW DYES. The extracting of these colours from lichens
is also a very old industry. Linnaeus found during his journey to
Lappland[1308], undertaken when he was quite a young man, that the women
in the northern countries made use of a brown lichen for dyeing which is
evidently _Parmelia omphalodes_ (Fig. 135). He describes it as a “rich
_Lichenoides_ of a brown stercoraceous colour,” and he has stated that
it grew in such abundance in the Island of Aland, that every stone was
covered, especially near the sea. In the _Plantae tinctoriae_[1309] there
is a record of six other lichens used for dyeing: _Lichen Roccella_,
_L. tartareus_, _L. saxatilis_, _L. juniperinus_, _L. parietinus_ and
_L. candelarius_. The value of _Lichen omphalodes_ was also emphasized
by Lightfoot; the women of Scotland evidently appreciated its dyeing
properties as much as other northern peoples.

A series of memoirs on the utility of lichens written by Willemet[1310],
Amoreux and Hoffmann, and jointly published at Lyons towards the end of
the eighteenth century, represents the views as to the economic value of
lichens held by scientific botanists of that time. All of them cite the
various dye-species, and Hoffmann, as already stated, gives illustrations
of colours that can be obtained. It has been once and again affirmed
that _Parmelia saxatilis_ yields a red colour, but Zopf[1311] denies
this. It contains saxatillic acid which is colourless when extracted but
on boiling gives a clear reddish-yellow to reddish-brown solution which
dyes wool and silk directly without the aid of a mordant. Zopf[1311]
observed the process of dyeing followed in South Tyrol: a layer of the
lichen was placed in a cooking pot, above this a layer of the material
to be dyed, then lichen and again the material until the pot was filled.
It was covered with water and boiled three to four hours, resulting in a
beautiful rust-brown and peculiarly fast dye.

[Illustration: Fig. 135. _Parmelia omphalodes_ Ach. (S. H., _Photo._).]

Reddish- or rust-brown dye is also obtained from _Haematomma ventosum_
and _H. coccineum_, a yellow-brown from _Parmelia conspersa_ (salazinic
acid), and other shades of brown from _Parmelia perlata_, _P. physodes_,
_Lobaria pulmonaria_ and _Cetraria islandica_.

Yellow lichens in general furnish yellow dyes, as for instance _Xanthoria
parietina_ which gives either brown or yellow according to treatment and
_Cetraria juniperina_ which forms a beautiful yellow colouring substance
on boiling. _Teloschistes flavicans_ and _Letharia vulpina_ yield very
similar yellow dyes, and from _Lecanora parella_ (Fig. 39), _Pertusaria
melaleuca_ and _Usnea barbata_ yellow colours have been obtained.
_Candelariella vitellina_ and _Xanthoria lychnea_ both contain yellow
colouring agents and have been employed by the Swedes for dyeing the
candles used in religious ceremonies.

_g._ COLLECTING OF DYE-LICHENS. Lauder Lindsay[1312] made exhaustive
studies of dye-lichens both in the field and in the laboratory, and
recorded results he obtained from the micro-chemical examination of
540 different specimens. He sought to revive and encourage the use of
their beautiful colour products among country people; he has given the
following practical hints to collectors:

1. That crustaceous dwarf pale-coloured species growing on rocks, and
especially on sea-coasts, are most likely to yield red and purple dyes
similar to orchil, cudbear or litmus; while on the other hand the
largest, most handsome foliaceous or fruticose species are least likely.

2. That the colour of the thallus is no indication of colorific power
(in orchil lichens), inasmuch as the red or purple colouring substances
are the result of chemical action on crystalline colorific “principles”
previously devoid of colour.

3. That alterations in physical characters, chemical composition and
consequently in dyeing properties are very liable to be produced by
modification in the following external circumstances:

  (i) Degree of moisture.
  (ii) Degree of heat.
  (iii) Degree of exposure to light and air.
  (iv) Climate.
  (v) Elevation above the sea.
  (vi) Habitat; nature of basis of support.
  (vii) Age.
  (viii) Seasons and atmospheric vicissitudes, etc.

August has been recommended as the best month for collecting dye-lichens:
_i.e._ just after the season of greatest light and heat when the
accumulation of acids will be at its maximum.

Some of the acids found useful in dyeing occur in the thalli of a large
number of lichens, many of which are too scantily developed to be of
any economic value. Thus salazinic acid which gives the effective
yellow-brown dye in _Parmelia conspersa_ was found by Zopf in 13 species
and varieties. It has since been located by Lettau[1313] in 72 different
lichens, many of them, however, with poorly developed or scanty thalli,
so that no technical use can be made of them.

_h._ LICHEN COLOURS AND SPECTRUM CHARACTERS. In a comparative study of
vegetable colouring substances, Sorby[1314] extracted yellow colouring
matters from various plants distinguished by certain spectrum characters.
He called them the “lichenoxanthine group” because, as he explains,
“these xanthines occur in a more marked manner in lichens than in plants
having true leaves and fronds.” Orange lichenoxanthine he found in
_Peltigera canina_, _Platysma glaucum_, etc., when growing well exposed
to the sun. Lichenoxanthine he obtained from the fungus _Clavaria
fusiformis_; it was difficult to separate from orange lichenoxanthine.
Yet another, which he terms yellow lichenoxanthine, he obtained most
readily from _Physcia_ (_Xanthoria_) _parietina_. The solutions of these
substances vary according to Sorby in giving a slightly different kind of
spectrum. He did not experiment on their dyeing properties.


F. LICHENS IN PERFUMERY

_a._ LICHENS AS PERFUMES. There are a few lichens that find a place in
Gerard’s[1315] _Herball_ and that are praised by him as being serviceable
to man. Among others he writes of a “Moss that partakes of the bark of
which it is engendered. It is to be used in compositions which serve
for sweet perfumes and that take away wearisomeness.” At a much later
date we find Amoreux[1316] recording the fact that _Lichen_ (_Evernia_)
_prunastri_, known as “Mousse de Chêne,” was used as a perfume plant.

Though lichens are not parasitic, the idea that they owed something of
their quality to the substratum was firmly held by the old herbalists.
It appears again and again in the descriptions of medicinal lichens, and
still persists in this matter of perfumes. Hue[1317] states in some notes
to a larger work, that French perfumers extract an excellent perfume from
_Evernia prunastri_ (Fig. 59) known as “Mousse des Chênes” (Oak moss),
and it appears that the plants which grow on oak contain more perfume
than those which live on other trees. The collectors often gather along
with _Evernia prunastri_ other species such as _Ramalina calicaris_ and
_R. fraxinea_, but these possess little if any scent. A still finer
perfume is extracted[1318] from _Lobaria pulmonaria_ called “moss from
the base of the oaks,” but as it is a rarer lichen than _Evernia_ it is
less used. Most of the Stictaceae, to which family _Lobaria_ belongs,
have a somewhat disagreeable odour, but this one forms a remarkable
exception, which can be tested by macerating the thallus and soaking it
in spirit: it will then be found to exhale a pleasant and very persistent
scent. These lichens are not, however, used alone; they are combined with
other substances in the composition of much appreciated perfumes. The
thallus possesses also the power of retaining scent and, for this reason,
lichens frequently form an ingredient of potpourri.

_b._ LICHENS AS HAIR-POWDER. In the days of white-powdered hair, use
was occasionally made of _Ramalina calicaris_ which was ground down and
substituted for the starch that was more commonly employed.

In older books on lichenology constant reference is made to a hair-powder
called “Pulvis Cyprius” or “Cyprus powder” and very celebrated in the
seventeenth century. It was believed to beautify and cleanse the hair by
removing scurf, etc. _Evernia prunastri_ was one of the chief ingredients
of the powder, but it might be replaced by _Physcia ciliaris_ or by
_Usnea_. The virtue of the lichens lay in their capacity to absorb and
retain perfume. The powder was for long manufactured at Montpellier
and was a valuable monopoly. Its composition was kept secret, but
Bauhin[1319] (J.) published an account of the ingredients and how to
mix them. Under the title “Pulvis Cyprius Pretiosius” a more detailed
recipe of the famous powder was given by Zwelser[1320], a Palatine
medical doctor. The lichen employed in his preparation, as in Bauhin’s,
is _Usnea_, but that may include both _Evernia_ and _Physcia_ as they
are all tree plants. He gives elaborate directions as to the cleaning of
the lichen from all impurities—it is to be beaten with a stick, washed
repeatedly with limpid and pure water, placed in a linen cloth and dried
in the sun till it is completely bleached and deprived of all odour and
taste.

When well dried it was placed in a basket in alternate layers with
freshly gathered, entire flowers of roses and jasmine (or flowers of
orange and citrus when possible). The whole was compressed by a heavy
weight, and each day the flowers were renewed until the “Usnea” was
thoroughly impregnated with a very fragrant odour. It was then reduced to
a fine powder and ready for other ingredients. To each pound should be
added:

1-1/2 oz. powdered root of white Iris.

1-1/2 oz. of _Cyperus_ (a sedge).

1 scruple or half drachm of musk reduced to a pulp with fragrant spirit
of roses.

1/2 drachm of ambergris dissolved in a scruple of genuine oil of roses,
or oil of jasmine or oranges as may be preferred.

Zwelser adds:

“This most fragrant royal powder when sprinkled on the head invigorates
by its remarkably pleasant odour; by its astringency and dryness it
removes all impurities, and, since it operates with no viscosity nor
sticks firmly either to skin or hair, it is easily removed from the hair
of the head.”


G. SOME MINOR USES OF LICHENS

The possibility of extracting gum or mucilage from lichens was
demonstrated by the Russian scientist, Professor Georgi[1321], and later
by Amoreux[1322], the method employed being successive boiling of the
plants. The larger foliose or fruticose forms were specially recommended.

At a later date, during the Napoleonic wars, the “ingenious Lord
Dundonald[1323],” of great fame as an inventor, published an account
of the extraction process and of the application of the gum to
calico-printing, staining and manufacture of paper, dressing and
stiffening silks. Lord Dundonald’s aim was to replace the gum Senegal,
then a monopoly of the French, who were in possession of the Settlement
of Senegambia. He took out a patent for his invention, but whether the
gum was successfully used is not recorded.

According to Henneguy[1324], lichen mucilage, as a substitute for gum
arabic, has been used at Lyons with advantage in the fabrication of dyed
materials.




APPENDIX

POSTSCRIPT TO CHAPTER VII[1325]


In a remarkable paper on _The Symbiosis of Lichens_[1326], Dr A.
Henry Church has presented a new and striking view of the origin and
development of lichens: he has sought to link them up with other classes
of vegetation that, in the great transmigration, passed from sea to
land. As we know from his _Thalassiophyta[1327] and the subaerial
transmigration_, he holds that primeval algae of advanced form and
structure were left exposed on dry land by the gradually receding waters,
and those that successfully adapted themselves to the changed conditions
formed the basis of the land flora. A certain number of the algae lost
their surface tissues containing chlorophyll and they had perforce
to secure from other organic sources the necessary carbohydrates:
they adopted a heterotrophic existence as saprophytic or parasitic
fungi. Fungi are a backward race (deteriorated according to Dr Church)
as regards their soma, but in number, distribution and variety of
spore-production, they are eminently successful plants.

Lichens are similarly regarded by Dr Church as derived from stranded
contemporaneous types of marine algae—crustaceous, foliose and
fruticose, that had also lost their chlorophyll, but by taking into
association green algal units of a lower grade they established a
vicarious photosynthesis. But, to quote his own words[1328], “as the
alga-lichen-fungus left the sea, so it remained: it might deteriorate,
but it certainly never advanced, once the sea factors which produced it
were eliminated, it simply stopped along these lines.”

And again[1329]: “Lichens thus present an interesting case of an algal
race deteriorating along the lines of a heterotrophic existence, yet
arrested, as it were, on the somatic down-grade, by the adoption of
intrusive algal units of lower degree to subserve photosynthesis (much
in the manner of the marine worm _Convoluta_). Thus arrested, they have
been enabled to retain more definite expression of more deeply inherent
factors of sea-weed habit and construction than any other race of fungi;
though closely paralleled by such types as _Xylaria_ (Ascomycete)
and _Clavaria_ (Basidiomycete), which have followed the full fungus
progression as holosaprophytic on decaying plant residues.”

Dr Church’s theory is of vivid interest and might be convincing were
there no possibility and no proof of advance within the symbiotic
plant, but in numbers of crustaceous thalli, there is evident,
by normal or abnormal[1330] development, the first advance to
the formation of rudimentary squamules, a condition diagnosed as
subsquamulose. “Deterioration” of the lichen plant—when it occurs owing
to unfavourable conditions—is a reversion to the leprose early stage
of the association; there is no evidence of reversion from fruticose
or foliose to squamulose. A glance at the table of lichen phyla[1331]
shows progression again and again from the crustaceous forms onwards.
In such a phylum as Physciaceae (with colourless polarilocular spores)
there is a clear example of a closely connected series; the different
types of thallus—crustaceous, squamulose, foliose and fruticose—are all
represented and form a natural sequence, being well delimited by the
unusual form of the spore and by the presence of parietin in thallus or
apothecium.

That there has been development seems absolutely certain, and that
along the lines sketched in the chapter on phylogeny. Progress has
been mainly in the thallus, but there has also been change and advance
in the reproductive organs, more especially in the spores which in
several families reach a size and septation unparalleled in fungi.
That association with green algal cells stimulated the fungus to new
development is the view taken of the lichen plant and emphasized in the
present volume. But it seems more in accordance with the polyphyletic
origin and recurring parallel development in the phyla that association
began at the elementary crustaceous stage, and that the lichen soma was
gradually evolved within what is after all a very limited and simple
structure.




ADDENDUM

FOOTNOTE TO PAGE 404


E. M. Holmes[1332] has published recently an account of a substance which
seems in some respects to answer to the description of manna (Exodus
xvi.; Numbers xi.) more nearly than the generally accepted _Lecanora
esculenta_. The information is quoted from Swann’s book: _Fighting the
slave-hunters in Central Africa_. The author writes (p. 116): “I was
shown a curious white substance similar to porridge. It was found early
in the morning before the sun rose. On examination it was found to
possess all the characteristics of the manna ... of the Israelites. In
appearance it resembled coriander seed, was white in colour like hoar
frost, sweet to the taste, melted in the sun and if kept over night was
full of worms in the morning. It required to be baked if you intended to
keep it for any length of time. It looked as if it was deposited on the
ground in the night.” The writer has suggested that “the substance might
be mushroom spawn as, on the spot where it melted tiny fungi sprung up
the next night.” Swann’s statement has been confirmed by Dr Wareham, a
medical missionary from the same district, who states, however, that it
is of rare occurrence.




FOOTNOTES


[1] E. Acton (1909) has described a primitive lichen _Botrydina
vulgaris_, in which there is no fruiting stage, and in which the fungus
seems to show affinity with a Hyphomycete.

[2] Luyken 1809.

[3] Hornschuch 1819.

[4] Raab 1819.

[5] Dillenius 1741, p. 200.

[6] Wallroth 1825.

[7] Agardh 1820.

[8] See p. 27.

[9] Schwendener 1867.

[10] Fink 1913.

[11] Lindsay 1876.

[12] Nylander 1869.

[13] Crombie 1891.

[14] Nylander 1891.

[15] Crombie 1874.

[16] Crombie 1877.

[17] Crombie 1885.

[18] Fink 1913.

[19] Elfving 1913.

[20] Moreau 1918.

[21] Peirce 1898.

[22] French 1898.

[23] Morison 1699.

[24] Tournefort 1694 and 1700.

[25] Dillenius 1741.

[26] Krempelhuber 1867-1872.

[27] _Grete Herball_ 1526.

[28] Ruel 1536.

[29] Dorstenius 1540.

[30] Camerarius 1586.

[31] Tabernaemontanus 1590.

[32] L’Obel 1576.

[33] Dodoens 1583.

[34] Gerard 1597.

[35] Schwenckfeld 1600.

[36] Colonna 1606.

[37] Bauhin 1623, pp. 360-2.

[38] Parkinson 1640.

[39] How 1650.

[40] Merrett 1666.

[41] Plot 1686.

[42] Morison 1699.

[43] Ray 1670.

[44] Ray 1686.

[45] Ray 1690.

[46] Petiver 1695.

[47] Plukenet 1691-1696.

[48] Malpighi 1686.

[49] Porta 1688.

[50] Tournefort 1694.

[51] Tournefort 1700.

[52] Rupp 1718.

[53] Buxbaum 1721.

[54] Vaillant 1727.

[55] Dillenius 1724 and 1741.

[56] Micheli 1729.

[57] Dillenius 1719.

[58] See Druce and Vines 1907.

[59] Crombie 1880.

[60] Haller 1742.

[61] Linnaeus 1753.

[62] Schneider 1897.

[63] Hill 1751. Hill’s genus _Collema_ is _Nostoc_, etc.

[64] Hill 1760.

[65] Watson 1759.

[66] Scopoli 1760.

[67] Adanson 1763.

[68] Hudson 1762 and 1778.

[69] Withering 1776.

[70] Lightfoot 1777.

[71] Dickson 1785.

[72] Weber 1780.

[73] Sibthorp 1794.

[74] Relhan 1785 and 1820.

[75] Smith 1790.

[76] Hoffmann 1798.

[77] Persoon 1794.

[78] Buxbaum 1728.

[79] Petiver 1712.

[80] Sloane 1796 and 1807.

[81] Swartz 1788 and 1791.

[82] Desfontaines 1798-1800.

[83] Georgi 1797.

[84] Willomet, etc. 1787.

[85] Acharius 1798.

[86] Acharius 1803.

[87] Acharius 1810.

[88] Acharius 1814.

[89] Hue 1908.

[90] De Candolle 1805.

[91] Flörke 1815-1819.

[92] Davies 1813.

[93] Forster 1816.

[94] S. F. Gray 1821.

[95] Carrington 1870.

[96] See _List of the Books_, etc. by John Edward Gray, p. 3, 1872.

[97] Hooker 1821.

[98] Hooker 1831.

[99] Greville 1823-1827.

[100] Greville 1824.

[101] Hooker 1833.

[102] Taylor 1836.

[103] Fries 1831.

[104] Fée 1824.

[105] Flörke 1828.

[106] Wallroth 1829.

[107] Delise 1822.

[108] Chevalier 1824.

[109] Wallroth 1825.

[110] Meyer 1825.

[111] Holle 1849.

[112] Koerber 1839.

[113] Michaux 1803.

[114] Mühlenberg 1813.

[115] Torrey 1819.

[116] Halsey 1824.

[117] Tuckerman 1839.

[118] Fée 1824.

[119] Martius 1833.

[120] Montagne 1851.

[121] Hooker 1841.

[122] Schaerer 1850.

[123] Eschweiler 1824.

[124] Fée 1824.

[125] De Notaris 1846.

[126] Massalongo 1852.

[127] Norman 1852.

[128] Koerber 1855.

[129] Mudd 1861.

[130] Lindsay 1856.

[131] Leighton 1851, etc.

[132] See Hue 1899.

[133] Nylander 1854 and 1855.

[134] Tulasne 1852.

[135] Lauder Lindsay 1859.

[136] Itzigsohn 1854-1855.

[137] Speerschneider 1853.

[138] Sachs 1855.

[139] Thwaites 1849.

[140] Schwendener 1863-1868.

[141] Leighton 1851.

[142] Leighton 1854.

[143] Leighton 1856.

[144] Mudd 1865.

[145] Th. Fries 1858.

[146] Schwendener 1867.

[147] Nylander 1874.

[148] Crombie 1885.

[149] Lett 1890.

[150] Fünfstück 1898.

[151] Zahlbruckner 1903-1907.

[152] Ventenat 1794, p. 36.

[153] Cassini 1817, p. 395.

[154] Agardh 1820.

[155] Scopoli 1760, p. 79.

[156] Persoon 1794, p. 17.

[157] Sprengel 1804, p. 325.

[158] Wallroth 1825, I.

[159] Wallroth 1825, I., p. 303.

[160] Fries 1831, pp. lvi and lvii.

[161] Kützing 1843.

[162] Thwaites 1849, pp. 219 and 241.

[163] Flotow 1850.

[164] Sachs 1855.

[165] Itzigsohn 1855.

[166] Itzigsohn 1854.

[167] Hicks 1860 and 1861.

[168] Speerschneider 1853.

[169] Famintzin and Baranetzky 1867.

[170] Baranetzky 1869.

[171] Itzigsohn 1867.

[172] Bayrhoffer 1851.

[173] Tulasne 1852.

[174] Speerschneider 1854.

[175] de Bary 1866, p. 242.

[176] Schwendener 1860, p. 125.

[177] de Bary 1866, p. 291.

[178] Nylander 1870.

[179] Elfving 1903 and 1913.

[180] See p. 133.

[181] Minks 1878 and 1879.

[182] Müller 1878 and 1884.

[183] Zukal 1884.

[184] Darbishire 1895¹.

[185] Schwendener 1860, etc.

