RESEARCH METHODS
                                   IN
                                ECOLOGY


                                   BY

                    FREDERIC EDWARD CLEMENTS, PH.D.

 ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY IN THE UNIVERSITY OF NEBRASKA


                             _ILLUSTRATED_


                           LINCOLN, NEBRASKA
                   THE UNIVERSITY PUBLISHING COMPANY
                                  1905




                            COPYRIGHT, 1905
              BY FREDERIC E. CLEMENTS AND IRVING S. CUTTER


                          All rights reserved


                    =Press of Jacob North & Company=
                           LINCOLN, NEBRASKA




                                PREFACE


The present volume is intended as a handbook for investigators and for
advanced students of ecology, and not as a text-book of the subject. An
elementary text-book covering the same field, but adapted to the needs
of undergraduate students, is in preparation. The handbook is
essentially an account of the methods used by the author in his studies
of the last eight years, during which a serious attempt has been made to
discover and to correlate the fundamental points of view in the vast
field of vegetation. No endeavor is made to treat any portion of the
subject exhaustively, since a discussion of general methods and general
principles is of much greater value in the present condition of ecology.
The somewhat unequal treatment given the different subjects is due to
the fact that it has been found possible to develop some of these more
rapidly than others. Finally, it must be constantly kept in mind that
ecology is still in a very plastic condition, and in consequence,
methods, fundamental principles, and matters of nomenclature and
terminology must be approached without prejudice in order that the best
possible development of this field may be attained.

Grateful acknowledgment for criticisms and suggestions is made to
Professor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound,
who have read the text. The author is under especial obligations to
Doctor Edith S. Clements for the drawings of leaf types, as well as for
reading and criticising the manuscript. Professor Goodwin D. Swezey,
Professor of Astronomy in the University of Nebraska, has kindly
furnished much material for the determination of the sun’s altitude, and
consequent light intensities, and has read the section devoted to light.
Mr. George A. Loveland, Director of the Nebraska Section of the U. S.
Weather Bureau, has contributed many helpful suggestions to the
discussion of meteorological instruments. To Nella Schlesinger, Alice
Venters, and George L. Fawcett, advanced students in experimental
ecology, the author is indebted for many experiments which have been
used in the discussion of adjustment and adaptation.

Acknowledgment is also made to the following for various cuts: Henry J.
Green, Brooklyn, New York; Julien P. Friez, Baltimore, Maryland; C. H.
Stoelting Co., Chicago, Illinois; Draper Manufacturing Co., New York
city; Gundlach-Manhattan Optical Co., Rochester, New York; Rochester
Optical Co., Rochester, New York; Bausch and Lomb Optical Co.,
Rochester, New York.

                                               FREDERIC EDWARD CLEMENTS.

  The University of Nebraska,
    May, 1905.




                                CONTENTS


                  CHAPTER I. THE FOUNDATION OF ECOLOGY

                          THE NEED OF A SYSTEM
                                                                    PAGE
        1. The scope of ecology                                        1
        2. Ecology and physiology                                      1
 Historical Development
        3. Geographical distribution                                   2
        4. The plant formation                                         2
        5. Plant succession                                            3
        6. Ecological phytogeography                                   4
        7. Experimental ecology                                        4
        8. Ecology of the habitat                                      5
        9. The evidence from historical development                    6
 Present Status of Ecology
       10. The lack of special training                                6
       11. Descriptive ecology                                         7
       12. The value of floristic                                      8
       13. Reconnaissance and investigation                            8
       14. Resident investigation                                      9
       15. The dangers of a restricted field                           9
 Applications of Ecology
       16. The subjects touched by ecology                            10
       17. Physiology and pathology                                   11
       18. Experimental evolution                                     11
       19. Taxonomy                                                   12
       20. Forestry                                                   14
       21. Physiography                                               15
       22. Soil physics                                               15
       23. Zoogeography                                               15
       24. Sociology                                                  16

                       THE ESSENTIALS OF A SYSTEM

       25. Cause and effect: habitat and plant                        16
       26. The place of function                                      17


                         CHAPTER II. THE HABITAT

                          CONCEPT AND ANALYSIS

       27. Definition of the habitat                                  18
       28. Factors                                                    18
 Classification of Factors
       29. The nature of factors                                      19
       30. The influence of factors                                   19
 Determination of Factors
       31. The need of exact measurement                              20
       32. The value of meteorological methods                        20
       33. Habitat determination                                      21
       34. Determinable and efficient differences                     21
 Instrumentation
       35. Methods                                                    22
       36. Method of simple instruments                               22
       37. Method of automatic instruments                            23
       38. Combined methods                                           23

                   CONSTRUCTION AND USE OF INSTRUMENTS

       39. The selection of instruments                               24
 Water-content
       40. Value of different instruments                             25
      Geotome methods
       41. The geotome                                                25
       42. Soil borers                                                26
       43. Taking samples of soil                                     26
       44. Weighing                                                   27
       45. Computation                                                28
       46. Time and location of readings                              28
       47. Location of readings                                       29
       48. Depth of samples                                           30
       49. Check and control instruments                              30
      Physical and Physiological Water
       50. The availability of soil water                             30
       51. Terms                                                      31
       52. Chresard determination under control                       32
       53. Chresard readings in the field                             33
       54. Chresard values of different soils                         34
      Records and Results
       55. The field record                                           35
       56. The permanent record                                       36
       57. Sums and means                                             36
       58. Curves                                                     37
 Humidity
       59. Instruments                                                37
      Psychrometers
       60. Kinds                                                      37
       61. The sling psychrometer                                     38
       62. Readings                                                   39
       63. Cog psychrometer                                           39
       64. Construction and use                                       40
       65. Hygrometers                                                40
      Psychrographs
       66. The Draper psychrograph                                    41
       67. Placing the instrument                                     42
       68. Regulating and operating the instrument                    43
       69. The weekly visit                                           44
      Humidity Readings and Records
       70. The time of readings                                       44
       71. Place and height                                           45
       72. Check instruments                                          45
       73. Humidity tables                                            46
       74. Sums, means, and curves                                    47
           Conversion scale for temperatures
       75. Records                                                    48
 Light
       76. Methods                                                    48
      The Photometer
       77. Construction                                               49
       78. Filling the photometer                                     50
       79. Making readings                                            50
       80. The Dawson-Lander sun recorder                             51
       81. The selagraph                                              52
      Standards
       82. Use                                                        53
       83. Making a standard                                          53
       84. Kinds of standards                                         54
      Readings
       85. Time                                                       55
           Chart for determining sun’s altitude                       57
       86. Table for determining apparent noon                        58
       87. Place                                                      59
           Table of intensity at various angles                       60
      Reflected and Absorbed Light
       88. The fate of incident light                                 60
       89. Methods of determination                                   61
       90. Leaf and epidermis prints                                  62
      Expression of Results
       91. Light records                                              63
       92. Light sums, means and curves                               63
 Temperature
       93.                                                            64
      Thermometers
       94. Air thermometers                                           64
       95. Soil thermometers                                          64
       96. Maximum-minimum thermometers                               65
       97. Radiation thermometers                                     67
       98. Thermographs                                               67
      Readings
       99. Time                                                       69
      100. Place and height                                           70
      Expression of Results
      101. Temperature records                                        70
      102. Temperature sums and means                                 70
      103. Temperature curves                                         71
      104. Plant temperatures                                         71
 Precipitation
      105. General relations                                          72
      106. The rain gauge                                             73
      107. Precipitation records                                      74
 Wind
      108. Value of readings                                          74
      109. The anemometer                                             75
      110. Records                                                    76
 Soil
      111. Soil as a factor                                           76
      112. The value of soil surveys                                  77
      113. The origin of soils                                        77
      114. The structure of soils                                     78
      115. Mechanical analysis                                        79
      116. Kinds of soils                                             79
      117. The chemical nature of soils                               80
 Physiography
      118. Factors                                                    80
      Altitude
      119. Analysis into factors                                      81
      120. The barometer                                              82
      Slope
      121. Concept                                                    83
      122. The clinometer                                             83
      123. The trechometer                                            84
      Exposure
      124. Exposure                                                   85
      125. Surface                                                    85
      126. Record of physiographic factors                            86
      127. Topography                                                 86
 Biotic Factors
      128. Influence and importance                                   86
      129. Animals                                                    87
      130. Plants                                                     87

                    METHODS OF HABITAT INVESTIGATION

      131.                                                            88
 Method of Simple Instruments
      132. Choice of stations                                         88
      133. Time of readings                                           89
      134. Details of the method                                      89
      135. Records                                                    91
 Method of Ecograph Batteries
      136.                                                            92
 Expression of Physical Factor Results
      137. The form of results                                        94
      Factor Records
      138.                                                            94
      Factor Curves
      139. Plotting                                                   95
      140. Kinds of curves                                            96
      141. Combinations of curves                                     96
      142. The amplitude of curves                                    98
      Factor Means and Sums
      143.                                                            98


                         CHAPTER III. THE PLANT

                          STIMULUS AND RESPONSE

 General Relations
      144. The nature of stimuli                                     100
      145. The kinds of stimuli                                      100
      146. The nature of response                                    101
      147. Adjustment and adaptation                                 102
      148. The measurement of response                               103
      149. Plasticity and fixity                                     104
      150. The law of extremes                                       105
      151. The method of working hypotheses                          106
 Hydroharmose
      Adjustment
      152. Water as a stimulus                                       107
      153. The influence of other factors upon water                 107
      154. Response                                                  108
      155. The measurement of absorption                             109
      156. The quantitative relation of absorption and
             transpiration                                           111
      157. Measurement of transpiration                              113
      158. Field methods                                             114
      159. Expression of results                                     116
      160. Coefficient of transpiration                              117
      Adaptation
      161. Modifications due to water stimuli                        118
      162. Modifications due to a small water supply                 118
      163. The decrease of water loss                                118
      164. The increase of water supply                              121
      165. Modifications due to an excessive water supply            121
      166. Plant types                                               122
      167. Xerophytic types                                          122
      168. Types of leaf xerophytes                                  123
      169. Types of stem xerophytes                                  125
      170. Bog plants                                                126
      171. Hydrophytic types                                         127
 Photoharmose
      Adjustment
      172. Light as a stimulus                                       129
      173. The reception of light stimuli                            131
      174. The response of the chloroplast                           132
      175. Aeration and translocation                                134
      176. The measurement of responses to light                     135
      Adaptation
      177. Influence of chloroplasts upon form and structure         138
      178. Form of leaves and stems                                  139
      179. Modification of the epidermis                             140
      180. The differentiation of the chlorenchym                    142
      181. Types of leaves                                           144
      182. Heliophytes and sciophytes                                144

                         EXPERIMENTAL EVOLUTION

      183. Scope                                                     145
      184. Fundamental lines of inquiry                              146
      185. Ancestral form and structure                              146
      186. Variation and mutation                                    147
      187. Methods                                                   149
 Method of Natural Experiment
      188. Selection of species                                      149
      189. Determination of factors                                  151
      190. Method of record                                          152
 Method of Habitat Cultures
      191. Scope and advantages                                      153
      192. Methods                                                   153
      193. Transfer                                                  154
      194. Modification of the habitat                               156
 Method of Control Cultures
      195. Scope and procedure                                       157
      196. Water-content series                                      158
      197. Light series                                              160


                        CHAPTER IV. THE FORMATION

                   METHODS OF INVESTIGATION AND RECORD

      198. The need of exact methods                                 161
 Quadrats
      199. Uses                                                      161
      200. Possible objections                                       163
      Kinds of Quadrats and Their Use
      201. Size and kinds                                            164
      202. Tapes and stakes                                          164
      203. Locating quadrats                                         165
      The List Quadrat
      204. Description                                               165
      205. Manner of use                                             166
      206. Table of abundance                                        166
      The Chart Quadrat
      207. Description and use                                       167
      208. The chart                                                 168
      209. Mapping                                                   168
      210. Factors and photographs                                   170
      The Permanent Quadrat
      211. Description and uses                                      170
      212. Manner of use                                             172
      The Denuded Quadrat
      213. Description                                               173
      214. Methods of denuding and recording                         174
      215. Physical factors                                          175
      Aquatic Quadrats
      216. Scope                                                     175
 Transects
      217. The transect                                              176
      The Line Transect
      218. Description and method                                    176
      219. The location and size                                     177
      The Belt Transect
      220. Details                                                   178
      The Permanent Transect
      221. Advantages                                                179
      222. Details                                                   179
      The Denuded Transect
      223.                                                           180
      The Layer Transect
      224.                                                           180
 Ecotone Charts
      225.                                                           181
 The Migration Circle
      226. Purpose                                                   182
      227. Location and method                                       182
      228. The denuded circle                                        183
      229. Photographs                                               183
 Cartography
      230. Value of cartographic methods                             183
      231. Standard scale                                            184
      232. Color scheme                                              184
      233. Formation and vegetation maps                             185
      234. Continental maps                                          187
 Photography
      235.                                                           188
      236. The camera and its accessories                            188
      237. The choice of a camera                                    190
      238. The use of the camera                                     191
      239. The sequence of details                                   192
      240. The time of exposure                                      193
      241. Developing                                                194
      242. Finishing                                                 195
 Formation and Succession Herbaria
      243. Concept and purpose                                       196
      244. Details of collecting                                     197
      245. Arrangement                                               197
      246. Succession herbaria                                       198

                        DEVELOPMENT AND STRUCTURE

      247. Vegetation an organism                                    199
      248. Vegetation essentially dynamic                            199
      249. Functions and structures                                  199
 Association
      250. Concept                                                   200
      251. Causes                                                    201
      252. Aggregation                                               203
      Kinds of Association
      253. Categories                                                204
      254. Stratum association                                       204
      255. Ground association                                        205
      256. Species guild association                                 206
      257. Light association                                         206
      258. Water-content association                                 208

                    THE DEVELOPMENT OF THE FORMATION

      259.                                                           210
 Invasion
      260.                                                           210
      Migration
      261.                                                           210
      262. Mobility                                                  211
      263. Organs for dissemination                                  211
      264. Contrivances for dissemination                            212
      265. Position of disseminules                                  214
      266. Seed production                                           215
      267. Agents of migration                                       216
      268. The direction of migration                                219
      Ecesis
      269. Concept                                                   220
      270. Germination of the seed                                   221
      271. Adjustment to the habitat                                 223
      Barriers
      272. Concept                                                   224
      273. Physical barriers                                         225
      274. Biological barriers                                       225
      275. Influence of barriers                                     226
      Endemism
      276. Concept                                                   227
      277. Causes                                                    228
      278. Significance                                              228
      Polyphylesis and Polygenesis
      279. Concept                                                   230
      280. Proofs of polygenesis                                     231
      281. Origin by polyphylesis                                    232
      Kinds of Invasion
      282. Continuous and intermittent invasion                      234
      283. Complete and partial invasion                             235
      284. Permanent and temporary invasion                          235
      Manner of Invasion
      285. Entrance into the habitat                                 236
      286. Influence of levels                                       238
      Investigation of Invasion
      287.                                                           239
 Succession
      288. Concept                                                   239
      289. Kinds of succession                                       240
      Primary Successions
      290.                                                           241
      291. Succession through elevation                              241
      292. Succession through volcanic action                        242
      293. Weathering                                                243
      294. Succession in residuary soils                             243
      295. Succession in colluvial soils                             244
      296. Succession in alluvial soils                              245
      297. Succession in aeolian soils                               246
      298. Succession in glacial soils                               247
      Secondary Successions
      299.                                                           247
      300. Succession in eroded soils                                247
      301. Succession in flooded soils                               248
      302. Succession by subsidence                                  249
      303. Succession in land slips                                  249
      304. Succession in drained or dried soils                      249
      305. Succession by animal agency                               250
      306. Succession by human agency                                250
      307. Succession in burned areas                                251
      308. Succession in lumbered areas                              252
      309. Succession by cultivation                                 253
      310. Succession by drainage                                    253
      311. Succession by irrigation                                  253
      312. Anomalous successions                                     254
      313. Perfect and imperfect successions                         254
      314. Stabilization                                             255
      Causes and Reactions
      315.                                                           256
      316. Succession by preventing weathering                       257
      317. Succession by binding aeolian soils                       258
      318. Succession by reducing run-off and erosion                259
      319. Succession by filling with silt and plant remains         260
      320. Succession by enriching the soil                          261
      321. Succession by exhausting the soil                         262
      322. Succession by the accumulation of humus                   263
      323. Succession by modifying atmospheric factors               264
      Laws of Succession
      324.                                                           264
      Classification and Nomenclature
      325. Basis                                                     267
      326. Nomenclature                                              267
      327. Illustrations                                             270
      Investigation of Succession
      328. General rules                                             270
      329. Method of alternating stages                              271
      330. The relict method                                         272

                     THE STRUCTURE OF THE FORMATION

      331.                                                           274
 Zonation
      332. Concept                                                   274
      Causes of Zonation
      333. Growth                                                    275
      334. Reactions                                                 276
      335. Physical factors                                          276
      336. Physiographic symmetry                                    278
      Kinds of Zonation
      337.                                                           279
      338. Radial zonation                                           280
      339. Bilateral zonation                                        280
      340. Vertical zonation                                         280
      341. Vegetation zones                                          281
 Alternation
      342. Concept                                                   283
      343. Causes                                                    284
      344. Competition                                               285
      345. Kinds of alternation                                      289
 The Formation in Detail
      346. The rank of the formation                                 292
      347. The parts of a formation                                  295
      348. Nomenclature of the divisions                             299
      349. The investigation of a particular formation               299
 Classification and Relationship
      350. Basis                                                     300
      351. Habitat classification                                    301
      352. Nomenclature                                              302
      353. Developmental classification                              304
      354. Regional classification                                   304
      355. Mixed formations                                          304

                         EXPERIMENTAL VEGETATION

      356. Scope and methods                                         306
 Method of Natural Habitats
      357. Natural experiments                                       307
 Method of Artificial Habitats
      358. Modification of habitat                                   307
      359. Denuding                                                  308
      360. Modification of the formation by transfer                 309
 Method of Control Habitats
      361. Competition cultures                                      310
      362. Details of culture methods                                311

 GLOSSARY                                                            314

 BIBLIOGRAPHY                                                        324




                      RESEARCH METHODS IN ECOLOGY




                  CHAPTER I. THE FOUNDATION OF ECOLOGY
                          THE NEED OF A SYSTEM


=1. The scope of ecology.= The clue to the field of ecology is found in
the Greek word, οἲκος, home. The point of view in the following treatise
is constantly that which is inherent in the term itself. Ecology is
therefore considered the dominant theme in the study of plants, indeed,
as the central and vital part of botany. This statement may at first
appear startling, if not unfounded, but mature reflection will show that
all the questions of botanical science lead sooner or later to the two
ultimate facts: plant and habitat. The essential truth of this has been
much obscured by detached methods of study in physiology, morphology,
and histology, which are too often treated as independent fields. These
have suffered incomplete and unsymmetric development in consequence of
extreme specialistic tendencies. Analytic methods have dominated
research to the exclusion of synthetic ones, which, in a greatly
diversified field, must be final, if botanical knowledge is something to
be systematized and not merely catalogued. Physiology in particular has
suffered at the hands of detached specialists. Originally conceived as
an inquiry into the origin and nature of plants, it has been developed
strictly as a study of plant activities. It all but ignores the physical
factors that control function, and the organs and tissues that reflect
it.

=2. Ecology and physiology.= There can be little question in regard to
the essential identity of physiology and ecology. This is evident when
it is clearly seen that the present difference between the two fields is
superficial. Ecology has been largely the descriptive study of
vegetation; physiology has concerned itself with function; but, when
carefully analyzed, both are seen to rest upon the same foundation. In
each, the development is incomplete: ecology has so far been merely
superficial, physiology too highly specialized. The one is chaotic and
unsystematized, the other too often a minute study of function under
abnormal circumstances. The greatest need of the former is the
introduction of method and system, of the latter, a broadening of scope
and new objectives. The growing recognition of the identity of the two
makes it desirable to anticipate their final merging, and to formulate a
system that will combine the good in each, and at the same time
eliminate superficial and extreme tendencies. In this connection, it
becomes necessary to point out to ecologist and physiologist alike that,
while they have been working on the confines of the same great field,
each must familiarize himself with the work and methods of the other,
before his preparation is complete. Both must broaden their horizons,
and rearrange their views. The ecologist is sadly in need of the more
intimate and exact methods of the physiologist: the latter must take his
experiments into the field, and must recognize more fully that function
is but the middleman between habitat and plant. It seems probable that
the final name for the whole field will be physiology, although the term
ecology has distinct advantages of brevity and of meaning. In this
event, however, it should be clearly recognized that, although the name
remains the same, the field has become greatly broadened by new
viewpoints and new methods.


                         HISTORICAL DEVELOPMENT

=3. Geographical distribution.= The systematic analysis of the great
field of ecology is essential to its proper development in the future. A
glance at its history shows that, while a number of essential points of
attack have been discovered, only one or two of these have been
organized, and that there is still an almost entire lack of correlation
and coordination between these. The earliest and simplest development of
the subject was concerned with the distribution of plants. This was at
first merely an offshoot of taxonomy, and, in spite of the work of
Humboldt and Schouw, has persisted in much of its primitive form to the
present time, where it is represented by innumerable lists and
catalogues. Geographical distribution was grounded upon the species, a
fact which early caused it to become stereotyped as a statistical study
of little value. This tendency was emphasized by the general practice of
determining distribution for more or less artificial areas, and of
instituting comparisons between regions or continents too little known
or too widely remote. The fixed character of the subject is conclusively
shown by the fact that it still persists in almost the original form
more than a half century after Grisebach pointed out that the formation
was the real unit of vegetation, and hence of distribution.

=4. The plant formation.= The corner-stone of ecology was laid by
Grisebach in 1838 by his recognition of the plant formation as the
fundamental feature of vegetation. Earlier writers, notably Linné (1737,
1751), Biberg (1749), and Hedenberg (1754), had perceived this relation
more or less clearly, but failed to reduce it to a definite guiding
principle. This was a natural result of the dominance of descriptive
botany in the 18th century, by virtue of which all other lines of
botanical inquiry languished. This tendency had spent itself to a
certain degree by the opening of the 19th century, and both plant
distribution and plant physiology began to take form. The stimulus given
the former by Humboldt (1807) turned the attention of botanists more
critically to the study of vegetation as a field in itself, and the
growing feeling for structure in the latter led to Grisebach’s concept
of the formation, which he defined as follows: “I would term a group of
plants which bears a definite physiognomic character, such as a meadow,
a forest, etc., a phytogeographic formation. The latter may be
characterized by a single social species, by a complex of dominant
species belonging to one family, or, finally, it may show an aggregate
of species, which, though of various taxonomic character, have a common
peculiarity; thus, the alpine meadows consist almost exclusively of
perennial herbs.” The acceptance of the formation as the unit of
vegetation took place slowly, but as a result of the work of Kerner
(1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze
(1882), Beck (1884), Drude (1889), and Warming (1889), this point of
view came to be more and more prevalent. It was not, however, until the
appearance of three works of great importance, Warming (1895), Drude
(1896), and Schimper (1898), that the concept of the formation became
generally predominant. With the growing recognition of the formation
during the last decade has appeared the inevitable tendency to
stereotype the subject of ecology in this stage. The present need, in
consequence, is to show very clearly that the idea of the formation is a
fundamental, and not an ultimate one, and that the proper superstructure
of ecology is yet to be reared upon this as the foundation.

=5. Plant succession.= The fact that formations arise and disappear was
perceived by Biberg as early as 1749, but it received slight attention
until Steenstrup’s study of the succession in the forests of Zealand
(1844 prox.). In the development of formations, as well as in their
recognition, nearly all workers have confined themselves to the
investigation of particular changes. Berg (1844), Vaupell (1851),
Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming
(1890), and others have added much to our detailed knowledge of
formational development. Notwithstanding the lapse of more than a half
century, the study of plant successions is by no means a general
practice among ecologists. This is a ready explanation of the fact that
the vast field has so far yielded but few generalizations. Warming
(1895) was the first to compile the few general principles of
development clearly indicated up to this time. The first critical
attempt to systematize the investigation of succession was made by
Clements (1904), though this can be considered as little more than a
beginning on account of the small number of successions so far studied.
Future progress in this field will be conditioned not only by the more
frequent study of developmental problems by working ecologists, but
also, and most especially, by the application of known principles of
succession, and by the working out of new ones.

=6. Ecological phytogeography.= Until recent years, the almost universal
tendency was to give attention to formations from the standpoint of
vegetation alone. While the habitat was touched here and there by
isolated workers, and plant functions were being studied intensively by
physiologists, both were practically ignored by ecologists as a class.
The appearance of Warming’s _Lehrbuch der oecologischen
Pflanzengeographie_ (1896) and of Schimper’s _Pflanzengeographie auf
physiologischer Grundlage_ (1898) remedied this condition in a measure
by a general discussion of the habitat, and by emphasizing the
importance of the ecological or physiological point of view. Despite
their frank recognition of the unique value of the habitat, the major
part of both books was necessarily given to what may be termed the
general description of formations. For this reason, and for others
arising out of an almost complete dearth of methods of investigation,
ecology is still almost entirely a floristic study in practice, although
there is a universal recognition of the much greater value of the
viewpoint which rests upon the relation between the formation and its
habitat.

=7. Experimental ecology.= Properly speaking, the experimental study of
ecology dates from Bonnier[1] (1890, 1895), though it is well understood
that experimental adjustment of plants to certain physical factors had
been the subject of investigation before this time. The chief merit of
Bonnier’s work, however, lies in the fact that it was done out of doors,
under natural conditions, and for these reasons it should be regarded as
the real beginning of this subject. Bonnier’s experiments were made for
the purpose of determining the effect of altitude. Culture plots of
certain species were located in the Alps and the Pyrenees, and the
results were compared with control cultures made in the lowlands about
Paris. In 1894 he also made a comparative study of certain polydemic
species common to the arctic islands, Jan Meyen and Spitzenberg, and to
the Alps. Both of these methods are fundamental to field experiment, but
the results are inconclusive, inasmuch as altitude is a complex of
factors. As no careful study was made of the latter, it was manifestly
impossible to refer changes and differences of structure to the definite
cause. In a paper that has just appeared, E. S. Clements (1905) has
applied the method of polydemic comparison to nearly a hundred species
of the Rocky mountains. In this work, the all-important advance has been
made of determining accurately the decisive differences between the two
or more habitats of the same species in terms of direct factors,
water-content, humidity, and light. In his own investigations of
Colorado mountain vegetation, the author has applied the method of field
cultures by planting seeds of somewhat plastic species in habitats of
measured value, and has thought to initiate a new line of research by
applying experimental methods to the study of vegetation as an organism.
In connection with this, there has also been developed a method of
control experiment in the plant house under definitely measured
differences of water and light.

=8. Ecology of the habitat.= Since the time of Humboldt, there have been
desultory attempts to determine the physical factors of habitats with
some degree of accuracy. The first real achievement in this line was in
the measurement of light values by Wiesner in 1896. In 1898 the writer
first began to study the structure of habitats by the determination of
water-content, light, humidity, temperature, wind, etc., by means of
instruments. These methods were used by one of his pupils, Thornber
(1901), in the study of a particular formation, and by another, Hedgcock
(1902), in a critical investigation of water-content. Two years later,
similar methods of measuring physical factors were put into operation in
connection with experimental evolution under control in the plant house.
E. S. Clements (1905), as already indicated, has made the use of factor
instruments the foundation of a detailed study of polydemic species, i.
e., those which grow in two or more habitats, and which are, indeed, the
most perfect of all experiments in the production of new forms. In a
volume in preparation upon the mountain vegetation of Colorado, the
writer has brought the use of physical factor instruments to a logical
conclusion, and has made the study of the habitat the basis of the whole
work. Out of this investigation has come a new concept of vegetation
(Clements 1904), namely, that it is to be regarded as a complex organism
with structures and with functions susceptible of exact methods of
study.

=9. The evidence from historical development.= This extremely brief
resume of what has been accomplished in the several lines of ecological
research makes evident the almost complete absence of correlation and of
system. The whole field not merely lacks system, but it also demands a
much keener perception of the relative value of the different tendencies
already developed. It is inevitable from the great number of tyros, and
of dilettante students of ecology in comparison with the few
specialists, that the surface of the field should have received all of
the attention. It is, however, both unfortunate and unscientific that
great lines of development should be entirely unknown to all but a few.
There is no other department of botany in which the superficial study of
more than half a century ago still prevails to the exclusion of better
methods, many of which have been known for a decade or more. It is
clear, then, that the imperative need of ecology is the proper
coordination of its various points of view, and the working out of a
definite system which will make possible a ready recognition of that
which is fundamental and of that which is merely collateral. The
historical development, as is well understood, can furnish but a slight
clue to this. It is a fact of common knowledge that the first
development of any subject is general, and usually superficial also.
True values come out clearly only after the whole field has been
surveyed. For these reasons, as will be pointed out in detail later, the
newer viewpoints are regarded as either the most important or the most
fundamental. Experimental ecology will throw a flood of light upon plant
structure and function, while exact methods of studying the habitat are
practically certain of universal application in the future.


                       PRESENT STATUS OF ECOLOGY

=10. The lack of special training.= The bane of the recent development
popularly known as ecology has been a widespread feeling that anyone can
do ecological work, regardless of preparation. There is nothing in
modern botany more erroneous than this feeling. The whole task of
ecology is to find out what the living plant and the living formation
are doing and have done in response to definite complexes of factors, i.
e., habitats. In this sense, ecology is practically coextensive with
botany, and the student of a local flora who knows a few hundred species
is no more competent to do ecological work than he is to reconstruct the
phylogeny of the vegetable kingdom, or to explain the transmission of
ancestral qualities. The comprehensive and fundamental character of the
subject makes a broad special training even more requisite than in more
restricted lines of botanical inquiry. The ecologist must first of all
be a botanist, not a mere cataloguer of plants, and he must also possess
a particular training in the special methods of ecological research. He
must be familiar with the various points of attack in this field, and he
must know the history of his subject thoroughly. Ecology affords the
most striking example of the prevalent evil of American botanical study,
i. e., an indifference to, or an ignorance of the literature of the
subject. The trouble is much aggravated here, however, by the breadth of
the field, and the common assumption that a special training is
unnecessary, if not, indeed, superfluous. Ignorance of the important
ecological literature has been a most fertile source of crude and
superficial studies, a condition that will become more apparent as these
fields are worked again by carefully trained investigators.

=11. Descriptive ecology.= The stage of development of the subject at
the present time may be designated as _descriptive ecology_, for
purposes of discussion merely. This is concerned with the superficial
description of vegetation in general terms, and results from the fact
that the development has begun on the surface, and has scarcely
penetrated beneath it. The organic connection between ecology and
floristic has produced an erroneous impression as to the relative value
of the two. Floristic has required little knowledge, and less
preparation: it lends itself with insidious ease to chance journeys or
to vacation trips, the fruits of which are found in vague descriptive
articles, and in the multiplication of fictitious formations. While
there is good reason that a record should be left of any serious
reconnaissance, even though it be of a few weeks’ duration, the
resulting lists and descriptive articles can have only the most
rudimentary value, and it is absurd to regard them as ecological
contributions at all. No statement admits of stronger emphasis, and
there is none that should be taken more closely to heart by botanists
who have supposed that they were doing ecological work. An almost
equally fertile source of valueless work is the independent treatment of
a restricted local area. The great readiness with which floristic lists
and descriptions can be made has led many a botanist, working in a small
area, or passing hurriedly through an extended region, to try his hand
at formation making. From this practice have resulted scores of
so-called formations, which are mere patches, consocies, or stages in
development, or which have, indeed, no existence other than in the minds
of their discoverers. The misleading definiteness which a photograph
seems to give a bit of vegetation has been responsible for a surplus of
photographic formations, which have no counterparts in nature.
Indispensable as the photograph is to any systematic record of
vegetation, its use up to the present time has but too often served to
bring it into disrepute. There has been a marked tendency to apply the
current methods of descriptive botany to vegetation, and to regard every
slightly different piece of the floral covering as a formation. No
method can yield results further from the truth. It is evident that the
recognition and limitation of formations should be left absolutely to
the broadly trained specialist, who has a thorough preparation by virtue
of having acquainted himself carefully with the development and
structure of typical formations over large areas.

=12. The value of floristic.= In what has been said above, there is no
intent to decry the value of floristic. The skilled workman can spare
the material which he is fashioning as readily as the ecologist can work
without an accurate knowledge of the genera and species which make up a
particular vegetation. Some botanists whose knowledge of ecology is that
of the study or the laboratory have maintained that it is possible to
investigate vegetation without knowing the plants which compose it.
Ecology is to be wrought out in the field, however, and the field
ecologist—none other, indeed, should bear the name—understands that
floristic alone can furnish the crude material which takes form under
his hands. It is the absolute need of a thorough acquaintance with the
flora of a region which makes it impossible for a traveler to obtain
anything of real ecological value in his first journey through a
country. As the very first step, he must gain at least a fair knowledge
of the floristic, which will alone take the major part of one or more
growing seasons. This information the student of a local flora already
has at the tip of his tongue; in itself it can not constitute a
contribution to ecology, but merely the basis for one. In this
connection, moreover, it can not be used independently, but becomes of
value only after an acquaintance with a wide field. Floristic study and
floristic lists, then, are indispensable, but to be of real value their
proper function must be clearly recognized. They do not constitute
ecology.