[186] Schwendener 1867.

[187] Schwendener 1868, p. 195.

[188] Schwendener 1869.

[189] Rees 1871.

[190] Bornet 1872.

[191] The authors quoted have been followed in their designation
of the various green algae that form lichen gonidia. It is however
now recognized (Wille 1913) that either _Protococcus viridis_ Ag.,
_Chlorella_ or other Protococcaceae may form the universal green coating
on trees, etc., and be incorporated as lichen gonidia. _Pleurococcus
vulgaris_ Naeg. and _Pleurococcus Naegeli_ Chod. are synonyms of
_Protococcus viridis_. In that alga there is no pyrenoid, and no
zoospores are formed.

The genus _Cystococcus_, according to Chodat (1913), is characterized
by the presence of a pyrenoid and by reproduction with zoospores and is
identical with _Pleurococcus vulgaris_ Menegh. (non Naeg.), though Wille
regards Meneghini’s species as of mixed content. Paulson and Hastings
(1920) now find that Chodat’s pyrenoid is the nucleus of the cell.

[192] Woronin 1872.

[193] Archer 1873, 1874, 1875.

[194] Bornet 1873 and 1874.

[195] Treub 1873.

[196] Borzi 1875.

[197] Stahl 1877.

[198] Bonnier 1886 and 1889.

[199] Bonnier was probably experimenting with an _Arthopyrenia_.
_Verrucaria_ species combine with _Protococcus_ or according to Chodat
with _Coccobotrys_ gen. nov.

[200] Nylander 1858.

[201] Fuisting 1868, p. 674.

[202] Winter 1876, p. 264.

[203] Stahl 1877.

[204] See p. 62.

[205] Wainio 1890, 2, p. 29.

[206] Reinke 1872, p. 108.

[207] Reinke 1873¹.

[208] Reinke 1873², p. 98.

[209] Frank 1876.

[210] de Bary 1879.

[211] Bornet 1873.

[212] Hedlund 1892.

[213] Peirce 1899.

[214] Hue 1915.

[215] Lindau 1895¹.

[216] Peirce 1899.

[217] Claassen 1914.

[218] Frank 1876.

[219] Lindau 1895.

[220] Bachmann 1913.

[221] Cunningham 1879.

[222] Ward 1884.

[223] Jennings 1895.

[224] Fitting 1910.

[225] Bornet 1873.

[226] Bonnier 1889².

[227] Schwendener 1867.

[228] Elenkin 1902¹ and 1904¹, 1904².

[229] Elenkin 1906².

[230] Danilov 1910.

[231] Paulson and Hastings 1920.

[232] Nienburg 1917.

[233] Zukal 1891.

[234] Sutherland 1915.

[235] Beyerinck 1890.

[236] Artari 1902.

[237] See p. 56.

[238] Artari 1902.

[239] Treboux 1912.

[240] Chodat 1913.

[241] See Paulson and Hastings 1920.

[242] Keeble 1910.

[243] Reinke 1872.

[244] Dufrenoy 1918.

[245] Artari 1899.

[246] Etard and Bouilhac 1898.

[247] Radais 1900.

[248] Artari 1901.

[249] Chodat 1913.

[250] Treboux 1905.

[251] Marshall Ward 1884.

[252] Uhlir 1915.

[253] Tobler 1911.

[254] Zopf 1907.

[255] Chambers 1912.

[256] Chodat 1913.

[257] See note Paulson and Hastings, p. 28.

[258] Chodat 1913.

[259] Gargeaune 1911.

[260] Servettaz 1913.

[261] See p. 65.

[262] Wettstein 1915.

[263] Meyer 1825.

[264] Holle 1849.

[265] Tulasne 1852.

[266] Bonnier 1889².

[267] Term coined by Lindau (1899) to describe the pseudo-cellular tissue
of lichens and fungi now referred to as “plectenchyma.”

[268] Wainio 1897.

[269] Möller 1887.

[270] Tobler 1909.

[271] Wahrlich 1893.

[272] Baur 1898.

[273] Darbishire 1899.

[274] Kienitz-Gerloff 1902.

[275] Meyer 1902.

[276] Salter 1902.

[277] Nylander (1866) gave the term “gonimia” to the blue-green algae of
the Phycolichens, retaining the term “gonidia” for the bright-green algae
of the Archilichens: the distinction is not now maintained.

[278] For further details see also the chapter on Classification.

[279] See p. 133.

[280] Krempelhuber 1873.

[281] Chodat 1913.

[282] Paulson and Hastings 1920.

[283] Paulson in litt.

[284] Acton 1909.

[285] Bialosuknia 1909.

[286] Hue 1905.

[287] Deckenbach 1893.

[288] In a comparative study of leaf algae from Ceylon and Barbadoes,
N. Thomas (1913) came to the conclusion that Marshall Ward’s alga in
its early stages is the same as _Phyllactidium tropicum_ Moebius; and
that the Barbadoes alga with which she was working represented the older
stages, it being then subcuticular in habit, forming rhizoids, barren and
sterile aerial hairs and subcuticular zoosporangia.

[289] De Toni 1889.

[290] Bornet 1873.

[291] Fünfstück 1899.

[292] Hedlund 1892.

[293] Zukal 1895, p. 19.

[294] Moebius 1888.

[295] Frank 1876, p. 158.

[296] Stahl 1877.

[297] Neubner 1893.

[298] Krabbe 1891.

[299] Forssell 1885.

[300] Hue 1910.

[301] Harmand 1913, p. 1050.

[302] Forssell 1886.

[303] See Chap. VII.

[304] Lindau 1895.

[305] Darbishire 1897.

[306] Nienburg 1917.

[307] Bonnier 1888 and 1889².

[308] Bonnier 1889.

[309] Forssell 1884, p. 34.

[310] Zahlbruckner 1902.

[311] Lindau 1899.

[312] Reinke 1895.

[313] Zukal 1895, p. 562.

[314] Zahlbruckner 1907.

[315] Hue 1899.

[316] Wainio has adopted this term for growing hyphae 1897, p. 33.

[317] Tulasne 1852.

[318] Zukal 1895.

[319] Zukal 1895.

[320] Schwendener 1866.

[321] Schwendener 1863.

[322] Hue 1906.

[323] See p. 83.

[324] Malinowski 1911.

[325] Steiner 1881.

[326] Fünfstück 1899.

[327] Bachmann 1913.

[328] See p. 215.

[329] Friedrich 1906.

[330] Bachmann 1907.

[331] Bachmann 1904.

[332] Bachmann 1904.

[333] Stahlecker 1906.

[334] Lang 1903.

[335] Fünfstück 1899.

[336] Darbishire 1897.

[337] Frank 1876.

[338] Bornet 1873, p. 81.

[339] Lindau 1895.

[340] Bitter 1899.

[341] See p. 83.

[342] Friedrich 1906.

[343] See p. 76.

[344] See p. 126.

[345] Schwendener 1860, 1863 and 1868.

[346] Zukal 1895, p. 1305.

[347] Hue 1906.

[348] Heber Howe 1912.

[349] Hue 1911.

[350] Schwendener 1863, p. 180.

[351] Darbishire 1897.

[352] Rosendahl 1907.

[353] See p. 96.

[354] See p. 133.

[355] Rosendahl 1907.

[356] Meyer 1902.

[357] See p. 52.

[358] Nylander 1858.

[359] Hue 1898.

[360] Rosendahl 1907.

[361] Darbishire 1912.

[362] Porter 1919.

[363] Darbishire 1897.

[364] Schwendener 1860.

[365] Rosendahl 1907.

[366] Meyer 1902.

[367] Reinke 1895, p. 186.

[368] Bitter 1901.

[369] Sernander 1901.

[370] Parfitt in Leighton 1871, p. 470.

[371] Galløe 1915.

[372] Bitter 1899.

[373] Sturgis 1890.

[374] See p. 108.

[375] Darbishire 1898.

[376] Darbishire 1895.

[377] Brandt 1906.

[378] Hue 1906.

[379] Haberlandt 1896.

[380] Schulte 1904.

[381] See p. 120.

[382] Lutz 1894.

[383] Peirce 1898.

[384] Zopf 1903.

[385] Lindau 1895.

[386] Brandt 1906.

[387] Porter 1916.

[388] Darbishire 1898.

[389] Wainio 1880.

[390] Krabbe 1891.

[391] Wainio 1897.

[392] Wainio 1880.

[393] Krabbe 1891.

[394] Krabbe 1891.

[395] Baur 1904.

[396] Wainio 1880.

[397] Chodat 1913.

[398] Wainio 1897.

[399] Baur 1904.

[400] Wainio 1897.

[401] Wainio 1897.

[402] Lindsay 1859, p. 171.

[403] Wainio 1897.

[404] Wainio 1897, p. 9.

[405] Wainio 1880.

[406] Krabbe 1891.

[407] Necker 1871.

[408] Persoon 1794.

[409] Acharius 1803.

[410] Wallroth 1829, p. 61.

[411] Tulasne 1852.

[412] Koerber 1855.

[413] Reinke 1894.

[414] Sättler 1914.

[415] Nienburg 1908.

[416] See p. 183.

[417] Wainio 1897.

[418] Baur 1904.

[419] Wolff 1905.

[420] Aigret 1901.

[421] Wainio 1897.

[422] Wainio 1890, p. 67.

[423] Reinke 1895.

[424] Nylander 1858, p. 63.

[425] Acharius 1810, p. 12.

[426] Haller 1768, p. 85.

[427] Schreber 1791, p. 768.

[428] Meyer 1825, p. 148.

[429] Delise 1822.

[430] Nylander 1858, p. 14.

[431] Nylander 1860, p. 333.

[432] Schwendener 1863, p. 169.

[433] Wainio 1890, I. p. 183.

[434] Schwendener 1863, p. 169.

[435] Stizenberger 1895.

[436] Zukal 1895, p. 1355.

[437] Wainio 1909.

[438] Schwendener 1863, p. 169.

[439] Jatta 1889, p. 48.

[440] Zukal 1895, p. 1357.

[441] Rosendahl 1907.

[442] Zukal 1895.

[443] Reinke 1895, p. 183.

[444] Darbishire 1901.

[445] Brandt 1906.

[446] Bitter 1899.

[447] Nylander 1874².

[448] Bitter 1901².

[449] Zukal 1895, p. 1348.

[450] Acharius 1803.

[451] Hue 1904 and 1910.

[452] Flörke 1815, IV. p. 15.

[453] Wallroth 1825, p. 678.

[454] Th. M. Fries 1858.

[455] Forssell 1884.

[456] Leighton 1869.

[457] Nylander 1878.

[458] Schneider 1897.

[459] Hue 1910.

[460] Hue 1910.

[461] Tuckerman 1875.

[462] Schneider 1897, p. 58.

[463] Nylander 1869.

[464] Bornet 1873, p. 72.

[465] Forssell 1885, p. 24.

[466] Riddle 1910.

[467] Babikoff 1878.

[468] Th. M. Fries 1866.

[469] Winter 1877.

[470] Schneider 1897.

[471] Etard and Bouilhac 1898.

[472] Hue 1910.

[473] Sernander 1907.

[474] Bitter 1904.

[475] Bitter 1904.

[476] Linkola 1913.

[477] Acharius, 1798, p. xix, and 1810, pp. 8 and 10.

[478] Malpighi, 1686, p. 50, pl. 27, fig. 106.

[479] Micheli 1729, pp. 73, 74.

[480] Linnaeus 1737, p. 325.

[481] Hedwig 1798.

[482] Sprengel 1807, Letter XXIII.

[483] Wallroth 1825, I. p. 595.

[484] Koerber 1841.

[485] Schwendener 1860.

[486] Meyer 1825, p. 170.

[487] Krabbe 1891.

[488] Bitter 1901.

[489] Schwendener 1860, p. 137.

[490] Wainio 1897, p. 32.

[491] Reinke 1895, p. 380.

[492] Bitter 1901.

[493] Lesdain 1910.

[494] Schwendener 1860.

[495] Nilson 1903.

[496] Darbishire 1897.

[497] Bitter 1901, p. 191.

[498] Krabbe 1891.

[499] Darbishire 1907.

[500] Tobler 1911², 11.

[501] Bitter 1901².

[502] Lindau 1895.

[503] Nilson 1903.

[504] Bitter 1904.

[505] Acharius 1798, pp. 2, 87.

[506] Fries 1825.

[507] Hooker 1833.

[508] Taylor 1836.

[509] Rosendahl 1907.

[510] Bitter 1899.

[511] Nilson 1903.

[512] Kajanus (Nilson) 1911.

[513] Zopf 1903.

[514] Zopf 1905².

[515] Bitter 1899.

[516] Swartz 1788.

[517] Gmelin 1791.

[518] Woodward 1797.

[519] Fries 1825.

[520] Nylander 1855.

[521] Mattirolo 1881.

[522] Johow 1884.

[523] Wainio 1890.

[524] Möller 1893.

[525] Malpighi 1686.

[526] Tournefort 1694.

[527] Morison 1699.

[528] Micheli 1729.

[529] Micheli, Pls. 52 and 56.

[530] Dillenius 1741.

[531] Linnaeus 1737.

[532] Necker 1771, p. 257.

[533] Scopoli 1772.

[534] Koelreuter 1777.

[535] Hoffmann 1784.

[536] Hedwig 1784.

[537] Acharius 1810.

[538] Hornschuch 1821.

[539] Wallroth 1825.

[540] Meyer 1825.

[541] Sprengel 1804.

[542] Luyken 1809.

[543] Persoon 1801.

[544] See also p. 166.

[545] Wainio 1890.

[546] Tulasne 1852.

[547] Fuisting 1868.

[548] Stahl 1877.

[549] Borzi 1878.

[550] Baur 1898.

[551] Fünfstück (1902) suggests that the lichen worked at by Baur is
_Collema cheileum_ Ach.

[552] Krabbe 1883.

[553] Mäule 1891.

[554] F. Bachmann 1912.

[555] This species of _Collema_ has been described as _Collemodes
Bachmannianum_ by Bruce Fink 1918.

[556] F. Bachmann 1913.

[557] Wainio I. 1890.

[558] Wolff 1905.

[559] Stahl 1877.

[560] Forssell 1885².

[561] Zukal 1887, p. 42.

[562] Borzi 1878.

[563] Lindau 1888.

[564] Mäule 1891.

[565] Darbishire 1900.

[566] Baur 1904.

[567] Lindau 1888.

[568] Darbishire 1900.

[569] See also p. 180.

[570] Nienburg 1908.

[571] Wainio I. 1890.

[572] Darbishire 1900.

[573] Baur 1901.

[574] Baur 1904.

[575] Nienburg 1908.

[576] Harper 1900.

[577] Guilliermond 1904, p. 60.

[578] Baur 1899.

[579] Sturgis 1890.

[580] Nienburg 1908.

[581] Schwendener 1864.

[582] Wahlberg 1902.

[583] Baur 1904.

[584] Brown 1911.

[585] Moreau 1916.

[586] Lindau 1888.

[587] Fünfstück 1884.

[588] Baur 1904.

[589] Nienburg 1908.

[590] Rosendahl 1907.

[591] Lindau 1888.

[592] Baur 1904.

[593] Wolff 1905.

[594] Fünfstück 1902.

[595] Krabbe 1882.

[596] Baur 1901.

[597] Fünfstück 1902.

[598] Darbishire 1897.

[599] See also p. 147.

[600] Wolff 1905.

[601] See Chap. VII.

[602] Krabbe 1883 and 1891.

[603] Baur 1904.

[604] Sättler 1914.

[605] See Chap. VII.

[606] Fuisting 1868.

[607] Stahl 1877.

[608] Baur 1904.

[609] Baur 1901.

[610] Krabbe 1882.

[611] Forssell 1883.

[612] Wainio 1890, p. x.

[613] Neubner 1893.

[614] Fünfstück 1884.

[615] Darbishire 1913.

[616] Moreau 1915.

[617] Sturgis 1890.

[618] Metzger 1903.

[619] Baur 1904.

[620] Moreau 1916.

[621] Krabbe 1882.

[622] Lindau 1899.

[623] Wolff 1905.

[624] Rosendahl 1907.

[625] Bitter 1901².

[626] Baur 1904.

[627] Nienburg 1908.

[628] Krabbe 1882.

[629] Schulte 1904.

[630] Wainio 1890.

[631] Schikorra 1909.

[632] Harper 1900.

[633] Fraser 1907.

[634] Thaxter 1912.

[635] Faull 1911.

[636] Dawson 1900.

[637] Brooks 1910.

[638] Blackman and Welsford 1912.

[639] Lindau 1899.

[640] Van Tieghem 1891.

[641] Zukal 1895.

[642] Wainio 1890.

[643] Steiner 1901.

[644] See also Chap. VII.

[645] F. Bachmann 1913.

[646] Cutting 1909.

[647] Darbishire 1900.

[648] Baur 1904.

[649] See p. 161.

[650] Stahl 1877.

[651] Baur 1898.

[652] F. Bachmann 1912 and 1913.

[653] Darbishire 1900.

[654] Fitzpatrick 1918.

[655] Harper 1900.

[656] Fünfstück 1902.

[657] Schwendener 1864.

[658] Hue 1906.

[659] Lindau 1899.

[660] Hedwig 1784.

[661] Acharius 1803.

[662] Sprengel 1807.

[663] Luyken 1809.

[664] Eschweiler 1824.

[665] Fée 1824.

[666] Mohl 1833.

[667] Dangeard 1894.

[668] Baur 1904.

[669] Nienburg 1908.

[670] Maire 1903.

[671] Bachmann 1913.

[672] Mohl 1833.

[673] Zukal 1895.

[674] Fée 1824.

[675] Meyer 1825.

[676] Holle 1849.

[677] Tulasne 1852.

[678] De Bary 1866-1867.

[679] Haberlandt 1887.

[680] Zopf 1905.

[681] Massalongo 1852.

[682] Koerber 1855.

[683] Wainio 1. 1890, p. 113.

[684] Harper 1899.

[685] Hue 1911².

[686] Tulasne 1852.

[687] Bornet 1873.

[688] Bonnier 1889².

[689] Maire 1905.

[690] Neubner 1893.

[691] Brefeld 1889.

[692] See p. 143.

[693] Bornet 1873.

[694] Bornet’s observations have not been repeated, and it is possible
that he may have been dealing with a parasitic hyphomycetous fungus.

[695] Steiner 1901.

[696] Müller 1881.

[697] Wainio 1890, II. p. 27.

[698] Fée 1873.

[699] Müller 1890.

[700] Tulasne 1851.

[701] Dillenius 1741.

[702] Hedwig 1784 and 1789.

[703] Acharius 1810.

[704] Fries 1831.

[705] Wallroth 1825.

[706] Schaerer 1823-1842.

[707] Flotow 1850.

[708] Itzigsohn 1850.

[709] Tulasne 1851.

[710] Tulasne 1852.

[711] Lindsay 1859 and 1872.

[712] Forssell 1885.

[713] Nienburg 1908.

[714] Möller 1887.

[715] Sturgis 1890.

[716] Nylander 1858, pp. 34, 35.

[717] Nylander, Crombie and others apply the term “sterigma” to the whole
spermatiophore. In the more usual restricted sense, it refers only to the
short process from which the spermatium is abstricted.

[718] Glück 1899.

[719] Steiner 1901.

[720] Gibelli 1866.

[721] Tulasne 1852.

[722] Nylander 1858.

[723] Corda 1839.

[724] Allescher 1901-3.

[725] Keiszler 1911.

[726] Nylander 1858, p. 37.

[727] Möller 1887.

[728] Istvanffi 1895.

[729] Möller 1888.

[730] Hedlund 1892.

[731] Tulasne 1852.

[732] Müller 1885.

[733] Lindsay 1859 and 1872.

[734] Laubert 1911.

[735] Blackman 1904.

[736] Istvanffi 1895.

[737] Plowright 1889.

[738] Sappin-Trouffy 1896.

[739] Brefeld 1891.

[740] De Bary 1866, p. 7.

[741] Gilson 1893.

[742] Winterstein 1893.

[743] Gilson 1894.

[744] The chemical formula of chitin is given as C₆₀H₁₀₀N₈O₃₈, that of
chitosan as C₁₄H₂₆N₂O₁₀.

[745] Escombe 1896.

[746] Wisselingh 1898.

[747] Wester 1909.

[748] Berzelius 1813.

[749] Guérin-Varry 1834.

[750] Mulder 1838.

[751] Berg 1873.

[752] Beilstein ex Errera 1882, p. 16 (note).

[753] Escombe 1896.

[754] Wiesner 1900.

[755] Lacour 1880.

[756] Wisselingh 1898.

[757] Stüde 1864.

[758] Czapek 1905, I. p. 515.

[759] Ulander 1905.

[760] Müller 1905.

[761] Stüde 1864.

[762] Wisselingh 1898.

[763] Schellenberg 1896.

[764] Escombe 1896.

[765] Wester 1909.

[766] Czapek 1905, I. p. 515.

[767] Moreau 1916.

[768] Errera 1882.

[769] Schwendener 1862, p. 231.

[770] De Bary 1866-1867, p. 211.

[771] Gautier 1899.

[772] Herissey 1898.

[773] Czapek 1905, II. p. 257.

[774] Ronceray 1904.

[775] Zopf in Schenk 1890, p. 448.

[776] Knop 1872.

[777] Zopf 1907.

[778] Hamlet and Plowright 1877.

[779] Braconnot 1825.

[780] Zopf 1907.

[781] Errera 1893.

[782] Euler 1908, p. 7.

[783] Rosendahl 1907.

[784] Knop 1872.

[785] Kratzmann 1913.

[786] Zukal 1895, p. 1311.

[787] Kerner and Oliver 1894, p. 235.

[788] Steiner 1881.

[789] Zukal 1884.

[790] Zukal 1886.

[791] Hulth 1891.

[792] Bachmann 1892.

[793] Fünfstück 1895.

[794] Fünfstück 1899.

[795] Bachmann 1904¹.

[796] Lang 1906.

[797] Lang 1906, p. 171.

[798] Bachmann 1892.

[799] Bachmann 1904¹.

[800] Bachmann 1904².

[801] Fünfstück 1895.

[802] Zukal 1895, p. 1372.

[803] Rosendahl 1907.

[804] Zukal 1895.

[805] Fünfstück 1896.

[806] Fünfstück 1899.

[807] Lang 1906.

[808] Beijerinck 1904.

[809] Wehmer 1891.

[810] Stahel 1911.

[811] See p. 218.

[812] Pfeffer 1877.

[813] Pfaff 1826.

[814] Herberger 1830.

[815] Knop and Schnederman 1846.

[816] Hesse 1904.

[817] Zopf 1907, p. 179.

[818] Zopf 1907.

[819] Senft 1907.

[820] Tobler 1909.

[821] Keegan 1907.

[822] Schwarz 1880, p. 264.

[823] Schwendener 1863, p. 180.

[824] Fünfstück 1902.

[825] Heber Howe 1913.

[826] Knowles 1913.

[827] West, W. 1905.

[828] Lettau 1914.

[829] Lettau 1914.

[830] Parietin differs chemically from chrysophanic acid of _Rheum_, etc.

[831] Stenhouse and Groves 1877.

[832] Volkard 1894.

[833] Nylander 1866.

[834] Jumelle 1892.

[835] Sievers 1908.

[836] Zukal 1895.

[837] Beckmann 1907.

[838] Herre 1911².

[839] Sievers 1908.

[840] Jumelle 1892.

[841] Schrenk 1898.

[842] Bonnier 1889².

[843] West 1905.

[844] Sandstede 1904.

[845] Friedrich 1906.

[846] Lindau 1895².

[847] See p. 109.

[848] Uloth 1861.

[849] Zopf 1903.

[850] Zopf 1907.

[851] Ohlert 1871.

[852] See p. 75.

[853] Uloth 1861.

[854] Egeling 1881.

[855] Buchet 1890.

[856] Bachmann 1904.

[857] Bachmann 1911.

[858] Lang 1903.

[859] Stahlecker 1906.

[860] Bitter 1899.

[861] Zukal 1879.

[862] Kihlman 1890.

[863] Jumelle 1892.

[864] Zopf 1890, p. 489.

[865] Jumelle 1892.

[866] Weir 1919.

[867] Wainio 1897, p. 16.

[868] Nienburg 1908.

[869] Metzger 1903.

[870] Bitter 1899.

[871] Wiesner 1895.

[872] Bitter 1901, p. 465.

[873] Fink 1909.

[874] Galløe 1908.

[875] Zukal 1896.

[876] Bitter 1901.

[877] Maheu 1906.

[878] Wiesner 1895.

[879] R. Paulson, ined.

[880] Krempelhuber 1861.

[881] Zukal 1896, p. 111.

[882] Zukal 1896.

[883] Hedlund 1892, p. 22.

[884] Zopf 1893.

[885] Zopf 1907.

[886] Zopf 1892.

[887] Knop 1872.

[888] Bachmann 1890.

[889] A similar reaction with nitric acid is produced on the blue
hypothalline hyphae of _Placynthium nigrum_.

[890] Knowles 1915.

[891] Rosendahl 1907.

[892] John 1819.

[893] Grimbel 1856.

[894] Molisch 1892.

[895] Nilson 1907.

[896] Meyer 1825, p. 44.

[897] Lindsay 1856.

[898] Berkeley 1857.

[899] Weddell 1869.

[900] Phillips 1878.

[901] Scott Elliot 1907.

[902] Vallot 1896.

[903] Bitter 1901.

[904] Heere 1904.

[905] Krabbe 1891, p. 131.

[906] Reinke 1894, p. 18.

[907] Bonnier, see p. 29.

[908] Darbishire, see p. 148.

[909] Tobler, see p. 148.

[910] Stahl 1877, p. 34.

[911] Paulson 1918.

[912] Paulson and Thompson 1913.

[913] Fink 1917.

[914] Baur 1901.

[915] Miyoshi 1901.

[916] Darbishire 1897, p. 657.

[917] Beckmann 1907.

[918] Bitter 1899.

[919] Peirce 1898.

[920] Schrenk 1898.

[921] Elenkin 1901.

[922] See Chap. X.

[923] Mereschkovsky 1918.

[924] Meyer 1825, p. 44.

[925] Paulson and Somerville Hastings 1914.

[926] Crombie 1872.

[927] See p. 35.

[928] Dufrenoy 1918.

[929] Arnold 1874.

[930] Kupfer 1894.

[931] See p. 236.

[932] Malme 1895.

[933] Bitter 1899.

[934] Hofmann 1906.

[935] Almquist 1880.

[936] Bitter 1899.

[937] See p. 237.

[938] Arnold 1874.

[939] Nylander 1852.

[940] Hue 1915.

[941] Th. Fries 1874, p. 343.

[942] Tobler 1911².

[943] Lindsay 1869².

[944] Winter 1877.

[945] _Abrothallus_ has been included in the lichen genus _Buellia_.

[946] Tulasne 1852.

[947] Lindsay 1856.

[948] Crombie 1894.

[949] Kotte 1910.

[950] Zopf 1896.

[951] Zopf 1898, p. 249.

[952] Tobler 1911².

[953] See p. 276.

[954] Elenkin 1901².

[955] Stahl 1904.

[956] Lindsay 1859, 1869, 1871.

[957] Zopf 1896.

[958] Zopf 1898.

[959] Moreau 1916³.

[960] Vouaux 1912, etc.

[961] Bitter 1904.

[962] Zukal 1893.

[963] Lister 1911.

[964] Zopf 1897.

[965] Zukal 1896, p. 258.

[966] Zukal 1896, p. 255.

[967] Cunningham 1879.

[968] Friedrich 1906, p. 401.

[969] See p. 78.

[970] Gleditsch 1775, p. 31.

[971] Lindau 1895, p. 53.

[972] Dufrenoy 1881.

[973] Porter 1917.

[974] Friedrich 1906.

[975] Waite 1893.

[976] Lesdain 1912.

[977] Zopf 1907.

[978] Lesdain 1910.

[979] Zukal 1896, p. 258.

[980] See p. 51.

[981] See p. 177 _et seq._

[982] Zahlbruckner 1903.

[983] Steiner 1896.

[984] Müller-Argau 1880.

[985] Wainio 1890, p. xxiii.

[986] Lloyd 1917.

[987] Rehm 1890.

[988] Reinke 1894.

[989] See p. 260.

[990] Wainio 1890.

[991] Müller-Argau 1862.

[992] Rehm 1890.

[993] Rehm 1890.

[994] Tobler 1911², p. 407.

[995] Lightfoot 1777, p. 965.

[996] See Chap. III.

[997] Forssell 1885.

[998] Hue 1911¹.

[999] See p. 133.

[1000] Zahlbruckner 1907.

[1001] Reinke 1895.

[1002] See p. 126.

[1003] Reinke 1895.

[1004] Neubner 1893.

[1005] Reinke 1895, p. 110.

[1006] Wainio 1890.

[1007] See Chap. III.

[1008] Wainio 1897.

[1009] See Chap. III.

[1010] Sättler 1914.

[1011] See p. 90.

[1012] Darbishire 1912.

[1013] See p. 101.

[1014] See p. 188.

[1015] Dr Church (1920) has published a new conception of the origin of
lichens. See postscript at the end of the volume, p. 421.

[1016] Tournefort 1694.

[1017] Morison 1699.

[1018] Dillenius 1741.

[1019] Linnaeus 1753.

[1020] Acharius 1803.

[1021] Acharius 1810.

[1022] Acharius 1814.

[1023] Wallroth 1825.

[1024] Meyer 1825.

[1025] Nylander 1854.

[1026] Reinke 1894, ’95, ’96.

[1027] Darbishire and Fischer-Benzon 1901.

[1028] Wainio 1887, ’94, ’97.

[1029] Wainio 1890.

[1030] Zahlbruckner 1907.

[1031] Massee 1887.

[1032] Fischer 1890.

[1033] Linnaeus 1753.

[1034] Steiner 1901.

[1035] See p. 56.

[1036] Norman 1872 and ’74.

[1037] Genera marked with an asterisk have not been found in the British
Isles.

[1038] Zukal 1890.

[1039] Hue 1914.

[1040] Hue 1909.

[1041] Hue 1905.

[1042] Zahlbr., in _Hedwigia_, LIX. p. 301, 1917.

[1043] Riddle 1917.

[1044] Bioret 1914.

[1045] Reinke 1895.

[1046] Darbishire 1898.

[1047] Hue 1909.

[1048] Nylander 1855.

[1049] Nylander 1883.

[1050] Lorrain Smith 1906.

[1051] _Neophyllis_ Wils. is synonymous with _Gymnoderma_.

[1052] Lindau 1899.

[1053] Stirton 1877, p. 164.

[1054] A. Zahlbruckner, in _Oesterr. bot. Zeitschr._ 1919, p. 163.

[1055] Bitter 1904².

[1056] Hue 1892.

[1057] Hue 1914.

[1058] Tuckerman 1872, p. 107.

[1059] See p. 188.

[1060] Hue 1908.

[1061] See p. 152.

[1062] Olivier 1907.

[1063] Th. Fries 1867.

[1064] Darbishire 1909.

[1065] Hue 1892.

[1066] Arnold 1890.

[1067] These genera are associated with _Trentepohlia_ algae which are
numerous and abundant in tropical climates, and their presence there may
possibly account for these particular lichens.

[1068] Wainio 1897.

[1069] Wainio 1909.

[1070] Elenkin 1906.

[1071] Darbishire 1905.

[1072] Hue 1915.

[1073] Darbishire 1912.

[1074] Lindsay 1870.

[1075] Calkins 1896.

[1076] Hue 1898.

[1077] Fink 1903.

[1078] Wainio 1896.

[1079] Comm. Heber Howe.

[1080] Herre 1910.

[1081] Nylander 1890.

[1082] Müller 1879.

[1083] Nylander and Crombie 1884.

[1084] Babington 1855.

[1085] Stirton 1875.

[1086] Nylander 1888.

[1087] Hellbom 1896.

[1088] Wilson 1892.

[1089] Müller-Argau 1884.

[1090] Stizenberger 1888-1895.

[1091] Steiner 1895.

[1092] Flagey 1892.

[1093] Wainio 1890.

[1094] Wainio 1890, II. p. 27 (recorded under _Lecidea_).

[1095] Elenkin and Woronichin 1908.

[1096] Jaczewski 1904.

[1097] Steiner 1919.

[1098] Müller 1892.

[1099] Nylander 1867.

[1100] Leighton 1869.

[1101] Nylander 1900.

[1102] Nylander 1891.

[1103] Schimper 1869, p. 145.

[1104] Lindsay 1879.

[1105] Braun 1840.

[1106] Muenster 1846, p. 26.

[1107] Eltingshausen and Debey 1857.

[1108] Engelhardt 1870 (Pl. I. figs. 1 and 2).

[1109] Goeppert 1845, p. 195.

[1110] See Schimper 1869, pp. 145, etc.

[1111] Goeppert and Menge 1883, t. 1, fig. 3.

[1112] Ludwig 1859, p. 61 (t. 9, figs. 1-4), 1859-61.

[1113] Schimper in Zittel 1890.

[1114] Goeppert and Menge 1883.

[1115] Sernander 1918.

[1116] See p. 392.

[1117] Moss 1913.

[1118] Macmillan 1894.

[1119] See p. 240 _et seq._

[1120] See p. 238.

[1121] West 1915.

[1122] Fink 1894.

[1123] Kihlman 1890.

[1124] Nilson 1907.

[1125] Lindsay 1869.

[1126] Sättler 1914.

[1127] Peirce 1898.

[1128] Schrenk 1898.

[1129] Nylander 1866.

[1130] Hue 1898.

[1131] Wheldon and Wilson 1915.

[1132] Paulson and Thompson 1911.

[1133] Paulson and Thompson 1912.

[1134] Chodat 1912.

[1135] Fée 1824.

[1136] Fries 1831.

[1137] Krempelhuber 1861.

[1138] Arnold 1891, etc.

[1139] Fink 1902.

[1140] Watson 1909.

[1141] Paulson 1919.

[1142] Lesdain 1912.

[1143] Fink 1896, etc.

[1144] Stahl 1877.

[1145] Fink 1902, etc.

[1146] Arnold 1891.

[1147] Mayfield 1916.

[1148] Paulson and Thompson 1913.

[1149] Lesdain 1910².

[1150] Lettau 1911.

[1151] Fink 1903.

[1152] Wheldon and Wilson 1907.

[1153] Arnold 1892, p. 34.

[1154] Wheldon and Wilson 1915.

[1155] See p. 358.

[1156] Aigret 1901.

[1157] Kieffer 1894.

[1158] Stahlecker 1906.

[1159] Link 1795.

[1160] Malinowski 1911.

[1161] See also p. 254.

[1162] Fink 1904.

[1163] Link 1789.

[1164] Watson 1918².

[1165] Wheldon and Wilson 1907.

[1166] Flagey 1901.

[1167] Bruce Fink 1902².

[1168] Forssell 1885.

[1169] Servit 1910.

[1170] Malinowski 1911.

[1171] Bachmann 1914.

[1172] West 1912.

[1173] Wheldon and Wilson 1913.

[1174] Stahlecker 1906.

[1175] Malinowski 1911.

[1176] Wheldon and Wilson 1915.

[1177] Wheldon and Wilson 1907.

[1178] Schade 1916.

[1179] Lesdain 1910.

[1180] Arnold 1858.

[1181] Richard 1877.

[1182] Darbishire 1909.

[1183] Cf. p. 234.

[1184] Paulson and Thompson 1913.

[1185] Wheldon and Wilson 1913.

[1186] Weddell 1875.

[1187] Knowles 1913.

[1188] The two morphologically similar plants _Ramalina cuspidata_ and
_R. scopulorum_ are here united under the older name _R. siliquosa_. The
distinction between the two is based on reaction tests with potash, which
give very uncertain results.

[1189] Nylander 1861.

[1190] Knowles 1915.

[1191] Wheldon and Wilson 1913.

[1192] Sandstede 1904.

[1193] Aigret 1901.

[1194] Wheldon and Wilson 1915.

[1195] Watson 1918¹.

[1196] Sandstede 1904.

[1197] McLean 1915.

[1198] Wheldon and Wilson 1915.

[1199] Wheldon and Wilson 1914.

[1200] Maheu 1887.

[1201] Leighton 1867.

[1202] Kihlman 1890.

[1203] Nilson 1907.

[1204] Darbishire 1909.

[1205] Flagey 1901.

[1206] Patouillard 1897.

[1207] Steiner 1895.

[1208] Bruce Fink 1909.

[1209] Herre 1911².

[1210] See p. 97.

[1211] Lindsay 1856.

[1212] Macmillan 1894.

[1213] Knowles _in litt._

[1214] Bruce Fink 1903.

[1215] Lettau 1911.

[1216] Wheldon and Wilson 1915.

[1217] Wheldon and Wilson 1913.

[1218] Linnaeus 1762.

[1219] Guembel 1856.

[1220] Goeppert 1860.

[1221] Salter 1856.

[1222] Bachmann 1911.

[1223] Bachmann 1913.

[1224] Braun 1917.

[1225] Treub 1888.

[1226] Brez 1791.

[1227] Persoon 1794.

[1228] Zukal 1895, p. 1317 (note).

[1229] Zukal 1895, p. 1315.

[1230] Zopf 1896.

[1231] Stahl 1904.

[1232] Hue 1915.

[1233] Zopf 1907.

[1234] Bitter 1899.

[1235] Petch 1913.

[1236] Paulson and Thompson 1913.

[1237] Michael 1884.

[1238] Zopf 1907.

[1239] Lesdain 1910.

[1240] Wheldon 1914.

[1241] See also p. 267.

[1242] Paulson and Thompson 1913.

[1243] Stone 1896.

[1244] Tutt 1900, p. 107.

[1245] Zopf 1907, p. 372.

[1246] Kihlman 1890.

[1247] Linnaeus 1762.

[1248] Johnson 1861.

[1249] Lindsay 1856.

[1250] Willemet 1787.

[1251] Keller 1866.

[1252] Proust 1906.

[1253] Johnson 1861.

[1254] Church 1880.

[1255] Brown 1898.

[1256] Hutchinson 1916.

[1257] Forskål 1875, p. 193.

[1258] Watt 1890.

[1259] Calkins 1892.

[1260] Miyoshi 1893.

[1261] See p. 422.

[1262] Eversmann 1825.

[1263] Berkeley 1849.

[1264] Visiani 1867.

[1265] Errera 1893.

[1266] Müller-Argau 1881, p. 526.

[1267] See p. 138.

[1268] From an examination of old figures of the _Muscus cranii_, Arnold
(1892, p. 53) has decided that several kinds of lichens or hepatics are
included in this designation.

[1269] Parkinson 1640, p. 1313.

[1270] Ray 1686, p. 117.

[1271] Amoreux 1787, p. 46.

[1272] Dillenius 1741, p. 202.

[1273] Lightfoot 1777, II. p. 846.

[1274] Dorstenius 1540.

[1275] Culpepper 1652.

[1276] Hill 1751.

[1277] Cordus 1561.

[1278] Sibbald 1684.

[1279] Ray 1686.

[1280] Linnaeus 1737.

[1281] Scopoli 1760.

[1282] Cramer 1880.

[1283] Kobert 1895.

[1284] Keegan 1905.

[1285] Henneguy 1883.

[1286] Kobert 1895.

[1287] Neubert 1893.

[1288] See p. 228.

[1289] Gmelin 1752, p. 425.

[1290] Léorier 1825.

[1291] Stenberg 1868.

[1292] Richard 1877.

[1293] Henneguy 1883.

[1294] Wainio 1887, p. 47.

[1295] Hellbom 1886, p. 72.

[1296] Hoffmann 1787.

[1297] Westring 1792 and 1793.

[1298] Westring 1805-1809.

[1299] Zopf 1907.

[1300] Ronceray 1904.

[1301] Zopf 1907.

[1302] Zahlbruckner (1905, p. 109) quotes from Czapek a statement that
orchil fermentation is brought about by an obligate aerobic bacillus.

[1303] Zopf 1907, p. 393.

[1304] Lindsay 1855.

[1305] Bohler 1835, N. 10.

[1306] Johnson 1861.

[1307] Lindsay 1855.

[1308] Linnaeus 1711.

[1309] Linnaeus 1760.

[1310] Willemet etc. 1787.

[1311] Zopf 1907.

[1312] Lindsay 1855.

[1313] Lettau 1914.

[1314] Sorby 1873.

[1315] Gerard 1597.

[1316] Amoreux 1787.

[1317] Hue 1889.

[1318] Hue 1900.

[1319] Bauhin 1650, p. 88.

[1320] Zwelser 1672.

[1321] Georgi 1779.

[1322] Amoreux 1787.

[1323] Dundonald 1801.

[1324] Henneguy 1883.

[1325] See p. 302.

[1326] _Journ. Bot._ LVIII. pp. 213-9; 262-7, 1920.

[1327] _Bot. Memoirs_, 3, Oxford, 1919.

[1328] Church _in litt._

[1329] _Journ. Bot._ _l.c._

[1330] See p. 271 _ante_.

[1331] See p. 302 _ante_.

[1332] _Chemist and Druggist_, XCII. pp. 25-26, 1920; _Bot. Abstracts_,
N. 903, p. 135, 1920.