=13. Reconnaissance and investigation.= In striving to indicate the true
value and worth of ecological study, it becomes necessary to draw a
definite line between what we may term _reconnaissance_ and
_investigation_. By the former is understood the preliminary survey of a
region, extending over one or two years. The objects of such a survey
are to obtain a comprehensive general knowledge of the topography and
vegetation of the region, and of its relation to the other regions about
it. The chief purpose, however, is to gain a good working acquaintance
with the flora: a reconnaissance to be of value must do this at all
events. Certain general facts will inevitably appear during this
process, but they will invariably need the confirmation of future study.
It would be an advantage to real ecology if reconnaissance were to
confine itself entirely to the matter of making a careful floristic
survey. Investigation begins when the inquiry is directed to the
habitat, or to the development and structure of the formation which it
bears, i. e., when it takes up the manifold problems of the οἶκος. Such
a study must be based upon floristic, but the latter becomes a part of
investigation only in so far as it leads to it. Standing by itself, it
is not ecological research: it is the preparation for it. This
distinction deserves careful thought. The numerous recruits to ecology
have turned their attention to what lay nearest to hand, with little
question as to its value, or to where it might lead. The result has been
to make reconnaissance far outweigh investigation in amount, and to give
it a value which properly belongs to the latter. Furthermore, this
mistaken conception has in many cases, without doubt, prevented its
leading to valuable research work.

=14. Resident investigation.= Obviously, if reconnaissance is a
superficial survey, and investigation thorough extensive study, an
important distinction between them is in the time required. While one
may well be the result of a journey of some duration, the other is
essentially dependent upon residence. In the past the great disparity
between the size of the field and the number of workers has made
resident study too often an ideal, but in the future it will be
increasingly the case that a particular region will be worked by a
trained ecologist resident in it. This may never be altogether true of
inaccessible and sterile portions of the globe. It may be pointed out,
however, that, between the tropics and the poles, residence during the
summer or growing period is in essence continuous residence. In the
ultimate analysis, winter conditions have of course some influence upon
the development of vegetation during the summer, but the important
problems which a vegetation presents must be worked out during the
period of development. For temperate, arctic, and alpine regions, then,
repeated study during the growing period for a term of years has
practically all the advantages of continuous residence. For all
practical purposes, it is resident study.

=15. The dangers of a restricted field.= In the resident study of a
particular region, the temptation to make an intensive investigation of
a circumscribed area is very strong. The limits imposed by distance are
alone a sufficient explanation of this, but it is greatly increased by
the inclination toward detailed study for which a small field offers
opportunity. This temptation can be overcome only by a general
preliminary study of the larger region in which the particular field is
located. The broader outlook gained in this way will throw needed light
upon many obscure facts of the latter, and at the same time it will act
as a necessary corrective of the tendency to consider the problems of
the local field in a detached manner, and to magnify the value of the
distinctions made and the results obtained. Such a general survey has
the purpose and value of a reconnaissance, and is always the first step
in the accurate and detailed investigation of the local area or
formation. Each corrects the extreme tendency of the other, and thorough
comprehensive work can be done only by combining the two methods. When
the field of inquiry is a large area or covers a whole region, the
procedure should be essentially the same. A third stage must be added,
however, in which a more careful survey is made of the entire field in
the light of the thorough study of the local area. The writer’s methods
in the investigation of the Colorado vegetation illustrate this
procedure. The summers of 1896, 1897, 1898 were devoted to
reconnaissance; those of 1899–1904 were given to detailed and
comprehensive study by instrument and quadrat of a highly diversified,
representative area less than 20 miles square, while the work of the
final summer will be the application of the results obtained in this
localized area to the region traversed from 1896–98. This is practically
the application of methods of precision to an area of more than 100,000
square miles. It also serves to call attention to another point not
properly appreciated as yet by those who would do ecological work. This
is the need of taking up field problems as a result of serious
forethought, and not as a matter of accident or mere propinquity. A
carefully matured plan of attack which contemplates an expenditure of
time and energy for a number of years will yield results of value, no
matter how much attention an area may have received. On the other hand,
an aimless or hurried excursion into the least known or richest of
regions will lead to nothing but a waste of time, especially upon the
part of the ecologist, who must read the articles which result, if only
for the purpose of making sure that there is nothing in them.


                        APPLICATIONS OF ECOLOGY

=16. The subjects touched by ecology.= The applications of ecological
methods and results to other departments of botany, and to other fields
of research are numerous. Many of these are both intimate and
fundamental, and give promise of affording new and extremely fruitful
points of view. It has already been indicated that ecology bears the
closest of relations to morphology and histology on the one side, and to
physiology on the other—that it is, indeed, nothing but a rational field
physiology, which regards form and function as inseparable phenomena.
The closeness with which it touches plant pathology follows directly
from this, as pathology is nothing more than abnormal form and
functioning. Experimental work in ecology is purely a study of
_evolution_, and the facts of the latter are the materials with which
_taxonomy_ deals. _Forestry_ has already been termed “applied ecology”
and in its scientific aspects, which are its foundation, it is precisely
the ecology of woody plants, and of the vegetation which they
constitute. Apart from botany, the physical side of ecology is largely a
question of _soil physics_, and of _physiography_. On the other hand,
vegetation is coming more and more to be regarded as a fundamental
factor in _zoogeography_ and in _sociology_. Furthermore, with respect
to the latter, it will be pointed out below that the principles of
association which have been determined for plants, viz., invasion,
succession, zonation, and alternation, apply with almost equal force to
man.

=17. Physiology and pathology.= The effect of ecology in emphasizing the
intrinsically close connection between physiology and morphology has
already been mentioned. Its influence in normalizing the former by
forcing it into the field as the place for experiment, and by directing
the chief attention to the plant as an organism rather than a complex of
organs, is also rapidly coming to be felt. Ecology will doubtless exert
a corrective influence upon pathology in the near future. This is
inevitable as the latter ceases to be the merely formal study of
specific pathogenic organisms, and turns its attention to the cause of
all abnormality, which is to be found in the habitat, whether this be
physical, as when the water-content is low, or biotic, when a parasitic
fungus is present. The relative ease with which specific diseases can be
studied has helped to obscure the essential fact that the approach to
pathology must be through physiology. Much indeed of the observational
physiology of the laboratories has been pathology, and it will be
impossible to draw a clear line between them until precise experiment in
the habitat has come into vogue.

=18. Experimental evolution.= As a result of the extremely fragmentary
character of the geological record, nothing is more absolute than that
there can be no positive knowledge of the exact origin of a form or
species, except in those rare cases of the present day, where the whole
process has taken place under the eye of a trained observer. The origin
of the plant forms known at present must forever lie without the domain
of direct knowledge. If it were possible, by a marvel of ingenuity and
patience to develop experimentally _Myosurus_ from _Selaginella_, this
would not be absolutely conclusive proof that _Myosurus_ was first
derived in this way. When all is said, however, this would be the very
best of presumptive evidence. It must also be recognized that this is
the nearest to complete proof that we shall ever attain, and with this
in mind it becomes apparent at once that evidence from experiment is of
paramount importance in the study of evolution (the origin of species).

The phase of experimental ecology which has to do with the plant has
well been called experimental evolution. While this is a field almost
wholly without development at present, there can be little question that
it is to be one of the most fertile and important in the future.
Attention will be directed first to those forms which are undergoing
modification at the present time. The cause and direction of change will
be ascertained, and its amount and rapidity measured by biometrical
methods. The next step will be to actually change the habitat of
representative types, and to determine for each the general trend of
adaptation, as well as the exact details. By means of the methods used
and the results obtained in these investigations, it will be possible to
attack the much more difficult problem of retracing the development of
species already definitely constituted. This will be accomplished by the
study of the derived and the supposed ancestral form, but owing to the
great preponderance of evolution over reversion, the study of the
ancestral form will yield much more valuable results.

The general application of the methods of experimental ecology will mark
a new era in the study of evolution. There has been a surplus of
literary investigation, but altogether too little actual experiment. The
great value of De Vries’ work lies not in the importance of the results
obtained, but in calling attention to the unique importance of
experimental methods in contributing to a knowledge of evolution. The
development of the latter has been greatly hindered by the dearth of
actual facts, and by a marked tendency to compensate for this by
verbiage and dogmatism. This is well illustrated by the present position
of the “mutation theory,” which, so far as the evidence available is
concerned, is merely a working hypothesis. An incredible amount of bias
and looseness of thought have characterized the discussion of evolution.
It is earnestly to be hoped that the future will bring more work and
less argument, and that the literary evolutionists will become less and
less reluctant to leave the relative merits of variation and mutation to
experiment.

=19. Taxonomy.= Taxonomy is classified evolution. It is distinct from
descriptive botany, which is merely a cataloguing of all known forms,
with little regard to development and relationship. The consideration of
the latter is peculiarly the problem of taxonomy, but the solution must
be sought through experimental evolution. The first task of the latter
is to determine the course of modification in related forms, and the
relationships existing between them. With this information, taxonomy can
group forms according to their rank, i. e., their descent. The same
method is applicable to the species of a genus, and, in a less degree,
perhaps, to the genera which constitute a family. The use to which it
may be put in indicating family relationships will depend largely upon
the gap existing between the families concerned. While interpretation
will always play a part in taxonomy, the general use of experiment will
leave much less opportunity for the personal equation than is at present
the case. Taxonomy, like descriptive botany, is based upon the species,
but, while there may exist a passable kind of descriptive botany, there
can be no real taxonomy as long as the sole criterion of a species is
the difference which any observer thinks he sees between one plant and
another. The so-called species of to-day range in value from mere
variations to true species which are groups of great constancy and
definiteness. The reasons for this are obvious when one recalls that
“species” are still the product of the herbarium, not of the field, and
that the more intensive the study, the greater the output in “species.”
It would seem that careful field study of a form for several seasons
would be the first requisite for the making of a species, but it is a
precaution which is entirely ignored in the vast majority of cases. The
thought of subjecting forms presumed to be species to conclusive test by
experiment has apparently not even occurred to descriptive botanists as
yet. Notwithstanding, there can be no serious doubt that the existing
practice of re-splitting hairs must come to an end sooner or later. The
remedy will come from without through the application of experimental
methods in the hands of the ecologist, and the cataloguing of slight and
unrelated differences will yield to an ordered taxonomy.

Experimental evolution will solve a taxonomic problem as yet untouched,
namely, the effect of recent environment upon the production of species.
It is well understood that some species grow in nature in various
habitats without suffering material change, while others are modified to
constitute a new form in each habitat. It is at once clear that these
forms (or ecads) are of more recent descent than the species, i. e., of
lower rank. It must also be recognized that a constant group and a
highly plastic one are essentially different. If constancy is made a
necessary quality of a species, one is a species, the other is not. If
both are species, then two different kinds must be distinguished. Among
the species of our manuals are found many ecads, alongside of constant
and inconstant species. These can be distinguished only by field
experiment, and their proper coordination is possible only after this
has been done. Indeed, the whole question of the ability or the
inability of environmental variation to produce constant species is one
that must be referred to repeated and long-continued experiment in the
field.

A minor service of considerable value can be rendered taxonomy by
working over the diagnosis from the ecological standpoint. Many
ecological facts are of real diagnostic value, while others are at least
of much interest, and serve to direct attention to the plant as a living
thing. The loose use of terms denoting abundance, which prevails in
lists and manuals, should be replaced by the exact usage which the
quadrat method has made possible for vegetation. The designation of
habitats could be made much more exact, and the formation, as well as
the habitat form or ecad, and the vegetation form or phyad, should be
indicated in addition. The general terms drawn from pollination,
seed-production, and dissemination might also be included to advantage.

=20. Forestry=, if the purely commercial aspects be disregarded, is the
ecology of a particular kind of vegetation, the forest. Therefore, in
pointing out the connection between them, it is only necessary to say
that whatever contributes to the ecology of the forest is a contribution
to forestry. There are, however, certain lines of inquiry which are of
fundamental importance. First among these, and of primary interest from
the practical point of view, are the questions pertaining to the
distribution of forests and their structure. Of even greater
significance are the problems of forest development, movement, and of
reforestation, which are comprised in succession. The gradual invasion
of the plains and prairies by the forest belt of the east and north is
in full conformity with the laws of invasion, and the ecological methods
to be employed here serve not merely to determine the actual conditions
at present, but also to forecast them with a great deal of accuracy. The
slow but certain development of forests on new soils, and their more
rapid reestablishment where the woody vegetation has been destroyed by
burning or lumbering, are ordinary phenomena of succession, for which
the ecologist has already worked out the laws, and determined the
methods of investigation. Having once ascertained the original and
adjacent vegetation and the character of the habitat, the ecologist can
indicate with accuracy not only the character of the new forest that
will appear, but also the nature of the antecedent formations. A full
knowledge of the character and laws of succession will prove of the
greatest value to the forester in all studies of forestation and
reforestation. Forests which now seem entirely unrelated will be seen to
possess the most intimate developmental connection, and the fuller
insight into the life history gained in this way will have a direct
bearing upon methods of conservation, etc. It will further show that the
forester must know other vegetations as well, since grassland and
thicket formations have an intimate influence upon the course of the
succession, as well as upon the advance of a forest frontier.

One of the greatest aids which modern ecology can furnish forestry,
however, is the method of determining the physical nature of the
habitat. So far, foresters have been obliged to content themselves with
a more or less superficial study of the structure of forest formations,
without being able to do more than guess at the physical causes which
control both structure and development. This handicap is especially
noticeable in the case of forest plantings in non-forested regions,
where it has been impossible to estimate the chances of success, or to
determine the most favorable areas except by actual plantations.
Equipped with the proper instruments for measuring water-content,
humidity, light and temperature, the ecologist is able to determine the
precise conditions under which reproduction is occurring, and to
ascertain what non-forested areas offer the most nearly similar
conditions. A knowledge of habitats and the means of measuring them
enables the forester to discover the causes which control the vegetation
with which he is already familiar, and to forecast results otherwise
hidden. Furthermore, it makes it possible for him to enter a new region
and to determine its nature and capabilities at a minimum of time and
energy.

=21. Physiography.= Physiographic features play an important part in
determining the quantity of certain direct factors of the habitat.
Perhaps a more important connection between physiography and ecology is
to be found in succession. The beginning of all primary, and of many
secondary successions is to be sought in the physiographic processes
which produce new habitats, or modify old ones. On the other hand, most
of the reactions which continue successions exert a direct influence
upon the form of the land. The most pronounced influence of terrestrial
successions is found in the stabilization which their ultimate stages
exert upon land forms, even where these are highly immature. The chief
effect of aquatic successions is to be found in the “silting up” and the
formation of new land, which result from the action of vegetation upon
silt-bearing waters. The closeness of the relation between succession
and the forms of the land has led to the application of the term
“physiographic ecology” to that part of the subject which deals with the
development of vegetation, i. e., succession.

=22. Soil physics.= This subject is as much a part of ecology as is
forestry. It is intrinsically that subdivision of ecology which deals
with the edaphic factors of the habitat, and their relation to the
plant. Since the basis is physics, there has been a general tendency to
overvalue the determination of soil properties, and to ignore the fact
that these are decisive only when considered with reference to the
living plant. As the soil contains the water which is the factor of
greatest importance to plants, soil physics is an especially important
part of ecology. Its methods are discussed under the habitat.

=23. Zoogeography.= Since animals are free for the most part, and hence
not confined so strictly to one spot as plants, their dependence upon
the habitat is not so evident. The relation is further obscured by the
fact that no physical factor has the direct effect upon them which water
or light exerts upon the plant. Vegetation, indeed, as the source of
food and protection, plays a more obvious, if not a more important part.
This is especially true of anthophilous insects, but it also holds for
all herbivorous animals, and, through them, for carnivorous ones. The
animal ecology of a particular region can only be properly investigated
after the habitats and plant formations have been carefully studied.
Here, as in floristics, a great deal can be done in the way of listing
the fauna, or studying the life habits of its species, without any
knowledge of plant ecology; but an adequate study must be based upon a
knowledge of the vegetation. Although animal formations are often poorly
defined, there can be no doubt of their existence. Frequently they
coincide with plant formations, and then have very definite limits. They
exhibit both development and structure, and are subject to the laws of
invasion, succession, zonation, and alternation, though these are not
altogether similar to those known for plants, a fact readily explained
by the motility of animals. Considered from the above point of view,
zoogeography is a virgin field, and it promises great things to the
student who approaches it with the proper training.

=24. Sociology.= In its fundamental aspects, sociology is the ecology of
a particular species of animal, and has in consequence, a similar close
connection with plant ecology. The widespread migration of man and his
social nature have resulted in the production of groups or communities
which have much more in common with plant formations than do formations
of other animals. The laws of association apply with especial force to
the family, tribe, community, etc., while the laws of succession are
essentially the same for both plants and man. At first thought it might
seem that man’s ability to change his dwelling-place and to modify his
environment exempts him in large measure from the influence of the
habitat. The exemption, however, is only apparent, as the control
exerted by climate, soil, and physiography is all but absolute,
particularly when man’s dependence upon vegetation, both natural and
cultural, is called to mind.


                       THE ESSENTIALS OF A SYSTEM

=25. Cause and effect: habitat and plant.= In seeking to lay the
foundation for a broad and thorough system of ecological research, it is
necessary to scan the whole field, and to discriminate carefully between
what is fundamental and what is merely collateral. The chief task is to
discover, if possible, such a guiding principle as will furnish a basis
for a permanent and logical superstructure. In ecology, the one relation
which is precedent to all others is the one that exists between the
habitat and the plant. This relation has long been known, but its full
value has yet to be appreciated. It is precisely the relation that
exists between cause and effect, and its fundamental importance lies in
the fact that all questions concerning the plant lead back to it
ultimately. Other relations are important, but no other is paramount, or
able to serve as the basis of ecology. Ecology sums up this relation of
cause and effect in a single word, and it may be that this advantage
will finally cause its general acceptance as the proper name for this
great field.

In the further analysis of the connection between the habitat and the
plant, it is evident that the causes or factors of the habitat act
directly upon the plant as an individual, and at the same time upon
plants as groups of individuals. The latter in no wise decreases the
importance of the plant as the primary effect of the habitat, but it
gives form to research by making it possible to consider two great
natural groups of phenomena, each characterized by very different
categories of effects. Ecology thus falls naturally into three great
fundamental fields of inquiry: habitat, plant, and formation (or
vegetation). To be sure, the last can be approached only through the
plant, but as the latter is not an individual, but the unit of a complex
from the formational standpoint, the formation itself may be regarded as
a sort of multiple organism, which is in many ways at least a direct
effect of the habitat. In emphasizing this fundamental relation of
habitat and vegetation, it is imperative not to ignore the fact that
neither plant nor formation is altogether the effect of its present
habitat. A third element must always be considered, namely, the
historical fact, by which is meant the ancestral structure. Upon
analysis, however, this is in its turn found to be the product of
antecedent habitats, and in consequence the essential connection between
the habitat and the plant is seen to be absolute.

=26. The place of function.= In the foregoing it is understood that the
immediate effect of the physical factors of the habitat is to be found
in the functions of the plant, and that these determine the plant
structure. Function has so long been the especial theme of plant
physiology that methods of investigation are numerous and well known,
and it is unnecessary here to consider it further than to indicate its
general bearing. The essential sequence in ecological research, then, is
the one already indicated, viz., habitat, plant, and formation, and this
will constitute the order of treatment in the following pages. That
portion of floristic which is not mere descriptive botany belongs to the
consideration of the formation, and in consequence there will be no
special treatment of floristic as a subdivision of ecology.




                        CHAPTER II. THE HABITAT


                          CONCEPT AND ANALYSIS

=27. Definition of the habitat.= The habitat is the sum of all the
forces or factors present in a given area. It is the exact equivalent of
the term environment, though the latter is commonly used in a more
general sense. As an ecological concept, the habitat refers to an area
much more definite in character, and more sharply limited in extent than
the habitat of species as indicated in the manuals. Since the careful
study of habitats has scarcely begun, it is impossible to recognize and
delimit them in an absolute sense. Visible topographic boundaries often
exist, but in many cases, the limit, though actual, is not readily
perceived. Contiguous habitats may be sharply limited, or they may pass
into each other so gradually that no real line of demarcation can be
drawn. Whatever variations they may show, however, all habitats agree in
the possession of certain essential factors, which are universally
present. On the other hand, a few factors are merely incidental and may
be present or absent. The relative value and amount of these is probably
similar for no two habitats, though the latter readily fall into groups
with reference to the amount of some particular factor.

=28. Factors.= The factors of a habitat are water-content, humidity,
light, temperature, soil, wind, precipitation, pressure, altitude,
exposure, slope, surface (cover), and animals. To these should be added
gravity and polarity, which are practically uniform for all habitats,
and may, in consequence, be ignored in this treatise. Length of season,
while it plays an important part in vegetation, is clearly a complex and
is to be treated under its constituents. Of the factors given, all are
regularly found in each habitat, though some are not constantly present.
The first five, water-content, humidity, light, temperature, and soil
are the most important, and any one may well serve as a basis for
grouping habitats into particular classes with reference to quantity. As
will be pointed out later, however, water-content and light furnish the
most striking differences between habitats, and offer the best means of
classification. As habitats are inseparable from the formations which
they bear, the discussion of the kinds of habitats is reserved for
chapter IV.


                       CLASSIFICATION OF FACTORS

=29. The nature of factors.= The factors of a habitat are arranged in
two groups according to their nature: (1) physical, (2) biotic. In the
strict sense, the physical factors constitute the habitat proper, and
are the real causative forces. No habitat escapes the influence of
biotic factors, however, as the formation always reacts upon it, and the
influence of animals is usually felt in some measure. Physical factors
are further grouped into (1) climatic and (2) edaphic, with respect to
source, or, better, the medium in which they are found. Climatic, or
atmospheric factors are humidity, light, temperature, wind, pressure,
and precipitation. Axiomatically, the stimuli which they produce are
especially related to the leaf. Edaphic or soil factors are confined to
the soil, as the term denotes, and are immediately concerned with the
functions of the root. Water-content is by far the most important of
these; the others are soil composition (nutrient-content), soil
temperature, altitude, slope, exposure, and surface. The last four are
of a more general character than the others, and are usually referred to
as physiographic factors. Cover, when dead, might well be placed among
these also, but as it is little different from the living cover in
effect, it seems most logical to refer it to biotic factors.

=30. The influence of factors.= While the above classification is both
obvious and convenient, a more logical and intimate grouping may be made
upon the influence which the factor exerts. On this basis, factors are
divided into (1) direct, (2) indirect, and (3) remote. Direct factors
are those which act directly upon an important function of the plant and
produce a formative effect: for example, an increase in humidity
produces an immediate decrease in transpiration. They are water-content,
humidity, and light. Other factors have a direct action: thus
temperature has an immediate influence upon respiration and probably
assimilation also, but it is not structurally formative. Wind has a
direct mechanical effect upon woody plants, but it does not fall within
our definition. Indirect factors are those that affect a formative
function of the plant through another factor; thus a change in
temperature causes a change in humidity and this in turn calls forth a
change in transpiration; or, a change in soil texture increases the
water-content, and this affects the imbibition of the root-hairs.
Indirect factors, then, are temperature, wind, pressure, precipitation,
and soil composition. Remote factors are, for the most part,
physiographic and biotic: they require at least two other factors to act
as middlemen. Altitude affects plants through pressure, which modifies
humidity, and hence transpiration. Slope determines in large degree the
run-off during a rainstorm, thus affecting water-content and the amount
of water absorbed. Earthworms and plant parts change the texture of the
soil, and thereby the water-content. Indirect factors often exert a
remote influence also, as may be seen in the effect which temperature
and wind have in increasing evaporation from the soil, and thus reducing
the water-content. This distinction between factors may seem
insufficiently grounded. In this event, it should be noted that it
centers the effects of all factors upon the three direct ones,
water-content, humidity, and light. If it further be recalled that these
are the only factors which produce qualitative structural changes, and
that the classification of ecads and formations is based upon them, the
validity of the distinction is clear.


                      THE DETERMINATION OF FACTORS

=31. The need of exact measurement.= Any serious endeavor to find in the
habitat those causes which are producing modification in the plant and
in vegetation can not stop with the factors merely. The next step is to
determine the quantity of each. It is not sufficient to hazard a guess
at this, or to make a rough estimate of it. Habitats differ in all
degrees, and it is impossible to institute comparisons between them
without an exact measure of each factor. Similarly, one can not trace
the adaptations of species to their proper causes unless the quantity of
each factor is known. It is of little value to know the general effect
of a factor, unless it is known to what degree this effect is exerted.
For this purpose it becomes necessary to appeal to instruments, in order
to determine the exact amount of each factor that is present in a
particular habitat, and hence to determine the ratio between the
stimulus and the amount of structural adjustment which results. The
employment of instruments of precision is clearly indispensable for the
task which we have set for ecology, and every student that intends to
strike at the root of the subject, and to make lasting contributions to
it, must familiarize himself with instrumental methods. One great
benefit will accrue to ecology as soon as this fact is generally
recognized. The use of instruments and the application of results
obtained from them demand much patience and seriousness of purpose upon
the part of the student. As a consequence, there will be a general
exodus from ecology of those that have been attracted to it as the
latest botanical fad, and have done so much to bring it into disrepute.

=32. The value of meteorological methods.= At the outset there must be a
very clear understanding that weather records and readings have only a
very general value. This is in spite of the fact that the instruments
employed are of standard precision. An important reason for this lack of
value is that readings are not made in a particular habitat; as a rule,
indeed, they are made in towns and cities, and hence are far removed
from masses of vegetation. They are usually taken at considerable
heights, and give but a general indication of the conditions at the
level of vegetation. The chief difficulty, however, is that the factors
observed at weather stations—temperature, pressure, wind, and
precipitation—are those which have the least value for the ecologist. It
is true that a knowledge of the temperature and rainfall of a great
region will afford some idea of the general character of its vegetation.
A proper understanding of such a vegetation is, however, to be gained
only through the exact study of its component formations. Ecology has
already incurred sufficient censure as a subject composed of very
general ideas, and the use of meteorological data, which can never be
connected definitely with anything in the plant or the formation, should
be discontinued. This must not be understood to mean that meteorological
instruments can not be used in the proper place and manner, i. e., in
the habitat.

=33. Habitat determination.= It is self-evident that determinations of
factors by instruments can only be of value in the habitat where they
are made. In other words, a habitat is a unit for purposes of measuring
its factors, and measures of one habitat have no exact value in another.
This fact can not be overstated. Thus, while it is perfectly legitimate,
and indeed highly desirable, to locate thermographs in different
mountain zones for ascertaining the rate at which temperature decreases
with altitude, the data obtained in this way are not directly applicable
in explanation of plant or formation changes, except when the same
species occurs at different altitudes. Special methods are valuable and
often absolutely necessary, but in view of the fact that the plant as
well as the formation is the definite product of a definite habitat, the
fundamental rule in instrumentation is that complete readings must be
made within a habitat for that habitat alone. This necessarily
presupposes a certain preliminary acquaintance with the habitat to be
investigated, as it is imperative that the station for making readings
be located well within the formation, in order to avoid transition
conditions. In vegetation, there are as many habitats as formations, and
in addition a large number of new and denuded habitats upon which
successions have not yet started; a knowledge of each formation or
succession must rest ultimately upon the factors of its particular
habitat.

=34. Determinable and efficient differences.= The instruments employed
in studying habitats can not be too exact, as there is no adequate
knowledge as yet concerning the real differences which exist between
related or contiguous formations. This is particularly true of
differences which are efficient in producing a recognizable structural
change in plant or formation. Investigations made by the writer have
shewn that standard instruments will measure differences of quantity
quite too small to produce a visible reaction. Efficient differences are
not the same for different factors, and perhaps also for the same factor
when found in various combinations. They vary widely for different
species, being in direct relation to the plasticity of the latter. The
point necessary to bear in mind in formulating methods for habitat
investigation and in making use of instruments is that standard
instruments should be used for the very reason that we do not yet know
the relation between determinable and efficient differences. On the
other hand, it is unnecessary to insist upon absolute exactness as soon
as it is found that the determinable difference lies well within the
efficient one. This by no means indicates that instruments are not to be
carefully standardized and frequently checked, or that accurate readings
should not be made. It means that a slight margin of error may be
permitted in a machine which registers well within the efficient
difference for that factor, and that instruments that read to the last
degree of nicety are not absolutely necessary. In the fundamental work
of determining efficient differences, however, instruments can not have
too great precision. Moreover, these differences must be based upon the
most plastic species of a formation, and the readings must be made under
normal conditions.


                            INSTRUMENTATION

=35. Methods.= In the field use of instruments two methods have been
developed. The first in point of time was the method of simple
instruments, devised especially for class work, and capable of being
used only where a number of trained students are available. The method
of automatic instruments was an immediate outgrowth of this, due to the
necessity which confronts the solitary investigator of being in
different habitats at the same time. In the gradual evolution of this
subject, it has become possible to combine the two methods in such a way
as to retain all the advantages of the automatic method, and most of
those of the method of simple instruments.

=36. Method of simple instruments.= By simple instruments are denoted
those that do not record, but must be read by the observer at the time.
They are standard instruments of precision, but possess the disadvantage
of requiring an observer for each one. They are well illustrated by the
thermometers and psychrometers used by the Weather Bureau. In the hands
of trained observers the results obtained are unimpeachable; in fact,
standard simple instruments must be constantly employed to check
automatic ones. As physical factors vary greatly through the day and
through the year, it is all-important that the readings in habitats
which are being compared should be made at the same instant. This
requires a number of observers; as many as twelve stations have been
read at one time, and there is of course no limit to the number. It is
very important, also, that observers be carefully trained in the
handling of instruments, and in reading them accurately and
intelligently at the proper moment. In practice it has been found
impossible to do such work in elementary classes, and, even in using
small advanced classes, prolonged drill has been necessary before
trustworthy results could be obtained. When a class has once been
thoroughly trained in making accurate simultaneous readings, there is
practically no limit, other than that set by time, to the valuable work
that can be done, both in instruction and investigation.

=37. Method of automatic instruments.= The solitary investigator must
replace trained helpers by automatic instruments or ecographs. These
have the very great advantages of giving continuous simultaneous records
for long periods, and of having no personal equation. They must be
regulated and checked, to be sure, but as this is all done by the same
person, the error is negligible. There is nothing more satisfactory in
resident investigation than a series of accurate recording instruments
in various habitats. Ecographs have two disadvantages. The chief perhaps
is cost. The expense of a single “battery” which will record light,
water-content, humidity, and temperature is about $250. Another
difficulty is that they can be used only within a few miles of the base,
since they require attention every week for regulation, change of
record, etc. While this means that ecographs in their present form are
not adapted to reconnaissance, this is not a real disadvantage, as the
scattered observations possible on such a journey can best be made by
simple instruments.

=38. Combined methods.= The best results by far are to be obtained by
the combined use of simple and automatic instruments. This is
particularly true in research, but it applies also to class instruction.
The ecographs afford a continuous, accurate basal record, to which a
single reading made at any time or place can be readily referred for
comparison. On the other hand, it is an easy matter to carry a full
complement of simple instruments on the daily field trips, and to make
accurate readings in a score or more of formations in a single day. An
isolated reading, especially of a climatic factor, has little or no
value in itself, but when it can be compared with a reading made at the
same time in the base station by an ecograph, it is the equivalent of an
automatic reading. This method renders a set of simple instruments more
desirable for a long trip or reconnaissance than a battery of automatic
ones. It is practically impossible to carry the latter into the field,
and in any event a continuous record is out of the question. As there
are other tasks at such times also, it becomes evident that the taking
of single readings which can be compared with a continuous record offers
the most satisfactory solution.


                  CONSTRUCTION AND USE OF INSTRUMENTS

=39. The selection of instruments.= In selecting and devising
instruments for the investigation of physical factors, emphasis has
first been laid upon accuracy. This is the result of a feeling that it
is better to have instruments that read too minutely than those which do
not make distinctions that are sufficiently close, particularly until
more has been learned about efficient differences. On the other hand, no
hesitation has been felt in employing instruments which are not
absolutely accurate, when it was clear that the error was less than the
efficient difference. Similarly, the margin of error practically
eliminates itself in the case of simultaneous comparative readings, when
the instruments have been checked to the same standard. Simplicity of
construction and operation are of great importance, especially in saving
time where a large number of instruments are in operation. Expense is
likewise to be carefully considered. It is impossible to have too many
instruments, but cost practically determines the number that can be
obtained. It is further necessary to secure or invent both simple and
automatic instruments for all factors, except such invariable ones as
altitude, slope, etc. Simple instruments must be of a kind that can be
easily carried, and so constructed that they can be used at a minimum of
risk. The sling psychrometer, for example, is very readily broken in
field use, and it has been replaced by a protected modification, the
rotating form.

In describing the construction and operation of the many factor
instruments, there has been no attempt to make the treatment exhaustive.
Those instruments which the author has found of greatest value in his
own work are given precedence, and the manner of using them is described
in detail. Other instruments of value are also considered, though with
greater brevity. Some of the most complex and expensive ones have been
ignored, as it is altogether improbable that they can come into general
use in their present form. While the conviction is felt that the methods
described below will enable the most advanced investigators to carry on
thorough work, it is hoped that they will be seen to be so fundamental,
and so attractive, that they will appeal to all who are planning serious
ecological study.