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INDEX


  _Abrothallus_ De Not., 267
    _A. Cetrariae_ Kotte, 264
    _A. oxysporus_ Tul., 263
    _A. Peyritschii_ Kotte, 264
    _A. Smithii_ Tul., 263

  _Acanthothecium_ Wain., 322

  _Acarinae_, 271, 397

  _Acarospora_ Massal., 183, 331, 390
    _A. chlorophana_ Massal., 374, 375, 390
    _A. glaucocarpa_ Koerb., 176
    _A. Heppii_ Koerb., 377
    _A. pruinosa_ (Sm.), 377
    _A. smaragdula_ Massal., 388, 393
    _A. xanthophana_ (Nyl.), 242

  Acarosporaceae, 310, 331

  _Acarus_, 395

  Acharius, 1, 10, 123, 126, 133, 141, 149, 156, 185, 192, 304

  _Acolium_, S. F. Gray, 277

  _Acrocordia gemmata_ Koerb., 152 (Fig. 90 B)

  _Acroscyphus_, Lév., 320
    _A. sphaerophoroides_ Lév., 289

  _Actinoplaca_ Müll.-Arg., 327

  Acton, xix, 57

  Adanson, 9

  Aesculus, 253

  Agardh, C. A., xx, 21

  _Agyrium flavescens_ Rehm, 266

  Aigret, 125, 371, 384

  _Alectoria_ Ach., 85, 94, 101, 103, 200, 257, 300, 340, 346, 350, 352
    _A. implexa_ Nyl., 227
    _A. jubata_ Ach., 3, 111, 401, 411
    _A. nigricans_ Nyl., 346, 389
    _A. ochroleuca_ Ach., 227, 389
    _A. thrausta_ Ach., 105 (Fig. 60)

  Alectoriaceae, 339

  _Allarthonia_ Nyl., 321

  _Allarthothelium_ Wain., 321

  Allescher, 201

  Almquist, 262

  Ambergris, 419

  Amoreux, 10, 407, 415, 418, 420

  _Amphidium_ Nyl., 335

  _Amphiloma_ Koerb., 325

  _Anabæna_ Bory, 41

  _Anaptychia_ Koerb., 341 (_see_ _Physcia_)

  _Anapyrenium_ Müll.-Arg., 315

  _Anema_ Nyl., 333, 373

  Angiocarpeae, 156

  _Anthoceros_ L., 41

  _Anthracothecium_ Massal., 316, 350

  _Anzia_ Stiz., 90, 299, 339
    _A. colpodes_ Stiz., 90
    _A. japonica_ Müll.-Arg., 90

  Archer, 28

  _Arctomia_ Th. Fr., 334

  _Argopsis_ Th. Fr., 105, 135, 297, 330

  Arnold, 18, 261, 342, 343, 364, 368, 370, 407

  _Arnoldia minutula_ Born., 190 (Fig. 108)

  Arnott, Walker, 15

  Artari, 39, 42

  _Arthonia_ Ach., 158, 203, 278, 305, 321, 343, 361
    _A. astroidea_ Ach., 202
    _A. cinnabarina_ Wallr. (_see_ _A. gregaria_), 349
    _A. dispersa_ Nyl., 365
    _A. gregaria_ Koerb., 247, 248
    _A. lecideella_ Nyl., 365
    _A. pruinosa_ Ach., 145
    _A. radiata_ Ach., 78, 365
    _A. subvarians_ Nyl., 262

  Arthoniaceae, 59, 278, 309, 321

  _Arthoniopsis_ Müll.-Arg., 321

  _Arthopyrenia_ Massal., 30, 316
    _A. fallax_ Arn., 365
    _A. halizoa_ A. L. Sm., 383
    _A. halodytes_ Oliv., 383
    _A. leptotera_ A. L. Sm., 383
    _A. macrospora_ Fink, 365
    _A. marina_ A. L. Sm., 383
    _A. punctiformis_ Arn., 346, 365
    _A. quinqueseptata_ Fink, 365

  _Arthotheliopsis_ Wain., 327

  _Arthothelium_ Massal., 321

  Ascolichens, 272, 273, 281, 308, 311

  Ascomycetes xix, 178 _et passim_

  _Ascophanus carneus_ Boud., 180

  _Aspergillus_ Micheli, 220

  _Aspicilia_ Massal., 133, 136, 140 (_see_ _Lecanora_)
    _A. atroviolacea_ (Flot.) Hue, 158
    _A. flavida_ (Hepp), 248

  Aspidoferae, 9

  _Aspidopyrenium_ Wain., 314

  _Aspidothelium_ Wain., 314

  _Asteristion_ Leight., 337

  _Asteroporum_ Müll.-Arg., 316

  _Asterothyrium_ Müll.-Arg., 327

  Astrotheliaceae, 309, 317, 352

  _Astrothelium_ Trev., 317

  Athalami, 305

  _Aulaxina_ Fée, 322

  _Azolla_ Laur., 41


  Babikoff, 138

  Babington, 18, 350

  Bachmann, E., 35, 75, 76, 215, 216, 235, 247, 347, 393

  Bachmann, Freda, 162, 179, 181, 186

  _Bacidia_, De Not., 329
    _B. acclinis_ (Flot.), 248
    _B. Beckhausii_ Koerb., 262
    _B. flavovirescens_ Anzi, 280
    _B. fuscorubella_ Arn., 249, 365
    _B. inundata_ Koerb., 372, 373, 377, 391, 392
    _B. muscorum_ Mudd, 248, 368, 370, 377
    _B. rubella_ Massal., 365

  _Bacotia sepium_, 399

  _Baeomyces_ Pers., 123, 293, 294, 330
    _B. paeminosus_ Krempelh., 55
    _B. placophyllus_ Ach., 293, 368
    _B. roseus_ Pers., 123, 167, 195, 218, 247, 362, 367, 368, 369
    _B. rufus_, DC., 123, 167, 177, 218, 237, 240, 362, 368, 369

  Baranetzky, 24

  Bary, de, 24, 31, 187, 209, 213

  Bauhin, J., 419

  Bauhin, K., 3

  Baur, 51, 115, 118, 124, 161, 165, 167, 168, 169, 170, 172, 173, 174,
      176, 177, 180, 181, 185, 255

  Beckmann, 230, 257

  Beechey, 15

  Beetle-mites, 397

  Beijerinck, 39, 220

  Beilstein, 211

  _Belonia_ Koerb., 316

  Berg, 211

  Berkeley, 252, 404

  Berzelius, 210

  _Betula nana_ L., 95

  Bialosuknia, 57

  _Biatora_ Koerb., 158, 279, 293 (_see also_ _Lecidea_), 391

  _Biatorella_ Th. Fr., 331
    _B. cinerea_ Th. Fr., 375
    _B. pruinosa_ Mudd, 217 (Fig. 119)
    _B. resinae_ Th. Fr., 355
    _B. simplex_ Br. and Rostr., 217 (Fig. 118)
    _B. testudinea_ Massal., 375

  _Biatorina_ Massal., 245, 291
    _B. Bouteillei_ Arn., 363
    _B. chalybeia_ Mudd, 386
    _B. coeruleonigricans_ A. L. Sm., 367
    _B. globulosa_ Koerb., 378
    _B. lenticularis_ Koerb., 383
    _B. prasina_ Syd., 33, 61
    _B._ (_denigrata_) _synothea_ Koerb., 33, 204

  _Bilimbia, aromatica_ Jatta, 349
    _B. incana_ A. L. Sm., 343
    _B. microcarpa_ Th. Fr., 262
    _B. obscurata_ Th. Fr., 262
    _B. sabulosa_ Massal., 370
    _B. sphaeroides_ Koerb., 385

  Bioret, 320

  Birger, _see_ Nilson

  Bitter, 64, 79, 94, 97, 131, 140, 143, 147, 148, 149, 151, 176, 240,
      242, 253, 257, 261, 267, 337, 397

  Blackman, 206

  Blackman and Welsford, 179

  _Blastenia_ Th. Fr., 340

  _Blastodesmia_ Massal., 316

  Bohler, 415

  _Bombyliospora_ De Not., 329

  Bonnier, 29, 36, 47, 65, 189, 232, 253

  Bornet, 27, 28, 32, 36, 61, 78, 136, 189

  Borrer, 12, 14

  _Borrera_, _see_ _Physcia_

  Borzi, 28, 161, 164

  _Botrydina vulgaris_ Bréb., xix, 57

  _Botrydium pyriforme_ Kütz., 45

  _Bottaria_ Massal., 317

  Bouilhac, 42, 140

  Braconnot, 214

  Brandt, 103, 130

  Braun, Fr., 354

  Braun, L., 393

  Brefeld, 189, 207

  Brez, 395

  Brooks, F. T., 64, 179

  Brown, E. W., 402

  Brown, W. H., 168

  _Bryopogon_, _see_ _Oropogon_

  _Bryum_ L., 392

  Buchet, 90

  Buddle, 4

  _Buellia_, De Not., 263, 280, 291, 302, 308, 341, 347
    _B. aethalea_ Th. Fr., 261
    _B. atrata_ Mudd, 245, 375
    _B. canescens_ De Not., 80, 366, 377, 380, 399
    _B. colludens_ Tuck., 382, 386
    _B. coracina_ Koerb., 375
    _B. discolor_ Koerb., 388
    _B. leptocline_ Koerb., 374
    _B. myriocarpa_ Mudd, 50, 346, 366, 369
    _B. parasema_ Th. Fr., 365, 367, 377
    _B. Parmeliarum_ Oliv., 263
    _B. punctiformis_, 50, 202, 207 (Fig. 118)
    _B. ryssolea_ A. L. Sm., 380, 382 (Fig. 125)
    _B. stellulata_ Mudd, 382, 388
    _B. triphragmia_ Th. Fr., 390
    _B. turgescens_ Tuck., 367
    _B. verruculosa_ Mudd, 261

  Buelliaceae, 311, 341

  Buxbaum, 6, 10

  _Buxus sempervirens_ L., 353


  Cactus, 325, 353

  _Calenia_ Müll.-Arg., 338

  Caliciaceae, 62, 115, 175, 189, 244, 288, 309, 319, 353, 366

  _Calicium_ De Not., 184, 201, 277, 319, 361
    _C. arenarium_ Nyl., 376
    _C. corynellum_ Ach., 376
    _C. hyperellum_ Ach., 349, 365
    _C. parietinum_ Ach., 202, 367
    _C. trachelinum_ Ach., 196, 202, 204

  Calkins, 348, 403

  _Callopisma_, _see_ _Placodium_

  _Calluna_ Salisb., 95, 355

  _Caloplaca_ Th. Fr. (_see_ _Placodium_), 340
    _C. aurantia_, var. _callopisma_ Stein., 190
    _C. gilvella_ (Nyl.), 276
    _C. interveniens_ Müll.-Arg., 276
    _C. pyracea_ Th. Fr., 34, 388

  Caloplacaceae, 311, 340

  _Calothricopsis_ Wain., 333

  _Calycidium_ Stirt., 289, 320
    _C. cuneatum_ Stirt., 350

  _Camellia_ L., 269

  Camerarius, 1

  _Camillea_ Fr., 276

  _Campylidium_, 191

  _Campylothelium_ Müll.-Arg., 317

  _Candelaria_ Massal., 339
    _C. concolor_ Wain., 365, 388, 399

  _Candelariella_ Müll.-Arg., 338
    _C. cerinella_ A. Zahlbr., 390
    _C. vitellina_ Müll.-Arg., 233, 237, 369, 377, 393, 417

  _Capnodium_ Mont., 179

  _Carpinus_ Tournef., 240

  Carrington, 12

  Carroll, 19

  Cassini, 21

  _Catillaria_ Th. Fr. (_see_ _Biatorina_), 329
    _C. Hochstetteri_ Koerb., 375

  Celidiaceae, 265

  _Cellidium stictarum_ Tul., 267

  _Cenomyce_ Th. Fr., 295

  _Cephaleuros_ Kunze (_see_ _Mycoidea_), 59, 288

  Cephaloidei, 303

  _Cepteus ocellatus_, 397

  _Cerania_ S. F. Gray, 340
    _C. vermicularis_ S. F. Gray, 194, 387

  _Cetraria_ Ach., 84, 94, 200, 210, 213, 225, 241, 264, 299, 346, 350,
      357, 358, 370, 388, 399
    _C. aculeata_ Fr., 211, 241, 262, 299, 300, 355, 369, 384, 385, 386,
      387
    _C. caperata_ Wain., 264
    _C. crispa_ Lamy, 387, 388
    _C. cucullata_ Ach., 201, 244, 389
    _C. diffusa_ A. L. Sm., 366
    _C. glauca_ Ach., 201, 231, 259, 264, 347, 388, 418
    _C. islandica_ Ach., 2, 94, 128, 195 (Fig. 112), 210, 212, 221, 227,
      231, 241, 338, 355, 387, 401 (Fig. 128), 406, 408, 409, 411, 416
    _C. juniperina_ Ach., 201, 246, 416
    _C. Laureri_ Kremp., 364
    _C. nivalis_ Ach., 201, 210, 389
    _C. pinastri_ S. F. Gray, 145, 246, 410
    _C. tristis_, _see_ _Parmelia_

  _Chaenotheca_ Th. Fr., 201, 319
    _C. chrysocephala_ Th. Fr., 265, 277, 288

  Chalice-Moss, 3

  Chambers, 43

  _Chasmariae_, 295

  Chevalier, 13

  _Chiodecton_ Müll.-Arg., 276, 320, 323, 351, 364

  Chiodectonaceae, 59, 278, 309, 323

  _Chlorella_ Beij., 56
    _Ch. Cladoniae_ Chod., 56
    _Ch. faginea_ Wille, 56 (Fig. 23 A)
    _Ch. lichina_ Chod., 56
    _Ch. miniata_ Wille, 56 (Fig. 23 A)
    _Ch. viscosa_ Chod., 56
    _Ch. vulgaris_ Beyer., 42, 56

  _Chlorococcus_ (?Chlorococcum Fr.), 24

  Chlorophyceae, xix, 51, 55-60, 61, 272, 324

  Chodat, 28, 30, 43, 44, 55, 115, 329

  Chroococcaceae, 25

  _Chroococcus_ Naeg., 24, 52, 82, 136, 153, 284, 311, 332, 373
    _Ch. giganteus_ West, 52 (Fig. 16)
    _Ch. Schizodermaticus_ West, 52 (Fig. 16)
    _Ch. turgidus_ Naeg., 52 (Fig. 16), 136

  _Chroolepus_ Ag., _see_ _Trentepohlia_
    _C. ebeneus_ Ag., 22

  Chrysothricaceae, 57, 310, 325

  _Chrysothrix_ Mont., 325, 353
    _C. noli tangere_ Mont., 325

  Church, A. Henry, 421

  Church, A. Herbert, 402

  _Cicinnobolus_ Ehrenb., 261

  _Cinchona_ L., 364
    _C. cordaminea_ Humb., 364
    _C. cordifolia_ Mutis, 364
    _C. oblongifolia_ Mutis, 364

  Claassen, 34

  _Cladina_ Leight., 112, 122, 253, 292

  _Cladonia_ Hill, 9, 13, 23, 38, 44, 55, 56, 80, 81, 95, 104, 106, 172,
      213, 237, 241, 242, 257, 262, 329, 344, 346, 347, 355, 358, 372,
      375, 385, 391, 399, 408
    _Cl. agariciformis_ Wulf., 368
    _Cl. aggregata_ Ach., 120
    _Cl. alcicornis_ Floerk., 385, 386
    _Cl. alpestris_ Rabenh., 125, 211, 349, 369
    _Cl. alpicola_ Wain., 122
    _Cl. amaurocrea_ Schaer., 118
    _Cl. bellidiflora_ Schaer., 119
    _Cl. botrytes_ Willd., 173
    _Cl. caespiticia_ Floerk., 115, 124, 294, 296
    _Cl. cariosa_ Spreng., 113, 120, 295, 296, 368
    _Cl. cartilaginea_ Müll.-Arg., 122
    _Cl. ceratophylla_ Spreng., 122
    _Cl. cervicornis_ Schaer., 113, 120, 122, 243, 384, 387
    _Cl. coccifera_ Willd., 113, 118, 368, 369, 370, 387
    _Cl. cristatella_ Tuck., 367, 369
    _Cl. decorticata_ Spreng., 172 (Fig. 98)
    _Cl. deformis_ Hoffm., 226
    _Cl. degenerans_ Floerk., 114, 117, 124
    _Cl. destricta_ Nyl., 387
    _Cl. digitata_ Hoffm., 113, 122, 371
    _Cl. divaricata_ Meng. and Goepp., 355
    _Cl. enantia_ f. _dilatala_ Wain., 112
    _Cl. endiviaefolia_ Fr., 384
    _Cl. fimbriata_ Fr., 51, 117, 120, 295, 296, 349, 367, 368, 370, 377;
      Subsp. _fibula_ Nyl., 119, 369
    _Cl. flabelliformis_ Wain., 371
    _Cl Floerkeana_ Fr., 173, 296, 362, 370
    _Cl. foliacea_ Willd., 112, 113, 120, 122, 240, 295, 296
    _Cl. furcata_ Schrad., 117 (Fig. 70), 118, 124, 194 (Fig. 109), 212,
      295, 297, 355, 368, 369, 377, 386
    _Cl. gracilis_ Hoffm., 115 (Fig. 68), 122, 124, 210, 297, 367, 369, 387
    _Cl. leptophylla_ Floerk., 295, 296
    _Cl. macilenta_ Hoffm., 362, 366, 367, 369, 378
    _Cl. miniata_ Mey., 112, 122
    _Cl. nana_ Wain., 112
    _Cl. Neo-Zelandica_ Wain., 112
    _Cl. papillaria_ Hoffm., 195, 296, 344
    _Cl. pityrea_ Floerk., 255, 366
    _Cl. pungens_ Floerk. (_see_ _Cl. rangiformis_)
    _Cl. pycnoclada_ Nyl., 345
    _Cl. pyxidata_ Hoffm., 2, 44, 110, 111 (Fig. 66), 113, 114, 117 (Fig.
      69), 118, 120, 124, 172, 227, 295, 346, 349, 362, 366, 368, 370,
      371, 377, 408
    _Cl. racemosa_ Hoffm., 387
    _Cl. rangiferina_ Web., 56, 95, 117, 119, 120, 210, 211, 215, 227, 231,
      237, 238, 253, 267, 293, 297, 349, 355, 357, 369, 386, 388, 400, 411
    _Cl. rangiformis_ Hoffm., 271, 295, 366, 368, 386
    _Cl. retepora_ Fr., 117, 120 (Fig. 71), 231, 351
    _Cl. rosea_ Ludw., 354
    _Cl. solida_ Wain., 114
    _Cl. squamosa_ Hoffm., 113, 115 (Fig. 67), 118, 210, 243, 295, 366, 368
    _Cl. sylvatica_ Hoffm., 95, 112, 117, 119, 271, 349, 366, 368, 369,
      385, 400
    _Cl. symphicarpia_ Tuck., 367
    _Cl. tophacea_ Hill, 8
    _Cl. turgida_ Hoffm., 369
    _Cl. uncialis_ Web., 112, 120, 369, 387, 389
    _Cl. verticillaris_ Fr., 122
    _Cl. verticillata_ Floerk., 114, 119, 120, 124, 349, 367, 369

  Cladoniaceae, 135, 292, 310, 329, 366, 370

  Cladoniodei, 306

  _Cladophora_ Kütz., 35, 59, 188
    _C. glomerata_ Kütz., 58 (Fig. 30)

  Cladophoraceae, 59

  _Clathrinae_, 117, 120

  _Clathroporina_ Müll.-Arg., 316

  _Clausae_, 295

  _Clavaria_ Vaill., 421

  _Cleora lichenaria_, 399

  _Cocciferae_, 295

  _Coccobotrys_ Chod., 30, 40, 56, 315
    _C. Verrucariae_ Chod., 57 (Fig. 24)

  _Coccocarpia_ Pers., 335
    _C. molybdaea_ Pers., 61
    _C. pellita_ Müll.-Arg., 166

  _Coccomyxa_ Schmidle, 56
    _C. Solorinae croceae_ Chod., 56
    _C. Solorinae saccatae_ Chod., 56
    _C. subellipsoidea_ Acton, 57 (Fig. 25)

  _Coccotrema_ Müll.-Arg., 316

  Coenogoniaceae, 59, 291, 310, 328

  _Coenogonium_ Ehrenb., 23, 35, 69, 182, 246, 291, 328, 351
    _C. ebeneum_ A. L. Sm., 22 (Fig. 3), 34, 59, 328, 350, 352, 363
    _C. implexum_ Nyl., 352
    _C. Linkii_ Ehrenb., 213

  Coenothalami, 303

  _Coleochaete_ Bréb., 178

  _Collema_ Wigg., 6, 9, 21, 23, 25, 30, 48, 69, 87, 132, 165, 173, 200,
      230, 284, 305, 334, 367, 392
    _C. ceranoides_ Borr., 385
    _C. cheileum_ Ach., 161
    _C. crispum_ Ach., 161, 180
    _C. flaccidum_ Ach., 365
    _C. fluviatile_ Sm., 392
    _C. granulatum_ Ach., 368
    _C. granuliferum_ Nyl., 69, 232, 243
    _C. Hildenbrandii_ Garov., 202 (_see_ _Leptogium_)
    _C. limosum_ Ach., xx, 21, 349
    _C. microphyllum_ Ach., 160 (Fig. 91), 161 (Fig. 92), 202
    _C. nigrescens_ Ach., 20 (Fig. 2), 161, 243, 245, 364
    _C. plicatile_, 409
    _C. pulposum_ Ach., 24, 162, 179, 186, 202, 266, 368, 385
    _C. pustulatum_ Ach., 373
    _C. pycnocarpum_ Nyl., 365
    _C. tenax_ Sm., 368

  Collemaceae, 27, 53, 69, 160, 241, 244, 266, 284, 306, 310, 334, 364,
      384, 396

  _Collemodes_ Fink, 162
    _C. Bachmannianum_ Fink, 162

  _Collemodium_, _see_ _Leptogium_

  _Collemopsidium_ Nyl., 333, 374

  _Collybia_, Quél., 105

  Colonna, 3

  _Combea_ De Not., 83

  _Conida_ Massal., 265, 267
    _C. rubescens_, Arn., 265

  _Conidella urceolata_ Elenk., 265

  Coniocarpi, 307

  Coniocarpineae, 267, 273, 274, 276, 288, 309, 319

  _Coniocarpon_ DC., 305

  _Coniocybe_ Ach., 277, 319, 366
    _C. furfuracea_ Ach., 246, 376

  _Conotrema_ Tuck., 326
    _C. urceolatum_ Tuck., 343

  _Convoluta roscoffensis_, 40

  _Cora_ Fr., 53, 246, 281, 311, 342, 352
    _C. Pavonia_ Fr., 88, 152 (Figs. 86, 87)

  Coralloides, 5, 6, 7, 303

  Corda, 200

  Cordus, 409

  _Cordyceps_ Fr., 261

  _Corella_ Wain., 153, 311, 342, 352
    _C. brasiliensis_ Wain., 154

  _Coriscium_ Wain., 285, 288, 319

  _Cornicularia_ (_Cetraria_) Schreb., 388
    _C. ochroleuca_ Ach., 355
    _C. subpubescens_ Goepp., 355
    _C. succinea_ Goepp., 355

  _Corylus_ Tournef., 240

  Cramer, 409

  Croall, 19

  _Crocynia_ Nyl., 325
    _C. gossypina_ Nyl., 325
    _C. lanuginosa_ Hue, 325, 373

  Crombie, xxi, 7, 18, 19, 197, 260, 262, 264, 306, 361

  Crottles, 415

  _Cruoria_ Fr., 73

  _Cryptothecia_ Stirton, 331

  _Cryptothele_ Nyl., 333

  Cudbear, 413, 415

  Culpepper, 409

  Cunningham, 35, 269

  Cuppe-Moss, 3

  Cupthongs, 9

  Curnow, 19

  Cutting, 180

  Cyanophili, 308, 310

  Cyanophyceae, 309; _see_ Myxophyceae

  _Cycas_ L., 40

  Cyclocarpineae, 273, 279, 290, 309, 324

  _Cyperus_, 419

  Cypheliaceae, 309, 320

  _Cyphelium_ Th. Fr., 276, 277, 288, 320

  _Cyphella aeruginascens_ Karst., 191

  _Cystococcus_ Chod., 55, 56
    _C. Cladoniae fimbriatae_ Chod., 56
    _C. Cladoniae pixidatae_ Chod., 56 (Fig. 56)

  _Cystococcus_ Naeg., 24, 26, 28, 34, 115, 229
    _C. humicola_ Naeg., 24, 27, 40, 55

  _Cystocoleus_ Thwaites, 23

  _Cytospora_ Ehrenb., 204

  Czapek, 211, 413


  _Dacampia_ Massal., 315

  _Dactylina_ Nyl., 340
    _D. arctica_ Nyl., 339, 346

  Dangeard, 185

  Danilov, 37

  Darbishire, 18, 26, 51, 64, 77, 86, 90, 92, 101, 103, 110, 130, 147,
      148, 166, 167, 171, 175, 180, 181, 253, 256, 299, 324, 342, 346,
      347, 377, 389

  Darbishire and Fischer-Benzon, 307

  _Darbishirella_ A. Zahlbr., 324

  Davies, 12, 14

  Dawson, 178

  De Candolle, 12

  Deckenbach, 59

  Deer, 401

  Delise, 13, 126

  _Dendrographa Darbish._, 324
    _D. leucophaea Darbish._, 103, 213

  _Dermatiscum_ Nyl., 331

  Dermatocarpaceae, 309, 314

  _Dermatocarpon_ Eschw., 80, 81, 276, 288, 315
    _D. aquaticum_ A. Zahlbr., 391, 392
    _D. cinereum_ Th. Fr., 368
    _D. hepaticum_ Th. Fr., 368, 388
    _D. lachneum_ A. L. Sm., 88, 368
    _D. miniatum_ Th. Fr., 56, 96 (Fig. 56), 173 (Fig. 99), 185, 241, 261,
      373, 391, 392, 403

  Desfontaines, 10

  Diatoms, 220

  _Dichodium_ Nyl., 334

  Dickson, 9

  _Dictyographa_ Müll.-Arg., 322

  _Dictyonema_ A. Zahlbr., 54, 153, 311, 342, 352

  _Didymella_ Sacc., 276

  _Didymosphaeria pulposi_ Zopf, 266

  Dillenius, xx, 1, 6, 155, 192, 262, 304, 407

  Dioscorides, 2

  _Diplogramma_ Müll.-Arg., 322

  Diplopodon, 270

  Diploschistaceae, 310, 326

  _Diploschistes_ Norm., 326
    _D. bryophilus_ Zahlbr., 374
    _D. ocellatus_ Norm., 247, 248, 374
    _D. scruposus_ Norm., 195, 214, 241, 243, 262, 368

  _Diplosphaera_ Bial., 57
    _D. Chodati_ Bial., 57

  _Dirina_ Fr., 73, 83, 290, 323

  Dirinaceae, 290, 309, 323

  _Dirinastrum_ Müll.-Arg., 290, 323

  Discocarpi, 307

  Discomycetes, 267, 273

  Dodoens, 3

  Dog-lichen, 408

  Domestic animals (Oxen, horses, etc.), 401

  Don, 14

  Doody, 4

  Dorstenius, 2, 408

  _Dothidea_ Fr., 317

  Dufour, 11

  _Dufourea_ Nyl., 340

  Dufrenoy, 42, 260, 269

  _Dumontia_ Lamour., 111

  Dundonald, Lord, 420


  Ectolechiaceae, 69, 310, 327, 352, 363

  Egeling, 234

  _Elaphomyces_ Nees, 261

  Elenkin, 36, 37, 258, 265, 347

  Elenkin and Woronichin, 353

  Elfving, xxi, 25

  _Encephalographa_ Massal., 322

  _Enchylium_ Massal., _see_ _Forssellia_

  _Endocarpon_ Hedw., 62, 88, 89, 197, 200, 261, 288, 315, 351, 389
    _E. monstrosum_ Massal., 373
    _E. pusillum_ Hedw., 28 (Figs. 5, 6)

  _Endocena_ Cromb., 339, 340

  _Endomyces scytonemata_ Zuk., 38

  Englehardt, 354

  _Enterodictyon_ Müll.-Arg., 323

  _Enterographa_ Fée, 320
    _E. crassa_ Fée, 350

  _Enterostigma_ Müll.-Arg., 323

  _Erioderma_ Fée, 335

  _Eolichen_ Zuk., 285, 319
    _E. Heppii_ Zuk., 319

  Ephebaceae, 54, 284, 310, 331

  _Ephebe_ Fr., 23, 25, 27, 30, 38, 68, 201, 284, 322
    _E. lanata_ Wain., _see_ _E. pubescens_
    _E. pubescens_ Nyl., 23 (Fig. 3)

  _Ephebeia_ Nyl., 332

  Epiconiaceae, 307

  Epiconiodei, 306

  _Epigloea_ Zuk., 313
    _E. bactrospora_ Zuk., 313

  Epigloeaceae, 57, 309, 313

  _Erica tetralix_ L., 95

  Errera, 213, 214, 405

  _Erysiphe_ Link, 188

  Eschweiler, 15, 184

  Escombe, 210

  Etard and Bouilhac, 42, 140

  Ettingshausen and Debey, 354

  Euler, 214

  _Eunephroma_ Stiz., 337

  _Euopsis granatina_ Nyl., 282, 387

  _Evernia_ Ach., 84, 95, 99, 200, 213, 340
    _E. furfuracea_ Mann, 24, 38, 94, 99, 108, 142 (Fig. 81), 151, 227,
      231, 233, 300, 366, 376, 403, 405
    _E. prunastri_ Ach., 2, 100 (Fig. 59), 108, 210, 211, 212, 227, 233,
      234, 238, 269, 300, 364, 384, 385, 396, 400, 403, 418, 419

  _Everniopsis_ Nyl., 339, 340

  Eversman, 404


  Famintzin, 24

  _Farriolla_ Norm., 319

  Faull, 178

  Fée, 13, 15, 184, 187, 192, 364

  Fink, Bruce, xx, 242, 254, 348, 358, 365, 367, 368, 369, 373, 389, 391

  Fischer, 308

  Fitting, 36

  Fitzpatrick, 181

  Flagey, 373, 389

  Florideae, 160, 177, 273

  Flörke, 12, 13, 133

  Flotow, 23, 192

  _Fontinalis_ L., 391

  _Forficula auricularia_, 396

  Forskål, 403

  Forssell, 63, 65, 133, 136, 163, 175, 282, 373

  _Forssellia_ A. Zahlbr., 284, 333, 373

  Forster, 12, 14

  Fossil Lichens, 353-355

  Frank, 31, 62, 78

  Fraser, 178

  French, xxiii

  Friedrich, 75, 233, 269, 270

  Fries, E., 13, 22, 149, 364

  Fries, Th. M., 17, 18, 133, 138, 152, 192, 263, 342

  _Fucus_ L., 281
    _F. spiralis_ L., 383

  Fuisting, 30, 159, 173

  Fünfstück, 18, 19, 61, 75, 76, 161, 169, 170, 171, 175, 181, 216, 218,
      219, 224, 342


  Gage, 14

  Galløe, 95, 242

  Gargeaune, 45

  Gasterolichens, 308

  Gautier, 213

  _Geisleria_ Nitschke, 314
    _G. sychnogonioides_ Nitschke, 370

  Georgi, 10, 420

  _Geosiphon_ Wettst., 45

  Gerard, John, 3, 418

  Gibelli, 200

  Gilson, 209

  Gleditsch, 269

  _Gloeocapsa_ Kütz., 23, 32, 55, 61, 68, 136, 195, 232, 284, 292, 332, 373
    _G. magma_ Kütz., 52 (Fig. 17), 60, 136
    _G. polydermatica_ Kütz., 53

  _Gloeocystis_ Naeg., 33, 57 (Fig. 28), 61, 133, 318

  Gloeolichens, 175, 282, 284, 373, 389

  _Glossodium_ Nyl., 330
    _G. aversum_ Nyl., 294

  Glück, 198

  _Glyphis_ Fée, 276, 323

  _Glypholecia_ Nyl., 331

  Gmelin, J. F., 152

  Gmelin, J. G., 411

  _Gnomonia erythrostoma_ Auersw., 178

  Goeppert, 354, 393

  Goeppert and Menge, 354

  _Gomphillus_ Nyl., 293, 330

  _Gongrosira_ Kütz., xxi

  _Gongylia_ Koerb., 314
    _G. viridis_ A. L. Sm., 368, 388

  _Gonohymenia_ Stein., 333

  _Gonothecium_ Wain., 31, 327

  Gordon, Cuthbert, 415

  _Gossypina Ulmi_, 399

  Grammophori, 307

  Graphidaceae, 59, 158, 309, 321, 351, 352, 364

  Graphideae, 13, 17, 27, 34, 62, 78, 79, 172, 348, 349, 351, 353, 364

  Graphidineae, 273, 278, 289, 309, 320, 365

  _Graphina_ Müll.-Arg., 322

  _Graphis_ Adans., 9, 211, 321, 322, 343, 349, 351, 355, 361, 364
    _G. elegans_ Ach., 30, 158 (Fig. 89), 172, 180, 397
    _G. scripta_ Ach., 50, 349, 354, 365, 366
    _G. scripta succinea_ Goepp., 355

  Gray, J. E., 12, 305

  Grete Herball, 2

  Greville, 12

  Grimbel, 250

  _Grimmia pulvinata_ Sm., 393
    _G. apocarpa_ Hedw., 393

  Guembel, 392

  Guérin-Varry, 210

  Guillermond, 167

  _Gunnera_ L., 31, 41

  _Gyalecta_ Ach., 191, 328
    _G. cupularis_ Schaer., 244
    _G. Flotovii_ Koerb., 244
    _G. geoica_ Ach., 254
    _G. rubra_ Massal., 249

  Gyalectaceae, 54, 59, 69, 310, 327

  _Gyalolechia_ Massal., 201
    _G. subsimilis_ (Th. Fr.) Darb., 378

  Gymnocarpeae, 156, 308, 318

  _Gymnoderma_ Nyl., 330
    _G. coccocarpum_ Nyl., 293

  _Gymnographa_ Müll.-Arg., 322

  _Gyrophora_ Ach., 88, 96, 184, 200, 227, 231, 241, 249, 268, 304, 331,
      346, 350, 376, 390, 393, 414
    _G. cylindrica_ Ach., 176, 184 (Fig. 103), 375, 387
    _G. erosa_ Ach., 330, 387
    _G. esculenta_ Miyosh., 403
    _G. flocculosa_ Turn. and Borr., 375
    _G. murina_ Ach., 94
    _G. polyphylla_ Hook., 387
    _G. polyrhiza_ Koerb., 94, 349, 404 (Fig. 129)
    _G. proboscidea_ Ach., 192, 346, 375
    _G. spodochroa_ Ach., 94
    _G. tonefacta_ Cromb., 375, 387
    _G. vellea_ Ach., 74, 176

  Gyrophoraceae, 291, 310, 330

  _Gyrostomum_ Fr., 326


  Haberlandt, 106, 188

  _Haematomma_ Massal., 230, 236, 338
    _H. coccineum_ Koerb., 214, 223, 226
    _H. elatinum_ Koerb., 201
    _H. ventosum_ Massal., 214, 225, 241, 252, 298, 375, 376, 388, 393

  _Hagenia ciliaris_, 24

  Haller, 7, 126

  _Halopyrenula_ Müll.-Arg., 318

  Halsey, 14

  Hamlet and Plowright, 213

  Harmand, 63

  Harper, 167, 178, 181, 188

  _Harpidium_ Koerb., 298, 338
    _H. rutilans_ Koerb., 298

  Harriman, 14

  _Hassea_ A. Zahlbr., 319

  Hedlund, 32, 61, 204, 245

  Hedwig, 142, 156, 184, 192

  _Helix hortensis_, 396
    _H. cingulata_, 396

  Hellbom, 350, 411

  _Helminthocarpon_ Fée, 322

  Henneguy, 410, 411, 420

  _Heppia_ Naeg., 81, 175, 285, 335, 348, 351, 389
    _H. Depreauxii_ Tuck., 368
    _H. Guepini_ Nyl., 80, 88, 96
    _H. virescens_ Nyl., 368

  Heppiaceae, 54, 285, 310

  Herberger, 221

  Herissey, 213

  Herre, 230, 253, 349

  Hesse, 12, 221, 224

  _Heterocarpon_ Müll.-Arg., 315

  _Heterodea_ Nyl., 339
    _H. Mülleri_ Nyl., 128, 299, 339, 350

  Heterogenei, 303

  _Heteromyces_ Müll.-Arg., 293, 330

  _Heufleria_ Trev., 317

  Hicks, 24

  _Hildenbrandtia_ Nardo, 73

  Hill, Sir John, 8, 409

  Hoffmann, 10, 154, 412, 415

  Hofmann, 261

  Holl, 19

  Holle, 14, 46, 187

  Holmes, 19, 422

  Homogenei, 303

  _Homopsella_ Nyl., 334

  Homothalami, 305

  _Homothecium_ Mont., 334

  Hooker, 12, 15, 149

  Hornschuch, xx, 156

  How, 3

  Howe, Heber, 85, 224, 348

  Hudson, 7, 9, 303

  Hue, 11, 16, 18, 33, 57, 63, 69, 73, 82, 85, 103, 133, 135, 136, 140,
      188, 262, 283, 315, 325, 339, 340, 342, 347, 348, 360, 396, 418

  Hulth, 215

  Hutchins, 14

  Hutchinson, 403

  _Hydrothyria_ Russ., 336
    _H. venosa_ Russ., 97, 175, 233, 286, 348, 390

  _Hymenobolina parasitica_ Zuk., 267, 399

  Hymenolichens, xix, 54, 152-154, 273, 281, 308, 311, 335, 342

  Hymenomycetes, xix, 153 _et passim_

  Hyphomycetes, xix, 191

  _Hypnum_ L., 392
    _H. cupressiforme_ L., 385

  _Hypogymnia_ Nyl., 94, 176

  _Hypoxylon_ Bull., 12

  Hysteriaceae, 273, 307

  _Hysterium_ Tode, 12


  Iceland Moss, 210, 401 _et passim_

  _Icmadophila_ Massal., 166, 338
    _I. aeruginosa_ Mudd, _see_ _I. ericetorum_
    _I. ericetorum_ A. Zahlbr., 196, 244, 370

  _Illosporium carneum_ Fr., 268

  _Ingaderia_ Darbish., 324

  Iris, white, 419

  _Isidium_ Ach., 149
    _I. corallinum_ Ach., 149
    _I. Westringii_ Ach., 149

  Istvanffi, 202, 206

  Itzigsohn, 17, 23, 24, 193


  Jaczewski, 353

  Jasmine, oil of, 419

  Jatta, 129

  _Jenmania_ Wächt., 333, 352

  Jennings, Vaughan, 60

  Jesuit’s bark, 10

  John, 250

  Johnson, C. P., 401, 402

  Johnson, W., 19

  Johow, 153

  _Jonaspis_ Th. Fr., 328

  Joshua, 19

  Jumelle, 230, 238


  Kajanus (Nilson), 151

  _Karschia_ Koerb., 280
    _K. destructans_ Tobl., 265
    _K. lignyota_ Sacc., 280

  Keeble, 41

  Keegan, 224, 410

  Keiszler, 201

  Keller, 402

  Kerner and Oliver, 215

  Kieffer, 371

  Kienitz-Gerloff, 51

  Kihlman, 237, 358, 388, 401

  Knop, 213, 247

  Knop and Schnederman, 221

  Knowles, 224, 249, 379, 384, 391

  Kobert, 409, 410

  Koelreuter, 155

  Koerber, 14, 123, 142, 188, 305

  _Koerberia_ Massal., 334

  Kotte, 264

  Krabbe, 63, 113, 114, 119, 122, 123, 124, 143, 147, 162, 170, 172, 174,
      176, 177, 253

  Kratzmann, 214

  Krempelhuber, 1, 55, 244, 364

  Kupfer, 261

  Kützing, 22


  _Laboulbenia_ Mont. and Robin, 178

  Laboulbeniaceae, 178, 274

  _Lachnea scutellata_ Gill., 168
    _L. stercorea_ Gill., 178

  Lacour, 211

  Lang, 76, 216, 235

  Larbalestier, 19

  Laubert, 206

  _Laudatea_ Joh., 154

  _Laurera_ Reichenb., 317

  Lecanactidaceae, 310, 325

  _Lecanactis_ Eschw., 204, 325

  _Lecania_ Massal., 136, 338
    _L. candicans_ A. Zahlbr., 80 (Fig. 43)
    _L. cyrtella_ Oliv., 377
    _L. erysibe_ Mudd, 377
    _L. holophaea_ A. L. Sm., 350