                             WATER-CONTENT

=40. Value of different instruments.= The paramount importance of
water-content as a direct factor in the modification of plant form and
distribution gives a fundamental value to the methods used for its
determination. Automatic instruments for ascertaining the water in the
soil are costly, in addition to being complicated, and often inaccurate.
For these reasons, much attention has been given to developing the
simpler but more reliable methods in which a soil borer or geotome is
used. The latter is simple, inexpensive, and accurate. It can be carried
easily upon daily trips or upon longer reconnaissances, and is always
ready for instant use. In the determination of physiological
water-content, it is practically indispensable. Indeed, the readiness
with which geotome determinations of water-content can be made should
hasten the universal recognition of the fact that it is the available,
and not the total amount of water in the soil, which determines the
effect upon the plant.


                           _Geotome Methods_

[Illustration: Fig. 1. Geotomes and soil can.]

=41. The geotome.= In its simplest form, the geotome is merely a stout
iron tube with a sharp cutting edge at one end and a firmly attached
handle at the other. The length is variable and is primarily determined
by the location of the active root surface of the plant. In xerophytic
habitats, generally a longer tube is necessary than in mesophytic ones.
The bore is largely determined by the character of the soil; for
example, a larger one is necessary for gravel than for loam. Tubes of
small bore also tend to pack the soil below them, and to give a
correspondingly incomplete core. The best results have been obtained
with geotomes of ½–1 inch tube. Each geotome has a removable rod,
flattened into a disk at one end, and bent at the other, for forcing out
the core after it has been cut from the soil. Sets of geotomes have been
made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12– and 15–inch
forms have been commonly used for herbaceous formations and layers. They
are marked in inches so that a sample of any lesser depth may be readily
taken. Such a device is very necessary for gravel soils and in mountain
regions, where the subsoil of rock lies close to the surface.

[Illustration: Fig. 2. Fraenkel soil borer.]

[Illustration: Fig. 3. American soil borer.]

=42. Soil borers.= There is a large variety of soil borers to choose
from, but none have been found as simple and satisfactory for relatively
shallow readings as the geotome just described. For deep-rooted plants,
many xerophytes, shrubs, and trees, borers of the auger type are
necessary. These are large and heavy, and of necessity slow in
operation. They can not well be carried in an ordinary outfit of
instruments, and the size of the soil sample itself precludes the use of
such instruments far from the base station, except on trips made
expressly for obtaining samples from deep-seated layers. For depths from
two to eight feet, the Fraenkel borer is perhaps the most satisfactory,
except for the coarser gravels: it costs $14 or $20 according to the
length. For greater depths, or when a larger core is desirable, the
Bausch & Lomb borer, number 16536, which costs $5.25, should be made use
of. This is a ponderous affair and can be employed only on special
occasions. On account of the size of samples obtained by these borers,
it is usually most satisfactory to take a small sample from the core at
different depths. Frequently, indeed, a hand trowel may be readily used
to obtain a good sample at a particular depth.

=43. Taking samples of soil.= In obtaining soil samples, the usual
practice is to remove the air-dried surface, noting its depth, and to
sink the geotome with a slow, gentle, boring movement, in order to avoid
packing the soil. This difficulty is further obviated by deep notches
with sharp, beveled edges which are cut at the lower end. In obtaining a
fifteen-inch core, there is also less compression if it be cut five
inches at a time. Repeated tests have shown, however, that the single
compressed sample is practically as trustworthy as the one made in
sections. The water-content of the former constantly fell within .5 per
cent of that of the latter, and both varied less than 1 per cent from
the dug sample used as a check. As soon as dug, the core is pressed out
of the geotome by the plunger directly into an air-tight soil can.
Bottles may be used as containers, but tin cans are lighter and more
durable. Aluminum cans have been devised for this purpose, but on
account of the expense, “Antikamnia” cans have been used instead. These
are tested, and those that are not water-tight are rejected, although it
has been found that, even in these, ordinary soils do not lose an
appreciable amount of water in twenty-four hours. The lid should be
screwed on as quickly as possible, and, as an added precaution, the cans
are kept in a close case until they have been weighed. The cans are
numbered consecutively on both lid and side in such a way that the
number may be read at a glance. The numbers are painted, as a label
wears off too rapidly, and scratched numbers are not quickly discerned.

[Illustration: Fig. 4. Field balance.]

=44. Weighing.= Although soil samples have been kept in tight cans
outside of cases for several days without losing a milligram of
moisture, the safest plan is to make it a rule to weigh cans as quickly
as possible after bringing them in from the field. Moreover, when
delicate balances are available, it is a good practice to weigh to the
milligram. At remote bases, however, and particularly in the field, and
on reconnaissance, where delicate, expensive instruments are out of
place, coarser balances, which weigh accurately to one centigram, give
satisfactory results. The study of efficient water-content values has
already gone far enough to indicate that differences less than 1 per
cent are negligible. Indeed, the soil variation in a single square meter
is often as great as this. The greatest difference possible in the third
place, i. e., that of 9 milligrams, does not produce a difference of .1
of 1 per cent in the water-content value. In consequence, such strong
portable balances as Bausch & Lomb 12308 ($2), which can be carried
anywhere, give entirely reliable results. The best procedure is to weigh
the soil with the can. Turning the soil out upon the pan or upon paper
obviates one weighing, but there is always some slight loss, and the
chances of serious mishap are many. After weighing, the sample is dried
as rapidly as possible in a water bath or oven. At a temperature of 100°
C. this is accomplished ordinarily in twenty-four hours; the most
tenacious clays require a longer time, or a higher temperature. High
temperatures should be avoided, however, for soils that contain much
leaf mould or other organic matter, in order that this may not be
destroyed. When it is necessary on trips, soil samples can be dried in
the sun or even in the air. This usually takes several days, however,
and a test weighing is generally required before one can be certain that
the moisture is entirely gone. The weighing of the dried soil is made as
before, and the can is carefully brushed out and weighed. The weight of
aluminum cans may be determined once for all, but with painted cans it
has been the practice to weigh them each time.

=45. Computation.= The most direct method of expressing the
water-content is by per cents figured upon the moist soil as a basis.
The ideal way would be to determine the actual amount of water per unit
volume, but as this would necessitate weighing one unit volume at least
in every habitat studied, as a preliminary step, it is not practicable.
The actual process of computation is extremely simple. The weight of the
dried sample, _w_^1, is subtracted from the weight of the original
sample, _w_, and the weight of the can, _w_^2, is likewise subtracted
from _w_. The first result is then divided by the second, giving the per
cent of water, or the physical water-content. The formula is: (_w_ −
_w_^1)/(_w_ − _w_^2) = _W_. The result is expressed preferably in grams
per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per cent of
water-content may readily be figured on the basis of dry or moist soil.

=46. Time and location of readings.= Owing to the daily change in the
amount of soil water due to evaporation, gravity, and rainfall, an
isolated determination of water-content has very little value. It is a
primary requisite that a basis for comparison be established by making
(1) a series of readings in the same place, (2) a series at practically
the same time in a number of different places or habitats, or (3) by
combining the two methods, and following the daily changes of a series
of stations throughout an entire season, or at least for a period
sufficient to determine the approximate maximum and minimum. The last
procedure can hardly be carried out except at a base station, but here
it is practically indispensable. It has been followed both at Lincoln
and at Minnehaha, resulting in a basal series for each place that is of
the greatest importance. When such a base already exists, or, better,
while it is being established, scattered readings may be used somewhat
profitably. As a practical working rule, however, it is most convenient
and satisfactory to make all determinations consecutively, i. e., in a
series of stations or of successive days. Under ordinary conditions, the
time of day at which a particular sample is taken is of little
importance, as the variation during a day is usually slight. This does
not hold for exposed wet soils, and especially for soils which have just
been wetted by rains. In all comparative series, however, the samples
should be taken at the same hour whenever possible. This is particularly
necessary when it is desired to ascertain the daily decrease of
water-content in the same spot. In the case of a series of stations,
these should be read always in the same order, at the same time of day,
and as rapidly as possible. When a daily station series is being run, i.
e., a series by days and stations both, the daily reading for each place
should fall at the same time. While there are certain advantages in
making readings either early or late in the day, they may be made at any
time if the above precautions are followed.

=47. Location of readings.= Samples should invariably be taken in spots
which are both typical and normal, especially when they are to be used
as representative of a particular area or habitat. A slight change in
slope, soil composition, in the amount of dead or living cover, etc.,
will produce considerable change in the amount of water present. Where
habitat and formation are uniform, fewer precautions are necessary. This
is a rare circumstance, and as a rule determinations must be made
wherever appreciable differences are in evidence. The problem is simpler
when readings are taken with reference to the structure or modifications
of a particular species, but even here, check readings in several places
are of great value. The variation of water in a spot apparently uniform
has been found to be slight in the prairies and the mountains. In taking
three samples in spots a few inches to several feet apart, the
difference in the amount of water has rarely exceeded 1 per cent, which
is practically negligible. Gardner[2] found that 16 samples taken to a
depth of 3 inches, in as many different portions of a carefully
prepared, denuded soil plot, showed a variation of 7½ per cent. This is
partially explained by the shallowness of the samples, but even then the
results of the two investigations are in serious conflict and indicate
that the question needs especial study. It should be further pointed out
that all readings should be made well within a particular area, and not
near its edge, and that, in the case of large diversified habitats, it
is the consocies and the society which indicate the obvious variations
in the structure of the habitat.

=48. Depth of samples.= The general rule is that the depth of soil
samples is determined by the layer to which the roots penetrate. The
practice is to remove the air-dried surface in which no roots are found,
and to take a sample to the proper depth. This method is open to some
objection, as the actively absorbing root surfaces are often localized.
There is no practical way of taking account of this as yet, except in
the case of deep-rooted xerophytes and woody plants. It is practicable
to determine the location of the active root area of a particular plant
and hence the water-content of the soil layer, but in most formations,
roots penetrate to such different depths that a sample which includes
the greater part of the distance concerned is satisfactory. Some
knowledge of the soil of a formation is also necessary, since shallow
soils do not require as deep samples as others. The same is true of
shaded soils without reference to their depth, and, in large measure, of
soils supplied with telluric water. In all cases, it is highly desirable
to have numerous control-samples at different depths. The normal cores
are 12 or 15 inches; control-samples are taken every 5 inches to the
depth desired, and in some cases 3–inch sections are made. It has been
found a great saving of time to combine these methods. A 5–inch sample
is taken and placed in one can, then a second one, and a third in like
manner. In this way the water-content of each 5–inch layer is
determined, and from the combined weight the total content is readily
ascertained.

=49. Check and control instruments.= A number of instruments throw much
light upon the general relations of soil water. The rain-gauge, or
ombrometer, measures the periodical replenishment of the water supply,
and has a direct bearing upon seasonal variation. The atmometer affords
a clue to the daily decrease of water by evaporation, and thus
supplements the rain-gauge. The run-off gauge enables one to establish a
direct connection between water-content and the slope and character of
the surface. The amount and rapidity of absorption are determined by
means of a simple instrument termed a rhoptometer. The gravitation water
of a soil is ascertained by a hizometer, and some clue to the
hygroscopic and capillary water may be obtained by an artificial osmotic
cell. All of these are of importance because they serve to explain the
water-content of a particular soil with especial reference to the other
factors of the habitat. It is evident that none of them can actually be
used in exact determinations of the amount of water, and they will be
considered under the factors with which, they are more immediately
concerned.


                   _Physical and Physiological Water_

=50. The availability of soil water.= The amount of water present in a
soil is no real index to the influence of water-content as a factor of
the habitat. All soils contain more water than can be absorbed by the
plants which grow in them. This residual water, which is not available
for use, varies for different soils. It is greatest in the compact
soils, such as clay and loam, and least in the loose ones, as sand and
gravel. It differs, but to a much less degree, from one species to
another. A plant of xerophytic tendency is naturally able to remove more
water from the same soil than one of mesophytic or hydrophytic
character. As the species of a particular formation owe their
association chiefly to their common relation to the water-content of the
habitat, this difference is of little importance in the field. In
comparing the structure of formations, and especially that of the plants
which are found in them, the need to distinguish the available water
from the total amount is imperative. Thus, water-contents of 15 per cent
in gravel and in clay are in no wise comparable. A coarse gravel
containing 15 per cent of water is practically saturated. The plants
which grow upon it are mesophytes of a strong hydrophytic tendency, and
they are able to use 14½ or all but .5 out of the 15 per cent of water.
In a compact clay, only 3½ of the 15 per cent are available, and the
plants growing in it are marked xerophytes. It is evident that a
knowledge merely of the physical water-content is actually misleading in
such cases, and this holds true of comparisons of any soils which differ
considerably in texture. After one has determined the physiological
water for the great groups of soils, it is more or less possible to
estimate the amounts in the various types of each. As an analysis is
necessary to show how close soils are in texture, this is little better
than a guess, and for accurate work it is indispensable that the
available water be determined for each habitat. Within the same
formation, however, after this has once been carefully ascertained, it
is perfectly satisfactory to convert physical water-content into
available by subtracting the non-available water, which under normal
conditions in the field remains practically the same.

The importance of knowing the available water is even greater in those
habitats in which salts, acids, cold, or other factors than the
molecular attraction of soil particles increase the amount of water
which the plant can not absorb. Few careful investigations of such soils
have yet been made, and the relation of available to non-available water
in them is almost entirely unknown. It is probable that the phenomena in
some of these will be found to be produced by other factors.

=51. Terms.= The terms, physiological water-content, and physical
water-content, are awkward and not altogether clear in their
application. It is here proposed to replace them by short words which
will refer directly to the availability of the soil water for absorption
by the plant. Accordingly, the total amount of water in the soil is
divided into the available and the non-available water-content. The
terms suggested for these are respectively, _holard_ (ὅλos, whole,
ἅpδov, water), _chresard_ (χοῆςις, use), and _echard_ (ἕχω, to
withhold).

=52. Chresard determinations under control.= The determination of the
chresard in the field is attended with peculiar difficulties. In
consequence, the method of obtaining it under control will first be
described. The inquiry may be made with reference to soils in general or
to the soil of a particular formation. In the last case, if the plants
used are from the same formation, the results will have almost the value
of a field determination. When no definite habitat is the subject of
investigation, an actual soil, and not an artificial mixture, should be
used, and the plants employed should be mesophytes. The individual
plants are grown from seeds in the proper soils, and are repotted
sufficiently often to keep the roots away from the surface. The last
transfer is made to a pot large enough to permit the plant to become
full-grown without crowding the roots. The pot should be glazed inside
and out in order to prevent the escape of moisture. This interferes
slightly with the aeration of the soil, but it will not cause any real
difficulty. The plant is watered in such a way as to make the growth as
normal as possible. After it has become well established, three soil
samples are taken in such a manner that they will give the variation in
different parts of the pot. One is taken near the plant, the second
midway between the plant and the edge of the pot, and the third near the
edge. The depth is determined by the size of the pot and the position of
the roots. The holard is determined for these in the usual way, but the
result is expressed with reference to 100 grams of dry soil; the average
is taken as representative. The soil is then allowed to dry out slowly,
as sudden drouth will sometimes impair the power of absorption and a
plant will wilt although considerable available water remains. Plants
often wilt in the field daily for several successive hot dry days, and
become completely turgid again during the night. If the drying out takes
place slowly, the plant will not recover after it has once begun to
wilt. The proper time to make the second reading is indicated by the
pronounced wilting of the leaves and shoots. Complete wilting occurs, as
a rule, only after the younger parts have drawn for some time upon the
watery tissues of the stem and root, by which time evaporation has
considerably deceased the water in the soil. It is a well-known fact
that young leaves do not wilt easily, especially in watery or succulent
plants. Three samples are again taken and the average water-content
determined as above. This is the non-available water or the echard. The
latter is then computed on the basis of 100 grams of dry soil, and this
result is subtracted from the holard to give the chresard in grams for
each 100 grams of dry weight. The chresard may also be expressed with
respect to 100 grams of moist soil. As a final precaution in basal work,
it is advisable to determine the chresard for six individuals of the
same species under as nearly the same conditions as possible. When it is
desired, however, to find the average chresard for a particular soil, it
is necessary to employ various species representing diverse phyads and
ecads. Such an investigation is necessarily very complicated, and must
be made the subject of special inquiry.

=53. Chresard readings in the field.= The especial difficulties which
must be overcome in the field are the exclusion of rain and dew and the
cutting off of the capillary water. It is evident, of course, that
experiments of this sort must also be entirely free from outside
disturbance. The choice of an area depends upon the scope of the study.
If the chresard is sought for a particular consocies, the block of soil
to be studied should show several species which are fairly
representative. In case the chresard of a certain species is to be
obtained, this species alone need be present, but it should be
represented by several individuals. Check plots are desirable in either
event, and at least two or three which are as nearly uniform as possible
should be chosen. The size and depth of the soil block depends upon the
plants concerned. It must be large enough that the roots of the
particular individuals under investigation are not disturbed. There is a
limit to the size of the mass that can be handled readily, and in
consequence the test plants must not be too large or too deeply rooted.
The task of cutting out the soil block requires a spade with a long
sharp blade. After ascertaining the spread and depth of the roots, the
block is cut so that a margin of several inches free from the roots
concerned is left on the sides and bottom. If the block is to be lifted
out of place, so that the sides are exposed to evaporation, this
allowance should be greater. In some cases, it may be found more
convenient to dig the plant up, place it in a large pot, and put the
latter back in the hole. As a general practice, however, this is much
less satisfactory.

After the block has been cut, it may be moved if the soil is
sufficiently compact, and then allowed to dry out in its own formation
or elsewhere. The results are most valuable in the first case, though it
is often an advantage to remove blocks cut from shade or wet formations
to dry, sunny stations where they will dry more rapidly. The most
satisfactory and natural method, however, is to leave the block in
place, and to prevent the reestablishment of capillary action by
enclosing it within plates. This is accomplished by slipping thin
sheet-iron plates into position along the cut surfaces. The plate for
the bottom should be somewhat wider than the block, and is slipped into
place by raising the block if the soil is not too loose; in the latter
event, it is carefully driven in. The side plates are then pushed down
to meet the former. The size of the plates depends upon the block; in
general, plates of 1, 2, and 3 feet square, with the bottom plates a
trifle larger, are the most serviceable. Access of rain and dew is
prevented by an awning of heavy canvas which projects far enough beyond
each side of the block to prevent wetting. The height will depend of
course upon the size of the plants. The awning must be used only when
rain or heavy dew is threatened, as the shade which it produces changes
the power of the plant to draw water from the soil.

The time necessary to cause wilting varies with the habitat and the
weather. When the block is large and in position, two or three weeks are
required. This period of drying incidentally furnishes an excellent
opportunity for determining the rate at which the particular soil loses
water. The holard sample is taken daily for several days before the
block is cut out, in order to obtain an average, care being taken of
course to avoid a period of extreme weather. The echard samples are
taken as soon as the wilting is sufficient to indicate that the limit of
available water is reached. The air-dry soil above the roots is first
removed. The treatment of the samples and the computation of the
chresard are as previously indicated.

=54. Chresard values of different soils.= The following table gives the
water-content values of six representative soils. The per cents of
holard (at saturation) and of echard are those determined by Hedgcock[3]
with six mesophytes as test plants for each soil. The chresard has been
computed directly from these.

                  ══════╤═════════╤═════════╤═════════
                        │ HOLARD  │ ECHARD  │CHRESARD
                  ──────┼────┬────┼────┬────┼────┬────
                  Sand  │14.3│12.6│  .3│ .25│  14│12.3
                  Clay  │47.4│32.5│ 9.3│ 6.3│38.1│26.2
                  Loess │59.3│37.1│10.1│ 6.4│49.2│30.7
                  Loam  │64.1│39.1│10.9│ 6.6│53.2│32.5
                  Humus │65.3│39.6│11.9│ 7.2│53.4│32.4
                  Saline│68.5│40.8│16.2│ 9.6│52.3│31.2
                  ──────┴────┴────┴────┴────┴────┴────


  The first column indicates the per cent based upon the dry weight, the
  second upon the weight of the moist soil.


While these can not be considered absolute for a particular soil other
than the ones investigated, they are found to correspond somewhat
closely to the results obtained for other soils of the respective
groups. For accurate research, the chresard must of course be
ascertained for each formation with respect to its peculiar plants and
soil. The influence of the ecad in more or less determining the echard
is also shown by Hedgcock, who found that floating plants wilt at 25 per
cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent,
and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat
higher for shade plants than for heliophytes.


                         _Records and Results_

=55. The field record.= It is superfluous to point out that a definite
form for field records saves much time and prevents many mistakes. The
exact form may be left to personal taste, but there are certain features
which are essential. Many of these are evident, while others may seem
unnecessary; all, however, have been proved by experience to have some
value in saving time or in preventing confusion. The two fundamental
maxims of field work are that nothing is too trivial to be of
importance, and that no detail should be entrusted to the memory. The
field record should contain in unmistakable terms all that the field has
yielded. These statements apply with especial force to water-content, in
many senses the most important of physical factors. The precise
character of the record depends upon the way in which the readings are
made, whether scattered or in series. As the day-station series is of
the greatest importance, the record is adapted for it especially, but it
will also serve for all readings. The record is chronological, since
this is the only convenient method for the field. A proper form for a
field record of water-content is the following:

 ═══╤══════╤═════════╤════════╤═══════════╤════════╤══════
 Can│ Date │Formation│Station │ Community │  Soil  │Sample
 No.│      │         │        │           │        │
 ───┼──────┼─────────┼────────┼───────────┼────────┼──────
  „ │  „   │    „    │   „    │     „     │   „    │  „
 ───┼──────┼─────────┼────────┼───────────┼────────┼──────
  „ │  „   │    „    │   „    │     „     │   „    │  „
 ───┼──────┼─────────┼────────┼───────────┼────────┼──────
 10 │2/8/04│Spruce   │Jack    │Mertensiare│    Loam│10
    │      │forest   │Brook   │           │        │
 17 │  „   │Spruce   │Milky   │Gentianare │  ¼ mold│10:2
    │      │forest   │Way     │           │¾ gravel│
 40 │  „   │Gravel   │Hiawatha│Asterare   │  Gravel│2:10
    │      │slide.   │        │           │        │
 ───┴──────┴─────────┴────────┴───────────┴────────┴──────
 ═══╤═══════════════════╤════════════════════╤════════╤═══════════════
 Can│      HOLARD       │       ECHARD       │Chresard│     NOTES
 No.│                   │                    │        │
 ───┼────────────────┬──┼─────────────────┬──┼────────┼──────┬────────
  „ │   Weighings    │% │    Weighings    │% │   „    │ Sky  │Rainfall
 ───┼─────┬────┬─────┼──┼─────┬─────┬─────┼──┼────────┼──────┼────────
  „ │ 1st │ 2d │ Can │„ │ 1st │ 2d  │ Can │„ │   „    │  „   │   „
 ───┼─────┼────┼─────┼──┼─────┼─────┼─────┼──┼────────┼──────┼────────
 10 │58.7 │50.1│25.52│ 2│58.7 │53.41│25.52│10│      16│Cloudy│       0
    │     │    │     │  │     │     │     │  │        │      │
 17 │64.25│57.5│21.35│16│64.25│59.6 │21.35│ 5│      11│Cloudy│       0
    │     │    │     │  │     │     │     │  │        │      │
 40 │78.55│74.3│22.85│ 8│78.55│74.85│22.85│ 1│       7│Cloudy│       0
    │     │    │     │  │     │     │     │  │        │      │
 ───┴─────┴────┴─────┴──┴─────┴─────┴─────┴──┴────────┴──────┴────────

A general designation of the soil composition is a material aid,
especially where there is a difference in the core. For example, in a
mountain forest or meadow, the upper layer will usually be mold, the
lower sand or gravel. A careful estimate of the relation between the two
throws much light upon the chresard. Under “sample” the number taken to
reach the desired depth, if more than one, is indicated by placing the
number before the depth, thus 2:10. When two or more full cores are
included in the same sample for a check, the order is 10:2. It has
already been shown, however, that these precautions are not necessary
for ordinary purposes. In computing the holard and echard, there is no
need to show the figuring, if the process is checked and then proved.
Notes upon sky conditions aid in explaining the daily decrease in
water-content. The amount of rain and the period during which it falls
are of great importance in understanding the fluctuations of the holard.
Under community it is highly desirable to have a list of all the
species, but it is impossible to include this in the table, and a glance
at the formation list will show them. The form indicated above serves
for a day-station series, a daily series in one station for any number
of check series in one spot, and for scattered readings. In many cases
the echard will not be determined, but on account of its primary
importance, there should be a space for it, especially since it may be
desirable to determine it at some later time.

=56. The permanent record.= This should be kept by formation, or if the
latter exhibits well-defined associations, the formational record may be
divided accordingly. This may seem an unnecessary expenditure of time,
but a slight experience in finding the water-content values of a
particular habitat, when scattered through a chronological field record,
will be convincing. The form of permanent record is the same as for the
field, except that the column for the formation and that for the society
are often unnecessary.

=57. Sums and means.= From the great difficulty of determining the
absolute water-content, and of obtaining a standard of comparison
between soils on account of the varying ratio between bulk and weight,
water-content sums are impracticable. For the same reasons, means of
actual water-content are practically impossible, and the mean
water-content must be expressed in per cents. Daily readings are not
essential to a satisfactory mean. In fact, a single reading at each
extreme enables one to approximate the real mean very closely; thus, the
average of 26 readings in the prairie formation is 18 per cent. The
extremes are 5 per cent and 28 per cent, and their average 16.5 per
cent. A few readings properly scattered through moist and dry periods
will give a reliable mean, as will also a series of daily readings from
one heavy rain through a long dry period. The one difficulty with the
last method is that such periods can not well be determined beforehand.
Means permit ready comparison between habitats, but in connecting the
modifications of a species with water-content as a cause, the extremes
are significant as indicating the range of conditions. Furthermore, the
extremes, i. e., 5 per cent and 28 per cent, make it possible to
approximate the mean, 18 per cent, while the latter gives little or no
clue to the extremes. It is hardly necessary to state that means and
extremes should be determined for a certain habitat, or particular area
of it, and that the results may be expressed with reference to holard
and chresard.

=58. Curves.= The value of graphic methods and the details of plotting
curves are reserved for a particular section. It will suffice in this
place to indicate the water-content curves that are of especial value.
Simple curves are made with regard to time, place, or depth. The day
curve shows the fluctuations of the water-content of one station from
day to day or from time to time. The station curve indicates the
variation in water from station to station, while the depth curve
represents the different values at various depths in the same station.
These may be combined on the same sheet in such a way that the station
curves of each day may be compared directly. Similar combinations may be
used for comparing the day curves, or the depth curves of different
stations, but these are of less importance. A combination of curves
which is of the greatest value is one which admits of direct comparison
between the station curves of saturation, holard, chresard, and echard.


                                HUMIDITY

=59. Instruments.= As a direct factor, humidity is intimately connected
with water-content in determining the structure and distribution of
plants. The one is in control of water loss; the other regulates water
supply. Humidity as a climatic factor undergoes greater fluctuation in
the same habitat, and the efficient difference is correspondingly
greater. Accordingly, simple instruments are less valuable than
automatic ones, since a continuous record is essential to a proper
understanding of the real influence of humidity. As is the rule,
however, the use of simple instruments, when they can be referred to an
ecographic basis, greatly extends the field which can be studied. In
investigation, both psychrometer and psychrograph have their proper
place. In the consideration of simple instruments for obtaining humidity
values, an arbitrary distinction is made between psychrometers and
hygrometers. The former consist of a wet and a dry bulb thermometer,
while the latter make use of a hygroscopic awn, hair, or other object.


                            _Psychrometers_

=60. Kinds.= There are three kinds of psychrometer, the sling, the cog,
and the stationary. All consist of a wet bulb and a dry bulb thermometer
set in a case; the first two are designed to be moved or whirled in the
air. The same principle is applied in each, viz., that evaporation
produces a decrease in temperature proportional to the amount of
moisture in the air. The dry bulb thermometer is an ordinary
thermometer, while the wet bulb is covered with a cloth that can be
moistened. The former indicates the normal temperature of the air, the
latter gives the reduced temperature due to evaporation. The relative
humidity of the air is ascertained by means of the proper tables, from
two terms, i. e., the air temperature and the amount of reduction shown
by the wet bulb. The sling and the cog psychrometers alone are in
general use. The stationary form has been found to be unreliable,
because the moisture, as it evaporates from the wet bulb, is not
removed, and, in consequence, hinders evaporation to the proper degree.

[Illustration: Fig. 5. Sling psychrometer.]

=61. The sling psychrometer.= The standard form of this is shown in the
illustration, and is the one used by the Weather Bureau. This instrument
can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien
P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of
a metal frame to which are firmly attached two accurately standardized
thermometers, reading usually from –30° to 130°. The frame is attached
at the uppermost end to a handle in such fashion that it swings freely.
The wet bulb thermometer is placed lower, chiefly to aid in wetting the
cloth more readily. The cloth for the wet bulb should be always of the
same texture and quality; the standard used by the Weather Bureau can be
obtained from the instrument makers. A slight difference in texture
makes no appreciable error, but the results obtained with different
instruments and by different observers will be more trustworthy and
comparable if the same cloth be used in all cases. The jacket for the
wet bulb may be sewed in the form of a close-fitting bag, which soon
shrinks and clings tightly. It may be made in the field by wrapping the
cloth so that the edges just overlap, and tying it tightly above and
below the bulb. In either case, a single layer of cloth alone must be
used. The cloth becomes soiled or thin after a few months’ constant use
and should be replaced. It is a wise precaution to carry a small piece
of psychrometer cloth in the field outfit.

[Illustration: Fig. 6. Cog psychrometer.]

=62. Readings.= All observations should be made facing the wind, and the
observer should move one or two steps during the reading to prevent the
possibility of error. The cloth of the wet bulb is moistened with water
by means of a brush, or, much better, it is dipped directly into a
bottle of water. Distilled water is preferable, as it contains no
dissolved material to accumulate in the cloth. Tap-water and the water
of streams may be used without appreciable error, if the cloth is
changed somewhat more frequently. The temperature of the water is
practically negligible under ordinary conditions. Readings can be made
more quickly, however, when the temperature is not too far from that of
the air. The psychrometer is held firmly and swung rapidly through the
air when the space is not too confined. Where there is danger of
breakage, it is swung back and forth through a short arc,
pendulum-fashion. As the reading must be made when the mercury of the
wet bulb reaches the lowest point, the instrument is stopped from time
to time and the position of the column noted. The lowest point is often
indicated by the tendency of the mercury to remain stationary; as a rule
it can be noted with certainty when the next glance shows a rise in the
column. In following the movement, and especially in noting the final
reading, great care must be taken to make the latter before the mercury
begins to rise. For this reason it is desirable to shade the
psychrometer with the body when looking at it, and to take pains not to
breathe upon the bulbs nor to bring them too near the body. At the
moment when the wet bulb registers the lowest point, the dry bulb should
be read and the results recorded.

=63. Cog psychrometer.= This instrument, commonly called the
“egg-beater” psychrometer, has been devised to obviate certain
disadvantages of the sling psychrometer in field work, and has entirely
supplanted the latter in the writer’s own studies. It is smaller, more
compact, and the danger of breaking in carriage or in use is almost nil.
It has the great advantage of making it possible to take readings in a
layer of air less than two inches in thickness, and in any position.
Fairly accurate results can even be obtained from transpiring leaves.
The instrument can readily be made by a good mechanic, at a cost for
materials of $1.75, which is less than half the price for the sling
form. A single drawback exists in the use of short, Centigrade
thermometers, inasmuch as tables of relative humidity are usually
expressed in Fahrenheit. It is a simple matter, however, to convert
Centigrade degrees into Fahrenheit, mentally, or the difficulty may be
avoided by the conversion table shown on page 47, or by constructing a
Centigrade series of humidity tables. The fact that the wet and dry
bulbs revolve in the same path has raised a doubt concerning the
accuracy of the results obtained with this instrument. Repeated
comparisons with the sling psychrometer have not only removed this doubt
completely, but have also proved that the standardization of the
thermometers has been efficient.

=64. Construction and use.= A convenient form of egg-beater is the Lyon
(Albany, New York), in which the revolving plates can be readily
removed, leaving the axis and the frame. The thermometers used are of
the short Centigrade type. They are 4½ inches long and read from –5° to
50°. Eimer and Amend, 205 Third Ave., New York city, furnish them at 75
cents each. The thermometers are carefully standardized and compared,
and then grouped in pairs that read together. Each pair is used to
construct a particular psychrometer. Each thermometer is strongly wired
to one side of the frame, pieces of felt being used to protect the tube
and increase the contact. The frame is also bent at the base angles to
permit free circulation of air about the thermometer bulbs. The bulb of
one thermometer is covered with the proper cloth, and the psychrometer
is finished. Since the frame revolves with the thermometers, it is
necessary to pour the water on the wet bulb, or to employ a pipette or
brush. The thermometer bulbs are placed in the layer to be studied, and
the frame rotated at an even rate and with moderate rapidity. The
observation is further made as in the case of the sling psychrometer. As
the circle of rotation is less than three inches in diameter, and the
layer less than an inch, in place of nearly three feet for the sling
form, the instrument should not be moved at all for extremely localized
readings, but it must be moved considerably, a foot or more, if it is
desirable to obtain a more general reading.

=65. Hygrometers.= While there are instruments designed to indicate the
humidity by means of a hygroscopic substance, not one of them seems to
be of sufficient accuracy for use in ecological study. The difficulty is
that the hygroscopic reaction is inconstant, rather than that the
instruments are not sufficiently sensitive. A number of hygrometers have
been tested, and in all the error has been found to be great, varying
usually from 10–20 per cent. In the middle of the scale they sometimes
read more accurately, but toward either extreme they are very inexact.
It seems probable that an accurate hygrometer can be constructed only
after the model of the Draper psychrograph. Its weight and bulk would
make it an impossible instrument for field trips, and the expense of one
would provide a dozen psychrometers. In consequence, it does not seem
too sweeping to say that no hygrometer can furnish trustworthy results.
Of simple instruments for humidity, the psychrometer alone can be
trusted to give reliable readings. Crova’s hygrometer, used by
Hesselmann, is not a hygrometer in the sense indicated. As it is much
less convenient to handle and to operate than the cog psychrometer, it
is not necessary to describe it.


                            _Psychrographs_

[Illustration: Fig. 7. Draper psychrograph.]

=66. The Draper psychrograph.= A year’s trial of the Draper psychrograph
in field and planthouse has left little question of its accuracy and its
great usefulness. Essentially, it consists of a band of fine catgut
strings, which are sensitive to changes in the moisture-content of the
air. The variations in the length of the band are communicated to a long
pointer carrying an inking pen. The latter traces the record in per cent
of relative humidity on a graduated paper disk, which is practically the
face of an eight-day clock. The whole is enclosed in a metal case with a
glass front. A glance at the illustration will show the general
structure of the instrument. Continued psychrometric tests demonstrate
that the margin of error is well within the efficient difference for
humidity, which is taken to be 5 per cent. In the field tests of the
past summer, two psychrographs placed side by side in the same habitat
did not vary 1 per cent from each other. The same instruments when in
different habitats did not deviate more than 1 per cent from the
psychrometric values, except when the air approached saturation. For
humidities above 90 per cent, the deviation is considerable, but as
these are temporary and incident upon rainfall, the error is not
serious. For humidities varying from 10–85 per cent, the psychrograph is
practically as accurate as the psychrometer. Per cents below 10 are
rare, and no tests have been made for them.

[Illustration: Fig. 8. Instrument shelter, showing thermograph and
psychrograph in position.]

=67. Placing the instrument.= The psychrograph should be located in a
place where the circulation of the air is typical of the station
observed. A satisfactory shelter will screen the instrument from sun and
rain, and at the same time permit the air to pass freely through the
perforations of the metal case. The form shown in figure 8 meets both of
these conditions. A desirable modification is effected by fastening a
strip about the cover of such depth as to prevent the sun’s rays from
striking the case except when the sun is near the horizon. A cross block
is fastened on the post of the shelter after being exactly leveled. The
psychrograph rests upon this block, which is three feet above the ground
in order to avoid the influence of radiation. The instrument is held in
position by slipping the eye over a small-headed nail driven obliquely.
It does not hang from the latter, but must rest firmly upon the cross
block. The post is set to a depth that prevents oscillation in the wind,
which is liable to obscure the record. In shallow mountain soils
stability is attained by fastening a broad board at the base of the post
before setting it. When two or more psychrographs are established in
different habitats, great pains are taken to set them up in exactly the
same way. The shelters are alike, the height above the soil the same,
and the instruments all face the south.

=68. Regulating and operating the instrument.= When two or more
psychrographs are to be used in series, they must be compared with each
other in the same spot for several days until they run exactly together
with respect to per cent of humidity and to time. During this comparison
they are checked by the psychrometer and so regulated that they register
the proper humidity. When a single instrument is used alone as the basis
to which simple readings may be referred, all regulating may well be
done after the instrument is in position. This is a simple process; it
is accomplished by obtaining the relative humidity beneath the shelter
and at the proper height by a psychrometer. The pen hand is then moved
to the proper line on the disk by means of the screws at its base. These
are reached by removing the lettered glass face. The thumbscrew on the
side opposite the direction in which the pen is to move is released, and
the opposite screw simultaneously tightened, until the pen remains upon
the proper line. Experience has proved that the record sheet should be
correctly labeled and dated before being placed on the disk. In the
press of field duties, records labeled after removal are liable to be
confused. It is likewise a great saving of time to write the date of the
month in the margin of each segment. Care is taken to place the sheet on
the disk in the same position each time; this can easily be done by
seeing that the sharp point on the disk penetrates the same spot on the
paper. A single drop of ink in the pen will usually give the most
satisfactory line. A thin line is read most accurately. If the pen point
is too fine, however, the ink does not flow readily, and the point
should be slightly blunted by means of a file. More often the line is
too broad and the pen must be carefully pointed. Occasionally the pen
does not touch the sheet, and it becomes necessary to bend the hand
slightly. This is a frequent difficulty if the records are folded or
wrinkled, and consequently the sheets should always be kept flat.

=69. The weekly visit.= Psychrographs must be visited, checked, rewound,
and inked every week. Whenever possible this should be done regularly at
a specified day and hour. This is especially desirable if the same
record sheet is used for more than one week. Time and energy are saved
by a fixed order for the various tasks to be done at each visit. After
opening the instrument the disk is removed, and the clock wound, and, if
need be, regulated. The record sheet is replaced, the disk again put on
the clock arbor, and the pen replenished with a drop of ink. A
psychrometer reading is made, and the results in terms of relative
humidity noted at the proper place on the disk sheet. If the
psychrograph vary more than 1 per cent, it is adjusted to read
accurately. In practice it has been found a great convenience to keep
each record sheet in position for three weeks, and the time may easily
be extended to four. In this event, the pen is carefully cleaned with
blotting paper at each visit, and is then refilled with an ink of
different color. To prevent confusion, the three different colored inks
are always used in the same order, red for the first week, blue for the
second, and green for the third. The advantages of this plan are
obvious: fewer records are used and less time is spent in changing them.
The records of several weeks are side by side instead of on separate
sheets, and in working over the season’s results, it is necessary to
handle but a third as many sheets.

The Draper psychrograph is made by the Draper Manufacturing Company, 152
Front St., New York city. The price is $30. A few record sheets and a
bottle of red ink are furnished with it. Additional records can be
obtained at 3 cents each. The inks are 25–50 cents per bottle, depending
upon the color.


                    _Humidity Readings and Records_

=70. The time of readings.= If simple instruments alone are used for
determining humidity, readings are practically without value unless made
simultaneously through several stations, or successively at one. When it
is possible to combine these, and to make psychrometer readings at
different habitats for each hour of the day, or at the same hour for
several days, the series is of very great value. Single readings are
unreliable on account of the hourly and daily variations of humidity,
but when these changes are recorded by a psychrograph, such readings at
once become of use, whether made in the same habitat with the recording
instrument or elsewhere. In the latter case, one reading will tell
little about the normal humidity of the habitat, but several make a
close estimate possible. When a series of psychrographs is in use,
accurate observations can be made to advantage anywhere at any time. As
a rule, however, it has been found most convenient to make simple
readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford
much evidence in regard to the daily range. A good time also is that at
which the temperature maximum occurs each day, but this is movable and
in the press of field work can rarely be taken advantage of. A very fair
idea of the daily mean humidity is obtainable by averaging the readings
made at the hours already indicated. The comparison of single readings
with the psychrograph record should not be made at a time when a rapid
change is occurring, as the automatic instrument does not respond
immediately. Such a condition is usually represented by a sudden rain,
and is naturally not a satisfactory time for single readings in any
event.

[Illustration: Fig. 9. Atmometer.]

=71. Place and height.= As stated above, the psychrograph is placed
three feet above the surface of the ground in making readings for the
comparison of stations. In low, herbaceous formations, the instrument is
usually placed within a few inches of the soil in order to record the
humidity of the air in which the plants are growing. In forest
formations, the moisture often varies considerably in the different
layers. This variation is easily determined by simultaneous psychrometer
readings in the several layers, or, if occasion warrants, a series of
psychrographs may be used. In field work the rule has been to make
observations with the psychrometer at 6 feet, 3 feet, and the surface of
the soil, but the reading at the height of 3 feet is ordinarily
sufficient. Humidity varies so easily that several readings in different
parts of one formation are often desirable. In comparing different
formations, the readings should be made in corresponding situations, for
example, in the densest portion of each.

=72. Check instruments.= Humidity is so readily affected by temperature,
wind, and pressure, that a knowledge of these factors is essential to an
understanding of its fluctuations. Pressure, disregarding daily
variation, is taken account of in the tables for ascertaining relative
humidity, and is determined once for all when the altitude of a station
has been carefully established. The temperature is obtained directly
from the dry bulb reading. Its value is fundamental, as the amount of
moisture in a given space is directly affected by it; like pressure, it
also is taken account of in the formula. The movement of the air has an
immediate influence upon moisture by mixing the air of different
habitats and layers. So far as the plant is concerned, it has
practically the effect of increasing or decreasing the humidity by the
removal of the air above it. Thus, while the anemometer can furnish no
direct evidence as to the amount of variation, it is of aid in
explaining the reason for it. Likewise, the rate of evaporation as
indicated by a series of atmometers, affords a ready method of
estimating the comparative effect of humidity in different habitats.
Potometers and other instruments for measuring transpiration throw much
light upon humidity values. Since they are concerned with the response
of the plant to humidity, they are considered in the following chapter.

=73. Humidity tables.= To ascertain the relative humidity, the
difference between the wet and dry bulb readings is obtained. This, with
the dry bulb temperature, is referred to the tables, where the
corresponding humidity is found. A variation in temperature has less
effect than a variation in the difference; in consequence, the dry bulb
reading is expressed in the nearest unit, and the difference reckoned to
the nearest .5. The humidity varies with the air pressure. Hence, the
altitude must be determined for the base station, and for all others
that show much change in elevation. Within the ordinary range of
growing-period temperatures, the effect of pressure is not great. For
all ordinary cases, it suffices to compute tables for pressures of 30,
29, 27, 25, and 23 inches. The following table indicates the decrease in
pressure which is due to altitude.

                     ALTITUDE           PRESSURE
                    Feet   Meters  Inches  Centimeters
                         0      0    30    76
                       910    277    29    73.5
                      1850    574    28    71
                      2820    860    27    68.5
                      3820   1165    26    66
                      4850   1477    25    63.5
                      5910   1792    24    61
                      7010   2138    23    58.5
                      8150   2485    22    56
                      9330   2845    21    53.5
                     10550   3217    20    51
                     13170   4016    18    46
                     16000   4880    16    41

The fluctuations of pressure due to weather are usually so slight that
their influence may be disregarded. An excellent series of tables of
relative humidity is found in Marvin’s Psychrometric Tables, published
by the U. S. Weather Bureau, and to be obtained from the Division of
Publications, Washington, D. C., for 10 cents. A convenient field form
is made by removing the portion containing the tables of relative
humidity, and binding it in stiff oilcloth.

[Illustration: Fig. 10. Conversion scale for temperatures.]

=74. Sums, means, and curves.= An approximate humidity sum can be
obtained by adding the absolute humidities for each of the twenty-four
hours, and expressing the results in grains per cubic foot. It is
possible to establish a general ratio between this sum and the
transpiration sum of the plant, but its value is not great at present.
Means of absolute and of relative humidity are readily determinable from
the psychrograph records; the latter are the most useful. The mean of
relative humidity for the twenty-four hours of a day is the average of
the twenty-four hour humidities. From these means the seasonal mean is
computed in the same manner. A close approximation, usually within 1
degree, may be obtained in either case by averaging the maximum and
minimum for the period concerned. Various kinds of curves are of value
in representing variation in humidity. Obviously, these must be derived
from the psychrograph, or from the psychrometer when the series is
sufficiently complete. The level curve indicates the variation in
different stations at the same time. These may be combined in a series
for the comparison of readings made at various heights in the stations.
The day or point curve shows the fluctuations during the day of one
point, and the station curve the variation at different heights in the
same station. The curves of successive days or of different stations may
of course be combined on the same sheet for comparison. Level and
station curves based upon mean relative humidities are especially
valuable.

=75. Records.= A field form is obviously unnecessary for the
psychrograph. The record sheets constitute both a field and permanent
record. The altitude and other constant features of the station and the
list of species, etc., are entered on the back of the first record
sheet, or, better, they are noted in the permanent formation record. For
psychrometer readings, whether single or in series, the following record
form is employed:

 ════════╤════╤═════════╤════════╤════════╤═══════════╤═══════╤════╤════
         │    │         │        │        │           │Height │Dry │Wet
   Date  │Hour│Formation│Station │Altitude│ Community │  of   │bulb│bulb
         │    │         │        │        │           │reading│    │
 ────────┼────┼─────────┼────────┼────────┼───────────┼───────┼────┼────
    „    │ „  │    „    │   „    │   „    │     „     │   „   │ „  │ „
 ────────┼────┼─────────┼────────┼────────┼───────────┼───────┼────┼────
 15/8/’04│6:20│Spruce   │Brook   │ 2500 m │Mertensiare│ 1 ft. │51° │46°
         │A.M.│         │bank    │        │           │       │    │
    „    │ „  │Half     │Hiawatha│   „    │Asterare   │   „   │56° │49°
         │    │gravel   │        │        │           │       │    │
    „    │6:45│Spruce   │Brook   │   „    │Mertensiare│   „   │54° │52°
         │P.M.│         │bank    │        │           │       │    │
    „    │ „  │Half     │Hiawatha│   „    │Asterare   │   „   │56° │52°
         │    │gravel   │        │        │           │       │    │
 ────────┴────┴─────────┴────────┴────────┴───────────┴───────┴────┴────

 ════════╤════╤═════════╤════════╤═════╤════╤════╤════╤═══════════════
         │    │         │        │     │Rel.│Base│Abs.│
   Date  │Hour│Formation│Station │Diff.│Hum.│Hum.│Hum.│     NOTES
         │    │         │        │     │    │    │    │
 ────────┼────┼─────────┼────────┼─────┼────┼────┼────┼─────┬────┬────
    „    │ „  │    „    │   „    │  „  │ „  │ „  │ „  │ Sky │Rain│Wind
 ────────┼────┼─────────┼────────┼─────┼────┼────┼────┼─────┼────┼────
 15/8/’04│6:20│Spruce   │Brook   │  5  │72% │63% │2.9 │Clear│ 0  │ 0
         │A.M.│         │bank    │     │    │    │    │     │    │
    „    │ „  │Half     │Hiawatha│  7  │64% │63% │3.0 │  „  │ 0  │ 0
         │    │gravel   │        │     │    │    │    │     │    │
    „    │6:45│Spruce   │Brook   │  2  │89% │69% │4.2 │  „  │ 2  │ 0
         │P.M.│         │bank    │     │    │    │    │     │cc. │
    „    │ „  │Half     │Hiawatha│  4  │79% │69% │4.0 │  „  │ 2  │ 0
         │    │gravel   │        │     │    │    │    │     │cc. │
 ────────┴────┴─────────┴────────┴─────┴────┴────┴────┴─────┴────┴────

On page 47 is given a table for the conversion of Centigrade into
Fahrenheit temperatures. This may be done mentally by means of the
formula _F_ = _C_/5 × 9 + 32°.


                                 LIGHT

=76. Methods.= All methods for measuring light intensity, which have
been at all satisfactory, are based upon the fact that silver salts
blacken in the light. The first photographic method was proposed by
Bunsen and Roscoe in 1862; this has been taken up by Wiesner and
variously modified. After considerable experiment by the writer,
however, it seemed desirable to abandon all methods which require the
use of “normal paper” and “normal black” and to develop a simpler one.
As space is lacking for a satisfactory discussion of the
Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited
below.[4] Simple photometers for making light readings simultaneously or
in series were constructed in 1900, and have been in constant use since
that time. An automatic instrument capable of making accurate continuous
records proved to be a more difficult problem. A sunshine recorder was
ultimately found which yields valuable results, and very recently a
recording photometer which promises to be perfectly satisfactory has
been devised. Since the hourly and daily variations of sunlight in the
same habitat are relatively small, automatic photometers are perhaps a
convenience rather than a necessity.


                            _The Photometer_

[Illustration: Fig. 11. Photometer, showing front and side view.]

=77. Construction.= The simple form of photometer shown in the
illustration is a light-tight metal box with a central wheel upon which
a strip of photographic paper is fastened. This wheel is revolved by the
thumbscrew past an opening 6 mm. square which is closed by means of a
slide working closely between two flanges. At the edge of the opening,
and beneath the slide is a hollow for the reception of a permanent light
standard. The disk of the thumbscrew is graduated into twenty-five
parts, and these are numbered. A line just beneath the opening coincides
with the successive lines on the disk, and indicates the number of the
exposure. The wheel contains twenty-five hollows in which the click
works, thus moving each exposure just beyond the opening. The metal case
is made in two parts, so that the bottom may be readily removed, and the
photographic strip placed in position. The water-photometer is similar
except that the opening is always covered with a transparent strip and
the whole instrument is water-tight. These instruments have been made
especially for measuring light by the C. H. Stoelting Co., 31 W.
Randolph street, Chicago, Ill. The price is $5.

=78. Filling the photometer.= The photographic paper called “solio”
which is made by the Eastman Kodak Company, Rochester, N. Y., has proved
to be much the best for photometric readings. The most convenient size
is that of the 8 × 10 inch sheet, which can be obtained at any supply
house in packages of a dozen sheets for 60 cents. New “emulsions,” i.
e., new lots of paper, are received by the dealers every week, but each
emulsion can be preserved for three to six months without harm if kept
in a cool, light-tight place. Furthermore, all emulsions are made in
exactly the same way, and it has been impossible to detect any
difference in them. To fill the photometer, a strip exactly 6 mm. wide
is cut lengthwise from the 8 × 10 sheet. This must be done in the dark
room, or at night in very weak light. The strip is placed on the wheel,
extreme care being taken not to touch the coated surface, and fixed in
position by forcing the free ends into the slit of the wheel by a piece
of cork 8–9 mm. long. The wheel is replaced in the case, turned until
the zero is opposite the index line, and the instrument is ready for
use.

=79. Making readings.= An exposure is made by moving the slide quickly
in such a way as to uncover the entire opening, and the standard if the
exposure is to be very short. Care must be taken not to pull the slide
entirely out of the groove, as it will be impossible to replace it with
sufficient quickness. The time of exposure can be determined by any
watch after a little practice. It is somewhat awkward for one person to
manage the slide properly when his attention is fixed upon a second
hand. This is obviated by having one observer handle the watch and
another the photometer, but here the reaction time is a source of
considerable error. The most satisfactory method is to use a stop-watch.
This can be held in the left hand and started and stopped by the index
finger. The photometer is held against it in the right hand in such a
way that the two movements of stopping the watch and closing the slide
may be made at the same instant. The length of exposure is that
necessary to bring the tint of the paper to that of the standard beside
it. A second method which is equally advantageous and sometimes
preferable does away with the permanent standard in the field and the
need for a stop-watch. In this event, the strip is exposed until a
medium color is obtained, since very light or very deep prints are
harder to match. This is later compared with the multiple standard. In
both cases, the date, time of day, station, number of instrument and of
exposure, and the length of the latter in seconds are carefully noted.
The instrument is held with the edge toward the south at the level to be
read, and the opening uppermost in the usual position of the leaf. When
special readings are desired, as for isophotic leaves, reflected light,
etc., the position is naturally changed to correspond. In practice, it
is made an invariable rule to move the strip for the next exposure as
soon as the slide is closed. Otherwise double exposures are liable to
occur. When a strip is completely exposed it is removed in the dark, and
a new one put in place. The former is carefully labeled and dated on the
back, and put away in a light-tight box in a cool place.

[Illustration: Fig. 12. Dawson-Lander sun recorder.]

=80. The Dawson-Lander sun recorder.= “The instrument consists of a
small outer cylinder of copper which revolves with the sun, and through
the side of which is cut a narrow slit to allow the sunshine to impinge
on a strip of sensitive paper, wound round a drum which fits closely
inside the outer cylinder, but is held by a pin so that it can not
rotate. By means of a screw fixed to the lid of the outer cylinder, the
drum holding the sensitive paper is made to travel endwise down the
outer tube, one-eighth of an inch daily, so that a fresh portion of the
sensitive surface is brought into position to receive the record.” The
instrument is driven by an eight-day clock placed in the base below the
drum. The slit is covered by means of a flattened funnel-shaped hood,
and the photographic strip is protected from rain by a perfectly
transparent sheet of celluloid. The detailed structure of the instrument
is shown in figure 12. This instrument may be obtained from Lander and
Smith, Canterbury, England, for $35.

In setting up the sunshine recorder, the axis should be placed in such a
position that the angle which it makes with the base is the same as the
altitude of the place where the observations are made. This is readily
done by loosening the bolts at either side. The drum is removed, the
celluloid sheet unwound by means of the key which holds it in place, the
sensitive strip put in position, and the sheet again wound up. Strips of
a special sensitive paper upon which the hours are indicated are
furnished by the makers of the instrument, but it has been found
preferable to use solio strips in order to facilitate comparison with
the standards. The drum is placed on the axis, and is screwed up until
it just escapes the collar at the top of the spiral. The clock is wound
and started, and the outer cylinder put on so that the proper hour mark
coincides with the index on the front of the base.

As a sunshine recorder, the instrument gives a perfect record, in which
the varying intensities are readily recognizable. Since the cylinder
moves one-half inch in an hour, and the slit is .01 of an inch, the time
of each exposure is 72 seconds. This gives a very deep color on the
solio paper, which results in a serious error in making comparisons with
the standard. On account of the hood, diffuse light is not recorded when
it is too weak to cast a distinct shadow. It seems probable that this
difficulty will be overcome by the use of a flat disk containing the
proper slit, and in this event the instrument will become of especial
value for measuring the diffuse light of layered formations. The
celluloid sheet constitutes a source of error in sunlight on account of
the reflection which it causes. This can be prevented by using the
instrument only on sunny days, when the protection of the sheet can be
dispensed with.

=81. The selagraph.= This instrument is at present under construction,
and can only be described in a general way. In principle it is a simple
photometer operating automatically. It consists of a light-tight box
preferably of metal, which contains an eight-day lever clock. Attached
to the arbor of the latter is a disk 7 inches in diameter bearing on its
circumference a solio strip 1 cm. wide and 59 cm. long. The opening in
the box for exposure is 6 mm. square and is controlled by a photographic
shutter. The latter is constructed so that it may be set for 5, 10, or
20 seconds, since a single period of exposure can not serve for both sun
and shade. The shutter is tripped once every two hours, by means of a
special wheel revolving once a day. Each exposure is 6 mm. square, and
is separated by a small space from the next one. Twelve exposures are
made every 24 hours, and 84 during the week, though, naturally, the
daytime exposures alone are recorded. Comparisons with the multiple
standard are made exactly as in the case of the simple photometer. The
selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.


                              _Standards_

=82. Use.= The light value of each exposure is determined by reference
to a standard. When the photometer carries a permanent standard, each
exposure is brought to the tint of the latter, and its value is
indicated by the time ratio between them. Thus, if the standard is the
result of a 5–second exposure to full sunlight at meridian, and a
reading which corresponds in color requires 100 seconds in the habitat
concerned, the light of the latter is twenty times weaker or more
diffuse. Usually, the standard is regarded as unity, and light values
figured with reference to it, as .05. With the selagraph such a use of
the standard is impossible, and often, also, with the photometer it is
unnecessary or not desirable. The value of each exposure in such case is
obtained by matching it with a multiple standard, after the entire strip
has been exposed. The further steps are those already indicated. After
the exact tint in the standard has been found, the length of the reading
in seconds is divided by the time of the proper standard, and the result
expressed as above.

=83. Making a standard.= Standards are obtained by exposing the
photometer at meridian on a typically clear day, and in the field where
there is the least dust and smoke. Exception to the latter may be made,
of course, in obtaining standards for plant houses located in cities,
though it is far better to have the same one for both field and control
experiment. Usable standards can be obtained on any bright day at the
base station. Indeed, valuable results are often secured by immediate
successive sun and shade readings in adjacent habitats, where the sun
reading series is the sole standard. Preferably, standards should be
made at the solstices or equinoxes, and at a representative station. The
June solstice is much to be preferred, as it represents the maximum
light values of the year. Lincoln has been taken as the base station for
the plains and mountains. It is desirable, however, that a national or
international station be ultimately selected for this purpose, in order
that light values taken in different parts of the world may be readily
compared.

=84. Kinds of standards.= The base standard is the one taken at Lincoln
(latitude 41° N.) at meridian June 20–22. This is properly the unit to
which all exposures are referred, but it has been found convenient to
employ the Minnehaha standard as the base for the Colorado mountains, in
order to avoid reducing each time. _Relative_ standards are frequently
used for temporary purposes. Thus, in comparing the light intensities of
a series of formations, one to five standards are exposed on the solio
strip before beginning the series of readings. _Proof_ standards are the
exposed solio strips, which fade in the light, and can, in consequence,
be kept only a few weeks without possibility of error. The fading can be
prevented by “toning” the strip, but in this event the exposures must be
fixed in like manner before they can be compared. This process is
inconvenient and time-consuming. It is also open to considerable error,
as the time of treatment, strength of solution, etc., must be exactly
equivalent in all instances. _Permanent_ standards are accurate
water-color copies of the originals obtained by the photometer. These
have the apparent disadvantage of requiring a double comparison or
matching, but after a little practice it is possible to reproduce the
solio tints so that the copy is practically indistinguishable from the
original. The most satisfactory method is to make a long stroke of color
on a pure white paper, since a broad wash is not quite homogeneous, and
then to reject such parts of the stroke as do not match exactly.
Permanent standards fade after a few month’s use, and must be replaced
by parts of the original stroke. _Single_ standards are made by one
exposure, while _multiple_ ones have a series of exposures filling a
whole light strip. These are regularly obtained by making the exposures
from 1–10 seconds respectively, and then increasing the length of each
successive exposure by 2 seconds. Single exposures of 1–5 seconds as
desired usually serve as the basis for permanent standards, but a
multiple standard may also be copied in permanent form. Exposures for
securing standards must be made only under the most favorable
conditions, and the length in seconds must be exact. The use of the
stop-watch is imperative, except where access may be had to an
astronomical clock with a large second hand, which is even more
satisfactory. The length of time necessary for the series desired is
reckoned beforehand, and the exposures begun so that the meridian falls
in the middle of the process.

Single standards are exceedingly convenient in photometer readings, but
they are open to one objection. In the sunshine it is necessary to make
instant decision upon the accuracy of the match, or the exposure becomes
too deep. In the shade where the action is slower, this difficulty is
not felt. For this reason it is usually desirable to check the results
by a multiple standard, and in the case of selagraph records, where the
various exposures show a wide range of tint, light values are obtainable
only by direct comparison with the multiple standard. The exact matching
of exposure and standard requires great accuracy, but with a little
practice this may be done with slight chance of error by merely moving
the exposure along the various tints of the standard until the proper
shade is found. The requisite skill is soon acquired by running over a
strip of exposures several times until the comparisons always yield the
same results for each. The margin of error is practically negligible
when the same person makes all the comparisons, and in the case of two
or three working on the same reading the results diverge little or not
at all. The efficient difference for light is much more of a variable
than is the case with water-content. It has been determined so far only
for a few species, all of which seem to indicate that appreciable
modification in the form or structure of a leaf does not occur until the
reduction in intensity reaches .1 of the meridian sunlight at the June
solstice. The error of comparison is far less than this, and
consequently may be ignored, even in the most painstaking inquiry.


                               _Readings_

=85. Time.= The intensity of the light incident upon a habitat varies
periodically with the hour and the day, and changes in accord with the
changing conditions of the sky. The light variations on cloudy days can
only be determined by the photometer. While these can not be ignored,
proper comparisons can be instituted only between the readings taken on
normal days of sunshine. The sunlight varies with the altitude of the
sun, i. e., the angle which its rays make with the surface at a given
latitude. This angle reaches a daily maximum at meridian. The yearly
maximum falls on June 22, and the angle decreases in both directions
through the year to a minimum on December 22. At equal distances from
either solstice, the angle is the same, e. g., on March 21 and September
23. At Lincoln (41° N. latitude) the extremes at meridian are 73° and
26°; at Minnehaha (39°) they are 75° and 28°. The extremes for any
latitude may be found by subtracting its distance in degrees north of
the two tropics from 90. Thus, the 50th parallel is 26.5° north of the
tropic of Cancer, and the maximum altitude of the sun at a place upon it
is 63.5°. It is 73.5° north of the tropic of Capricorn, and the minimum
meridional altitude is 16.5°.

The changes in the amount of light due to the altitude of the sun are
produced by the earth’s atmosphere. The absorption of light rays is
greatest near the horizon, where their pathway through the atmosphere is
longest, and it is least at the zenith. The absorption, and,
consequently, the relative intensity of sunlight, can be determined at a
given place for each hour of any sunshiny day by the use of chart 13.
This chart has been constructed for Lincoln, and will serve for all
places within a few degrees of the 40th parallel. The curves which show
the altitude of the sun at the various times of the day and the year
have been constructed by measurements upon the celestial globe. Each
interval between the horizontal lines represents 2 degrees of the sun’s
altitude. The vertical lines indicate time before or after the apparent
noon, the intervals corresponding to 10 minutes. If the relative
intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent
noon for this day must first be determined. A glance at the table shows
that the sun crosses the meridian on this day at 9 minutes 53 seconds
past noon at the 90th meridian. The apparent noon at Lincoln is found by
adding 26 minutes 49 seconds, the difference in time between Lincoln and
a point on the 90th meridian. When the sun is fast, the proper number of
minutes is taken from 26 minutes 49 seconds. The apparent noon on March
12 is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23
minutes later. The sun’s altitude is accordingly 36°. If the intensity
of the light which reaches the earth’s surface when the sun is at zenith
is taken as 1, the table of the sun’s altitudes gives the intensity at
3:00 P.M. on March 12 as .85.

For places with a latitude differing by several degrees from that of
Lincoln, it is necessary to construct a new table of altitude curves
from the celestial globe. It is quite possible to make a close
approximation of this from the table given, since the maximum and
minimum meridional altitude, and hence the corresponding light
intensity, can be obtained as indicated above. For Minnehaha, which is
on the 105th meridian, and for other places on standard meridians, i.
e., 60°, 75°, 90°, and 120° W., the table of apparent noon indicates the
number of minutes to be added to 12 noon, standard time, when the sun is
slow, and to be subtracted when the sun is fast. The time at a place
east or west of a standard meridian is respectively faster or slower
than the latter. The exact difference in minutes is obtained from the
difference in longitude by the equation, 15° = 1 hour. Thus, Lincoln,
96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is
consequently 26 minutes 49 seconds slower, and this time must always be
added to the apparent noon as determined from the chart. At a place east
of a standard meridian, the time difference is, of course, subtracted.

[Illustration: Fig. 13. Chart for the determination of the sun’s
altitude, and the corresponding light intensity.]

The actual differences in the light intensity from hour to hour and day
to day, which are caused by variations in the sun’s altitude, are not as
great as might be expected. For example, the maximum intensity at
Lincoln, June 22, is .98; the minimum meridional intensity December 22
is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00 A.M.
and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.M. the range
in intensity is from .90 to .98 merely. On December 22, the greatest
intensity is .52, the least .20 (the latter at 8:00 A.M. and 4:00 P.M.
approximately). If the growing season be taken as beginning with the 1st
of March and closing the 1st of October, the greatest variation in light
intensity at Lincoln within a period of 10 hours with the meridian at
its center (cloudy days excepted) is from .33 to .98. In a period of 8
hours, the extremes are .65 to .98, i. e., the greatest variation, .3,
is far within the efficient difference, which has been put at .9. For
the growing period, then, readings made between 8:00 A.M. and 4:00 P.M.
on normal sunshiny days may be compared directly, without taking into
account the compensation for the sun’s altitude. Until the efficient
difference has been determined for a large number of species, however,
it seems wise to err on the safe side and to compensate for great
differences in time of day or year. In all doubtful cases, the intensity
obtained by the astronomical method should also be checked by
photometric readings. A slight error probably enters in, due to
reflection from the surface of the paper, and to temperature, but this
is negligible.