  _Lecaniella_ Wain., 327

  _Lecanora_ Ach., 78, 88, 200, 298, 305, 338, 347, 349, 351, 353, 364,
      365, 372, 390
    _L. aquatica_ Koerb., 391
    _L. aspidophora_ f. _errabunda_ Hue, 262
    _L. atra_ Ach., 63, 225, 249, 375, 380, 382 (Fig. 125), 384, 386, 393
    _L. atriseda_ Nyl., 261
    _L. atroflava_, _see_ _Placodium_
    _L. aurella_ (Hoffm.), 262
    _L. badia_ Ach., 79, 375, 386
    _L. caesiocinerea_ Nyl., 218, 384
    _L. calcarea_ Somm., 218 (Fig. 120), 373, 396
    _L. campestris_ B. de Lesd., 361, 384
    _L. cenisia_ Ach., 375
    _L. cinerea_ Somm., 229, 349, 375
    _L. citrina_ Ach., _see_ _Placodium_
    _L. coilocarpa_ Nyl., 30
    _L. crassa_ Ach., 79, 81, 201, 218, 367, 368, 373, 389
    _L. crenulata_ Hook., 361, 377
    _L. Dicksonii_ Nyl., 250, 375
    _L. dispersa_ Nyl., 261, 369, 377, 384
    _L. effusa_ Ach., 204
    _L. epanora_ Ach., 246
    _L. epibryon_ Ach., 378, 389
    _L. epulotica_ Nyl., 392
    _L. esculenta_ Eversm., 211, 257, 265, 298, 389, 404 (Fig. 130), 422
    _L. exigua_, _see_ _Rinodina_
    _L. ferruginea_ Nyl., 30
    _L. galactina_ Ach., 254, 262, 360, 369, 377, 384, 386
    _L. gelida_ Ach., 135, 136, 137 (Fig. 77), 140, 375
    _L. gibbosa_ Nyl., 375, 384, 386
    _L. glaucoma_ Ach., _see_ _L. sordida_; var. _corrugata_ Nyl., 84
      (Fig. 46)
    _L. Hageni_ Ach., 366, 367, 369, 377, 383
    _L. hypnorum_ Ach., _see_ _Psoroma_
    _L. lacustris_ Th. Fr., 233, 250, 391, 392
    _L. lentigera_ Ach., 81, 90, 298, 367
    _L. muralis_ Schaer., 242
    _L. ochracea_ Nyl., 373
    _L. pallescens_ Mudd, 213
    _L. pallida_ Schaer., 78
    _L. parella_ Ach., 72, 375, 382, 384, 417
    _L. peliocypha_ Nyl., 375
    _L. picea_ Nyl., 374
    _L. piniperda_ Koerb., 204
    _L. polytropa_ Schaer., 237, 376, 394
    _L. prosechoides_ Nyl., 383, 384
    _L. rubina_ Wain., 390
    _L. rugosa_ Nyl., 366
    _L. Sambuci_ Nyl., 204
    _L. saxicola_ Ach., 79, 80, 81, 233, 252, 349, 369, 384, 386, 393, 396
    _L. simplex_ Nyl. (_see_ _Biatorella_), 75, 77, 382
    _L. smaragdula_ Nyl., 382
    _L. sophodes_ Ach., 30; _see_ _Rinodina_
    _L. sordida_ Th. Fr., 194, 236, 261, 374, 375, 380, 382
    _L. squamulosa_ Nyl., 374
    _L. subfusca_ Ach., 22, 30, 49, 65 (Fig. 34), 70 (Fig. 57), 157 (Fig.
      88), 164, 166, 167, 168, 236, 347, 365, 366
    _L. sulphurea_ Ach., 226, 238, 376, 384
    _L. tartarea_ Ach., 57, 147, 183 (Fig. 102), 224, 225, 227, 237, 262,
      346, 358, 359, 371, 375, 387, 389, 414 (Fig. 134)
    _L. umbrina_ Massal., 377, 385
    _L. upsaliensis_ Nyl., 387
    _L. urbana_ Nyl., 361
    _L. varia_ Ach., 227, 346, 360, 361, 362, 366, 367, 377
    _L. ventosa_, _see_ _Haematomma_
    _L. verrucosa_ Laur., 378
    _L. xantholyta_ Nyl., 373

  Lecanoraceae, 136, 311, 337, 353

  Lecanorales, 297

  _Lecidea_ Ach., 78, 184, 261, 279, 292, 304, 308, 328, 346, 347, 349,
      351, 353, 364, 365, 372, 373, 385, 390
    _L. aglaea_ Somm., 375
    _L. albocoerulescens_ Ach., 392
    _L. alpestris_ Somm., 387
    _L. arctica_ Somm., 387
    _L. aromatica_ (_see_ _Bilimbia_)
    _L. atrofusca_ Nyl., 248, 387
    _L. auriculata_ Th. Fr., 375
    _L. Berengeriana_ Th. Fr., 387
    _L. coarctata_ Nyl., 247
    _L. coeruleonigricans_ Schaer., 373
    _L. colludens_ Nyl., 384; _see_ _Buellia_
    _L. confluens_ Ach., 375, 388; f. _oxydata_ Leight., 250
    _L. consentiens_ Nyl., 134, 135
    _L. contigua_ Fr., 375, 376, 388, 392; var. _flavicunda_ Nyl., 250
    _L. crustulata_ Koerb., 369
    _L._ (_Bilimbia_) _cuprea_ Somm., 387
    _L. cupreiformis_ Nyl., 387
    _L. decipiens_ Ach., 291, 367, 368
    _L. decolorans_ Floerk, _see_ _L. granulosa_
    _L. demissa_ Th. Fr., 369, 387
    _L. diducens_ Nyl., 375
    _L. enteroleuca_ Nyl., 164, 168, 365
    _L. fumosa_ Ach., 159
    _L. fuscoatra_ Ach., 200 (Fig. 114), 375
    _L. gelatinosa_ Floerk., 368
    _L. granulosa_ Schaer., 218, 237, 269, 291, 362, 369, 370, 377
    _L. grisella_ Floerk., 243
    _L. helvola_ Th. Fr., 245
    _L. herbidula_ Nyl., xxi
    _L. illita_ Nyl., 136
    _L. immersa_ Ach., 217 (Fig. 117), 398
    _L. inserena_ Nyl., 375
    _L. insularis_ Nyl., 236, 261
    _L. irregularis_ Fée, 192
    _L. Kochiana_ Hepp, 375
    _L. lapicida_ Ach., 375
    _L. lavata_ Nyl., 384
    _L. limosa_ Ach., 387
    _L. lucida_ Ach., 246, 376
    _L. lurida_ Ach., 79, 195, 241, 367
    _L. mesotropa_ Nyl., 375
    _L. Metzleri_ Th. Fr., 398
    _L. nigroclavata_ Nyl., 384
    _L. ostreata_ Schaer., 79, 145, 291, 366
    _L. pallida_ Th. Fr., 135
    _L. panaeola_ Ach., 134, 135, 136, 375
    _L. parasema_ Ach., 183 (Fig. 101), 366
    _L. pelobotrya_ Somm., 135, 136
    _L. phylliscocarpa_ Nyl., 31
    _L. phyllocaris_ Wain., 31, 327
    _L. plana_ Nyl., 375
    _L. platycarpa_ Ach., 375
    _L. pycnocarpa_ Koerb., 375
    _L. quernea_ Ach., 236, 349, 386
    _L. rivulosa_ Ach., 374, 375, 376
    _L. sanguinaria_ Ach., 187 (Fig. 105), 248
    _L. sanguineoatra_ Ach., 370
    _L. stellulata_ Tayl., 376
    _L. sulphurella_ Hedl., 242
    _L. sylvicola_ Flot., 372
    _L. testacea_ Ach., 195 (Fig. 111)
    _L. tricolor_ Nyl. (_Biatorina Griffithii_), 362
    _L. tumida_ Massal., 375
    _L. uliginosa_ Ach., 254, 291, 370, 385, 387
    _L. vernalis_ Ach., 66 (Fig. 35)

  Lecideaceae, 135, 241, 279, 291, 298, 310, 327, 328, 341, 346, 353

  Lecideales, 290, 308

  _Leciophysma_ Th. Fr., 334

  Leighton, 16, 17, 18, 19, 134, 306, 342, 353, 388

  _Leiosoma palmicinctum_, 397

  Lemming rats, 401

  _Lemmopsis_ A. Zahlbr., 334

  _Lenzites_ Fr., 261, 371

  Léorier, 411

  _Lepidocollema_ Wain., 81, 336

  Lepidoptera, 399

  _Lepolichen_ Trevis., 318
    _L. coccophora_ Hue, 57, 318
    _L. granulatus_ Müll.-Arg., 318

  _Lepra_ Hall., 143
    _L. viridis_ Humb., 23

  _Lepraria_ Ach., 143, 237, 305
    _L. botryoides_, xx
    _L. chlorina_, 376

  Leprieur, 15

  _Leprocollema_ Wain., 285, 354

  _Leproloma_ Nyl., 325

  _Leptodendriscum_ Wain., 284, 332

  _Leptogidium_ Nyl., 284, 332, 350
    _L. dendriscum_ Nyl., 332

  _Leptogium_ S. F. Gray, 69, 84, 87, 232, 285, 335, 370
    _L. Burgessii_ Mont., 245
    _L. byssinum_ Nyl., 368
    _L. Hildenbrandii_ Nyl., 364
    _L. lacerum_ S. F. Gray, 243, 254, 373
    _L. myochroum_ Nyl., 365
    _L. scotinum_ Fr., 385
    _L. turgidum_ Nyl., 385

  _Leptorhaphis_ Koerb., 263, 316

  Lesdain, Bouly de, 140, 270, 271, 366, 369, 376, 398

  _Letharia_ A. Zahlbr., 84, 340
    _L. vulpina_ Wain., 95, 105, 226, 228, 246, 265, 349, 364, 410, 417

  Lett, 19

  Lettau, 225, 227, 369, 391, 417

  Lichen, xxvi, 1, 5, 9, 303

  _Lichen albineus_ Ludw., 354

  _Lichen candelarius_ L., 371, 415

  _Lichen cinereus terrestris_, 407

  _Lichen dichotomus_ Engelh., 354

  _Lichen diffusus_ Ludw., 354

  _Lichen gelatinosus_ Rupp, 6

  _Lichen juniperinus_ L., 415

  _Lichen orbiculatus_ Ludw., 354

  _Lichen parietinus_ L., 371, 415

  _Lichen Roccella_ L., 415

  _Lichen saxatilis_ L., 415

  _Lichen tartareus_ L., 415

  _Lichen tenellus_ Scop., 371

  Lichenacei, 306

  _Lichenes Coralloidei_ etc. Hall., 7

  _Lichenodium_ Nyl., 334

  Lichenoides, 1, 6, 7, 304, 415

  _Lichenophoma_ Keisz., 201

  Lichenoxanthine, 418

  _Lichina_ Ag., 163, 195, 201, 233, 281, 284, 334, 383
    _L. confinis_ Ag., 383, 384
    _L. pygmaea_ Ag., 195, 201, 383

  Lichinaceae, 55, 99, 310, 333

  _Lichinella_ Nyl., 354

  Lightfoot, 9, 280, 303, 407, 415

  _Limax_, 396

  Lindau, 18, 34, 36, 48, 64, 67, 78, 108, 149, 164, 168, 170, 176, 178,
      184, 233, 269, 330

  Lindsay, xx, 16, 17, 19, 120, 193, 203, 252, 262, 266, 348, 354, 358,
      391, 401, 415, 417

  Link, 371

  Linkola, 141

  Linnaeus, 7, 142, 154, 304, 312, 392, 401, 409, 415

  Lister, 267

  _Listerella paradoxa_ Jahn, 267

  _Lithographa_ Nyl., 322

  _Lithoicea_ Massal., _see_ _Verrucaria_
    _L. lecideoides_ Massal., 373

  _Lithothelium_ Müll.-Arg., 317

  Litmus, 413

  _Lobaria_ Schreb., 136, 182, 287, 336
    _L. laciniata_ Wain., 133, 134
    _L. laetevirens_ A. Zahlbr., 2, 196
    _L. pulmonaria_ Hoffm., 2, 3, 10, 90, 96, 126 (Fig. 127), 130, 195,
      252, 267, 336, 400, 406, 408, 411, 416, 418
    _L. scrobiculata_ DC., 130, 143

  L’Obel, 2

  _Lopadiopsis_ Wain., 327

  _Lopadium_ Koerb., 191, 329

  _Lophothelium_ Stirt., 319

  Loxa (_Cinchona_), 364

  Ludwig, 354

  _Luffia lapidella_, 399

  Lung-wort, 406, 409

  Lutz, 108

  Luyken, xx, 156, 184

  Lycoperdaceae, 307

  Lyell, 14

  _Lyngbya_ Ag., 136


  Mackay, 13

  McLean, 385

  Macmillan, 357, 391

  Maheu, 243, 387

  Maire, 185, 186, 189

  Malinowski, 74, 371, 374

  Malme, 261

  Malpighi, 5, 142, 155

  Manna, 404, 422

  _Marchantia_ L., 1, 5

  _Maronea_ Massal., 331

  Martindale, 19

  Martius, 15

  _Massalongia_ Koerb., 287, 335

  Massalongo, 16, 188, 305

  Massee, 308

  _Mastoidea_ Hook. and Harv., 315

  Mastoidiaceae, 60, 309, 315

  Mattirolo, 152

  Mäule, 162, 164

  Mayfield, 368

  _Mazosia_ Massal., 59, 323

  Mead, Richard, 407

  _Megalospora_ Mey. and Flot., 329

  _Melampydium_ Müll.-Arg., 325

  _Melanotheca_ Müll.-Arg., 317

  _Melaspilea_ Nyl., 321, 322

  Mereschkovsky, 258

  Merrett, 3

  Metzger, 176, 240

  Meyer, 13, 46, 51, 126, 143, 156, 187, 252, 258, 305

  _Micarea_ Fr., _see_ _Biatorina_ Massal.

  Michael, 397

  Michaux, 14

  Micheli, 1, 6, 142, 155

  _Microcystis_ Kütz., 52, 319

  _Microglaena_ Lönnr., 314

  _Micrographa_ Müll.-Arg., 322

  _Microphiale_ A. Zahlbr., 328

  _Microthelia_ Koerb., 316

  _Microtheliopsis_ Müll.-Arg., 318

  Minks, 26

  _Minksia_ Müll.-Arg., 323

  Mites, 395, 397

  Miyoshi, 256, 403

  _Mnium hornum_ L., 65 (Fig. 35)

  Moebius, 62

  Mohl, 185, 186

  Molisch, 250

  Möller, 49, 154, 196, 202, 203

  _Moma orion_, 399

  _Monas Lens_, xx

  _Monascus_, Van Teigh., 178

  Montagne, 15

  Moreau, xxi, 168, 175, 176, 212, 266

  _Moriola_ Norm., 313

  Moriolaceae, 309, 313

  Morison, 1, 4, 5, 155, 304

  Moss, 356

  Mousse des Chênes, 418

  Mudd, 16, 17, 19

  Muenster, 354

  Mühlenberg, 14

  Mulder, 210

  Müller(-Argau), 18, 26, 191, 192, 205, 278, 307, 353, 405

  Müller, K., 212

  Müllerella Hepp, 275

  _Musco-fungus_, 1

  _Muscus_, 1

  _Muscus cranii humani_, 413

  Musk, 419

  Mycetozoon on Lichens, 267

  _Mycoblastus_ Norm., 329
    _M. sanguinarius_ Th. Fr., 188; _see_ _Lecidea_

  _Mycocalicium_ Rehm, 277
    _M. parietinum_ Rehm, 277

  _Mycoconiocybe_ Rehm, 277

  _Mycoidea_ Cunningh., 35, 59, 309, 318, 352, 363
    _M. parasitica_ Cunningh., 36, 59 (Fig. 31), 60

  Mycoideaceae, 59

  Mycoporaceae, 309, 318, 352

  Mycoporellum Zahlbr., 159, 318

  Mycoporum Flot., 159, 276, 318

  _Mycosphaerella_ Johans., 39

  Mycosphaerellaceae, 275

  Myriangiacei, 306

  _Myxodictyon_ Massal., 338

  Myxophyceae, xix, 51, 52-55, 60, 68, 272, 324, 385


  _Narcyria monilifera_, 399

  Necker, 123, 154

  Nees von Esenbeck, xxiv

  _Neophyllis_ Wils., 330, 351

  _Nephroma_ Ach., 63, 135, 136, 169, 244, 286, 337, 348
    _N. expallidum_ Nyl., 139 (Fig. 79)

  _Nephromium_ Nyl., 63, 158, 175, 200, 222, 244, 283, 286, 337, 349, 351
    _N. laevigatum_ Nyl., 195
    _N. lusitanicum_ Nyl., 228, 246
    _N. tomentosum_ Nyl., 87, 128, 169

  _Nephromopsis_ Müll.-Arg., 158, 244, 339

  Neubert, 410

  Neubner, 62, 175, 189, 288

  _Neuropogon_ Flot. and Nees, 346

  Nienburg, 38, 64, 123, 166, 167, 168, 169, 177, 185, 196, 240

  Nilson, 147, 151, 250, 358, 389

  Norman, 16, 313

  _Normandina_ Nyl., _see_ _Coriscium_

  _Normandina_ Wain., 315

  _Nostoc_ Vauch., 20, 21, 23, 24, 26, 27, 32, 42, 53, 61, 63, 69, 136,
      138, 232, 246, 266, 285, 309 _et seq._, 396
    _N. coerulescens_ Lyngb., 53 (Fig. 18)
    _N. lichenoides_ Kütz., xx, 54
    _N. Linckia_ Born., 53 (Fig. 18)
    _N. sphaericum_ Vauch., 54
    _N. symbioticum_, 45

  Nostocaceae, 25, 53

  Notaris, De, 1, 15, 16

  _Notaspis lutorum_, 397

  _Nyctalis_ Fr., 261

  Nylander, xxi, 7, 8, 16, 18, 25, 30, 52, 126, 131, 135, 136, 152, 197,
      228, 262, 306, 325, 350, 353, 360, 383

  _Nylanderiella_ Hue, 315


  _Obryzum_ Wallr., 263

  _Ocellularia_ Spreng., 326

  _Ochrolechia_ Massal., 338
    _O. pallescens_ Koerb., 187 (Fig. 106), 213

  _Ochrophaeae_ Wain., 295

  Officinal barks, 15

  Ohlert, 234

  Oidia, 189

  Olivier, 342

  _Omphalaria_ Dur. and Mont., 348, 373, 393
    _O. Heppii_ Müll., 63
    _O. pulvinata_ Nyl., 373

  _Oniscus_, 396

  _Oospora_ Wallr., 45

  _Opegrapha_ Ach., 11, 13, 35, 184, 304, 321, 322, 353, 354, 361
    _O. atra_ Pers., 15, 202
    _O. calcarea_ Turn., 383
    _O. endoleuca_ Nyl., 243
    _O. hapalea_ Ach., 243
    _O. saxicola_ Ach., 216, 219
    _O. subsiderella_ Nyl., 50, 202, 349
    _O. Thomasiana_ Goepp., 354
    _O. varia_ Pers., 354, 365
    _O. vulgata_ Ach., 30
    _O. zonata_ Koerb., 392

  _Opegraphella_ Müll.-Arg., 322

  _Orbilia coccinella_ Karst., 261

  Orchil lichen, 412, 416

  _Oribata Parmeliae_, 397

  Oribatidae, 397

  _Oropogon_ Fr., 340
    _O. loxensis_ Th. Fr., 130, 210, 352

  _Orphniospora_ Koerb., 329

  _Orthidium_, 191

  Orthoptera, 397

  _Oscillaria_ Bosc., 24


  _Pachyphiale_ Lönnr., 328

  _Padina Pavonia_ Gaillon, 153

  _Palmella_ Lyngb., 24, 57, 232, 278, 282, 289, 309, 321, 338, 353
    _P. botryoides_ Kütz., 313

  _Pannaria_ Del., 61, 79, 81, 135, 168, 175, 336, 392
    _P. brunnea_ Massal., 244, 370
    _P. microphylla_ Massal., 81, 244
    _P. pezizoides_ Leight., 63
    _P. rubiginosa_ Del., 283
    _P. triptophylla_ Nyl., 244

  Pannariaceae, 54, 135, 285, 287, 311, 335

  _Pannoparmelia_ Darbish., 338
    _P. anzioides_ Darbish., 90 (Fig. 51)

  Paracelsus, 407

  Paratheliaceae, 309, 316, 352

  _Parathelium_ Müll.-Arg., 317

  Parfitt, 95

  Parkinson, 3, 407

  Parmelei, 353

  _Parmelia_ Ach., 84, 86, 93, 94, 95, 133, 200, 213, 227, 231, 238, 241,
      242, 249, 260, 264, 267, 269, 299, 300, 305, 346, 347, 348, 349,
      351, 354, 364, 372, 414
    _P. acetabulum_ Dub., 30, 167, 169 (Fig. 96), 170, 180, 195 (Fig.
      111), 231, 255, 259, 360
    _P. adglutinata_ Floerk., 365
    _P. aleurites_ Ach., 364
    _P. alpicola_ Fr., 18, 350, 387
    _P. aspidota_ Rosend. (_see_ _P. exasperata_), 92 (Fig. 53), 170, 338
    _P. Borreri_ Turn., 265; _see_ _P. dubia_
    _P. caperata_ Ach., 88 (Fig. 49), 253, 255, 365, 366, 395
    _P. cetrata_ Ach., 92
    _P. conspersa_ Ach., 194, 241, 242, 355, 369, 376, 416, 417
    _P. crinita_ Nyl., 365
    _P. dubia_ Schaer., 377
    _P. encausta_ Ach., 268, 388, 393
    _P. enteromorpha_ Ach., 131
    _P. exasperata_ Carroll, 62, 129 (Fig. 74), 132, 196
    _P. farinacea_ Bitt., 131, 143
    _P. fuliginosa_ Nyl., 247, 361, 376, 386
    _P. glabra_ Nyl., 87, 170, 176
    _P. glabratula_ Lamy, 170
    _P. glomellifera_ Nyl., 249, 251
    _P. hyperopta_ Ach., 261
    _P. isidiophora_ A. Zahlbr., 66
    _P. Kamtschadalis_ Eschw., 300
    _P. lacunosa_ Meng. and Goepp., 355
    _P. lanata_ Wallr., _see_ _P. pubescens_
    _P. locarensis_ Zopf., 249
    _P. molliuscula_ Ach., 265
    _P. Mougeotii_ Schaer., 375
    _P. obscurata_ DC., 64, 131, 176, 242
    _P. olivacea_ Ach., 247, 365
    _P. omphalodes_ Ach., 3, 260, 375, 387, 415 (Fig. 135)
    _P. papulosa_ Rosend., 150, 214, 219
    _P. perforata_ Hook. (?), 365
    _P. perlata_ Ach., 92, 114, 213, 237, 243, 262, 353, 363, 403, 416
    _P. pertusa_ Schaer., 131
    _P. physodes_ Ach., 64, 91, 144 (Fig. 83), 146 (Fig. 84), 156, 194,
      234, 237, 242, 253, 262, 299, 355, 361, 363, 366, 384, 385, 416
    _P. pilosella_ Hue, 92
    _P. proboscidea_ Tayl., 92, 150
    _P. prolixa_ Carroll, 241, 249, 382
    _P. pubescens_ Wain., 85, 299, 300, 350, 375, 387
    _P. revoluta_ Floerk., 247, 259 (Fig. 121)
    _P. saxatilis_ Ach., 169, 170, 242, 243, 253, 260, 355, 361, 365, 366,
      375, 386, 387, 393, 407 (Fig. 131), 416
    _P. scortea_ Ach., 150, 366
    _P. stygia_ Ach., 130, 299, 350, 375, 387, 393
    _P. subaurifera_ Nyl., 143, 226, 246, 377
    _P. sulcata_ Tayl., 144, 361
    _P. tiliacea_ Ach., 164, 170, 252, 365
    _P. tristis_ Wallr., 88, 130, 247, 375, 387
    _P. verruculifera_ Nyl., 87, 143, 214
    _P. vittata_ Nyl., 131, 143

  Parmeliaceae, 200, 287, 298, 311

  Parmeliales, 308

  _Parmeliella_ Müll.-Arg., 81, 286, 336

  _Parmeliopsis_ Nyl., 339

  _Parmentaria_ Fée, 317

  _Patellaria_ Fr., 280

  Patellariaceae, 278

  _Patinella_ Sacc., 279
    _P. atroviridis_ Rehm, 278

  Patouillard, 389

  _Paulia_ Fée, 284, 333, 352

  Paulson, 244, 254, 366

  Paulson and Hastings, 28, 38, 44, 56, 260

  Paulson and Thompson, 254, 361, 369, 377, 397

  _Peccania_ Forss., 284, 333, 373

  Peirce, xxiii, 33, 34, 108, 258, 359

  Peltati, 305

  _Peltidea_ Ach., 63, 286; _see_ _Peltigera_

  _Peltigera_ Willd., 3, 42, 53, 61, 63, 88, 135, 136, 137, 168, 175, 186,
      204, 212, 213, 222, 232, 242, 257, 266, 283, 286, 337, 346, 349,
      355, 367, 384, 385, 392
    _P. americana_ Wain., 351
    _P. aphthosa_ Willd., 26, 87, 133, 138 (Fig. 78 A, B), 141, 211, 262,
      347, 359, 370, 406
    _P. canina_ Willd., 24, 51, 84 (Fig. 47), 87, 89 (Fig. 50), 93 (Figs.
      54, 55), 97, 185, 213, 254, 262, 359, 367, 370, 394, 396, 407, 418
    _P. horizontalis_ Hoffm., 169, 244
    _P. lepidophora_ (Nyl.) Bitt., 140
    _P. leptoderma_ Nyl., 351
    _P. malacea_ Fr., 169, 370
    _P. polydactyla_ Hoffm., 51, 244, 266, 368
    _P. rufescens_ Hoffm., 169, 386
    _P. spuria_ Leight., 268, 369
    _P. spuriella_ Wain., 351
    _P. venosa_ Hoffm., 244, 347

  Peltigeraceae, 54, 135, 283, 286, 287, 311, 336

  _Pelvetia canaliculata_ Dec. and Thur., 39

  _Pentagenella_ Darbish., 83, 324

  _Perforaria_ Müll.-Arg., 337

  Persio, 413

  Persoon, 10, 21, 123, 156, 395

  _Pertusaria_ DC., 34, 73, 85, 86, 88, 170, 180, 186, 213, 246, 253,
      337, 414
    _P. amara_ Ach., 148, 236, 243, 349, 361, 366, 408 (Fig. 132)
    _P. communis_ DC., 50, 202, 214 (Fig. 116), 255, 269, 366, 393
    _P. concreta_ Nyl., 382
    _P. corallina_ (Ach.) Bachm., 374
    _P. dactylina_ Nyl., 387
    _P. dealbata_ Cromb., 215, 375, 376
    _P. faginea_ Leight., 396
    _P. globulifera_ Nyl., 33 (Fig. 12), 236, 237, 262, 357, 366
    _P. glomerata_ Schaer., 387
    _P. lactea_ Nyl., 374, 376
    _P. leioplaca_ Schaer., 365
    _P. lutescens_ Lamy, 226
    _P. melaleuca_ Dub., 417
    _P. oculata_ Th. Fr., 387
    _P. velata_ Nyl., 265
    _P. Wulfenii_ DC., 226, 366

  Pertusariaceae, 147, 311

  Petch, 397

  Petiver, 4, 10

  _Petractis_ Fr., 327
    _P. exanthematica_ Fr., 61, 75, 215, 216

  _Peziza_ Dill., 157, 213, 307
    _P. resinae_ Fr., 355

  Pfaff, 221

  Pfeffer, 220

  _Phacopsis vulpina_ Tobl., 265

  _Phaeographina_ Müll.-Arg., 322

  _Phaeographis_ Müll.-Arg., 322
    _Ph. Lyellii_ A. Zahlbr., 350

  _Phaeotrema_ Müll.-Arg., 326

  _Phalena_, 395

  _Phascum cuspidatum_ Schreb., 45

  _Phialopsis rubra_ Koerb., 174, 249; _see_ _Gyalecta_

  Phillips, 252

  _Phleopeccania_ Stein., 284, 333, 352

  _Phlyctella_ Müll.-Arg., 338

  _Phlyctidia_ Müll.-Arg., 338

  _Phlyctis_ Wallr., 338
    _P. agelaea_ Koerb., 174

  Phycolichens, 22, 282, 283, 285

  _Phycopeltis_ Millard., 59, 278, 318, 321, 322, 323, 327, 352, 363
    _P. expansa_ Jenn., 35 (Fig. 13), 60 (Fig. 32)

  _Phyllactidium_ Moeb., 59, 62, 288, 309, 310, 318, 327, 363
    _P. tropicum_ Moeb., 59

  _Phylliscidium_ Forss., 333

  _Phylliscum_ Nyl., 286, 333

  _Phyllobathelium_ Müll.-Arg., 318

  _Phyllophora_ Grev., 111

  _Phyllophthalmaria_ A. Zahlbr., 326, 352
    _Ph. coccinea_ A. Zahlbr., 352

  _Phylloporina_ Müll.-Arg., 318

  _Phyllopsora_ Müll.-Arg., 329
    _P. furfuracea_ A. Zahlbr., 329

  Phyllopsoraceae, 310, 329

  Phyllopyreniaceae, 309, 318

  Phymaloidei, 304

  _Physcia_ Schreb., 90, 94, 166, 186, 238, 301, 351, 372, 399
    _P. aipolia_ Nyl., 20 (Fig. 1), 249
    _P. aquila_ Nyl., 380, 382, 384
    _P. ascendens_ Bitt., 270, 369, 377
    _P. caesia_ Nyl., 226, 369, 384
    _P. chrysophthalma_, _see_ _Teloschistes_
    _P. ciliaris_ DC., 3, 46, 84 (Fig. 48), 92, 94, 99, 103, 155, 165
      (Fig. 94), 166, 167, 182 (Fig. 100), 184, 185 (Fig. 104), 187, 189,
      192, 243, 246, 247, 355, 360, 411, 419
    _P. granulifera_ Nyl., 365
    _P. hispida_ Tuck., 29, 92, 146, 164, 166, 169, 194 (Fig. 110), 241,
      271, 360, 366
    _P. hypoleuca_ Tuck., 399
    _P. intricata_ Schaer., 301
    _P. leucomelas_ Mich., 99
    _P. obscura_ Nyl., 243, 360, 365, 369, 377
    _P. parietina_ (_see_ _Xanthoria_), 29 (Figs. 7, 8)
    _P. picta_ Nyl., 349, 353
    _P. pulverulenta_ Nyl., 28, 164 (Fig. 93), 166, 181, 248, 360, 365,
      366, 377, 399
    _P. puncticulata_ Hue, 33
    _P. sciastrella_ Hann., 369
    _P. stellaris_ Nyl., 29, 365, 384
    _P. stellaris_ var. _tenella_ Cromb., _see_ _P. hispida_ Tuck.
    _P. subobscura_ A. L. Sm., 384
    _P. tenella_ Bitt., 366, 384, 386
    _P. tribacia_ Nyl., 365
    _P. villosa_ Dub., 268

  Physciaceae, 136, 200, 267, 300, 308, 311, 341

  _Physcidia_ Tuck., 299, 339
    _Ph. Wrightii_ Nyl., 352

  _Physma_ Massal., 163, 284, 334, 341
    _P. chalazanum_ Arn., 32 (Fig. 9)
    _P. compactum_ Koerb., 163, 266
    _P. franconicum_ Massal., 263

  Pilocarpaceae, 310, 325

  _Pilocarpon_ Wain., 325, 353; _see_ _Pilophorus_
    _P. leucoblepharum_ Wain., 325, 363

  _Pilophorus_ Th. Fr., 17, 125, 133, 135, 201, 292, 294, 297, 330
    _P. robustus_ Th. Fr., 136

  _Pinus sylvestris_ L., 94, 271

  _Piptocephalis_ De Bary, 261

  _Placidiopsis_ Beltr., 288

  _Placodium_ DC., 80, 339, 340, 346, 360, 372
    _P. atroflavum_ A. L. Sm., 386
    _P. aurantiacum_ Hepp, 365
    _P. bicolor_ Tuck., 136
    _P. callopismum_ Mér., 349
    _P. cerinum_ Hepp, 262, 365, 366, 367
    _P. citrinum_ Hepp, 224, 271, 349, 373, 377, 386, 393
    _P. decipiens_ Leight., 218, 369, 383
    _P. elegans_ DC., 225, 241, 347, 369, 390
    _P. ferrugineum_ Hepp, 346, 384
    _P. flavescens_ A. L. Sm., 377
    _P. fruticulosum_ Darbish., 347
    _P. fulgens_ S. F. Gray, 367
    _P. lacteum_ Lesd., 377
    _P. lobulatum_ A. L. Sm., 379, 382, 384
    _P. luteoalbum_ Hepp, 301
    _P. murorum_ DC., 42, 80 (Fig. 42), 227, 241, 243, 347, 369, 380
    _P. nivale_ Tuck., 301
    _P. pyraceum_ Anzi, 369, 377
    _P. rupestre_ Br. and Rostr., 301
    _P. subfruticulosum_ Elenk., 347
    _P. sympageum_, _see_ _P. flavescens_
    _P. tegularis_ (Ehrh.) Darbish., 379, 384
    _P. teicholytum_ DC., 369

  _Placodium_ Hill (non DC.), 8

  _Placodium_ Web. (non DC.), 9
    _P. Garovagli_ (Koerb.) Fried., 81
    _P. saxicolum_ S. F. Gray, 146, 168; _see_ _Lecanora_

  _Placolecania_ Zahlbr., 338

  _Placothelium_ Müll.-Arg., 285, 319

  _Placynthium_ Ach., 336
    _P. nigrum_ S. F. Gray, 248, 373

  _Plagiothecium sylvaticum_ Buch. and Schimp., 237

  _Plagiotrema_ Müll.-Arg., 317

  _Platygrapha_ Nyl., 325

  _Platysma_ Nyl., 8, 200, 257; _see_ _Cetraria_
    _P. commixtum_ Nyl., 375
    _P. corniculatum_ Hill, 8
    _P. Fahlunense_ Nyl., 375
    _P. glaucum_ Nyl., 10, 375, 376, 418
    _P. lacunosa_ Nyl., 375

  _Pleospora collematum_ Zuk., 163, 266

  _Pleurococcus_ Menegh. (?), 22, 29, 62
    _P. Naegeli_ Chod., 28
    _P. vulgaris_ Menegh., 28, 55 (Fig. 22), 223
    _P. vulgaris_ Naeg., 28

  _Pleurocybe_ Müll.-Arg., 320
    _P. madagascarea_ A. Zahlbr., 289

  _Pleurothelium_ Müll.-Arg., 317

  _Pleurotrema_ Müll.-Arg., 317

  Plot, 4

  Plowright, 207

  Plukenet, 5

  _Poa compressa_ L., 393

  _Poduridae_, 256

  _Polyblastia_ Massal., 48, 314
    _P. catalepta_ (Ach.) Fuist., 30
    _P. Vouauxi_ Lesd., 378

  _Polyblastiopsis_ Nyl., 316

  _Polycauliona_ Hue, 339, 340, 346
    _P. regale_ Hue, 339, 346

  Polycaulionaceae, 339

  _Polychidium_ A. Zahlbr., 284, 332

  _Polycoccus_ Kütz., 24
    _P. punctiformis_ Kütz., 24, 54, 61

  _Polyporus_ Mich., 261

  _Polystictus versicolor_ (Fr.), 152

  _Polystigma rubrum_ DC., 178, 207

  _Polystroma_ Clem., 326
    _P. Ferdinandezii_ Clem., 326

  _Polytrichum_ L., 392
    _P. commune_ L., 237

  _Polyxenus_, 270

  _Porina_ Ach., 204, 316
    _P. lectissima_ A. Zahlbr., 249, 251, 392
    _P. olivacea_ A. L. Sm., 159 (Fig. 90 A)

  _Porocyphus_ Koerb., 332

  _Poronia_ Willd., 13, 178

  Porta, 5

  Porter, 109, 270

  _Prasiola_ Ag., 60, 309, 315
    _P. parietina_ Wille, 60 (Fig. 33)

  Prasiolaceae, 60

  Propagula, 11

  Protocaliceaceae, 277

  Protococcaceae, 55, 288, 291, 309, 310, 313 _et seq._, 353, 363

  _Protococcus_ Ag., xx, 28, 56, 62, 63, 65, 287
    _P. botryoides_ Kirchn., 65
    _P. viridis_ Ag., 22, 28, 44, 48 (Fig. 15), 55 (Fig. 22), 65, 313

  _Pseudopyrenula_ Müll.-Arg., 316

  _Psocus_, 397

  _Psora_ (_Lecidea_) _decipiens_ Hook., 388

  _Psorella_ Müll.-Arg., 329

  _Psoroglaena_ Müll.-Arg., 315

  _Psoroma_ S. F. Gray, 136, 285, 286, 335
    _P. hypnorum_ S. F. Gray, 63, 81, 88, 89, 135, 246, 283, 370

  _Psoromaria_ Nyl., 285, 286, 335

  _Psorotichia_ Massal., 68, 163, 333, 373
    _Ps. lugubris_ Dal. Tor. and Sarnth., 375
    _Ps. lutophila_ Arn., 368

  Psychides, 399

  _Pterygiopsis_ Wain., 332

  _Pterygium_ Nyl., 333
    _Pt. Kenmorensis_ A. L. Sm., 392

  _Ptychographa_ Nyl., 321, 322

  Pulteney, 4, 14

  Pulvis antilyssus, 407

  Pulvis Cyprius, 419

  _Pycnothelia_ (_Cladonia_) _papillaria_ Duf., 369

  _Pyrenastrum_ Eschw., 317

  Pyrenidiaceae, 53, 54, 275, 285, 309, 319

  _Pyrenidium_ Nyl., 285, 319
    _P. actinellum_ Nyl., 99

  Pyrenocarpeae, 158, 273, 308

  Pyrenocarpei, 306, 307, 353

  Pyrenocarpineae, 273, 275, 288, 308

  _Pyrenocollema_ Reinke, 334

  _Pyrenographa_ Müll.-Arg., 323

  Pyrenolichens 159, 241, 276, 352, 391

  Pyrenomycetes 158, 267, 273

  Pyrenopsidaceae, 282, 284, 310, 352

  _Pyrenopsidium_ Forss., 333

  _Pyrenopsis_ Nyl., 60, 68, 163, 175, 333
    _P. haematopis_ Th. Fr., 195
    _P. impolita_ Forss., 175
    _P. phaeococca_ Tuck., 175

  _Pyrenothamnia_ Tuck., 99, 315
    _P. Spraguei_ Tuck., 288

  Pyrenothamniaceae, 309, 315

  _Pyrenothea_ Ach., 192

  _Pyrenothrix_ Riddle, 319

  _Pyrenula_ Ach., 200, 316
    _P. cinerella_ Fink, 365
    _P. leucoplaca_ Koerb., 365
    _P. nitida_ Ach., 174, 194, 240, 255, 350, 354, 364, 365
    _P. thelena_ Fink, 365

  Pyrenulaceae, 50, 276, 309, 316, 365

  _Pyrgidium_ Nyl., 319
    _P. bengalense_ Nyl., 353

  _Pyrgillus_ Nyl., 289, 320

  _Pyronema_ Carus., 167
    _P. confluens_ Tul., 178

  _Pyxidium_ Hill, 8

  _Pyxine_ Nyl., 301, 341
    _P. Cocoës_ Nyl., 353
    _P. Meissnerii_ Tuck., 353


  _Quercus alba_, 359
    _Q. chrysolepis_, 359
    _Q. Douglasii_, 359


  _Racodium_ Pers., 35, 328
    _R. rupestre_ Pers., 291, 328

  Radais, 42

  _Ramalina_ Ach., 3, 84, 103, 110, 195, 213, 238, 244, 257, 270, 305,
      340, 347, 348, 351, 359, 361, 363
    _R. calicaris_ Fr., 3, 104, 147, 210, 353, 355, 365, 366, 418, 419
    _R. ceruchis_ De Not., 103
    _R. Curnowii_ Cromb., 104, 109
    _R. cuspidata_ Nyl., 225, 271 (_see_ _R. siliquosa_), 384
    _R. dilacerata_ Hoffm., 106, 130
    _R. Eckloni_ Mont., 130
    _R. evernioides_ Nyl., 103, 300
    _R. farinacea_ Ach., 10, 239, 269, 271, 353, 366, 400, 411
    _R. fastigiata_ Ach., 109, 365, 366, 400, 411
    _R. fraxinea_ Ach., 104, 106, 130 (Fig. 75 A), 155, 164, 170, 195,
      200, 212, 215, 300, 355, 365, 366, 400, 411, 418
    _R. gracilenta_ Ach., 349
    _R. homalea_ Ach., 103
    _R. Landroensis_ Zopf, 109, 130
    _R. minuscula_ Nyl., 103 (Fig. 62), 147
    _R. pollinaria_ Ach., 109, 227, 349, 366
    _R. reticulata_ Krempelh., 33 (Fig. 11), 99, 106 (Fig. 64), 253, 257,
      359
    _R. scopulorum_ Ach., _see_ _R. siliquosa_
    _R. siliquosa_ A. L. Sm., 104, 109 (Fig. 65), 130, 224, 225, 271, 300,
      379 (Fig. 122), 381 (Figs. 123, 124)
    _R. strepsilis_ Zahlbr., 104, 130 (Fig. 75 B)
    _R. subfarinacea_ Nyl., 380
    _R. tertiaria_ Engelh., 354

  Ramalinaceae, 339

  _Ramalinites lacerus_ Braun, 354

  Ramalodei, 306

  _Ramonia_ Stizenb., 328

  Rathapu, 403

  Ray, 4, 407, 409

  Rees, 27

  Rehm, 277

  Reindeer, 401

  Reindeer moss, 400 _et passim_

  Reinke, 18, 31, 41, 68, 123, 125, 130, 144, 253, 277, 284, 291, 307, 324

  _Reinkella_ Darbish., 83, 324

  Relhan, 9

  Rhabdopsora Müll.-Arg., 319

  _Rhizina undulata_ Fr., 181

  _Rhizocarpon_ Ramond, 248, 302, 329, 341
    _R. alboatrum_ Th. Fr., 365, 369, 373, 383
    _R. concentricum_, _see_ _R. petraeum_
    _R. confervoides_ DC., 71 (Fig. 38 A, B), 369, 386
    _R. distinctum_ Th. Fr., 261
    _R. epipolium_ (Ach.), 265
    _R. geographicum_ DC., 73, 74 (Figs. 40, 41), 226, 236, 243, 246, 249,
      252, 261, 264, 291, 346, 372, 374, 376, 380
    _R. obscuratum_ Massal., 392
    _R. Oederi_ Koerb., 375
    _R. petraeum_ Koerb. (?), 374
    _R. petraeum_ Massal., 171 (Fig. 97), 375, 392
    _R. viridiatrum_ Koerb., 249, 375, 376