86. _Table for determining apparent noon_

               ════════════╤═══════════════╤═════════════
                   DATE    │ TIME EQUATION │LINCOLN NOON
               ─────────┬──┼───────────────┼──────┬──────
                        │  │  _Sun slow_: +│  26m.│ 49s.
               January  │ 1│    3m.│   47s.│ 12:31│ P.M.
               „        │ 6│      6│      7│   :33│
               „        │11│      8│     12│   :35│
               „        │16│     10│      3│   :37│
               „        │21│     11│     35│   :38│
               „        │26│     12│     48│   :40│
               „        │31│     13│     41│   :40│
               February │10│     14│     27│   :41│
               „        │20│     13│     56│   :41│
               March    │ 2│     12│     18│   :39│
               „        │ 7│     11│     10│   :38│
               „        │12│      9│     53│   :37│
               „        │17│      8│     29│   :36│
               „        │22│      6│     59│   :34│
               „        │27│      5│     27│   :32│
               April    │ 1│      3│     55│   :31│
               „        │ 6│      2│     27│   :29│
               „        │11│      1│      3│   :28│
                        │  │  _Sun fast_: −│      │
               „        │16│      0│     13│   :27│
               „        │21│      1│     20│   :25│
               „        │26│      2│     16│   :24│
               May      │ 1│      3│      0│   :24│
               „        │16│      3│     48│   :23│
               „        │31│      2│     33│   :24│
               June     │ 5│      1│     45│   :25│
               „        │10│      0│     49│   :26│
                        │  │  _Sun slow_: +│      │
               „        │15│      0│     13│   :27│
               „        │20│      1│     18│   :28│
               „        │25│      2│     22│   :29│
               „        │30│      3│     22│   :30│
                        │  │  _Sun slow_: +│  26m.│ 49s.
               July     │ 5│    4m.│   19s.│ 12:31│ P.M.
               „        │10│      5│      7│   :32│
               „        │20│      6│      6│   :33│
               August   │ 4│      5│     53│   :33│
               „        │14│      4│     30│   :31│
               „        │19│      3│     28│   :30│
               „        │24│      2│     13│   :29│
               „        │29│      0│     48│   :28│
                        │  │  _Sun fast_: −│      │
               September│ 3│      0│     45│   :26│
               „        │ 8│      2│     25│   :24│
               „        │13│      4│      9│   :23│
               „        │18│      5│     55│   :21│
               „        │23│      7│     41│   :19│
               „        │28│      9│     23│   :17│
               October  │ 3│     10│     59│   :16│
               „        │ 8│     12│     26│   :14│
               „        │13│     13│     43│   :13│
               „        │18│     14│     48│   :12│
               „        │23│     15│     37│   :11│
               November │ 2│     16│     20│   :10│
               „        │12│     15│     45│   :11│
               „        │17│     14│     54│   :12│
               „        │22│     13│     44│   :13│
               „        │27│     12│     14│   :15│
               December │ 2│     10│     25│   :16│
               „        │ 7│      8│     21│   :18│
               „        │12│      6│      5│   :21│
               „        │17│      3│     41│   :23│
               „        │22│      1│     12│   :26│
                        │  │  _Sun slow_: +│      │
               „        │27│      1│     17│   :28│
               ─────────┴──┴───────┴───────┴──────┴──────

=87. Place.= The effect of latitude upon the sun’s altitude, and the
consequent light intensity have been discussed in the pages which
precede. Latitude has also a profound influence upon the duration of
daylight, but the importance of the latter apart from intensity is not
altogether clear. The variation of intensity due to altitude has been
greatly overestimated; it is practically certain, for example, that the
dwarf habit of alpine plants is not to be ascribed to intense
illumination, since the latter increases but slightly with the altitude.
It has been demonstrated astronomically that about 20 per cent of a
vertical ray of sunlight is absorbed by the atmosphere by the time it
reaches sea level. At the summit of Pike’s Peak, which is 14,000 feet
(4,267 meters) high, the barometric pressure is 17 inches, and the
absorption is approximately 11 per cent. In other words, the light at
sea level is 80 per cent of that which enters the earth’s atmosphere; on
the summit of Pike’s Peak it is 89 per cent. As the effect of the sun’s
altitude is the same in both places, the table of curves on page 57 will
apply to both. Taking into account the difference in absorption, the
maximum intensity at sea level and at 14,000 feet on the fortieth
parallel is .98 and 1.09 respectively. The minimum intensities between
8:00 A.M. and 4:00 P.M. of the growing period are .64 and .71
respectively. The correctness of these figures has been demonstrated by
photometer readings, which have given almost exactly the same results.
Such slight variations are quite insufficient to produce an appreciable
adjustment, particularly in structure. They are far within the efficient
difference, and Reinke[5] has found, moreover, that photosynthetic
activity in _Elodea_ is not increased beyond the normal in sunlight
sixty times concentrated. In consequence, it is entirely unnecessary to
take account of different altitudes in obtaining light values.

The slope of a habitat exerts a considerable effect upon the intensity
of the incident light. If the angle between the slope and the sun’s ray
be 90°, a square meter of surface will receive the maximum intensity, 1.
At an angle of 10°, the same area receives but .17 of the light. This
relation between angle and intensity is shown in the table which
follows. The influence of the light, however, is felt by the leaf, not
by the slope. Since there is no connection between the position of the
leaf and the slope of the habitat, the latter may be ignored. In
consequence, it is unnecessary to make allowances for the direction of a
slope, viz., whether north, east, south, or west, in so far as light
values are concerned. The angle which a leaf makes with its stem
determines the angle of incidence, and hence the amount of light
received by the leaf surface. This is relatively unimportant for two
reasons. This angle changes hourly and daily with the altitude of the
sun, and the intensity constantly swings from one extreme to the other.
Moreover, the extremes 1.00 and 0.17, even if constant, are hardly
sufficient to produce a measurable result. When the angle of the leaf
approaches 90°, there is the well-known differentiation of leaf surfaces
and of chlorenchym, but this has no relation to the angle of incidence.


                 _Table of Intensity at Various Angles_

                            ANGLE INTENSITY
                             90        1.00
                             80         .98
                             70         .94
                             60         .87
                             50         .77
                             40         .64
                             30         .50
                             20         .34
                             10         .17

In the sunlight, it makes no difference at what height a light reading
is taken. In forest and thicket as well as in some herbaceous
formations, the intensity of the light, if there is any difference, is
greatest just beneath the foliage of the facies. In forests especially,
the light is increasingly diffuse toward the ground, particularly where
layers intervene. In woodland formations, moreover, the exact spot in
which a reading is made must be carefully chosen, unless the foliage is
so dense that the shade is uniform. A very satisfactory plan is to take
readings in two or more spots where the shade appears to be typical, and
to make a check reading in a “sunfleck,” a spot where sunlight shows
through. In forests and thickets, the sunflecks are fleeting, and the
light value is practically that of the shade. In passing into open
woodland and thicket, the sunflecks increase in size and permanence,
until finally they exceed the shade areas in amount and become typical
of the formation.


                     _Reflected and Absorbed Light_

=88. The fate of incident light.= The light present in a habitat and
incident upon a leaf is not all available for photosynthesis. Part is
reflected or screened out by the epidermis, and a certain amount passes
through the chlorenchym, except in very thick leaves. The light absorbed
is by far the greatest in the majority of species. Many plants with
dense coatings of hairs reflect or withhold more light than they absorb,
and the amount of light reflected by a thick cuticule is likewise great.
As light is imponderable, the actual amount absorbed or reflected by the
leaf can not be determined. It is possible, however, to express this in
terms of the total amount received, by means of readings with solio
paper, and the knowledge thus obtained is of great importance in
interpreting the modifications of certain types of leaves. For example,
a leaf with a densely hairy epidermis may receive light of the full
intensity, 1; the amount reflected or screened out by the hairs may be
95 per cent of this, the amount absorbed 5 per cent, and that
transmitted, nil. In the majority of cases, however, the absorbed light
is considerably more than the amount reflected or transmitted.

[Illustration: Fig. 14. Leaf print: exposed 10 m., 11 A.M. August 20.
The leaves are from sun and shade forms of _Bursa bursa-pastoris_, _Rosa
sayii_, _Thalictrum sparsiflorum_, and _Machaeranthera aspera_. In each
the shade leaf prints more deeply.]

=89. Methods of determination.= If results are to be of value, reflected
and transmitted light must be determined in the habitat of the plant
simultaneously with the total light which a leaf receives. An
approximation of the light reflected from a leaf surface is secured by
placing the photometer so that the light reflected is thrown upon the
solio strip. A much more satisfactory method, however, is to determine
it in connection with the amount of light transmitted through the
epidermis. This is done by stripping a piece of epidermis from the upper
surface of the leaf and placing it over the slit in the photometer for
an exposure. An exposure in the full light of the habitat is made
simultaneously with another photometer, or immediately afterward upon
the same strip. When the epidermis is not too dense, both exposures are
permitted to reach the same tint, and the relation between them is
precisely that of their lengths of exposure. Ordinarily the two
exposures are made absolutely simultaneous by placing the epidermis over
half of the opening, leaving the other half to record the full light
value, and the results, or _epidermis prints_, are referred to a
multiple standard. The difference between the two values thus obtained
represents the amount of reflected light together with that screened by
the epidermis. The amount of light transmitted through the leaf may be
measured in the same way by using the leaf itself in place of the
epidermis alone. The time of exposure is necessarily long, however, and
it has been found practicable to obtain leaf prints by exposing the leaf
in a printing frame, upon solio paper, at the same time that the
epidermis print is made. In a few species both the upper and lower
epidermis can be removed and the amount of light absorbed determined
directly by exposing the strip covered with the chlorenchym. Generally,
however, this must be computed by subtracting the sum of the per cents
of reflected and transmitted light from 100 per cent, which represents
the total light.

[Illustration: Fig. 15. Leaf print: exposure as before. Sun and shade
leaves of _Achillea lanulosa_, _Capnoides aureum_, _Antennaria
umbrinella_, _Galium boreale_, and _Potentilla propinqua_.]

=90. Leaf and epidermis prints.= In diphotic leaves the screening effect
of the lower epidermis may be ignored. Isophotic sun leaves, i. e.,
those nearly upright in position or found above light-colored,
reflecting soils, are usually strongly illuminated on both sides, and
the absorbed light can be obtained only by measuring the screening
effect of both epiderms. Shade leaves and submerged leaves often contain
chloroplasts in the epidermis, and the above method can not be applied
to them. In fact, in habitats where the light is quite diffuse,
practically all incident light is absorbed. The rare exceptions are
those shade leaves with a distinct bloom. In addition to their use in
obtaining the amount of light absorbed, both leaf and epidermis prints
are extremely interesting for the direct comparison of light relations
in the leaves of species belonging to different habitats. The relative
screening value of the upper and lower epidermis, or of the
corresponding epiderms of two ecads or two species, is readily
ascertained by exposing the two side by side in sunshine, over the slit
in the photometer. For leaf prints fresh leaves are desirable, though
nearly the same results can be obtained from leaves dried under
pressure. The leaves are grouped as desired on the glass of a printing
frame, and covered with a sheet of solio. They are then exposed to full
sunlight, preferably at meridian, and the prints evaluated by means of
the multiple standard. This method is especially useful in the
comparison of ecads of one species. These differences due to transmitted
light are very graphic, and can easily be preserved by “toning” the
print in the usual way.


                        _Expression of Results_

=91. Light records.= The actual photographic records obtained by
photometer and selagraph can at most be kept but a few months, unless
they are toned or fixed. “Toning” modifies the color of the exposure
materially, and changes its intensity so that it can not be compared
with readings not fixed. It would involve a great deal of inconvenience
to make all comparisons by means of toned strips and standard, even if
it were not for the fact that it is practically impossible to obtain
exactly the same shade in lots toned at different times. The field
record, if carefully and neatly made, may well take the place of a
permanent one. The form is the following:

 ═══════╤══════════╤═════════╤═══════╤════════╤════════╤══════════╤═══════
   Day  │   Hour   │Formation│Station│Altitude│Exposure│  Group   │Height
        │          │         │       │        │        │          │
        │          │         │       │        │        │          │
 ───────┼──────────┼─────────┼───────┼────────┼────────┼──────────┼───────
 14/9/04│ 12:00 M. │ Spruce  │ Milky │2600 m. │N.E. 20°│Opulaster │1 foot
        │          │         │  Way  │        │        │          │
    „   │12:05 P.M.│ Spruce  │ Moss  │2500 m. │ Level  │Streptopus│   „
        │          │         │ Glen  │        │        │          │
    „   │12:15 P.M.│  Brook  │Grotto │2500 m. │ E. 3°  │  Filix   │Surface
        │          │  b’nk   │       │        │        │          │
 ───────┴──────────┴─────────┴───────┴────────┴────────┴──────────┴───────

 ═══════╤════╤════════╤════════╤═════╤═════╤═════════╤════════╤════════
   Day  │No. │ Length │Standard│Light│Base │Reflected│Transm’d│Absorbed
        │    │   of   │        │value│value│  light  │ light  │ light
        │    │exposure│        │     │     │         │        │
 ───────┼────┼────────┼────────┼─────┼─────┼─────────┼────────┼────────
 14/9/04│2:10│ 160 s. │  3 s   │.019 │     │         │        │
        │    │        │        │     │     │         │        │
    „   │2:12│ 240 s. │  3 s.  │.012 │     │         │        │
        │    │        │        │     │     │         │        │
    „   │2:13│ 360 s. │  3 s.  │.008 │     │         │        │
        │    │        │        │     │     │         │        │
 ───────┴────┴────────┴────────┴─────┴─────┴─────────┴────────┴────────

=92. Light sums, means, and curves.= Owing to the fact that the
selagraph has not yet been used in the field, no endeavor has been made
to determine the light value for every hour of the day in different
habitats. Consequently there has been no attempt to compute light sums
and means. Photometer readings have sufficed to interpret the effect of
light in the structure of the formation, and of the individual, but they
have not been sufficiently frequent for use in ascertaining sums and
means. The latter are much less valuable than the extremes, especially
when the relative duration of these is indicated. Means, however, are
readily obtained from the continuous records. Light sums are probably
impracticable, as the factor is not one that can be expressed in
absolute terms. The various kinds and combinations of light curves are
essentially the same as for humidity. The level curve through a series
of habitats is the most illuminating, but the day curve of hour
variations is of considerable value. The curve of daily duration, based
upon full sunlight, is also of especial importance for plants, and
stations which receive both sun and shade during the day.


                              TEMPERATURE

=93.= In consequence of its indirect action, temperature does not have a
striking effect upon the form and structure of the plant, as is the case
with water and light. Notwithstanding, it is a factor of fundamental
importance. This is especially evident in the character and distribution
of vegetation. It is also seen in the germination and growth of plants,
in the length of season, and in the important influence of temperature
upon humidity, and hence upon water-content. Because of its intimate
relation with the comfort of mankind, the determination of temperature
values has received more attention than that of any other factor, and
excellent simple and recording instruments are numerous. For plants, it
is also necessary to employ instruments for measuring soil temperatures.
The latter unquestionably have much less meaning for the plant than the
temperatures of the air, but they have a direct influence upon the
imbibition of water, and upon germination.


                             _Thermometers_

=94. Air thermometers.= The accurate measurement of temperature requires
standard thermometers. Reasonably accurate instruments may be
standardized by determining their error, but they are extremely
unsatisfactory in practice, since they result in a serious waste of
time. Accurate thermometers which read to the degree are entirely
serviceable as a rule, but instruments which read to a fraction of a
degree are often very much to be desired. The writer has found the
“cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be
an exceedingly satisfactory instrument. The best numbers for general use
are 247 and 251, which read from –15° to 50° C. and are graduated
in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and
$3.50. These instruments are delicate and require careful handling, but
even in class work this has proved to be an advantage rather than
otherwise. In making readings of air temperatures with such
thermometers, constant precautions must be taken to expose the bulb
directly to the wind and to keep it away from the hand and person.

=95. Soil thermometers.= The thermometer described above has been used
extensively for soil temperatures. The determination of the latter is
conveniently combined with the taking of soil samples, by using the hole
for a temperature reading. When carefully covered, these holes can be
used from day to day throughout the season without appreciable error,
even in gravel soils. Repeated tests of this have been made by
simultaneous readings in permanent and newly made holes, and the results
have always been the same. It has even been found that the error is
usually less than 1 degree when the hole is left uncovered, if it is
more than 9 inches deep. A slight source of error lies in the fact that
the thermometer must be raised to make the reading. With a little
practice, however, the top of the column of mercury may be raised to the
surface and read before the change of temperature can react upon it.
This is especially important in very moist or wet soils where the bulb
becomes coated with a film of moisture. This evaporates when the bulb is
brought into the air, and after a moment or two the mercury slowly
falls.

[Illustration: Fig. 16. Soil thermometer]

Regular soil thermometers are indispensable when readings are desired at
depths greater than 12–18 inches. They possess several disadvantages
which restrict their use almost wholly to permanent stations. It is
scarcely possible to carry them on field trips, and the time required to
place them in the soil renders them practically useless for single
readings. Moreover, the instruments are expensive, ranging in price from
$7 for the two-foot thermometer, to $19 for the eight-foot one. When it
is recognized that deep-seated temperatures are extremely constant and
that the slight fluctuations affect, as a rule, only the relatively
stable shrubs and trees, it is evident that such temperatures are of
restricted importance. Still, in any habitat, they must be ascertained
before they can well be ignored, though it is unwise to spend much time
and energy in their determination. Soil thermometers of the form
illustrated may be obtained from H. J. Green, Brooklyn.

=96. Maximum-minimum thermometers.= These are used for determining the
range of temperature within a given period, usually a day. Since they
are much cheaper than thermographs, they can replace these in part,
although they merely indicate the maximum and minimum temperatures for
the day, and do not register the time when each occurs. The maximum is a
mercurial thermometer with a constriction in the tube just above the
bulb; this allows the mercury to pass out as it expands, but prevents it
from running back, thus registering the maximum temperature. The minimum
thermometer contains alcohol. The column carries a tiny dumbbell-shaped
marker which moves down with it, but will not rise as the liquid
expands. This is due to the fact that the fluid expands too slowly to
carry the marker upward, while the surface tension causes it to be drawn
downward as the fluid contracts. The minimum temperature is indicated by
the upper end of the marker. In setting up the thermometers, they are
attached by special thumbscrews to a support which holds them in an
oblique position. The minimum is placed in a special holder above the
maximum which rests on a pin that is used also for screwing the
pivot-screw into position. The support is screwed tightly to the
cross-piece of a post, or in forest formations it is fastened directly
to a board nailed upon a tree trunk. A shelter has not been used in
ecological work, although it is the rule in meteorological observations.
The minimum thermometer is set for registering by raising the free end,
so that the marker runs to the end of the column. The mercury of the
maximum is driven back into the bulb by whirling it rapidly on the
pivot-screw after the pin has been taken out. This must be done with
care in order that the bulb may not be broken. As soon as the instrument
comes to rest, it is raised and the pin replaced, great care being taken
to lift it no higher than is necessary. When the night maximum is
sought, the thermometer should be whirled several times in order to
drive the column sufficiently low. Usually, in such cases, a record is
made of this point to make sure that the maximum read is the actual one.
If the pivot-screw is kept well oiled, less force will be required to
drive the mercury back. In practice, the thermometers have been observed
at 6:00 A.M. and 6:00 P.M. each day, thus permitting the reading of the
maximum-minimum for both day and night. Pairs of maximum-minimum
thermometers are to be obtained from H. J. Green, 1191 Bedford Ave.,
Brooklyn, or Julien P. Friez, Baltimore, Maryland, at a cost of $8.25.

[Illustration: Fig. 17. Maximum-minimum thermometer.]

[Illustration: Fig. 18. Terrestrial radiation thermometer.]

[Illustration: Fig. 19. Draper thermograph.]

=97. Radiation thermometers.= These are used to determine the radiation
in the air, and from the soil, i. e., for solar and terrestrial
radiation. The latter alone has been employed in the study of habitats,
chiefly for the purpose of ascertaining the difference in the cooling of
different soils at night. The terrestrial radiation thermometer is
merely a special form of minimum thermometer, so arranged in a support
that the bulb can be placed directly above the soil or plant to be
studied. It is otherwise operated exactly like the minimum thermometer,
and the reading gives the minimum temperature which the air above the
plant or soil reaches, _not_ the amount of radiation. As a consequence,
these instruments are valuable only where read in connection with a pair
of maximum-minimum thermometers in the air, or when read in a series of
instruments placed above different soils or plants.

=98. Thermographs.= Two types of standard instruments are in general use
for obtaining continuous records of air temperatures. These are the
Draper thermograph, made by the Draper Manufacturing Company, 152 Front
St., New York city ($25 and $30), and the Richard thermograph sold by
Julien P. Friez, Baltimore ($50). After careful trial had demonstrated
that they were equally accurate, the matter of cost was considered
decisive, and the Draper thermograph has been used exclusively in the
writer’s own work. This instrument closely resembles the psychrograph
manufactured by the same company. It is made in two sizes, of which the
larger one is the more satisfactory on account of the greater distance
between the lines of the recording disk. The thermometric part consists
of two bimetallic strips, the contraction and expansion of which are
communicated to a hand carrying a pen. The latter traces a line on the
record sheet which is attached to a metal disk made to revolve by an
eight-day clock. In practice the thermograph is set up in the shelter
which contains the psychrograph, and in exactly the same manner. The
clock is wound, the record put in place, and the pen inked in the same
way also. The proper position of the pen is determined by making a
careful thermometer reading under the shelter, and then regulating the
pen hand by means of the screws at the base of it. A similar test
reading is also made each week, when the clock is rewound. A record
sheet may be left in position for three weeks, the pen being filled each
week with a different ink. The fixed order of using the inks is red,
blue, and green as already indicated.

[Illustration: Fig. 20. Shelter for thermograph.]

Owing to the fact that they are practically stationary, soil
thermographs are of slight value, except at base stations. Here, the
facts that they are expensive, that the soil temperatures are of
relatively little importance, and that they can be determined as easily,
or nearly so, by simple thermometers, make the use of such instruments
altogether unnecessary, if not, indeed, undesirable. In a perfectly
equipped research station, they undoubtedly have their use, but at
ordinary stations, and in the case of private investigators, their value
is in no wise commensurate with their cost.


                               _Readings_

=99. Time.= The hourly and daily fluctuations of the temperature of the
air render frequent readings desirable. It is this variation, indeed,
which makes single readings, or even series of them, inconclusive, and
renders the use of a recording instrument almost imperative in the base
station at least. Undoubtedly, a set of simultaneous readings at
different heights in one station, or at the same height in different
stations, especially if made at the maximum, have much value for
comparison, but their full significance is seen only when they are
referred to a continuous base record. Such series, moreover, furnish
good results for purposes of instruction. In research work, however, it
has been found imperative to have thermographs in habitats of widely
different character. With these as bases, it is possible to eke them out
with considerable satisfaction by means of maximum-minimum thermometers
in less different habitats, or in different parts of the same habitat.
Naturally these are less satisfactory, and are used only when expense
sets a limit to the number of thermographs. In a careful analysis of a
single habitat, more can be gained by one base thermograph supplemented
by three pairs of maximum-minimum thermometers in dissimilar areas of
the habitat than by two thermographs, and the cost is the same.

[Illustration: Fig. 21. Richard thermograph.]

=100. Place and height.= For general air temperatures, thermograph and
thermometer readings are made at a height of 3 feet (1 meter). Soil
temperatures are regularly taken at the surface and at a depth of 1
foot. When a complete series of simultaneous readings is made in one
station, the levels are 6 feet and 3 feet in the air, the surface of the
soil, and 5, 10, and 15 inches in the soil. When sun and shade occur
side by side in the same formation, as is true of many thickets and
forests, surface readings are regularly made in both. Similarly,
valuable results are obtained by making simultaneous readings on the
bare soil, on dead cover, and upon a leaf, while the influence of cover
is readily ascertained by readings upon it and beneath it. A full series
of station readings made at the same time upon north, east, south, and
west slopes is of great importance in studying the effects of exposure.


                        _Expression of Results_

=101. Temperature records.= Neither field nor permanent form is required
for thermographic records, other than the record sheet itself, which
contains all the necessary information in a fairly convenient form.
Although the temperature of a particular hour and day can not be read at
a mere glance, it can be obtained so easily that it is a waste of time
to make a tabular copy of each record sheet. For thermometer readings,
either single or in series, the following form is used:

 ═══════╤═════════╤═════════╤════════╤════════╤════════╤═══════════
   Day  │  Hour   │Formation│Station │Altitude│Exposure│ Community
 ───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
    „   │    „    │    „    │   „    │   „    │   „    │     „
 ───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
 17/8/04│6:30 A.M.│ Spruce  │  Jack  │2550 m. │N.E. 5° │Mertensiare
        │         │         │ Brook  │        │        │
    „   │    „    │  Half   │Hiawatha│2550 m. │N.E. 7° │ Asterare
        │         │ gravel  │        │        │        │
    „   │6:30 P.M.│ Spruce  │  Jack  │2550 m. │N.E. 5° │Mertensiare
        │         │         │ Brook  │        │        │
    „   │    „    │  Half   │Hiawatha│2550 m. │N.E. 7° │ Asterare
        │         │ gravel  │        │        │        │
 ───────┴─────────┴─────────┴────────┴────────┴────────┴───────────

 ═══════╤═════════╤═══════════════════╤═══════════╤══════╤════
   Day  │  Hour   │POSITION OF READING│Thermograph│ Sky  │Wind
 ───────┼─────────┼──────┬─────┬──────┼───────────┼──────┼────
    „   │    „    │3 feet│Surf.│12 in.│     „     │  „   │ „
 ───────┼─────────┼──────┼─────┼──────┼───────────┼──────┼────
 17/8/04│6:30 A.M.│  9°  │ 9°  │ 9.8° │    10°    │Clear │ 0
        │         │      │     │      │           │      │
    „   │    „    │11.2° │11.2°│14.8° │    10°    │Clear │ 0
        │         │      │     │      │           │      │
    „   │6:30 P.M.│11.4° │11.4°│ 9.8° │    11°    │Cloudy│ 0
        │         │      │     │      │           │      │
    „   │    „    │ 12°  │13.8°│16.4° │    11°    │Cloudy│ 0
        │         │      │     │      │           │      │
 ───────┴─────────┴──────┴─────┴──────┴───────────┴──────┴────

=102. Temperature sums and means.= The amount of heat, i. e., the number
of calories received within a given time by a definite area of plant
surface, can be determined by means of a calorimeter. From this the
temperature sum of a particular period may be obtained by simple
addition. In the present condition of our knowledge, it is impossible to
establish any exact connection between such results and the functional
or growth effect that can be traced directly to heat. As a consequence,
temperature sums do not at present contribute anything of value to an
understanding of the relation between cause and effect. The mean daily
temperature is readily obtained by averaging twenty-four
hour-temperatures recorded by the thermograph. The method employed by
Meyen[6], of deriving the mean directly from the maximum and minimum for
the day, is not accurate; from a large number of computations, the error
is always more than two degrees. On the other hand, the mean obtained by
averaging the maximum and minimum for the day and night has been found
to deviate less than 1 degree from the mean proper. This fact greatly
increases the value of maximum-minimum instruments if they are read
daily at 6:00 A.M. and 6:00 P.M.

=103. Temperature curves.= The kinds and combinations of temperature
curves are almost without number. The simple curves of most interest are
those for a series of stations or habitats, based upon the level of
three feet, or the surface, or the daily mean. The curves for each
station representing the different heights and depths and the season
curve of the daily means for a habitat are also of much importance. One
of the most illuminating combinations is that which groups together the
various level curves for a series of habitats. Other valuable
combinations are obtained by grouping the curves of daily means of
different habitats for the season, or the various station curves.

=104. Plant temperatures.= The direct effects of temperature as seen in
nutrition and growth can be ascertained only by determining the
temperature of plant tissues. The temperatures of the air and of the
soil surface have an important effect upon humidity, and water-content,
and through them upon the plant, but heat can influence assimilation,
for example, only in so far as it is absorbed by the assimilating
tissue. The temperatures of the leaf, as the most active nutritive organ
of the plant, are especially important. While it is a well-known fact
that internal temperatures follow those of the air and soil closely,
though with varying rapidity of response, this holds less for leaves
than for stems and roots. Owing to the very obvious difficulties,
practically nothing has yet been done in this important field. A few
preliminary results have been obtained at Minnehaha, which serve to show
the need for such readings. Gravel slide rosettes in an air temperature
of 24° C. and a surface temperature of 40° C. gave the following surface
readings: _Parmelia_, 40°, _Eriogonum_, 38.6°, _Arctostaphylus_, 35°,
_Thlaspi_, 31.8°, and _Senecio_, 31°. The leaf of _Eriogonum flavum_,
which is smooth above and densely hairy below, indicated a temperature
of 31.8° when rolled closely about the thermometer bulb with the smooth
surface out, and 28° when the hairy surface was outside. The surface
readings of the same leaf were .5°–1° higher when made upon the upper
smooth surface. This immediately suggests that the lower surface may be
modified to protect the leaf from the great heat of the gravel, which
often reaches 50° C. (122° F.).


                             PRECIPITATION

=105. General relations.= As the factor which exerts the most important
control upon water-content and humidity, rainfall must be carefully
considered by the ecologist. It is such an obvious factor, and is
usually spoken of in such general terms that the need of following it
accurately is not evident at once. When it is recognized that the
fluctuations of water-content are directly traceable to it, it becomes
clear that its determination is as important as that of any indirect
factor. This does not mean, however, that the amount of yearly rainfall
is to be taken from the records of the nearest weather station, and the
factor dismissed. Like other instruments, the rain gauge must be kept at
the base station of the area under study, and when this is extensive or
diverse, additional instruments should be put into commission. While the
different parts of the same general climatic region may receive
practically the same amount of precipitation during the year, it is not
necessarily true that the rainfall of any particular storm is equally
distributed, especially in the mountains. Nothing less than an exact
knowledge of the amount of rain that falls in the different areas will
make it possible to tell how much of the water-content found at any
particular time in these represents merely the chance differences of
precipitation.

The forms of precipitation are rain, dew, hail, snow, and frost. Of
these, hail is too infrequent to be taken into account, while frost
usually occurs only at the extremes of the growing season, and in its
effect is rather to be reckoned with temperature. Snow rarely falls
except during the period of rest, and, while it plays an important part
as cover, it is merely one of several factors that determine the
water-content of the soil at the beginning of spring. The influence of
dew is not clearly understood. It is almost always too slight in amount
and too fleeting to affect the water-content of the soil. It seems
probable that it may serve by its own evaporation to decrease in some
degree the water loss from the soil, and from bedewed plants. If,
however, the dew is largely formed by the water of the soil and of the
plant, as is thought by some, then it is negligible as a reinforcement
of water-content. From the above, it is evident that rainfall alone
exerts a profound effect upon the habitat, and it is with its
measurement that the ecologist is chiefly concerned.

[Illustration: Fig. 22. Rain gauge showing construction.]

=106. The rain gauge=, as the illustration shows, is a cylindrical
vessel with a funnel-shaped receiver at the top, which is 8 inches in
diameter. The receiver fits closely upon a narrower brass vessel or
measuring tube in which the rain collects. The ratio of surface between
receiver and tube is 10 to 1. For readings covering a general area, the
rain-gauge is placed in the open, away from buildings or other
obstructions, and is sunken in the ground sufficiently to keep it
upright. In localities where winds are strong, it is usually braced at
the sides also or supported by a wooden frame. In measuring the amount
of rain in the measuring tube, the depth is divided by ten in order to
ascertain the actual rainfall. The depth is measured by inserting the
measuring-rod through the hole in the funnel until it touches the
bottom. It is left for a second or so, quickly withdrawn, and the limit
of the wetted portion noted. In the case of standard rods, the actual
rainfall is read directly in hundredths, so that the division by ten is
unnecessary. After each reading, the measuring tube is carefully
drained, replaced, and the receiver put in position. No regular time for
making readings is necessary. During a rainy period, it is customary to
make a measurement each day, but it has been found more satisfactory for
ecological purposes to measure each shower, and to record its duration.
These two facts furnish a ready clue to the relative amount of run-off
in each fall of rain. The measurement of snowfall is often made merely
by determining its depth. For comparison with rainfall, the rain gauge
with receiver and tube withdrawn is used. The snow which falls is
melted, poured into the measuring tube, and measured in the ordinary
way. The U. S. Weather Bureau standard rain gauge, with measuring stick,
may be obtained of H. J. Green, or of J. P. Friez for $5.25.

=107. Precipitation records.= From the periodic character of
precipitation, rainfall sums, means, and curves have little importance
in the careful study of the habitat. The rainfall curve for the growing
season is an aid in explaining the curve of water-content, and the mean
rainfall of a region gives some idea of its vegetation, though even here
the matter of its distribution is of primary importance. The rain and
snow charts published by the U. S. Weather Bureau furnish data of some
importance for the general study of vegetation, but it is evident that
they can play little part in a system which is founded upon the habitat.
Precipitation records, for reasons of brevity and convenience, are
united with wind records, and the form will be found under the
discussion of this factor.


                                  WIND

[Illustration: Fig. 23. Simple anemometer.]

=108. Value of readings.= On account of its direct effect upon humidity,
and its consequent influence upon water-content, the part which wind
plays in a habitat can not be ignored in a thorough investigation. It is
an important element in exposure, and accordingly has a marked
mechanical effect upon the vegetation of exposed habitats, alpine
slopes, seacoasts, plains, etc. Owing to its inconstancy and its extreme
variation in velocity, single wind readings are absolutely without
value. When read in series, anemometers give some information upon the
comparative air movement in different habitats, but the chance of error
is great, except when the breeze is steady. Anemographs alone give real
satisfaction. Accurate results, however, are not obtainable without a
series of two or more in different habitats, and it is still an open
question whether the results obtained justify the expense. For a
completely equipped base station, anemometer, anemograph, and wind vane
are desirable instruments, but the study of the habitat has by no means
reached the stage of precision in which their general use is necessary.

[Illustration: Fig. 24. Standard anemometer.]