  _Rhizomorpha_ Roth, 12

  _Rhymbocarpus punctiformis_ Zopf, 264

  _Ricasolia_ De Not., 94 (_see_ _Lobaria_), 168, 175
    _R. amplissima_ De Not., 133, 134 (Fig. 76), 195, 197, 357
    _R. laetevirens_ Leight., 357

  Richard, 377, 411

  Richardson, Dr, 6

  Richardson, Sir John, 388

  Riddle, 137

  _Rinodina_ S. F. Gray, 301, 302, 341, 372
    _R. archaea_ Wain., 346
    _R. Conradi_ Koerb., 370
    _R. exigua_ S. F. Gray, 366, 367, 377, 383, 384
    _R. isidioides_ Oliv., 301
    _R. oreina_ Wain., 301, 374, 390
    _R. sophodes_ Th. Fr., 367
    _R. turfacea_ Th. Fr., 262, 377

  _Rivularia_, 55, 136, 138, 284, 333
    _R. Biasolettiana_, 54 (Fig. 21)
    _R. minutula_ Born. and Fl., 54 (Fig. 21)
    _R. nitida_ Ag., 55

  Rivulariaceae, 54

  _Roccella_ DC., 3, 34, 35, 83, 103, 200, 225, 233, 242, 278, 292, 324,
      351, 359, 363
    _R. fuciformis_ DC., 83 (Fig. 45), 98 (Fig. 57), 101, 110, 227, 228,
      349, 350, 412
    _R. fucoides_ Wain., 349, 350
    _R. Montagnei_ Bél., 213, 413
    _R. peruensis_ Kremp., 413
    _R. phycopsis_ Ach., 110; _see_ _R. fucoides_
    _R. portentosa_ Mont., 413
    _R. sinuensis_ Nyl., 413
    _R. tinctoria_ DC., 213, 215, 227, 349, 350, 413 (Fig. 133)

  Roccellaceae, 59, 83, 110, 279, 290, 309, 323

  _Roccellaria_ Darbish., 323, 324

  _Roccellina_ Darbish., 83, 290, 323, 324

  _Roccellographa_ Stein., 83, 290, 323, 324

  Rock tripe, 404

  Roebuck, 401

  Ronceray, 213, 413

  Rosendahl, 86, 90, 93, 129, 170, 176, 214, 218, 249

  Roses, spirit of, 419

  Roy, 411

  Ruel, 2

  Rupp, 5

  _Russula_ Pers., 161


  Sachs, 17, 23

  _Sagedia_, _see_ _Verrucaria_
    _S. declivum_ Am., 251

  _Sagiolechia_ Massal., 328

  _Salix repens_ L., 357

  Salter, 51, 393

  Sandstede, 233, 384, 385

  Sappin-Trouffy, 207

  _Sarcographa_ Fée, 323

  _Sarcographina_ Müll.-Arg., 323

  _Sarcogyne_ (= _Biatorella_) _latericola_ Stein., 76

  _Sarcopyrenia_ Nyl., 314

  Sättler, 123, 173, 296, 358

  Schade, 376

  Schaerer, 15, 192

  Schellenberg, 212

  Schenk, 213

  Schikorra, 178

  Schimper, 354, 355

  _Schismatomma_ Flot., 325

  _Schizopelte_ Th. Fr., 83, 324

  Schneider, 7, 135, 136, 139

  Schreber, 126

  Schrenk, 231, 258, 359

  Schulte, 104, 105, 106, 177

  Schwarz, 224

  Schweinfurth, 405

  Schweinitz, 15

  Schwenckfeld, 3

  Schwendener, xx, 2, 16, 17, 18, 25, 27, 36, 71, 82, 86, 92, 126, 128,
      129, 142, 147, 168, 213, 224, 307

  _Sclerophyton_ Eschw., 323
    _S. circumscriptum_ A. Zahlbr., 322

  Scopoli, 8, 21, 154, 409

  Scott-Elliot, 253

  Scutellati, 305

  _Scutovertes maculatus_, 397

  _Scytonema_ Ag., 54, 57, 61, 68, 75, 136, 153, 216, 232, 281, 284, 309
      _et seq._, 318
    _S. mirabile_ Thur., 53 (Fig. 19)

  Scytonemaceae, 54

  _Secoliga_ (_Gyalecta_) _bryophaga_ Koerb., 368

  _Segestria_, _see_ _Porina_

  Senft, 223

  _Septoria_ Fr., 204

  Sernander, 94, 140, 355

  Servettaz, 45

  Servit, 374

  Sherard, 4, 6, 7

  Sibbald, 409

  Sibthorp, 9

  Sievers, 230

  _Simonyella_ Steiner, 324

  _Siphula_ Fr., 340

  _Sirosiphon pulvinatus_ Bréb., 54

  Sloane, 10

  Smith, Lorrain, 328

  Smith, Sir J. E., 10

  _Solorina_ Ach., 56, 63, 85, 94, 135, 136, 168, 175, 176, 183, 287,
      337, 392
    _S. bispora_ Nyl., 135
    _S. crocea_ Ach., 63, 88, 140, 210, 228, 246, 287, 346, 388
    _S. octospora_ Arn., 85
    _S. saccata_ Ach., 155, 244, 388
    _S. spongiosa_ Carroll, 135, 186, 368

  _Solorinella_ Anzi, 337

  Sorby, 418

  Sowerby, James, 10

  Speerschneider, 17, 25

  _Sphaeria_ Hall., 192, 213

  Sphaeriaceae, 307

  _Sphaerocephalum_ Web., 9

  Sphaerophoraceae, 135, 309, 320

  _Sphaerophoropsis_ Wain., 291, 329
    _S. stereocauloides_ Wain., 292

  _Sphaerophorus_ Pers., 83, 105, 184, 277, 289, 320, 361, 375, 387, 393
    _S. coralloides_ Pers., 83 (Fig. 44) (_see_ _S. globosus_), 355, 375,
      387, 388, 389
    _S. fragilis_ Pers., 375, 387
    _S. globosus_ A. L. Sm., 346
    _S. stereocauloides_ Nyl., 135

  _Sphagnum_ Dill., 231, 355

  _Spheconisca_ Norm., 313

  _Sphinctrina_ Fr., 277, 319, 353

  _Sphyridium byssoides_, 177
    _S. fungiforme_ Koerb., 177

  _Spilonema_ Born., 68, 333

  _Spirographa_ A. Zahlbr., 322

  _Spirogyra_ Link, 188

  _Splachnum_ L., 5

  _Sporocladus lichenicola_ Corda, 200

  _Sporodinia_ Link, 188

  _Sporopodium_ Mont., 327, 352
    _S. Caucasium_ Elenk. and Woron., 353

  Sprengel, 21, 142, 156, 184

  _Squamaria_ DC., 200, 298
    _S. saxicola_, _see_ _Lecanora_

  Stahel, 220

  Stahl, 28, 30, 62, 160, 163, 173, 266, 395

  Stahlecker, 76, 235, 371, 374

  _Staurothele_ Norm., 31, 62, 76, 314
    _S. clopima_ Th. Fr., 391
    _S. clopismoides_ Anzi, 249
    _S. hymenogonia_ A. Zahlbr., 361
    _S. umbrinum_ A. L. Sm., 373, 393

  _Steganosporium cellulosum_ Corda, 201

  Steiner, 75, 179, 190, 198, 215, 276, 312, 353, 389

  _Steinera_ A. Zahlbr., 333

  Stenberg, 411

  Stenhouse and Groves, 228

  _Stenocybe_ Nyl., 177, 319

  _Stereocaulon_ Schreb., 17, 23, 83, 105, 125, 133, 135, 176, 201, 283,
      292, 294, 297, 330, 346, 358, 361, 387
    _S. alpinum_ Laur., 137, 346, 387
    _S. condensatum_ Hoffm., 319, 388
    _S. coralloides_ Fr., 125, 375
    _S. Delisei_ Borg., 375
    _S. denudatum_ Floerk., 137, 375, 387
    _S. evolutum_ Graewe, 375
    _S. paschale_ Fr., 211, 372, 385, 391, 401
    _S. ramulosum_ Ach., 125, 136
    _S. salazinum_ Borg., 227
    _S. tomentosum_ Fr., 125, 136, 387

  _Stereochlamys_ Müll.-Arg., 316

  _Stichococcus_ Naeg., 62
    _S. bacillaris_ Naeg., 42

  _Sticta_ Schreb., 13, 63, 85, 86, 94, 136, 138, 200, 283, 287, 336, 350,
      351, 364, 392
    _St. aurata_ Ach., 126, 128, 223, 226, 246, 350
    _St. crocata_ Ach., 128, 246
    _St. damaecornis_ Nyl., 127 (Fig. 73), 128, 210, 350
    _St. Dufourei_ Del., 128
    _St. fuliginosa_ Ach., 126, 128, 223
    _St. intricata_ Del., 128
    _St. limbata_ Ach., 128
    _St. oregana_ Tuck., 136, 139
    _St. sylvatica_ Ach., 128
    _St. Wrightii_ Nyl., 349

  Stictaceae, 96, 136, 286, 311, 336, 347, 350, 418

  Stictidaceae, 278

  _Stictina_ Nyl., 63, 168, 175, 287

  _Stictis_ Pers., 278

  _Stigmatea_ Fr., 275

  _Stigonema_ Ag., 23, 26, 54 (Fig. 20), 68, 136, 283, 284, 310 _et seq._,
      317
    _S. panniforme_ Kirchn., 54

  Stigonemaceae, 54

  Stirton, 331, 350

  Stizenberger, 18, 128

  Stone, 399

  _Streptothrix_ Cohn, 45

  _Strigula_ Fr., 60, 65, 288, 318, 353, 363
    _S. Buxi_ Chod., 363
    _S. complanata_ Mont., 35, 42, 59, 205, 260, 269
    _S. elegans_ Müll.-Arg., 205

  Strigulaceae, 59, 60, 204, 309, 318, 363

  Stüde, 211

  Sturgis, 97, 168, 175, 197, 289

  _Suaeda fruticosa_ Forsk., 387

  Swartz, 10, 152

  Swedish moss, 415

  Symbiosis, 31

  _Synalissa_ Fr., 32, 33, 61, 284, 333, 373
    _S. symphorea_ Nyl., 33 (Fig. 10)

  _Synarthonia_ Müll.-Arg., 321


  Tabernaemontanus, 2

  _Tapellaria_ Müll.-Arg., 327

  Taylor, 13, 149

  _Tegeocranus labyrinthicus_, 328

  Teloschistaceae, 311, 341

  _Teloschistes_ Norm., 85, 301, 341
    _T. chrysophthalmus_ Th. Fr., 92, 365, 367
    _T. flavicans_ Norm., 3, 301, 341, 417

  _Teras literana_, 399

  _Termes monoceros_, 397

  Termites, 397

  _Tetranychus lapidus_, 398 (Fig. 126)

  Tetrasporaceae, 57

  _Thamnolia_ Ach., 83, 101 (_see_ _Cerania_), 246, 340, 389
    _Th. vermicularis_ Schaer., 346, 377

  _Thamnonia_ Tuck., 339

  Thaxter, 178

  _Thelenidia_ Nyl., 314

  _Thelephora_ Ehrh., 281, 342

  Thelephoraceae, 152, 273

  _Thelidea_ Hue, 335
    _Th. corrugata_ Hue, 335

  _Thelidium_ Massal., 314
    _Th. microcarpum_ A. L. Sm., 361
    _Th. minutulum_ Koerb., 253, 367

  _Thelocarpon_ Nyl., 331
    _Th. prasinellum_ Nyl., 367
    _Th. turficolum_ Arn., 370

  _Thelopsis_ Nyl., 316

  _Thelotrema_ Ach., 326, 343
    _Th. lepadinum_ Ach., 397

  Thelotremaceae, 59, 302, 310, 326, 351, 352

  Thelotremei, 353

  Theophrastus, 1, 2, 411

  _Thermutis_ Fr., 68, 284, 332

  _Tholurna_ Norm., 320
    _Th. dissimilis_ Norm., 289

  Thomas, N., 59

  _Thrambium_ Wallr., 192, 314
    _T. epigaeum_ Wallr., 254, 367, 368

  Thwaites, 17

  _Thyrea_ Massal., 284, 333

  _Thysanothecium_ Berk. and Mont., 330
    _T. Hookeri_ Berk. and Mont., 294

  _Ticothecium_ Flot., 275, 319
    _T. pygmaeum_ Koerb., 267

  Tieghem, Van, 179

  Tobler, 43, 50, 148, 224, 253, 263, 265, 280

  _Tomasiella_ Müll.-Arg., 317

  Toni, De, 60

  _Toninia_ Th. Fr., 329

  Torrey, 14

  Tournefort, 1, 5, 155, 304

  Tournesol, 413

  Treboux, 40, 42

  _Trematosphaeropsis_ Elenk., 266

  _Tremotylium_ Nyl., 326

  _Trentepohlia_ Born., 26, 30, 34, 59, 75, 78, 232, 246, 276, 278, 287,
      289, 291, 309, 316 etc., 343, 352, 365
    _T. abietina_ Hansg., 65, 66
    _T. aurea_ Mart., 34, 35, 58 (Fig. 29 A), 59
    _T. jolithus_, 223
    _T. umbrina_ Born., 22, 34, 58 (Fig. 29 B), 59, 62, 216

  Trentepohliaceae, 59, 288, 289

  Treub, 28, 394

  Treveris, Peter, 2

  _Tricothelium_ Müll.-Arg., 318

  _Trimmatothele_ Norm., 314

  Tripe de Roche, 404

  Trypetheliaceae, 309, 317

  _Trypethelium_ Spreng., 276, 317, 351, 364

  _Tubercularia_ Web., 9

  Tuckerman, 15, 136, 339

  Tulasne, 17, 25, 46, 70, 123, 159, 187, 189, 192, 193, 200, 204, 263

  Turner, Dawson, 14

  Tutt, 399

  _Tylophorella_ Wain., 320

  _Tylophoron_ Nyl., 289, 320


  Uhlir, 43

  Ulander, 211

  Uloth, 233

  _Umbilicaria_, 17, 82, 200, 241, 262, 268, 331
    _U. pustulata_ Hoffm., 86, 96, 150, 195, 214, 240, 257, 414

  Unguentum Armarium, 407

  Unguentum sympatheticum, 407

  _Urceolaria_ Ach., 48; _see_ _Diploschistes_

  _Urococcus_ Kütz., 57, 133, 318

  _Usnea_ Dill., 1, 3, 7, 9, 83, 111, 195, 213, 233, 257, 268, 269, 300,
      304, 305, 340, 347, 348, 351, 361, 408, 419
    _U. articulata_ Hoffm., 210, 268
    _U. barbata_ Web., 25, 99 (Fig. 58), 104 (Fig. 63 A), 130, 143 (Fig.
      82), 167 (Fig. 95), 168, 177, 200, 211, 215, 226, 234, 239, 246,
      339, 348, 364, 417
    _U. ceratina_ Ach., 227
    _U. compressa_ Hill, 8
    _U. dasypoga_ Stiz., 359
    _U. florida_ Web., 91 (Fig. 52), 92, 210, 213, 348, 363, 411
    _U. hirta_ Hoffm., 348, 355, 366
    _U. laevis_ Nyl., 177
    _U. longissima_ Ach., 85, 99, 102 (Fig. 61), 105 (Fig. 63 B), 106,
      215, 348
    _U. macrocarpa_ Arn., 177
    _U. melaxantha_ Ach., 346
    _U. plicata_ Web., 359
    _U. Taylori_ Hook., 104

  Usneaceae, 299, 311, 339


  Vaillant, 6

  Vallot, 253

  _Valsa_ Fr., 317

  _Varicellaria_ Nyl., 337
    _V. microsticta_ Nyl., 77, 92, 187 (Fig. 126)

  _Variolaria_ Ach. (_see_ _Pertusaria_), 64, 171, 237

  _Vaucheria sessilis_ DC., 65 (Fig. 34)

  Ventenat, 21

  _Verrucaria_ Web. (non Pers.), 9, 174, 200, 275, 314, 364
    _V. aethiobola_ Wahlenb., 391, 392
    _V. anceps_ Koerb., 377
    _V. aquatilis_ Mudd, 383
    _V. calciseda_ DC., 176, 215, 219, 241, 373, 398
    _V. Dufourii_ DC., 173
    _V. fuscella_ Ach., 373
    _V. Hoffmanni_ Hepp; f. _purpurascens_ Arn., 251
    _V. hydrela_ Ach., 391, 392
    _V. lecideoides_ Koerb., 373
    _V. maculiformis_ Krempelh., 379
    _V. margacea_ Wahlenb., 391, 392
    _V. maura_ Wahlenb., 245, 383, 384, 386
    _V. memnonia_ Flot., 383
    _V. microspora_ Nyl., 256, 383, 386
    _V. mucosa_ Wahlenb., 73, 383
    _V. muralis_ Ach., 30, 46 (Fig. 14), 70, 243, 255, 361, 393
    _V. nigrescens_ Pers., 56, 254, 369, 377, 392
    _V. papillosa_ Ach., 377
    _V. prominula_ Nyl., 383
    _V. rupestris_ Schrad., 215, 243, 361
    _V. scotina_ Wedd., 383
    _V. striatula_ Wahlenb., 383
    _V. viridula_ Ach., 391

  Verrucariaceae, 249, 309, 314, 353, 367

  _Verrucarites geanthricis_ Goepp., 354

  _Verrucula_ Stein., 265, 276
    _V. aegyptica_ Stein., 276
    _V. cahirensis_ Stein., 276

  Visiani, 405

  Volkard, 228, 410

  Vouaux, 267


  Wahlberg, 11, 168

  Wahrlich, 51

  Wainio, 31, 48, 70, 112, 114, 118, 120, 122, 123, 124, 125, 126, 128,
      144, 153, 159, 163, 166, 175, 177, 179, 188, 191, 240, 276, 277,
      292, 294, 308, 344, 346, 348, 411

  Waite, 270

  Wallroth, xx, 13, 21, 22, 123, 133, 142, 156, 192, 305

  Ward, Marshall, 35, 42, 59

  Watson, Sir W., 8

  Watson, 365, 373, 385

  Watt, 403

  Weber, 1, 9

  Weddell, 252, 379

  Wehmer, 220

  Weir, 239

  West, G. F., 52, 54, 55, 56

  West, W., 225, 233, 357, 374

  Wester, 211

  Westring, 412

  Wettstein, 45

  Wheldon, 398

  Wheldon and Wilson, 360, 370, 373, 374, 379, 384, 385, 387, 391, 392

  Wiesner, 211, 241, 244

  Wilde, 395

  Wille, 28

  Willemet, 10, 401, 415

  Wilson, 350

  Winter, 30, 138, 263

  Winterstein, 209

  Wisselingh, 211

  Withering, 9

  Wolff, 124, 163, 170, 172, 176

  Woodward, 152

  Woronin, 28

  _Woronina_ Cornu, 261


  _Xanthocapsa_ (Sect. of _gloeocapsa_), 52, 63, 284, 332, 373

  _Xanthoria_ Th. Fr., 166, 246
    _X. lychnea_ Th. Fr., 233, 252, 365, 417
    _X. parietina_ Th. Fr., 3, 22, 24, 27, 28, 38, 42, 48 (Fig. 15), 50,
      56, 65, 67 (Fig. 36), 86, 164, 176, 189, 195, 200, 224, 225, 227,
      231, 232, 241, 242, 253, 269, 270, 301, 341, 348, 351, 360, 369,
      373, 376, 380, 383, 384, 386, 397, 406, 416, 418
    _X. polycarpa_ Oliv., 365, 390

  _Xylaria_ Hill, 12, 421

  _Xylographa_ Fr., 278, 322
    _X. spilomatica_ Th. Fr., 145

  _Xyloschistes_ Wain., 322


  Zahlbruckner, A., 19, 59, 60, 66, 69, 275, 284, 308, 335, 413

  Zopf, 19, 43, 108, 151, 188, 213, 221, 233, 238, 246, 264, 265, 266,
      268, 270, 395, 396, 398, 400, 412, 417

  Zukal, 18, 26, 38, 61, 68, 70, 82, 128, 129, 130, 163, 179, 187, 215,
      219, 230, 237, 244, 267, 268, 271, 313, 395

  Zwelser, 419

CAMBRIDGE: PRINTED BY J. B. PEACE, M.A., AT THE UNIVERSITY PRESS