=109. The anemometer= in its simplest form is adapted only to readings
made under direct observation, as a sudden change in the direction of
the wind reverses the movement of the indicator needle. This simple wind
gauge, shown in figure 23, has been used for instructional purposes, and
to a slight extent, also, in ascertaining the effect of cover. In
constant winds, successive single readings are found to have value, but,
ordinarily, the observations must be simultaneous. Careful tests of this
simple instrument show that it is essentially accurate. It may be
obtained from the C.H. Stoelting Company, 31 W. Randolph St., Chicago,
for $25. The standard anemometer (Fig. 24) is practically a recording
instrument up to 1,000 miles, but as the dials run on without any
indication of the total number of revolutions, it must be visited and
read each day. This renders its use difficult for habitats which are
some distance apart. When exact determinations of wind values become
necessary, the most successful method is to establish a series of three
standard anemometers. One of these should be placed upon the most
exposed part of a typically open habitat, the second in the most
protected part of the same habitat, while the third is located in the
midst of a representative forest formation. If the two habitats are
close together, the daily visits can be made without serious
inconvenience. The reading of the registering dials requires detailed
explanation, and for this the reader is referred to the printed
directions which accompany the instrument. In setting up the anemometer
it must be borne in mind that the ecologist desires the wind velocity
for a particular habitat. In consequence, the precautions which the
meteorologist takes to place the instrument at a certain height and well
away from surrounding obstructions do not hold here. Standard
anemometers are furnished by H. J. Green, and J. P. Friez for $25 each.

The anemograph is an anemometer electrically connected with an automatic
register. It is the only instrument adapted to continuous weekly records
in different habitats, but the price, $75 ($25 for the anemometer and
$50 for the register) is practically prohibitive, at least until a
complete series of ecographs for other factors has been obtained.

=110. Records.= The following form is used as a combined record for
precipitation and wind:

 ═══════╤══════════╤═════════╤════════╤════════╤════════╤═════════
   Day  │   Time   │Formation│Station │Altitude│Exposure│Community
 ───────┼──────────┼─────────┼────────┼────────┼────────┼─────────
    „   │    „     │    „    │   „    │   „    │   „    │    „
 ───────┼──────────┼─────────┼────────┼────────┼────────┼─────────
 29/8/04│ 6:30 P.M.│  Half   │Hiawatha│2550 m. │N.E. 17°│Asterare
        │          │ gravel  │        │        │        │
 31/8/04│ 5:45 P.M.│    „    │   „    │   „    │   „    │    „
  2/9/04│ 4:00 P.M.│    „    │   „    │   „    │   „    │    „
  3/9/04│10:00 P.M.│    „    │   „    │   „    │   „    │    „
 ───────┴──────────┴─────────┴────────┴────────┴────────┴─────────

 ═══════╤══════════╤═══════════════╤════╤═════════════════════════╤════
   Day  │   Time   │   RAINFALL    │Base│          WIND           │Base
 ───────┼──────────┼──────┬────────┼────┼────────┬──────┬─────────┼────
    „   │    „     │Inches│Duration│ „  │Velocity│Heig’t│Direction│ „
 ───────┼──────────┼──────┼────────┼────┼────────┼──────┼─────────┼────
 29/8/04│ 6:30 P.M.│  1   │8 hours │    │   5    │3 ft. │  N. W.  │
        │          │      │        │    │        │      │         │
 31/8/04│ 5:45 P.M.│Trace │10 min. │    │   12   │  „   │    „    │
  2/9/04│ 4:00 P.M.│  .2  │2 hours │    │   7    │  „   │   W.    │
  3/9/04│10:00 P.M.│Trace │        │    │   18   │  „   │    „    │
 ───────┴──────────┴──────┴────────┴────┴────────┴──────┴─────────┴────


                                  SOIL

=111. Soil as a factor.= In determining the value of the soil as a
factor in a particular habitat, it must be clearly recognized that its
importance lies solely in the control which it exerts upon water-content
and nutrient-content. The former is directly connected with the texture
or fineness of the soil, the latter with its chemical nature.
Accordingly, the structure of the soil and its chemical composition are
the fundamental points of attack. These are not at all of equal value,
however. Water is both a food, and a solvent for the nutrient salts of
the soil. Furthermore, the per cent of soluble salts, as determined in
mechanical analyses, is practically the same for all ordinary soils.
Indeed, the variations for the same soil types are as great as for
entirely different types. For these reasons, soluble salt-content may be
ignored except where it is readily seen to be excessive, as in alkaline
soils; and determinations of chemical composition are necessary only in
those soils which contain salts or acids to an injurious degree, e. g.,
alkaline soils, peat bogs, humus swamps, etc. The structure of the soil,
on the other hand, in the usual absence of excessive amounts of solutes,
absolutely controls the fate of the water that enters the ground, in
addition to its influence upon the run-off. It determines the amount of
gravitation water lost by percolation, as well as the water that can be
raised by capillarity. The resultant of these, the total soil water or
holard, is hence an effect of structure, while the size and compactness
of the particles are conclusive factors in controlling the chresard. It
must be recognized, however, that these are all factors which enable us
to interpret the amount of holard or chresard found in a particular
soil. They have no direct important effect upon the plant, but influence
it only in so far as they affect the water present.

=112. The value of soil surveys.= The full appreciation of the
preeminent value of water-content, particularly of the chresard, greatly
simplifies the ecological study of soils. The ecologist is primarily
concerned with soil water only in its relation to the plant, and while a
fair knowledge of soil structure is essential to a proper understanding
of this, he has little concern with the detailed study of the problems
of soil physics. For the sake of a proper balance of values, he must
avoid the tendency noted elsewhere of ignoring the claims of the plant,
and of studying the soil simply as the seat of certain physical
phenomena. Accordingly, it is felt that mechanical and chemical
analyses, determinations of soluble salt-content, etc., have much less
value than has been commonly supposed. The usual methods of soil survey,
which pay little or no attention to water-content, and none at all to
available water, are practically valueless for ecological research. This
statement does not indicate a failure to appreciate the importance of
the usual soil methods for many agricultural problems, such as the use
of fertilizers, conservation of moisture, etc., though even here to
focus the work upon water-content would give much more fundamental and
serviceable results. For these reasons, slight attention will be paid to
methods of mechanical and chemical analysis. In their stead is given a
brief statement of the origin, structure, and character of soils with
especial reference to water-content.

=113. The origin of soils.= Rocks form soils in consequence of
weathering, under the influence of physical and biotic factors.
Weathering consists of two processes, disintegration, by which the rock
is broken into component particles of various sizes, and decomposition,
in which the rock or its fragments are resolved into minute particles in
consequence of the chemical disaggregation of its minerals, or of some
other chemical change. These processes are usually concomitant,
although, as a rule, one is more evident than the other. The relation
between them is dependent upon the character of the rock and the forces
which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a
rule disintegrate more rapidly than they decompose; sedimentary rocks,
on the other hand, tend to decompose more rapidly than they
disintegrate. In many cases the two processes go hand in hand. This
difference is the basis for the distinction, first proposed by Thurmann,
between those rocks which weather with difficulty and those which
weather readily. The former were called dysgeogenous, the latter
eugeogenous. Thurmann restricted the application of the first term to
those rocks which produce little soil, but it seems more logical to
apply dysgeogenous to those in which disintegration is markedly in
excess of decomposition, and eugeogenous to those rocks that break down
rather readily into fine soils. With respect to the general character of
the soil formed, rocks are _pelogenous_, clay-producing, _psammogenous_,
sand-forming, or _pelopsammogenous_, producing mixed clay and sand. The
first two are divided into _perpelic_, _hemipelic_, _oligopelic_,
_perpsammic_, etc., with reference to the readiness with which they are
weathered, but this distinction is not a very practicable one. The
grouping of soils into silicious, calcareous, argillaceous, etc., with
reference to the chemical nature of the original rock, is of no value to
the ecologist, apart from the general clue to the physical properties
which it furnishes.

=114. The structure of soils.= The water capacity of a soil is a direct
result of the fineness of the particles. Since the water is held as a
thin surface film by each particle or group of them, it follows that the
amount of water increases with the water-holding surface. The latter
increases as the particles become finer and more numerous, and thus
produce a greater aggregate surface. The upward and downward movements
of water in the soil are likewise in immediate connection with the size
of particles. The upward or capillary movement increases as the
particles become finer, thus making the irregular capillary spaces
between them smaller, and magnifying the pull exerted. On the contrary,
the downward movement of gravitation water, i. e., percolation, is
retarded by a decrease in the size of the soil grains and hastened by an
increase. Hence, the two properties, capillarity and porosity, are
direct expressions of the structure of the soil, i. e., of its texture
or fineness. Capillarity, however, increases the water-content of the
upper layers permeated by the roots of the plant, while porosity
decreases it. On the basis of these properties alone, soils would fall
into two groups, capillary soils and porous soils, the former
fine-grained and of high water-content, the latter coarse-grained and
with relatively little water. A third factor, however, of great
importance must be taken into account. This is the pull exerted upon
each water film by the soil particle itself. This pull apparently
increases in strength as the film grows thinner, and explains why it
finally becomes impossible for the root-hairs to draw moisture from the
soil. This property, like capillarity, is most pronounced in
fine-grained soils, such as clays, and is least evident in the coarser
sands and gravels. It seems to furnish the direct explanation of
non-available water, and, in consequence, to indicate that the chresard
is an immediate result of soil texture.

[Illustration: Fig. 25. Sieves for soil analysis.]

=115. Mechanical analysis.= From the above it is evident that, with the
same rainfall, coarse soils will be relatively dry, and fine soils
correspondingly moist. However, this difference in holard is somewhat
counterbalanced by the fact that the chresard is much greater in the
former than in the latter. The basis of these relations can be obtained
only from a study of the texture of the soil. The usual method of doing
this is by mechanical analysis. This is far from satisfactory, since the
use of the sieves often brings about the disaggregation of groups of
particles which act as units in the soil. Furthermore, the analysis
affords no exact evidence of the compactness of the soil in nature, and
tests of capillarity and porosity made with soil samples out of position
are open to serious error. Nevertheless, mechanical analyses furnish
results of some value by making it possible to compare soils upon the
basis of texture. For ecological purposes, minute analyses are
undesirable; their value in any work is doubtful. A separation of soil
into gravel, sand, and silt-clay is sufficient, since the relative
proportion of these will explain the holard and chresard of the soil
concerned. The latter are also affected in rich soils, especially of
forests, by the organic matter present. If this is in a finely divided
condition, the amount is determined by calcining. When a definite layer
of leafmold is present, as in forests and thickets, its water-value is
found separately, since its power of retaining water is altogether out
of proportion to its weight.

=116. Kinds of soils.= It is very doubtful whether it is worth while to
attempt to distinguish soils upon the basis of mechanical analysis.
Unquestionably, the most satisfactory method is to distinguish them with
respect to holard and chresard, and to regard texture as of secondary
importance. A series of soil classes which comprise various soil types
has been proposed by the U. S. Bureau[7] of Soils as follows: (1) stony
loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine
sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam,
(11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based
entirely upon mechanical analyses, and in some cases are too closely
related to be useful. The line between them can nowhere be sharply
drawn. Indeed, the variation within one class is so great that soils
have frequently been referred to the wrong group. Thus, Cassadaga sand
(gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per
cent) is more closely related to Oxnard sandy loam (26–37–18–12) and to
Afton fine sandy loam (28–43–18–8) than to Coral sand (61–29–3–4),
Galveston sand (6–91–1–1), or Salt Lake sand (84–15–1–0). Elsinore sandy
loam (8–38–35–10) is much nearer to Hanford fine sandy loam (9–36–33–14)
than to Billings sandy loam (1–60–22–11) or to Utuado sandy loam
(48–23–19–8). The soil types are much more confused, and for ecological
purposes at least are entirely valueless. Lake Charles fine sandy loam
has the composition, 1–34–52–9; Vernon fine sandy loam, 1–37–54–7, while
many other so-called types show nearly the same degree of identity.

=117. The chemical nature of soils.= The effect of alkaline and acid
substances in the soil upon water-content and the activities of the
plant is far from being well understood. It is generally recognized that
salts and acids tend to inhibit the absorptive power of the root-hairs.
In the case of saline soils, this inhibitive effect seems to be
established, but the action of acids in bogs and swamps is still an open
question. It is probable that the influence of organic acid has been
overestimated, and that the curious anomaly of a structural xerophyte in
a swamp is to be explained by the stability of the ancestral type and by
the law of extremes. Apart from the effect which excessive amounts of
acids and salts may have in reducing the chresard, the chemical
character of the soil is powerless to produce structural modification in
the plant. Since Thurmann’s researches there has been no real support of
the contention that the chemical properties of the soil, not its
physical nature, are the decisive factors in the distribution and
adaptation of plants. It is not sufficient that the vegetation of a
silicious soil differs from that of a calcareous one. A soil can modify
the plants upon it only though its water-content, or the solutes it
contains. Hence, the chemical composition of the original rock is
immaterial, except in so far as it modifies these two factors. Humus,
moreover, while an important factor in growth, has no formative
influence beyond that which it exerts through water-content.


                              PHYSIOGRAPHY

=118. Factors.= The physiographic factors of a definite habitat are
altitude, exposure, slope, and surface. In addition, topography is a
general though less tangible factor of regions, while the dynamic forces
of weathering, erosion and sedimentation play a fundamental role in the
change of habitats. It is evident, however, that these, except where
they affect the destruction of vegetation directly, can operate upon the
plant only through more direct factors, such as water, light, and
temperature. While they are themselves not susceptible of measurement,
they can often be expressed in terms of determinable factors, i. e.,
slope, exposure, and surface. Fundamentally, they constitute the forces
which change one habitat into another, and, in consequence, are really
to be considered as the factors which produce succession. The static
features of physiography, altitude, etc., lend themselves readily to
determination by means of precise instruments. These factors, though by
no means negligible, are remote, and consequently their mere measurement
is insufficient to indicate the nature or extent of their influence upon
the plant. It is necessary to determine also the manner and degree in
which they affect other factors, a task yet to be done. Readings of
altitude, slope, and exposure are so easily made that the student must
carefully avoid the tendency to let them stand at their own value, which
is slight. Instead, they should be made the starting point for
ascertaining the differences which they produce in water-content,
humidity wind, and temperature.


                               _Altitude_

=119. Analysis into factors.= Of all physiographic features, altitude is
the most difficult to resolve into simple factors. Because of general
geographic relations, it has a certain connection with rainfall, but
this is vague and inconstant. Obviously, in its influence upon the
plant, altitude is really pressure, and in consequence its effect is
exerted upon the climatic and not the edaphic factors of the habitat.
Theoretically, the decrease of air pressure in the increased altitude
directly affects humidity, light, and temperature. Actually, while there
is unquestionably a decrease in the absorption of the light and heat
rays owing to the fact that they traverse less atmosphere, which is at
the same time less dense, this seems to be negligible. Photometric
readings at elevations of 6,000 and 14,000 feet have so far failed to
show more than slight differences, which are altogether too small to be
efficient. The effect upon humidity is greater, but the degree is
uncertain. Continuous psychrographic records at different elevations for
a full season, at least, will be necessary to determine this, since the
psychrometric readings so far made, while referred to a base
psychrograph, are too scattered to be conclusive. Finally, the length of
the season, itself a composite, is directly dependent upon the altitude.
This relation, though obscure, rests chiefly upon the rarefaction of the
air which prevents the accumulation of heat in both the soil and the
air.

[Illustration: Fig. 26. Aneroid barometer.]

=120. The barometer.= To secure convenience and accuracy in the
determination of altitude, it is necessary to use both a mercurial and
an aneroid barometer. The latter is by far the most serviceable for
field work, but it requires frequent standardizing by means of the
former. The mercurial form is much more accurate and should be read
daily in the base station. It is practically impossible to carry it in
the field, except in the so-called mountain form, which is of great
service in establishing the altitudes of a series of stations. In use
the aneroid barometer may be checked daily by the mercurial standard, or
it may be set at the altitude of the base station, thus giving a direct
reading. After the normal pressure at the base has once been
ascertained, however, the most satisfactory method is to set the aneroid
each day by the standard, at the same time noting the pressure deviation
in feet of elevation (see p. 46). The absolute elevation of the various
stations of a series may be determined either by adding or subtracting
this deviation from the actual reading at the station, or by noting the
change from the base station, and then adding or subtracting this from
the normal of the latter. When it is impossible to check the aneroid by
means of a mercurial barometer, the average of a series of readings made
at different days at one station, especially if taken during settled
weather, will practically eliminate the daily fluctuations, and yield a
result essentially accurate. Even in this event, the accuracy of the
aneroid should be checked as often as possible, since the mechanism may
go wrong at any time. The barograph, while a valuable instrument for
base stations, is not at all necessary. These instruments can be
obtained from all makers of meteorological apparatus, such as H. J.
Green, and J. P. Friez. Aneroid barometers reading to 16,000 feet cost
about $20; the price of the Richards aneroid barograph is $45. Ordinary
observatory barometers cost $30–$40; the standard instrument sells at
$75–$100. The mountain barometer, which is altogether the most
serviceable for the ecologist, ranges from $30–$55, depending upon
accessories, etc.


                                _Slope_

[Illustration: Fig. 27. Mountain barometer: (_a_) in carrying case;
(_b_) set up for use.]

=121. Concept.= This term is used in the ordinary sense to indicate the
relation of the surface of a habitat to the horizon. Although it is a
complex of factors, or rather influences several factors, these are
readily determinable. The primary effect of slope is seen in the control
of run-off and drainage, and consequently of water-content, although
these are likewise affected by soil texture and by surface. Slope,
moreover, as a concomitant of exposure, has an important bearing upon
light and heat by virtue of determining the angle of incidence, and also
upon wind, and, through it, upon the distribution of snow. At present,
while it can be expressed definitely in degrees, it has not yet been
connected quantitatively with more direct factors. This is, however, not
a difficult task, and it is probable that we shall soon come to express
slope principally in amount of run-off, and of incident heat.

=122. The clinometer.= In the simplest form, this instrument is merely a
semicircle of paper, with each half graduated from 1–90°. It is mounted
on a board and placed base upward, upon a wooden strip, 2 feet long and
2 inches wide, which has a true edge. At the center of the circle is
attached a line and plummet for reading the perpendicular. A more
convenient form is shown in figure 28, which is both clinometer and
compass. This also necessitates the use of a basing strip to eliminate
the inequalities of the surface. The dial face is graduated to show
inches of rise per yard, as well as the number of degrees, but the
latter, as the simpler term, is preferable for ecological work. In
making a reading, the basing strip is placed upon a representative area
of the slope, and pressed down firmly to equalize slight irregularities.
The clinometer is moved slightly along the upper edge, causing the
marker to swing freely. After the latter comes to rest, the instrument
is carefully turned upon its back, when the angle of the slope in
degrees may be read directly. Two or three such readings in different
areas will suffice for the entire habitat, unless it be extremely
irregular. The clinometer with compass may be obtained from the Keuffel
and Esser Company, 111 Madison St., Chicago, Illinois, for $5.

[Illustration: Fig. 28. Combined clinometer and compass.]

=123. The trechometer.= For measuring the effect of slope upon run-off,
a simple instrument called the trechometer (τρέχω, to run off) has been
devised. This consists merely of a metal tank, 3 × 4 × 12 inches,
holding 144 cubic inches of water, with an opening ¼ × 12 inches at the
base in front, closed by a tight-fitting slide. Three metal strips, 2 ×
12 inches, are fastened to the front of the tank in such a way as to
enclose a square foot of soil into which the strips penetrate an inch.
In the front strip is an opening, 1 inch square, provided with a drip
from which the run-off is collected in a measuring vessel. In use, the
instrument is put in position with the metal rim forced down 1 inch into
the soil; the tank is filled, the graduate put in place, and the slide
raised. The run-off for a square foot is the amount of water caught by
the graduate, and is represented in cubic inches per square foot. For
obtaining results which express slope alone, comparisons must be made
upon the same soil, from which all cover, dead and living, has been
removed. They must be as closely together in time as possible, at least
during the same day, as rain or evaporation will cause considerable
error. It is obvious that with the same slope or on a level the
trechometer may also be used to advantage to determine the absorptive
power of soils of different texture. It serves well a similar purpose
when used in different habitats to measure the composite action of
slope, soil, and cover in dividing the rainfall into run-off and
absorbed water.


                               _Exposure_

=124. Exposure= refers primarily to the direction toward which a slope
faces, i. e., its exposition or insolation with respect to sun and wind.
It is not altogether separable from slope, however, inasmuch as the
angle of the slope has some effect upon the degree of exposure. The
chief influence of exposure is exerted through temperature, since slopes
longest exposed to the sun’s rays receive the most heat. This is
supplemented in an important degree by the fact that a group of rays 1
foot square will occupy this area only on slopes upon which they fall at
right angles. In all other cases the rays are spread over a longer area,
with a consequent reduction in the amount of heat received. This effect
is felt principally in evaporation from the soil, and in soil
temperatures. For the leaf, it is largely if not entirely negligible,
since the angle of incidence is determined by the position of the leaf,
which is the same for each species whether on the level or upon a slope.
On this account, exposure has little or no bearing upon light, except
that the total amount of light received by the aggregate vegetation of a
slope will be greater than for a level area of the same size. The effect
of wind varies with the exposure. It is naturally most pronounced in
those directions from which the prevailing dry or cold winds blow, and
it is greatly emphasized by the fact that the opposite exposure is
correspondingly protected. The influence of wind, especially in
producing evaporation from the plant and the soil, increases with the
slope, since the mutual protection of the plants, or that afforded the
soil by the cover, is much reduced. Finally, the distribution of the
snow by the wind, a matter of considerable importance for early spring
vegetation, is largely determined by exposure.

Exposure is expressed directly in terms of direction, to which is added
the angle of the slope. A good field compass, reading to twelve points,
is sufficient. It should be checked, of course, by the declination of
the needle at the place under observation. A convenient instrument is
the one already mentioned, in which compass and clinometer are combined,
since these are regularly used at the same time.

=125. Surface.= The most important consideration with respect to surface
is the presence or absence of cover, and the character of the latter.
With the exception of snow, cover is, however, a question of vegetation,
living and dead, and consequently is to be referred to the discussion of
biotic factors. The surface of the soil itself often shows
irregularities which must be taken into account. Such are the rocks of
boulder and rock fields, the hummocks of meadows and bogs, the mounds of
prairie dog towns, the innumerable minute gullies and ridges of bad
lands, the raised tufts of sand-hills, etc. The influence of these is
not profound, but they do have an appreciable effect upon the run-off,
temperature, and wind. In many cases, this is distinctly measurable, but
as a rule little more can be done than to indicate that the surface is
even or uneven, and to describe the degree and kind of unevenness.

=126. Record of physiographic factors.= Altitude, slope, exposure, and
surface are essentially constant factors, and are determined once for
all, after a few check readings have been made, except in those
relatively rare habitats in which dynamic forces are very active. The
form of record used is the following:

 ═══════╤═════════╤════════╤════════════╤════════╤═════╤════════╤═══════
  DATE  │FORMATION│STATION │   GROUP    │ALTITUDE│SLOPE│EXPOSURE│SURFACE
 ───────┼─────────┼────────┼────────────┼────────┼─────┼────────┼───────
 10/7/02│Gravel   │Golf    │Eriogonare  │2700 m. │  23°│ N.N.W. │Even
        │slide    │Links   │            │        │     │        │
    „   │Brook    │Jack    │Violare     │2550 m. │   5°│ E.N.E. │   „
        │bank     │Brook   │            │        │     │        │
    „   │Half     │Hiawatha│Achilleare  │2600 m. │  14°│   E.   │Uneven
        │gravel   │        │            │        │     │        │
    „   │Spruce   │Milky   │Opulasterare│2625 m. │  12°│   N.   │Even
        │         │Way     │            │        │     │        │
 ───────┴─────────┴────────┴────────────┴────────┴─────┴────────┴───────

=127. Topography.= As heretofore indicated, questions pertaining to the
form and development of the land concern groups of habitats within which
each habitat is the unit of investigation after the manner already laid
down. A knowledge of topography is essential to the accurate mapping of
a region, for which the simple methods of plane table and contour work
are employed, while the geology of the surface is of primary importance
in the study of successions.


                             BIOTIC FACTORS

=128. Influence and importance.= Biotic factors are animals and plants.
With respect to influence they are usually remote, rarely direct.
Nevertheless, they often play a decisive part in the vegetation. Their
effect is, as a rule, felt directly by the formation rather than the
habitat, but in either case the one reacts upon the other. Such factors
are not themselves susceptible of exact measurement, but their influence
upon the habitat is usually measurable in terms of the physical factors
affected. In the case of biotic factors, it must be distinctly
understood that these are not properly factors of the habitat as a
physical complex, but that they are rather to be considered as reactions
exerted by the effect, or formation, upon the cause or habitat. This is
most especially true of plants.

=129. Animals.= The activities of man fall into two classes: (1) those
that destroy vegetation, and (2) those that modify it. There are rare
instances also where the work of man has changed a new or already
denuded habitat. In the cases where the vegetation is destroyed, the
habitat itself is sufficiently changed to permit the effect to be
measured by physical factor instruments. Otherwise, the influence is
felt only by the formation, as when man makes possible the migration of
weeds, and it can be measured in terms of invasion by the quadrat alone.
It becomes especially evident, then, in the case of man’s activities,
that where they produce a denuded habitat they are to be regarded as
factors in the habitat; when they merely affect the formation, this is
not strictly true. The changes wrought by other animals are essentially
the same as those produced by man. They are not so marked nor so
important, but their relation to habitat and formation is the same. As a
rule, however, they affect the habitat much less than they do the
formation.

=130. Plants.= As a dead cover, vegetation is a factor of the habitat
proper, but it has relatively little importance, since it occurs
regularly during the resting period. Its chief effects are in modifying
soil temperature, and in holding snow and rain, and thereby increasing
the water-content. By its gradual decay, moreover, it not only adds
humus to the soil, but it thereby increases the water-retaining capacity
of the latter also. The cover of living vegetation reacts upon the
habitat in a much more vital fashion, exerting a powerful effect upon
every physical factor of the habitat. The factors thus affected are
distinctly measurable though it is often impossible to determine just
how much of the factor is directly traceable to the vegetation. This is
a simple problem in the case of most aerial factors, especially light,
but it is extremely difficult for soil factors, such as water-content
and soil texture. In the case of all habitats covered with formations,
by far the great majority, it is impossible as well as unnecessary to
separate the physical factors of the habitat proper from the reaction
upon them which the plant covering exerts. Indeed, the great
differentiation of habitats is largely due to the universal principle
that in vegetation the effect or formation always reacts upon the cause
or habitat in such a way as to modify it. As fundamental causes of
succession, the discussion of the various reactions of vegetation is
reserved for another place.


                    METHODS OF HABITAT INVESTIGATION

=131.= The use of the various instruments previously described depends
largely upon the preponderance of simple instruments or recording ones.
The former necessitate a number of well-trained assistants; the latter
require only a part of the time of one investigator. For the most
satisfactory results, however, an assistant is all but indispensable.
Since simple instruments are most easily obtained because of their
cheapness, and are especially adapted to purposes of instruction, the
method of using them will be described first, and then that of ecograph
batteries.


                    THE METHOD OF SIMPLE INSTRUMENTS

[Illustration: Fig. 29. Series of stations: I, at Minnehaha; II, at
Lincoln in the prairie formation.]

=132. Choice of stations.= This method is based upon simultaneous
readings by means of simple instruments in a series of habitats, or of
stations in a single habitat. Such readings are necessary for the
variable atmospheric factors, humidity, light, temperature, and wind.
Frequent readings suffice for water-content and precipitation, while
only two or three determinations, enough to check out the error, are
necessary for the constant factors, altitude, slope, exposure, and
surface. An account of the exact procedure employed in class study at
Lincoln and Minnehaha will best serve to illustrate the use of this
method. The series of stations chosen at Lincoln were primarily within a
single formation, for the purpose of determining the physical factor
variation in different areas. One series was located in the
prairie-grass formation (_Koelera-Andropogon-psilium_), and consisted of
the following stations: (1) low prairie, (2) crest of ridge I, (3)
northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of
ridge II, (6) bare crest of ridge II, (7) thicket ravine. The
other series was established in the bur-oak-hickory forest
(_Quercus-Hicoria-hylium_) at the following stations: (1) thicket, (2)
woodland, (3) knoll in forest, (4) depression in forest, (5) level
forest floor, (6) nettle thicket, (7) brook bank. At Minnehaha the
series was primarily one of different formations: (1) the pine
formation (_Pinus-xerohylium_), (2) the gravel slide formation
(_Pseudocymopterus-Mentzelia-chalicium_), (3) east slope of spruce
forest (_Picea-Pseudotsuga-hylium_), (4) ridge in the spruce forest, (5)
north slope of spruce forest, (6) brook bank in forest, (7) the thicket
formation (_Quercus-Cercocarpus-lochmodium_), (8) the aspen formation
(_Populus-hylium_). When permanent or temporary quadrats are
established, they are ordinarily used as regular stations, since this
enables one to refer the physical factor readings to a few definite
individual plants, as well as to the entire formation. The transects in
figure 29 illustrate two of the above series of stations.

=133. Time of readings.= The frequency of simple readings and the times
at which they are made must be regulated largely by opportunity and
convenience. In addition to making readings once or twice a week
throughout the season, the series should be read at least once every day
for a representative week or two. It is also very desirable to have a
series for each hour of a typical day, or of two days, one of which is
clear, the other cloudy. When a single daily reading is made, it should
be taken at or as near meridian as possible. The usual series is the one
obtained by simultaneous observations at the same level in different
stations. An important series is also secured by simultaneous readings
at the various levels of the same station, though it is not necessary to
take this series frequently.

[Illustration: Fig. 30. A denuded station in the aspen formation.]

=134. Details of the method.= After the stations have been selected by a
careful preliminary survey of the habitat or series of habitats, their
location is indicated by a small flag bearing a number, in case there is
no danger of these being disturbed. Otherwise, less conspicuous stakes
are used. The ordinary practice is to visit each station of the series,
and to take readings of water-content, altitude, slope, and exposure. On
the first trip these are all made by the instructor, but after a short
time the determination of each factor may be assigned in rotation to
each of the students. After these constant factors have been read and
recorded, one student is equipped with photometer, thermometer, and
psychrometer, and, if desirable, anemometer, and left at the first
station. At each succeeding station the same plan is followed, so that
at the end of the series the constant factors have all been read, and
there is an observer at each station prepared to make readings of the
variable ones. The task of acquainting the students with the operation
of photometer, psychrometer, etc., can best be done in class or at a
previous field period, as it is evident that they must be familiar with
the instruments before they can use them accurately in the field series.
The details of operation have already been given and need not be
repeated here. The task of obtaining readings at the same moment may be
met by supplying each observer with a watch, which runs exactly with all
the others, or by making observations upon signal. The second means has
been found most successful in practice, since the signal fixes the
attention at the exact moment. The best plan is for the instructor to
occupy a commanding position somewhere near the middle of the series,
and to give the signals by shout or whistle at the proper interval.
Considerable care and experience are necessary to do the last
satisfactorily. Sufficient time must be given for the operation of the
instrument and the making of the record. In addition, a period must be
permitted to elapse which is long enough for every instrument to reach
the proper reading. For example, in a series which contains a gravel
slide and a forest, the thermometer which has just been used for an air
reading will require four or five times as long an interval to respond
to the temperature of the gravel as to that of the cool forest floor. In
such series, the instructor should regularly take his place in the
station where the response is slowest or greatest. He must record the
exact time of each signal, and note any general changes of sky or wind
that produce temporary fluctuations at the time of reading. When the
readings extend over a whole day, the usual plan is to begin at the last
station and take a second series of water-content samples, noting the
exact time in order that the rate of water loss may be determined. A
check series of physiographic factors may be made at this time also, or
this may be left for future visits. While it is unnecessary to take soil
samples oftener than once a day, it is important to make at least one
series at each visit. Sometimes it becomes desirable to know the rate of
water loss in different stations during the day, and in this event,
samples are taken at one or two hour intervals for the entire day.

In making simultaneous readings at the different levels of one station,
the observers are grouped in one spot in such a way that they do not
interfere with the correct reading of each instrument. Readings of this
sort are most valuable in the case of temperature, which shows greater
differences at the various levels. Important differences of humidity and
wind also are readily obtained, and, in layered formations, marked
variations in the amount of light. In the open, the ordinary levels for
temperature are 6 feet, 3 feet, surface, 5, 10, and 15 inches in the
ground, and for wind and humidity, 6 feet, 3 feet, and surface. In
forests the same levels are used for comparison with formations in the
open, but a more desirable series for light especially is secured by
making readings at the height of, or better, just below the various
layers. Series of this sort are likewise made on signal. The best time
of day is that of a period in which the middle station is read near
meridian, since the variation due to time is sufficiently small to
permit fairly accurate comparisons between the readings for the
different stations.

=135. Records.= The form used for recording the observations made by
means of simple instruments is shown below. It is hardly necessary to
state that it may be readily modified to suit the needs of different
investigators. Ordinarily, each sheet is used for the records of one
habitat or series alone, but for convenience sake, the records of two
different series are here combined. The figures given are taken from
records for the prairie and forest formations at Lincoln.


                      _Koelera-Andropogon-psilium_

                   April 25, 1901. Clear. South wind.

 ════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════
     │         TEMPERATURE         │   LIGHT   │ HUMIDITY  │WATER-CONTENT │   WIND
 ────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬─────
 TIME│3:20 │3:24 │3:30 │3:35 │3:40 │     │     │3:45 │3:55 │      %       │2:40 │3:05
 P.M.│     │     │     │     │     │     │     │     │     │              │     │
 ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼─────
 STA-│1½ m.│Surf.│  5  │ 10  │ 15  │1½ m.│Surf.│1½ m.│Surf.│ 5  │ 10 │ 15 │1½ m.│Surf.
 TION│     │     │     │     │     │     │     │     │     │    │    │    │     │
 ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼─────
  1. │27.8 │29.6 │17.8 │15.8 │10.9 │     │     │ 57  │ 59  │17.7│14.6│17.4│  740│  280
  2. │26.5 │31.3 │18.3 │16.6 │12.8 │     │     │ 59  │ 59  │17.9│12.2│14.3│ 1100│  510
  3. │26.9 │28.5 │18.2 │14.2 │13.5 │     │     │ 58  │ 59  │16.5│16.9│19  │  980│  520
  4. │26.2 │30   │16.8 │13.2 │11.6 │     │     │ 63  │ 66  │24.4│21.3│24.8│  920│  460
  5. │28   │32.4 │18.6 │14.6 │14.2 │     │     │ 59  │ 60  │10.7│17  │17.2│ 1080│  490
  6. │28   │40.8 │23.8 │16   │15   │     │     │ 51  │ 51  │ 5  │8.3 │10.3│ 1010│  410
  7. │26.4 │30.8 │16   │13   │11   │     │     │ 68  │ 70  │27  │24.3│21.4│  680│   52
 ────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────


                        _Quercus-Hicoria-hylium_

                 April 20, 1901. Clear. Southeast wind.

 ════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════
     │         TEMPERATURE         │   LIGHT   │ HUMIDITY  │WATER-CONTENT │   WIND
 ────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬─────
 TIME│10:40│10:46│10:50│10:55│11:00│12:00│12:05│11:10│11:20│      %       │11:30│11:45
 A.M.│     │     │     │     │     │     │     │     │     │              │     │
 ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼─────
 STA-│1½ m.│Surf.│  5  │ 10  │ 15  │1½ m.│Surf.│1½ m.│Surf.│ 5  │ 10 │ 15 │1½ m.│Surf.
 TION│     │     │     │     │     │     │     │     │     │    │    │    │     │
 ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼─────
  1. │16   │25   │  9.6│8.4  │7.8  │ .08 │ .06 │ 73  │ 81  │24.2│19.2│19.5│  298│    0
  2. │16.2 │30.5 │  8.5│8.4  │7.8  │ .11 │ .09 │ 73  │ 86  │22  │22.5│19.4│  375│    2
  3. │16.2 │17.8 │  7.6│7.8  │8    │ .08 │ .06 │ 73  │ 95  │22.1│20.4│21.6│  640│    6
  4. │15.6 │26.2 │ 10.6│8.4  │8.2  │ .06 │ .03 │ 81  │ 95  │25.4│23.1│22.4│  275│   12
  5. │17.6 │25.4 │  7.6│7.4  │7.2  │ .03 │ .02 │ 90  │ 95  │27.2│19.8│18.8│  178│    2
  6. │16.2 │20.2 │  8.4│7    │6.2  │ .02 │ .01 │ 82  │ 90  │27.6│20.8│18.8│  115│    4
  7. │15.8 │17.2 │  6.4│6.4  │6.1  │ .05 │ .04 │ 82  │ 90  │23.8│19  │19.3│   60│    0
 ────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────


                      METHOD OF ECOGRAPH BATTERIES

=136.= A battery of recording instruments consists of a selagraph, a
psychrograph, and a thermograph, to which an anemograph is added when
possible. As stated before, the determination of water-content by the
geotome method is more satisfactory than by any automatic instrument yet
devised. When the base station is located where the sunlight is
unobstructed, which should be the case whenever possible, it is
unnecessary to include a selagraph in those batteries placed in
similarly exposed stations, since the light values will be the same. As
a rule, batteries are established within different zones or different
habitats, except where a highly diversified habitat is made the subject
of special inquiry. Such a restriction arises from the fact that
expense, care of operation, etc., place a limit upon the number of
batteries, and, in such case, the task of primary importance is to
establish the physical character of representative habitats. For these
reasons, the first series of thermographs established in 1903 was
located with respect to altitude, the instruments being placed at
Manitou 2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In
1904, the stations established for the record of temperature and
humidity were situated with respect to habitats representing the four
formations: gravel slide, half gravel slide, spruce forest, and brook
bank.

The batteries are located and set up according to the directions already
given. A 2–meter quadrat with the battery as the middle is staked and
mapped. Within this, all readings of water-content, soil temperature,
and physiographic factors are made. Altitude, slope, exposure, and cover
are recorded when each battery is located, and a soil sample is taken
for mechanical analysis. When the position of the batteries permits it,
water-content readings should be made frequently, once or twice a week
at least. In addition, a complete series of samples should be taken
daily for a period sufficient to indicate the ordinary extremes of
water-content.

The ecograph battery of each habitat constitutes a standard to which the
results obtained by simple instruments may be referred with accuracy. It
not only does this, but it also serves as a basis for interpreting the
readings of simple instruments in distant habitats of the same
character. In this way a few batteries judiciously placed make possible
the exact physical investigation of a large number of habitats, covering
a considerable area. The only limit, indeed, upon this method is that
placed by time. The proportionate use of batteries and of simple
instruments must be largely determined by the conditions which confront
the investigator. It is obvious that, where expense is not a decisive
factor, the gain in time and in completeness of results is enormously in
favor of the battery. There is an additional value in the automatic and
continuous record which can not be overlooked. When the use of
instruments in the study of habitat and formation becomes universal, the
importance of the ecograph will be immeasurably enhanced. It will be
possible to secure duplicate records of batteries located in the most
remote and diverse regions, from the equator to the poles, and
comparative phytogeography upon a scientific basis will for the first
time be possible. This opens an alluring vista of the future when
ecologists the world over will cooperate in such a way that the results
obtained by ecograph batteries anywhere on the globe will permit of
exact comparison.


               THE EXPRESSION OF PHYSICAL FACTOR RESULTS

=137. The form of results.= It is almost inevitable that the general
adoption of precise methods of measuring the habitat will result in a
common form for expressing the physical character of the latter. An
actual diagnosis of each habitat is not a difficult matter, after the
factors are carefully measured, and will unquestionably lead to very
desirable definiteness and precision. The accurate investigation of the
physical factors of a number of habitats for one growing season
furnishes the necessary material for a diagnosis based upon the mean for
the growing season. Similar results for two or three seasons will yield
a diagnosis as accurate and as final as that of a formation, or, indeed,
as that of many species. The author’s investigations have not yet gone
far enough to warrant proposing a final form for this, but the following
diagnosis is offered as a suggestion:

_Elymus-Muhlenbergia-chalicium._ Habitat: holard 9 per cent, chresard 8
per cent, relative humidity 40 per cent, light 0.6, soil colluvial
gravel (gravel 70 per cent, sand 27 per cent, silt 3 per cent), air
temperature 65°, surface 82°, soil 59°, wind 10 miles, rainfall 8
inches, altitude 2,800 m., slope 23°, exposure south, surface even,
cover open, no active biotic agencies.

The detailed comparison of habitats is made most readily by the graphic
method of curves, which constitute the most desirable form of expression
in connection with the original record upon which they are based. Factor
means are particularly desirable for diagnostic purposes, and they
furnish valuable curves also. Factor sums are impracticable at present,
and it seems doubtful that they will ever be of much value. It is by no
means impossible, however, that a more detailed and exact knowledge of
the physiology of adaptation, coupled with methods of precision in the
habitat, will render them necessary.


                            _Factor Records_

=138.= Experience has shown that the practice of making hasty and often
formless records in the field is unwise and is apt to be inaccurate as
well. The time saved in the field is more than counterbalanced by that
consumed in copying the results into the permanent form. The danger of
error in field notes rapidly taken is very grave, and the chance of
confusion and the waste of time in deciphering them are great. Moreover,
the task of checking a copy with the original, which is absolutely
necessary for accuracy, involves a further expenditure of time and
energy. For these reasons the field record should be made in permanent
form. Definite record sheets are used, and the invariable rule is made
that all readings are to be noted in ink at the time and spot where they
are taken. On a long journey, or in the face of many observations, the
tendency to take notes or to record observations rapidly is very great,
but this will correct itself after a few attempts to use such notes. The
record forms for various factors have been indicated in the proper
place, as well as the one for simultaneous readings. Ecograph sheets are
carefully filed, and constitute permanent records. With a little
practice they may be read almost as easily as tables, and any attempt to
put them into tabular form is a mere waste of time. For purposes of
study and of publication, it often becomes necessary to bring together
all the results obtained for a particular habitat, both by simple
instruments and by ecographs. The form of record used for this is
essentially that already indicated for simultaneous readings on page 92,
since general features and constant factors can not well be included in
the table. Record sheets of this type have been printed at a cost of $5
per thousand, and the various factor records can be obtained at about
the same rate. The size of sheet used is 9½ × 7¾ inches. The record book
is the usual notebook cover, which has been found neither too large nor
too small. It is protected from dirt and rain by a covering of oilcloth
which overlaps the edges. Record books should be carefully labeled, and
each one should contain a single year’s records.


                            _Factor Curves_

=139. Plotting.= The paper employed is divided into centimeter squares
which are subdivided into 2–millimeter units. For ordinary curves the
size of sheet is 9½ × 7¾ inches, which makes it possible for curve
sheets to be filed in the record book. Tablets containing 60 of these
sheets can be obtained for 20 cents each from the Central School Supply
House, Chicago. For curves longer than 9 inches special sizes of sheets
must be used. On account of their inconvenience large sheets are avoided
whenever possible. This can usually be accomplished by increasing the
numerical value of the intervals. The inks employed in plotting are the
waterproof inks of Chas. Higgins & Co., Brooklyn, New York. These are
made in ten or more colors, black, violet, indigo, blue, green, yellow,
orange, brown, brick red, carmine, and scarlet, and cost 25 cents per
bottle. In addition to being waterproof, they make it possible to
combine curves in all conceivable ways without destroying their
identity. Furthermore, it is a great advantage to use the same color
invariably for the same kind of curve: thus, it has been the practice to
indicate the 3–foot, surface, 5, 10, and 15–inch temperature curves by
violet, green, yellow, blue, and carmine respectively. A fine-pointed
pen, such as the Spencerian No. 1, is most satisfactory for inking;
drawing pens, such as Gillott’s Crowquill, are too finely pointed for
ordinary use.

In plotting a curve, it is first necessary to determine the value of the
interval, and the extreme range of the curve or combination. For
example, in the case of temperature, it is most convenient to assign a
value of 1° Centigrade to each centimeter, since the thermometers used
read to one-fifth of a degree, which corresponds exactly to the
2–millimeter units of each square. The length of the sheet permits a
range of 22 degrees Centigrade, and the actual limits must be determined
for the particular results to be employed. For the same region, it is
very desirable that the unit interval and the range be the same, in
order that all curve sheets may admit of direct comparison. Indeed, it
is greatly to be hoped that in the future ecologists will agree to a
uniform system of curve-plotting, cartography, etc., as the geographers
are beginning to do in the construction of maps. The major intervals are
written, or, better, typewritten, at both sides of the sheet, and the
time or space intervals are indicated at the top. Each curve sheet is
properly labeled, and essential data indicated. The readings are taken
from the field record, and their proper positions indicated by a dot.
These are connected first by a pencil line, the curves being made abrupt
rather than flowing; and the line, after having been carefully checked,
is traced in ink.

=140. Kinds of curves.= Curves are named both with reference to the
factor concerned and the position or sequence of the readings. The
factors which lend themselves most readily to this method of
representation are the variable ones, water-content, humidity, light,
temperature, and wind, and corresponding curves are distinguished.
Altitude and slope may likewise be shown by means of curves, but the use
of cross section or contour lines serves the same purpose and is more
natural. With regard to time and position, curves are distinguished as
level, station, and point curves. A level curve is one based upon
readings made at the same level through a series of stations or of
habitats, e. g., the level curve of surface temperature. The station
curve represents the various levels or points at which readings are made
in a single station. The point curve has for a basis the hourly or daily
variation of a factor at a particular point or level in a station. All
of these may be simple curves, when established upon a single reading
for a series, or mean curves when they are based upon the mean of a
number of readings. Curves which show the extremes of a factor, i. e.,
the maximum and minimum, are also extremely valuable, though a
combination of the two for comparison is preferable.

=141. Combinations of curves= are invaluable for bringing similar curves
together, and permitting ready comparison of them. For this, and also
because they save space, they are regularly employed to the almost
complete exclusion of single curves. Combinations are made simply by
tracing the curves to be compared upon the same sheet, it being
understood that dissimilar curves, e. g., level and station, can not be
combined. Colored inks are an absolute necessity in combining; the
primary principle underlying their use is that curves that approach
closely or cross each other must be traced in inks that contrast
sharply. As elsewhere stated, it has been made the invariable rule to
use the same color for the same level or point. This applies especially
to temperature, but holds also for humidity, light, wind, and
water-content, so that the color always indicates the level. For the
same reason, it is applied to a combination of point curves for one
station, though it is inapplicable to a series of point curves when
these lie in the same level. Light readings above 6 feet and
water-content readings below 15 inches necessitate the use of additional
colors.

Combinations may be made of the curves of a single factor for purposes
of comparison, or they may consist of curves of different factors in
order to aid in interpreting or indicating their relation to each other.
Curves of the same factor may be combined to form various series. The
level series consists of all the level curves for the stations under
observation, e. g., the six levels for temperature, three levels for
wind, etc. Similarly, the station series is a combination of all the
station curves, and a corresponding arrangement may be made for point
curves with reference either to station or to level. An extremely
valuable combination of curves is that of the holard and chresard for a
series of stations. The most important combinations of the curves of
different factors are naturally those based upon factors intimately
related to each other or to the plant. The grouping of water-content and
humidity curves is of great value, especially when the transpiration
curve is added. Light and temperature curves make an interesting
combination, while a humidity, temperature, and wind series is of much
aid in tracing the connection between these factors. Finally, it is
altogether feasible to arrange the curves of water-content, humidity,
light, temperature, and wind upon the same sheet in such fashion as to
give a graphic representation of the whole physical nature of a single
habitat or a series. In all combinations of curves representing
different factors, it must be borne in mind that the position of a curve
does not represent a definite value with reference to the others, since
some are based upon per cents, others upon degrees, etc. The comparison
must be based upon the character of the curves, but even then it is an
important aid. An instructive grouping has been employed where series of
readings on the same day, or on two successive days in forest and in
prairie have yielded the usual level series of curves. The series for
the two habitats are arranged on the same page, one at the right and the
other at the left, and permit direct comparison of corresponding level
or factor curves, both with respect to position and character.

=142. The amplitude of all the curves= described above is determined by
the unit values of the factors concerned, while the length is dependent
upon the number of stations, points, or times. The value assigned the
latter upon the plotting paper is purely arbitrary, but it is most
convenient to fix this at the centimeter square. The unit value for
temperature is 1° Centigrade per square, each subdivision of the latter
representing 0.2, and the range being 22 degrees. For water-content
curves, each square represents a value of 2 per cent, the smaller square
being 0.4 per cent, and the range 2–48 per cent. The unit value for
humidity is taken as 5 per cent, making each small square 1 per cent,
and giving room on the sheet for the entire range from 1–100 per cent.
Owing to the anemometer used, curves of wind velocity have been based
upon the number of feet per minute. One hundred feet is taken as the
unit value, and the range is from 0–2200 feet. The unit value for the
curve of light intensity is .005. Each small square is .001, which
permits a range from .001 to .01 on one sheet. Consequently, when it is
desired to plot the curve of a series of habitats with a range in
intensity greater than this it is necessary to use a double sheet. This
is the usual device when the range of curves is too great, except where
the excess is slight. In this case the curve is left open at the top,
and the value which the crest attains is indicated. All curves in
combination are labeled at the beginning or left to indicate the level,
station, or point, and at the end or right to show the time, or day, if
this is not the basis of the curve or series.

The discussion that precedes deals exclusively with curves representing
factors determined in the field. It applies with equal force to results
obtained by instruments in control houses. In these, however, all
factors except those directly experimented with, usually water-content
and light, are practically equalized, and the curves based upon them are
used chiefly to show how nearly equal they have become. The important
curves are those of the water-content series, both holard and chresard,
and of the shade tents. Where several houses are differentiated with
respect to temperature or humidity, curve series of both these factors
are necessary.


                        _Factor Means and Sums_

=143.= It has been shown elsewhere that the daily mean of temperature
can be closely approximated from the maximum and minimum of both day and
night. Maximum-minimum instruments for the other factors are lacking,
however, and for light, humidity, and wind these values can only be
obtained from the ecograph which makes it possible to get the exact mean
from the sum of all the hour readings. When it comes to the seasonal
mean, the ecograph is even more necessary, exception being made for
water-content, in which case a number of readings on various days
through the season will suffice. The value of factor means for diagnosis
and for curves has already been sufficiently commented upon, and the
feasibility of factor sums already indicated.




                         CHAPTER III. THE PLANT
                         STIMULUS AND RESPONSE


                           GENERAL RELATIONS

=144. The nature of stimuli.= Whatever produces a change in the
functions of a plant is a stimulus. The latter may be a force or a
material; it may be imponderable or ponderable; effect, not character,
determines a stimulus. Consequently, reaction or response decides what
constitutes a stimulus. The presence of the latter can be recognized
only through an appreciable or visible response, since it is impossible
to discriminate between an impact which produces no reaction and one
which produces a merely latent one. From this it is evident that
quantity is decisive in determining whether the impact becomes a
stimulus. Plants grow constantly under the influence of many stimuli,
all varying from time to time in amount. Small changes in these are so
frequent that, in many cases at least, the plant no longer appreciably
reacts to them. Such changes, though usually measurable, are not
stimuli. Furthermore, it must be clearly recognized that plants which
are in constant response to stimuli are stimulated anew by an efficient
increase or decrease in the amount of any one of these. As is well
known, however, such increase or decrease is a stimulus only within
certain limits, and the degree of change necessary to produce a response
depends upon the amount of the factor normally present. The entire
absence of a force usually present, moreover, often constitutes a
stimulus, as is evident in the case of light. The nature of the plant
itself has a profound bearing upon the factors that act as stimuli. Many
species are extremely labile, and react strongly to relatively slight
stimuli; others are correspondingly stable, and respond only to stimuli
of much greater force. Some light is thrown upon the nature of this
difference by the behavior of ecads. A form which has grown under
comparatively uniform conditions for a long time seems to respond less
readily, and is therefore less labile than one which is subject to
constant fluctuation. In many cases this is not true, however, and the
degree of stability, i. e., of response, can only be connected in a
general way with taxonomic position.

=145. The kinds of stimuli.= The factors of a habitat are external to
the plant, and consequently are termed _external_ stimuli. Properly
speaking, all stimuli are external, but since the response is often
delayed or can not be clearly traced, it may be permissible to speak of
_internal_ stimuli, i. e., those which appear to originate within the
plant. These, however, are extremely obscure, and it is hardly possible
to deal with them until much more is known of the action of external
stimuli. Of the latter, certain forces, gravity and polarity, act in a
way not at all understood, and as they are essentially alike for all
plants and all habitats, they can here be ignored. Stimuli are
imponderable when, like light and heat, they are measured with reference
to intensity, and ponderable, when, as in the case of water-content,
humidity, and salt-content, they can be expressed in mass or weight. It
is undesirable to insist upon this distinction, however, since the real
character of a stimulus is determined by its effect, and the latter is
not necessarily dependent upon whether the stimulus is one of force or
one of material. There is, however, a fundamental difference between
factors with respect to their relation to the plant. Direct factors
alone are stimuli, since indirect factors must always act through them.
For example, the wind, its mechanical influence excepted, can affect the
plant only in so far as it is converted into the stimulus of increased
or decreased humidity. Consequently, the normal stimuli of the plants of
a formation are: (1) water-content, (2) solutes, (3) humidity, (4)
light, (5) temperature, (6) wind. Soil, pressure, physiography, and
biotic factors influence plants only through these, and are not stimuli,
though exceptions must be made of biotic factors in the case of
sensitive, insectivorous, and gall-producing plants.

=146. The nature of response.= Since plants have no special organs for
the perception of stimuli, nor sensory tracts for their transmission, an
external stimulus acting upon a plant organ is ordinarily converted into
a response at once. The latter as a rule becomes evident immediately; in
some cases it is latent or imperceptible, or some time elapses before
the chain of responses finds visible expression. A marked decrease in
humidity calls forth an immediate increase of transpiration, but the
ultimate response is seen in the closing of the stomata. A response to
decreased light intensity, on the other hand, is much less rapid and
obvious. This difference is largely due to the fact that the functional
response is more marked, or at least more perceptible in one case than
in the other.

Response is the reaction of the plant to a stimulus; it begins with the
impact of an efficient factor, and ends only with the consequent final
readjustment. The immediate reaction is always functional. The nature
and intensity of the stimulus determine whether this functional response
is followed by a corresponding change in structure. The consideration of
this theme consequently gains in clearness if a functional and a
structural response be distinguished. The chief value of this
distinction lies in the fact that many reactions are functional alone;
it serves also to emphasize the absolute interdependence of structure
and function, and the imperative need of considering both in connection
with the common stimulus. For these reasons, the logical treatment is to
connect each stimulus with its proper functional change, and, through
this, with the corresponding modification of structure. For the sake of
convenience, the term _adjustment_ is used to denote response in
function, and _adaptation_, to indicate the response in structure.

=147. Adjustment and adaptation.= The adjustment of a plant to the
stimuli of its habitat is a constant process. It is the daily task, seen
in nutrition and growth. So long as these take place under stimulation
by factors which fall within the normal variation of the habitat, the
problems belong to what has long been called physiology. When the
stimuli become unusual in degree or in kind, by a change of habitat or a
modification in it, adjustment is of much greater moment and is recorded
in the plant’s structure. These structural records are the foundation of
proper ecological study. Since they are the direct result of adjustment,
however, this affords further evidence that a division of the field into
ecology and physiology is illogical and superficial. Slight or
periodical adjustment may concern function alone; it may be expressed in
the movement of parts or organs, such as the closing of stomata or
changes in the position of leaves, in growth, or in modifications of
structure. This expression is fundamentally affected by the nature of
the factor and is in direct relation to the intensity of the latter.
Adaptation comprises all structural changes resulting from adjustment.
It includes both growth and modification. The latter is merely growth in
response to unusual stimuli, but this fact is the real clue to all
evolution. Growth is periodic, and in a sense quantitative; it results
from the normal continuous adjustment of the plant to the stimuli of its
proper habitat. In contrast, modification is relatively permanent and
qualitative; it is the response to stimuli unusual in kind or intensity.
A definite knowledge of the processes of growth is indispensable to an
understanding of modification. In the fundamental task of connecting
plant and habitat, it is the modification of the plant, and not its
growth, which records the significant responses to stimuli. For this
reason the discussion of adaptation in the pages that follow is
practically confined to modification of structure. This is particularly
desirable, since growth has long been the theme of physiological study,
while modification has too often been considered from the structural
standpoint alone. The comparatively few studies that have taken function
into account have been largely empirical; in them neither stimulus nor
adaptation has received anything approaching adequate treatment.

=148. The measurement of response.= The amount of response to a stimulus
is proportional to the intensity of the factor concerned. This does not
mean that the same stimulus produces the same response in two distinct
species, or necessarily in two plants of one species. In these cases the
rule holds only when the plants or species are equally plastic. For each
individual, however, this quantitative correspondence of stimulus and
response is fundamental. It is uncertain whether an exact or constant
ratio can be established between factor and function; the answer to this
must await the general use of quantitative methods. There can be no
doubt, however, that within certain limits the adjustment is
proportional to the amount of stimulus, whereas reaction is well known
to be abnormal or inhibited beyond certain extremes. It is quite
erroneous to think that reaction is independent of quantity of stimulus,
or to liken the stimulating factor to “the smallest spark (which) by
igniting a mass of powder, produces an enormous mechanical effect.”[8]
Such a statement is only apparently true of the action of mechanical
stimuli upon the few plants that may properly be said to possess
irritability, such as sensitive plants and certain insectivorous ones.
Of the normal relation of response to direct factors, water, light,
etc., it is entirely untrue. Axiomatically, there is ordinarily an
essential correspondence, also, between the amount of adjustment and of
adaptation. This correspondence is profoundly affected, however, by the
structural stability of the plant.

From the preceding it follows that the measurement of response and the
relating it to definite amounts of direct factors as stimuli are two of
the most fundamental tasks of ecology. The exact determination of
physical factors has no value apart from its use for this purpose. It is
perfectly clear that precise methods of measuring stimuli call for
similar methods in determining the amount of adjustment and of
adaptation. The problem is a difficult one, and it is possible at
present only to indicate the direction which its development should
take, and to describe a few methods which will at least serve as a
beginning. To cover the ground adequately it is necessary to measure
response by adjustment and by adaptation separately, and in the latter
to find a measure for the individual and one for the species. The one is
furnished by the methods of morphology and the other by biometry.

A primary requisite for any method for measuring adjustment is that it
be applicable to field conditions. Many instruments for measuring
transpiration, for example, are valueless, not because they are
inaccurate, but because the plant studied is under abnormal conditions.
To avoid the latter is absolutely necessary, a fact which makes it
peculiarly difficult to devise a satisfactory field method. After the
latter has been found and applied, it becomes possible to check other
methods by it, and to give them real value. The final test of a field
method is three-fold: (1) the plant must be studied while functioning
normally in its own habitat; (2) the method must give accurate results;
and (3) it must permit of extensive and fairly convenient application in
the field. Until methods of this character, some of which are described
later, have been employed for some time, it is impossible to connect
definite intensities of factor stimuli with measured amounts of
adjustment. Ultimately, it seems certain that researches will regularly
take this form.

Adaptation is primarily indicated by changes in the arrangement and
character of the cells of the plant. Since these determine the form of
each organ, morphology also furnishes important evidence in regard to
the course of adaptation, but form can be connected certainly with
adjustment only through the study of cellular adaptation. In tracing the
modifications of cell and of tissue, the usual methods of histology,
viz., sectioning and drawing, suffice for the individual. It is merely
necessary to select plants and organs which are as nearly typical as can
be determined. The question of quantity becomes paramount, however,
since it often gives the clue to qualitative changes, and hence it is
imperative that complete and accurate measurements of cells, tissues,
and organs be made. These measurements, when extended to a sufficiently
large number of plants, serve to indicate the direction of adaptation in
the species. They constitute the materials for determining biometrically
the mean of adaptation for the species and the probable evolution of the
latter. In its present development, biometry contains too much
mathematics, and too little biology. This has perhaps been unavoidable,
but it is to be hoped that the future will bring about a wise sifting of
methods, which will make biometry the ready and invaluable servant of
all serious students of experimental evolution. This condition does not
obtain at present, and in consequence it seems unwise to consider the
subject of biometry in this treatise.

=149. Plasticity and fixity.= As the product of accumulated responses,
each species is characterized by a certain ability or inability to react
to stimuli. Many facts seem to indicate that the degree of stability is
connected with the length of time during which the species is acted upon
by the same stimuli. It seems probable that plants which have reacted to
sunlight for hundreds of years will respond less readily to shade than
those which have grown in the sun for a much shorter period. This
hypothesis is not susceptible of proof in nature because it is
ordinarily impossible to distinguish species upon the basis of the time
during which they have occupied one habitat. Evidence and ultimate
proof, perhaps, can be obtained only by field and control experiments,
in which the time of occupation of any habitat is definitely known. Even
in this case, however, it is clear that antecedent habitats will have
left effects which can neither be traced nor ignored. Additional support
is given this view by the fact that extreme types, both ecological and
taxonomic, are the most stable. Intense xerophytes and hydrophytes are
much more fixed than mesophytes, though the intensity of the stimulus
has doubtless as great an influence as its duration. Composites,
labiates, grasses, orchids, etc., are less plastic than ranals, rosals,
etc., but there are many exceptions to the apparent rule that fixity
increases with taxonomic complexity. At present it seems quite
impossible to suggest an explanation of the rule. Recent experiments
indicate that there may be ancestral fixity of function, as well as of
structure. It has been found, for example, that the flowers of certain
species always react normally to the stimuli which produce opening and
closing, while others make extremely erratic response. If further work
confirms this result and extends it to other functions, the necessity of
arriving at a better understanding of fixity will be greatly emphasized.

It is impossible to make progress in the study of adaptation without
recognizing the fundamental importance of ancestral fixity as a factor.
E. S. Clements[9] has shown that a number of species undergo pronounced
changes in habitat without showing appreciable modification.
Consequently, it is incorrect to assume that each habitat puts a
structural impress upon every plant that enters it. For this reason, the
writer feels that the current explanation of xerophytic bog plants,
etc., is probably wrong, and that the discrepancy between the nature of
the habitat and the structure of the plant is to be explained by the
persistence of a fixed ancestral type. The anomaly is scarcely greater
than in cases that have proved capable of being explained.

=150. The law of extremes.= When a stimulus approaches either the
maximum or minimum of the factor for the species concerned, response
becomes abnormal. The resulting modifications approach each other and in
some respects at least become similar. Such effects are found chiefly in
growth, but they occur to some degree in structure also. It is
imperative that they be recognized in nature as well as in field and
control experiment, since they directly affect the ratio between
response and stimulus. The data which bear upon the similarity of
response to extremes of different factors are too meager to permit the
formulation of a rule. It is permissible, however, to suggest the
general principle that extreme stimuli produce similar growth responses,
and to emphasize the need of testing its application to adaptation
proper.

=151. The method of working hypotheses.= In the study of stimulus and
response, where the unimpeachable facts are relatively few, and their
present correlation slight, the working hypothesis is an indispensable
aid. “The true course of inductive procedure ... consists in
_anticipating nature_, in the sense of forming hypotheses as to the laws
which are probably in operation, and then observing whether the
combinations of phenomena are such as would follow from the laws
supposed. The investigator begins with facts and ends with them. He uses
such facts as are in the first place known to him in suggesting probable
hypotheses; deducing other facts which would happen if a particular
hypothesis is true, he proceeds to test the truth of his notion by fresh
observations or experiments. If any result prove different from what he
expects, it leads him either to abandon or to modify his hypothesis; but
every new fact may give some new suggestion as to the laws in action.
Even if the result in any case agrees with his anticipations, he does
not regard it as finally confirmatory of his theory, but proceeds to
test the truth of the theory by new deductions and new trials.”[10] In
the treatment of adjustment and adaptation which follows, the method of
multiple working hypotheses is uniformly employed. No apology is felt to
be necessary for this, since the whole endeavor is to indicate the
proper points of attack, and not to distinguish between that which is
conjectural and that which is known. If an hypothesis occasionally seem
to be stated too strongly, it is merely that it appears, after a survey
of the problem from all sides, to explain the facts most satisfactorily.
The final proof of any hypothesis, however, rests not only upon its
ability to explain all the facts, but also upon the inability of other
hypotheses to meet the same test. The discovery and examination of all
possible hypotheses, and the elimination of those that prove inadequate
are the essential steps in the method of working hypotheses.


                              HYDROHARMOSE


                              _ADJUSTMENT_

=152. Water as a stimulus.= Plants are continually subjected to the
action of the water of the soil and of the air; exception must naturally
be made of submerged plants. The stimulus of soil water acts upon the
absorbing organ, the root, while that of humidity affects the part most
exposed to the air, viz., the assimilative organ, which is normally the
leaf. But since both are simultaneous water stimuli, a clearer
conception is gained of this operation if they are viewed as two phases
of the same stimulus. This point of view receives further warrant from
the essential and intimate relation of humidity and water-content as
determined by the plant. They are in fact largely compensatory, as is
shown at some length later. In determining the intensity of the two, a
significant difference between them must be recognized. The total
humidity of the air at any one time constitutes a stimulus to the leaf
which it touches. This is not true of the total soil water. Part of the
latter is not available under any circumstances, and can not affect the
plant, at least directly. The chresard alone can act as a stimulus, but
even this is potential in the great majority of cases, since the actual
stimulus is not the water available but the water absorbed. The latter,
moreover, contains many nutrient salts which are in themselves stimuli,
but as they normally have little bearing upon the action of water as a
stimulus they are to be considered only when present in excessive
amounts.

=153. The influence of other factors upon water.= The amount of humidity
is modified directly by temperature, wind, precipitation, and pressure,
and, through these, it is affected by altitude, slope, exposure, and
cover. Naturally, also, the evaporation of soil water has a marked
influence. In determining water-content, atmospheric factors, with the
exception of precipitation, are usually subordinate to edaphic ones.
Soil texture, slope, and precipitation act directly in determining soil
water, while temperature, wind, and pressure can operate only through
humidity. This is likewise true of altitude, exposure, and cover, though
the latter has in addition a profound effect upon run-off. Biotic
factors can affect humidity or water-content only through the medium of
another factor. Light in itself has no action upon either, but through
its conversion into heat within the chloroplast, it has a profound
effect upon transpiration. The following table indicates the general
relation between water and the other physical factors of the habitat.
The order of the signs, ±, denotes that the water increases and
decreases with an increase and decrease of the factor, or the reverse,
∓.

                    _Humidity_ ±   _Water-content_ ±
                   Temperature ∓   Temperature ∓
                   Wind ∓          Wind ∓
                   Precipitation ± Precipitation ±
                   Pressure ±      Pressure ∓
                   Soil texture 0  Soil texture
                   Altitude ∓       Porosity ∓
                                    Capillarity ±
                   Slope ∓         Slope ∓
                   Exposure ∓      Exposure ∓
                   Cover ±         Cover ±

=154. Response.= The normal functional responses to water stimuli are
absorption, diffusion, transport, and transpiration. Of these,
absorption and transpiration alone are the immediate response to soil
water and humidity, respectively. Consequently they are the critical
points of attack in studying the fundamental relation of the plant to
the water of its habitat. In determining the pathway of the response, it
is necessary to trace the steps in diffusion and transport, but, as
these are essentially alike for all vascular plants, this task lies
outside the scope of the work in hand. As previously suggested, the
relation between absorption and transpiration is strictly compensatory,
though, for obvious reasons, the amount of water transpired is usually
somewhat less than the amount absorbed. Absorption falls below
transpiration when extreme conditions cause temporary or permanent
wilting; the two activities are essentially equal after a growing plant
reaches maturity. In all cases, however, the rule is that an increase or
decrease in water loss produces a corresponding change in the amount of
water absorbed, and, conversely, variation in absorption produces a
consequent change in transpiration. This is strictly true only when the
stimuli are normal. For example, a decrease in humidity causes increased
water loss, which, through diffusion and transport, is compensated by
increased activity of the root surface. Frequently the water supply is
insufficient to compensate for a greater stimulus, and the proper
balance can be attained only by the closing of the stomata. In the case
of excessive stimuli, neither compensation suffices, and the plant dies.
Many mesophytes and all xerophytes have probably resulted from stimuli
which regularly approached the limit of compensation for each, and often
overstepped, but never permanently exceeded it. For hydrophytes, the
danger arises from excessive water supply, not water loss. There is a
limit to the compensation afforded by transpiration, which is naturally
dependent upon the amount of plant surface exposed to the air. No
compensation occurs in the case of submerged plants; floating
hydrophytes possess a single transpiring leaf surface, while the leaves
of amphibious plants behave as do those of mesophytes. The whole
question of response to water stimuli thus turns upon the compensation
for water loss afforded by water supply where the latter is moderate or
precarious, and upon the compensation for water supply furnished by
water loss where the supply is excessive, submerged plants excepted.

=155. The measurement of absorption.= As responses to measured stimuli
of water-content and humidity, it is imperative that the amount of
absorption and of transpiration be determined quantitatively. It is also
extremely desirable that this be done in the normal habitat of the
plant. A careful examination of the problems to be met quickly discloses
the great difficulty of obtaining a direct and accurate measure of
absorption under normal conditions, especially in the field. For this
purpose, the ordinary potometric experiments by means of cut stems are
valueless. The use of the entire plant in a potometer yields much more
trustworthy results, though the fact that the root is under abnormal
conditions can not be overlooked, especially in the case of mesophytes
and xerophytes. While potometric conditions are less abnormal for
amphibious plants, the error is not wholly eliminated, since the roots
normally grow in the soil. The potometer can be made of value for
quantitative work only by checking the results it gives by means of an
instrument or a method in which the plant functions normally. In
consequence, the potometer can not at present be used to measure
absorption directly, though, as is further indicated in the discussion
of transpiration, it is a valuable supplementary instrument, after the
check mentioned has been applied to its use with a particular species.

An estimate of the amount of absorption may be obtained either in the
field or in the control house by taking samples from the protected soil
at different times. Since it is impossible to determine the weight of
the area in which the roots lie, and since the soil water is often
unequally distributed, this method can not yield exact results. An
accurate method of measuring absorption under essentially normal
conditions has been devised and tested in the control house. The
essential feature of the process is the placing a plant in a soil
containing a known quantity of water, and removing it after it has
absorbed water from the soil for a certain period. In carrying out the
experiment, a soil consisting of two parts of sod and one of sand was
used, since the aeration is more perfect and the particles are more
easily removed from the roots. The soil was completely dried out in a
water bath and then placed in a five-inch battery jar. The latter,
together with the rubber cloth used later to prevent evaporation, was
weighed to the decigram. A weighed quantity of water was added, and the
whole again weighed as a check. Two plants of _Helianthus annuus_ were
taken from the pots in which they had grown, and the soil was carefully
washed from the roots. Each plant was weighed with its roots in a dish
of water to prevent wilting, and then carefully potted, one in each
battery jar. A thistle tube was placed in the soil of each jar to
facilitate aeration, as well as the addition of weighed amounts of
water, when necessary, and the rubber cloth attached in the usual manner
to prevent evaporation. The entire outfit was weighed again, and the
weighing repeated at 8:00 A.M. and 5:00 P.M. for five days, in order to
determine the amount of transpiration and its relation to the water
absorbed. The plants were kept in diffuse light to prevent excessive
water loss while the roots were becoming established. At the close of
the experiment, the jar and its contents were weighed finally. The
plants were removed and weighed, the soil particles being shaken from
the roots into the jar, which was also weighed. The results obtained
were as follows:

 ══╤═════════╤═══════════════════╤════════╤════════╤════════╤══════════
   │ Wt. of  │Wt. of pot and wet │ Total  │_H_{2}O_│_H_{2}O_│ _H_{2}O_
   │ pot and │       soil        │_H_{2}O_│  left  │absorbed│transpired
   │dry soil │                   │        │        │        │
 ──┼─────────┼─────────┬─────────┼────────┼────────┼────────┼──────────
   │         │    I    │   II    │        │        │        │
  I│1846.0 g.│2218.0 g.│2174.3 g.│372.0 g.│328.3 g.│ 43.7 g.│   43.7 g.
 II│1886.7 g.│2253.2 g.│2221.6 g.│366.5 g.│334.9 g.│ 31.6 g.│   31.6 g.
 ──┴─────────┴─────────┴─────────┴────────┴────────┴────────┴──────────

The amount of water absorbed may be obtained directly by subtracting the
final weight of the jar and moist soil from their first weight, but a
desirable check is obtained by taking the dry weight of jar and soil
from the first, and the final weight of these, and subtracting the one
from the other as indicated in the table. A second check is afforded by
daily weighings, from which the amount of water transpired is
determined. Since the two sunflower plants made practically no growth
during the period of experiment, the exact correspondence between water
absorbed and water lost is not startling, though it can not be expected
that the results will always coincide.

This method has certain slight sources of error, all of which, it is
thought, have been corrected in a new and more complete series of
experiments now being carried on. The aeration of the soil is not
entirely normal, as is also true of the capillary movements of the
water, on account of the nonporous glass jar and the rubber cloth. Since
the latter are necessary conditions of all accurate methods for
measuring absorption and transpiration, the resulting error must be
ignored. It can be reduced, however, by forcing air through the thistle
tube from time to time. Sturdy plants, such as the sunflower, are the
most satisfactory, since they recover more quickly from the shock of
transplanting. Almost any plant can be used, however, if repotted in a
loose sandy soil often enough. This permits the root system to develop
normally, and also makes it possible to wash the soil away without
injury to the root. The method is so recent that there has been no
opportunity to test it in the field. It would seem that it can be
applied without essential change to plants in their normal habitats.
Very large herbs or plants with extensive root systems could not be used
to advantage, and to be practicable the experiments would need to be
carried on near the base station. The great value of the method,
however, lies in its use as a check in determining the accuracy of other
methods, and in practice it will often be found convenient and
time-saving to use the latter, after they have once been carefully
checked for different groups of species. This matter is further
considered under measures of transpiration.

[Illustration: Fig. 31. Absorption and transpiration of _Helianthus
annuus_. I and II, plants repotted in soil of known weight and
water-content; III, plant undisturbed in the original soil; IV,
potometer containing plant with cut stem; V, potometer with entire
plant.]

=156. The quantitative relation of absorption and transpiration.=
Burgerstein[11] has summarized the results of various investigators in
the statement “that between the quantitative absorption of water on the
one hand and emission on the other there exists no constant parallelism
or proportion,” and he has cited the work of Kröber, and of Eberdt in
proof. This statement holds, however, only for short periods of a few
hours, or more rarely, a day, and even here its truth still remains to
be conclusively demonstrated. The discrepancy between absorption and
transpiration for a short period is often greater than for a longer
time, but it is evident that a transient change in behavior or a small
error in the method would inevitably produce this result. Eberdt found
the discrepancy for a few hours to be 1–2 ccm. in an entire plant of
_Helianthus annuus_, while for a whole day the water absorbed was 33.57
ccm. and the water lost 33.98 ccm. Kröber’s experiments with cut
branches of _Asclepias incarnata_ showed a maximum difference for 12
hours of 2.5 ccm., but the discrepancy for the first 24 hours was 1 ccm.
and for the second 1.9 ccm. In both cases, the potometer was employed.
Consequently, as will be shown later, Eberdt’s results are not entirely
trustworthy, while those of Kröber, made with cut stems, are altogether
unreliable. Hence, it is clear that the discrepancy is slight for a
period of several days or weeks, and that it may be ignored without
serious error, except in a few plants that retain considerable water as
cell-sap, in consequence of extremely rapid growth. Accordingly, the
amount of transpiration, which may be readily and accurately determined,
can be employed as a measure of absorption that is sufficiently accurate
for nearly all purposes. The truth of this statement may be easily
confirmed. It is evident that the amount of water absorbed equals the
amount transpired plus that retained by the plant as cell-sap, or used
in the manufacture of organic compounds. In plants not actively growing,
the amount lost equals that absorbed, as already shown in the experiment
with _Helianthus_. According to Gain[12], Dehérain has found that a
plant rooted in ordinary soil transpired 680 kg. of water for each
kilogram of dry substance elaborated. In _Helianthus annuus_, the dry
matter is 10 per cent of the weight of the green plant. A well-grown
plant weighing 1,000 grams, therefore, consists of 100 grams of dry
matter and 900 of water. The length of the growing period for such a
plant is approximately 100 days, during which it transpires 68 kilograms
of water. Assuming the rate of transpiration and of growth to be
constant, the plant transpires 680 grams daily, adds 9 grams to its
cell-sap, and 1 gram to its dry weight. The amount of water in a gram of
cellulose and its isomers is about ⅗. Consequently, the total water
absorbed daily by the plant is 689.6 grams. The 680 grams transpired are
98.6 per cent of the amount absorbed; in other words, only 1.4 per cent
of the water absorbed is retained by the plant. From this it is evident
that the simplest and most convenient measure of absorption under normal
conditions can be obtained through transpiration, since the discrepancy
between absorption and transpiration is scarcely larger than the error
of any method applicable to the field. Conversely, the measure of
absorption obtained by the process described in the preceding section
serves also as a measure of transpiration. The determination of the
latter in the field is so much simpler, however, that it is rarely
desirable to apply the absorption method.

=157. Measurement of transpiration.= The water loss of a plant may be
determined absolutely or relatively. Absolute or quantitative
determinations are by (1) weighing, (2) collecting, or (3) measuring the
water absorbed; relative values are indicated by hygroscopic substances.
A number of methods have been employed more or less generally for
measuring transpiration. The great majority of these can be used to
advantage only in the laboratory, and practically all fail to meet the
fundamental requirement for successful field work, namely, that the
plant be studied under normal conditions in its own habitat. The
following is a summary of the various methods, the details of which may
be found in Burgerstein.

1. _Weighing._ This is the most satisfactory of all methods for
determining water loss. It is more accurate than any other, and is
unique in that it does not place the plant under abnormal conditions. On
the score of convenience, moreover, it excels every other method capable
of yielding quantitative results. Various modifications of weighing are
employed, but none of these have all the advantages of a direct, simple
weighing of the plant in its own soil.

2. _Collecting the water transpired._ This may be done by collecting and
weighing the water vapor exhaled by a plant placed within a bell jar, or
by weighing a deliquescent salt, such as calcium chloride, which is used
to absorb the water of transpiration. The decisive disadvantage of these
methods is that transpiration is carried on in an atmosphere far more
humid than normal. If an excessive amount of salt is used, the air is
abnormally dry. In both cases, the water loss decreases until it reaches
a point much below the usual amount. Finally, all methods of this kind
are open to considerable error, and are inconvenient, especially in
field work. They are of relatively slight value in comparison with
weighing.

3. _Potometers._ It has already been shown that the amount of water
absorbed is a close measure of the amount transpired. In consequence,
the potometer can be used to determine the amount of transpiration
provided the absorption is not abnormal. It is rarely and only with much
difficulty that this condition can be met. The use of cut stems and
branches does not meet it, and even in the case of plants with roots,
the results must be compared with those obtained from absorption
experiments made with plants rooted in soil before they can be relied
upon. This necessity practically puts the potometer out of commission
for accurate work, unless future study may show a somewhat constant
ratio between the absorption of a plant in its own soil and that of a
plant placed in a potometer.

4. _Measuring absolute humidity._ The cog psychrometer makes it possible
to determine the increased relative humidity produced within a glass
cylinder or special tin chamber by a transpiring plant. From this result
the absolute humidity is readily obtained, and by means of the latter
the actual amount of water given off. The evident drawback to this
method is that the increasing humidity within the chamber gives results
entirely abnormal for the plant concerned.

5. _Self-registering instruments._ There are various methods for
registering the amount of transpiration, based upon weighing, or upon
the potometer. The Richard recording evaporimeter has all the advantages
of weighing, inasmuch as the water loss is measured in this way, and in
addition the amount is recorded upon a revolving drum, obviating the
necessity of repeated attention in case it is desirable to know the
exact course of transpiration. On the other hand, methods which depend
upon the potometer, while graphic, are not sufficiently accurate to be
of value.

6. _The use of hygroscopic materials._ Hygroscopic substances change
their form or color in response to moisture. As they indicate
comparative water loss alone, they are of value chiefly in the study of
the stomatic surfaces of leaves. F. Darwin[13] has used strips of horn,
awns of _Stipa_, and epidermis of _Yucca_ to construct small hygroscopes
for this purpose. In these instruments the error is large, but as no
endeavor is made to obtain exact results, it is negligible. Filter paper
impregnated with a 3–5 per cent aqueous solution of cobalt chloride is
deep blue when dry. If a strip of cobalt paper is placed upon a leaf and
covered with a glass slip it turns bright rose color, the rapidity of
the change affording a clue to the amount of transpiration.

=158. Field methods.= The conditions which a satisfactory field method
of measuring transpiration must fulfill have already been discussed;
they are accuracy, simplicity, and normality. These conditions are met
only by weighing the plant in its own soil and habitat. This has been
accomplished by means of the sheet-iron soil box, already described
under the determination of the chresard. The method is merely the
familiar one of pot and balance, slightly modified for field use. The
soil block, which contains the plant to be studied, is cut out, and the
metal plates put in position as indicated in section 53. Indeed, it is a
great saving of time and effort to determine transpiration and chresard
in the same experiment; this is particularly desirable in view of the
close connection between them. In this event, the soil block must be
small enough not to exceed the load of a field balance. After the block
is cut and encased, all the plants are removed, except the one to be
studied. If several individuals of the same species are present, it is
an advantage to leave all of them, since the error arising from
individual variations of water loss may, in this way, be almost
completely eliminated. A sheet of rubber or rubber cloth is carefully
tied over the box to prevent evaporation from the soil. A broad band is
passed under the box to aid in lifting it upon the scales. The latter
must be of the platform type, and should have a capacity as great as
consistent with the need for moving it about in the field. Weighings are
made in the usual way, care being taken to free the surface of the box
from soil. The aeration of the soil block is kept normal by removing the
rubber for a few minutes from time to time, or by forcing air through a
thistle tube. Water is also added through the latter, when it is desired
to continue the experiment for a considerable period. After the study of
transpiration is concluded, the rubber cloth is removed, soil samples
taken, and the soil allowed to dry out until the plant becomes
thoroughly wilted. If the box is weighed again, the difference
represents the amount of available water. The per cent of chresard is
also obtained in the usual way by taking samples for ascertaining the
echard, and subtracting this from the holard. Field determinations of
water loss yield the most valuable results when different habitat forms,
or ecads, of the same species are used. There is little profit in
comparing the transpiration of a typical sun plant, such as _Touterea
multiflora_, with that of a shade plant, such as _Washingtonia obtusa_.
But the simultaneous study of plants like _Chamaenerium angustifolium_,
_Gentiana acuta_, _Scutellaria brittonii_ etc., which grow in several
different habitats, furnishes direct and fundamental evidence of the
course of adjustment and adaptation.

Hesselmann[14], in his study of open woodlands in Sweden, has employed a
method essentially similar to the preceding. Young plants of various
species were transferred to pots in the field, where they were allowed
to grow for several months before a series of weighings was made to
determine the amount of transpiration. Since weighing is the measure
used in each, both methods are equally accurate. The one has a certain
advantage in that the pots are, perhaps, more easily handled, while the
other has the advantage of maintaining the normal relation of soil and
roots, a condition more or less impossible in a pot. In both instances
the weighing should be done in the habitat, which was not the case in
Hesselmann’s researches.

The slight value of the potometer, which has had a vogue far beyond its
merits, is indicated by the following table. These results were obtained
from three plants of _Helianthus annuus_; III was left undisturbed in
the pot where it had been growing, IV was placed in a potometer, after
the root had been cut off, and V was an entire plant placed in a
potometer. The amount of transpiration is indicated in grams per square
decimeter of leaf surface. The plants were kept in diffuse light, except
for a period of two hours (8:00 to 10:00 A.M.) on the last day, when
they were in full sunshine at a temperature of 75° F. Plant IV wilted so
promptly in the sunshine that it was found necessary to conclude the
experiment in diffuse light.

 ═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════╤═════
      │  8  │  5  │  8  │  5  │  8  │  5  │  8  │ 10  │  5  │  8  │Total
      │A.M. │P.M. │A.M. │P.M. │A.M. │P.M. │A.M. │A.M. │P.M. │A.M. │
 ─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────
   III│ 2.9 │ 7.3 │ 2.4 │ 6.0 │ 1.7 │ 1.6 │ 2.0 │ 3.4 │ 2.0 │ 1.8 │31.1
    IV│ 4.7 │ 7.2 │ 2.9 │ 2.3 │ 1.0 │ 0.6 │ 0.9 │ 0.5 │ 0.5 │ 0.4 │21.0
     V│ 3.7 │ 5.3 │ 3.2 │ 4.8 │ 2.5 │ 1.6 │ 3.0 │ 2.6 │ 1.6 │ 2.6 │30.9
 ─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────

The cut plant, IV, lost more water the first day than either of the
others, but the water loss soon decreased, and at the end of the period
was almost nil. The total transpiration for III and V is much the same,
but the range of variation for periods of 12 hours is from +2 to –1
gram. This experiment is taken as a fair warrant that the use of cut
stems in potometers can not give accurate results. It is inconclusive,
however, as to the merits of potometric values obtained by means of the
entire plant, and further studies are now being made with reference to
this point.

=159. Expression of results.= From the previous discussion of the
relation between them, it follows that an expression of the amount of
transpiration likewise constitutes an expression of absorption. It is
very desirable also that the latter be based upon root surface and
chresard, but the difficulty of determining the former accurately and
readily is at present too great to make such a basis practicable. In
expressing transpiration in exact terms, the fact that plants of the
same species or form are somewhat individual in their behavior must be
constantly reckoned with. In consequence, experiments should be made
upon two or three individuals whenever possible, in order to avoid the
error arising from this source.

Water loss may be expressed either in terms of transpiring surface or of
dry weight. Since there is no constant relation between surface and
weight, the terms are not interchangeable or comparable, and in practice
it is necessary to use one to the exclusion of the other. Obviously,
surface furnishes by far the best basis, on account of its intimate
connection with stomata and air-spaces, a conclusion which Burgerstein
(_l. c._, p. 6) has shown by experiment to be true. For the best
results, the whole transpiring surface should be determined. This is
especially necessary in making comparisons of different species. In
those studies which are of the greatest value, viz., ecads of the same
species, it is scarcely desirable to measure stem and petiole surfaces,
unless these organs show unusual modification. The actual transpiring
surface is constituted by the walls of the cells bordering the
intercellular spaces, but, since it is impossible to determine the
aggregate area of these, or the humidity of the air-spaces themselves,
the leaf surface must be taken as a basis. Since the transpiration
through the stomata is much greater than that through the epidermal
walls, the number of stomata must be taken into account. Since they are
usually less abundant on the upper surface, their number should be
determined for both sides of the leaf. The errors arising from more or
less irregular distribution are eliminated by making counts near the
tip, base, and middle of two or three mature leaves. The most convenient
unit of leaf surface is the square decimeter. The simplest way to
determine the total leaf area of a plant is to outline the leaves upon a
homogeneous paper, or to print them upon a photographic paper. The
outlines are then cut out and weighed, and the leaf area obtained in
square decimeters by dividing the total weight by the weight of a square
decimeter of the paper used. The area may also be readily determined by
means of a planimeter.

=160. Coefficient of transpiration.= At present it does not seem
feasible to express the transpiration of a plant in the form of a
definite coefficient, but it is probable that the application of exact
methods to each part of the problem will finally bring about this
result. Meanwhile the following formula is suggested as a step toward
this goal: _t_ = _g_(_u/l_)_LHT_, in which _t_, the transpiration
relation of a plant, is expressed by the number of grams of water lost
per hour, on a day of sunshine, by one square decimeter of leaf,
considered with reference to the stomata of the two surfaces, and the
amount of the controlling physical factors, light, humidity, and
temperature, at the time of determination. For _Helianthus annuus_, this
formula would appear as follows: _t_ = 2(²⁰⁰⁄₂₅₀) : 1 : 50 : 75°. To
avoid the large figures arising from the extent of surface considered,
the number of stomata per square decimeter is divided by 10,000. This
amounts to the number per square millimeter, and time may consequently
be saved by using this figure directly. While this formula obviously
leaves much to be desired, it has the great advantage of making it
possible to compare ecads of one species, or species of the same habitat
or of different habitats, upon an exact basis of factor, function, and
structure.


                              _ADAPTATION_

=161. Modifications due to water stimuli.= In adaptation, the great
desideratum is to connect each modification quantitatively with the
corresponding adjustment. This is even more difficult than to ascertain
the quantitative relation between stimulus and functional response, a
task still beset with serious obstacles. At the present time, little
more can be done than to indicate the relation of marked adaptations of
organs and tissues to the direct factors operating upon them, and to
attempt to point out among the functions possibly concerned the one
which seems to be the most probable connection between the probable
stimulus and the structure under investigation. In the pages that
follow, no more than this is attempted. The general changes of organs
and tissues produced by water are first discussed, and after this is
given a summary of the structural features of the plant types based upon
water-content.

=162. Modifications due to a small water supply.= A water supply which
may become deficient at any time is compensated either by changes which
decrease transpiration, or by those that increase the amount of water
absorbed or stored. These operate upon the form and size of the organs
concerned, as well as upon their structure. Modifications of the form of
leaf and stem are alike in that they lessen transpiration by a reduction
of the amount of surface exposed to the air. Structural adaptations, on
the other hand, bring about the protection of epidermal cells and
stomata, and often internal cells also, from the factors which cause
transpiration, or they anticipate periods of excessive transpiration by
the storage of water in specialized cells or tissues. In certain extreme
types the epidermis is itself modified for the absorption of water vapor
from the air.

=163. The decrease of water loss.= The following is a summary of the
contrivances for reducing transpiration.

1. _Position of the leaf._ Since the energy of a ray of sunlight is
greatest at the sun’s highest altitudes, those leaves transpire least
which are in such a position during midday that the rays strike them as
obliquely as possible. A leaf at right angles to the noonday sun
receives ten times as much light and heat upon a square decimeter of
surface as does one placed at an angle of 10 degrees. This device for
reducing the intensity of insolation is best developed in the erect or
hanging leaves of many tropical trees. In temperate zones, it is found
in such plants as _Silphium laciniatum_ and _Lactuca scariola_, and in
species with equitant leaves. In such plants as _Helianthus annuus_, the
effect is just the opposite, since the turning of the crown keeps the
leaves for a long time at a high angle to the incident rays. In the case
of mats, it is the aggregation of plants which brings about the mutual
protection of the leaves from insolation and wind.

2. _Rolling of the leaf._ Many grasses and ericaceous plants possess
leaves capable of rolling or folding themselves together when drouth
threatens. In other cases, the leaves are permanently rolled or folded.
The advantage of this device arises not only from the reduction of
surface, but also from the fact that the stomata come to lie in a
chamber more or less completely closed. In the case of those mosses
whose leaves roll or twist, a reduction of surface alone is effected.

3. _Reduction of leaf._ The transpiring surface of a plant is reduced by
decreasing the number of leaves, by reducing the size of each leaf, or
by a change in its form. In so far as the stem is a leaf, a decrease in
size or a change in shape brings about the same result. The final
outcome of reduction in size or number is the complete loss of leaves,
and more rarely, of the stem. Such marked decrease of leaf area is found
only in intense xerophytes, though it occurs in all deciduous trees as a
temporary adaptation. Changes in leaf form are nearly always accompanied
by a decrease in size. Of the forms which result, the scale, the linear
or cylindrical leaf, and the succulent leaf are the most common. Leaves
which show a tendency to divide often increase the number of lobes or
make them smaller.

4. _Epidermal modifications._ Excretions of wax and lime by the
epidermis have a pronounced effect by increasing the impermeability of
the cuticle, and, hence, decreasing epidermal transpiration. It seems
improbable that a coating of wax on the lower surface of a diphotic leaf
can have this purpose. The thickening of the outer wall of epidermal
cells to form a cuticle is the most perfect of all contrivances for
decreasing permeability and reducing transpiration. In many desert
plants, the greatly thickened cuticle effectually prevents epidermal
transpiration. In these also the cuticle is regularly developed in such
a way as to protect the guard cells, and even to close the opening
partially. An epidermis consisting of two or more layers of cells is an
effective, though less frequent device against water loss. When combined
with a cuticle, as is usually the case, the impermeability is almost
complete. Hairs decrease transpiration by screening the epidermis so
that the amount of light and heat is diminished, and the access and
movement of dry air impeded. While hairs assume the most various forms,
all hairy coverings serve the same purpose, even when, as in the case of
_Mesembryanthemum_, they are primarily for water-storage. Hairs protect
stomata as well as epidermal cells: the greater number of the former on
the lower surface readily explains the occurrence of a hairy covering on
this surface, even though absent on the more exposed upper side. In some
cases, hairs are developed only where they serve to screen the stomata.

The modifications of the stomata with respect to transpiration are
numerous, yet all may be classed with reference to changes of number or
level. With the exception of aquatic and some shade plants, the number
of stomata is normally greater on the less exposed, i. e., lower
surface. The number on both surfaces decreases regularly as the danger
of excessive water loss increases, but the decrease is usually more
rapid on the upper surface, which finally loses its stomata entirely. It
has been shown by many observers that species growing in dry places have
fewer stomata to the same area than do those found in moist habitats.
This result has been verified experimentally by the writer in the case
of _Ranunculus sceleratus_, in which, however, the upper surface
possesses the larger number of stomata. Plants of this species, which
normally grow on wet banks, were grown in water so that the leaves
floated, and in soils containing approximately 10, 15, 30, and 40 per
cent of water. The averages for the respective forms were: upper 20,
lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8;
upper 10, lower 6. Reduction of number is effective, however, only under
moderate conditions of dryness. As the latter becomes intense, the guard
cells are sunken below the epidermis, either singly or in groups. In
both cases, the protection is the same, the guard cells and the opening
between them being withdrawn from the intense insolation and the dry
air. The sun rays penetrate the chimney-shaped chambers of sunken
stomata only for a few minutes each day, and they are practically
excluded from the stomatal hollows which are filled with hairs. The
influence of dry winds is very greatly diminished, as is also true,
though to a less degree, for leaves in which the stomata are arranged in
furrows. Sunken stomata often have valve-like projections of cuticle
which reduce the opening also. Finally, in a few plants, water loss in
times of drouth is almost completely prevented by closing the opening
with a wax excretion.

5. _Modifications in the chlorenchym._ A decrease in the size and number
of the air passages in the leaf renders the movement of water-laden air
to the stomata more difficult, and effects a corresponding decrease in
transpiration. The increase of palisade tissue, though primarily
dependent upon light, reduces the air-spaces, and consequently the
amount of water lost. The development of sclereids below the epidermis
likewise hinders the escape of water. Finally the character of the cell
sap often plays an important part, since cells with high salt-content or
those containing mucilaginous substances give up their water with
reluctance.

=164. The increase of water supply.= Plants of dry habitats can increase
their absorption only by modifying the root system so that the absorbing
surfaces are carried into the deep-seated layers of soil, and the
surfaces in contact with the dry soil are protected by means of a
cortex. Exception must be made for epiphytes and a few other plants that
absorb rain water and dew through their leaves, and for those desert
plants that seem to condense the moisture of the air by means of
hygroscopic salts, and absorb it through the epidermis of the leaf. The
storage of water in the leaf is a very important device; it increases
the water supply by storing the surplus of absorbed water against the
time of need. Modifications for water-storage are occasionally found in
roots and stems, but their chief development takes place in the leaf.
The epidermis frequently serves as a reservoir for water, either by the
use of the epidermal cells themselves, by the formation of hypodermal
water layers, or by means of superficial bulliform cells. The water
cells of the chlorenchym regularly appear in the form of large clear
cells, scattered singly or arranged in groups. In this event, they occur
either as transverse bands, or as horizontal layers, lying between the
palisade and sponge areas, and connecting the bundles. A few plants
possess tracheid-like cells which also serve to store water. In the case
of succulent leaves, practically the whole chlorenchym is used for
storing water, though they owe their ability to withstand transpiration
to a combination of factors.

=165. Modifications due to an excessive water supply.= Water plants with
aerial leaf surfaces are modified in such manner as to increase water
loss and to decrease water supply, but the resulting modifications are
rarely striking. There is a marked tendency to increase the exposed
surface. This is indicated by the fact that, while the leaves of mud and
floating forms become larger, they change little or not at all in
thickness. The lobing of leaves is also greatly reduced, or the lobes
come to overlap. Leaves of water plants are practically destitute of all
modifications of epidermis and stomata, which could serve to hinder
transpiration. The stomata are usually more numerous on the upper
surface, and in the same species their number is greater in the forms
grown in wet places. These facts explain in part the extreme development
of air-passages in water plants, though this is, in large measure, a
response to the increasing difficulty of aeration. The increase of
air-spaces is correlated with reduction of the palisade, and a decided
increase in the sponge. An increase in water supply is indicated by the
absence of storage tissues, and the reduction of the vascular system,
which, however, is more closely connected with a diminished need for
mechanical support.

[Illustration: Fig. 32. Mesophyll of _Pedicularis procera_ (chresard,
15%, light, 1). × 130.]

=166. Plant types.= The necessity for decreasing or increasing water
loss in compensation of the water supply has made it possible to
distinguish two fundamental groups of plants upon the twofold basis of
habitat and structure. These familiar groups, xerophytes and
hydrophytes, represent two extremes of habitat and structure, between
which lies a more or less vague, intermediate condition represented by
mesophytes. These show no characteristic modifications, and it is
consequently impossible to arrange them in subgroups. Xerophytes and
hydrophytes, on the other hand, exhibit marked diversity among
themselves, a fact that makes it desirable to recognize subgroups, which
correspond to fundamental differences of habitat or adaptation. It is
hardly necessary to point out that these types are not sharply defined,
or that a single plastic species may be so modified as to exhibit
several of them. The extremes are always clearly defined, however, and
they indicate the specific tendency of the adaptation shown by other
members of the same group.

=167. Xerophytic types.= With the exception of dissophytes, all
xerophytes agree in the possession of a deep-seated root system, adapted
to withdraw water from the lower moist layers, and to conserve from loss
from the upper dry layers. Reservoirs are developed in the root,
however, in relatively few cases. The stem follows the leaf more or less
closely in its modification, except when the leaf is greatly reduced or
disappears, in which event the stem exhibits peculiar adaptations. While
the leaf is by far the most strikingly modified, it is a difficult task
to employ it satisfactorily as the basis for distinguishing types.
Several adaptations are often combined in the same leaf, and it is only
where one of these is preeminently developed, as in the case of
succulence, that the plant can be referred to a definite type. The
latter does not happen in many species of the less intensely xerophytic
habitats, and, consequently, it is difficult, if not undesirable, to
place such xerophytes under a particular group. The best that can be
done is to recognize the types arising from extreme or characteristic
modification, and to connect the less marked forms as closely as
possible with these. Halophytes differ from xerophytes only in the fact
that the chresard is determined by the salt-content of the habitat, and
not by the texture of the soil. In consequence, they should not be
treated as a distinct group.

[Illustration: Fig. 33. Staurophyll of _Bahia dissecta_, showing extreme
development of palisade (chresard, 3–9%; light, 1). × 130.]

=168. Types of leaf xerophytes.= In these, adaptation has acted
primarily upon the leaf, while the stem has remained normal for the most
part. Even when the leaves have become scale-like, they persist
throughout the growing season, and continue to play the primary part in
photosynthesis. The following types may be distinguished:

1. _The normal form._ The leaf is of the usual dorsiventral character.
In place of a reduction in size, structural modifications are used to
decrease transpiration. With respect to the protective feature that is
predominant, three subtypes may be recognized. The _cutinized_ leaf
compensates for a low water-content by means of a thick cuticle, often
reinforced by a high development of palisade tissue. Such leaves are
more or less leathery, and they are often evergreen also.
_Arctostaphylus_ and many species of _Pentstemon_ are good examples.
Lanate leaves, i. e., those with dense hairy coverings on one or both
surfaces, as _Artemisia_, _Antennaria_, etc., regularly lack both
cuticle and palisade tissue. The protection against water loss, however,
is so perfect that the chlorenchym often assumes the loose structure of
a shade leaf. _Storage_ leaves usually have a well-developed cuticle and
several rows of palisade cells, but their characteristic feature is the
water-storage tissue, which maintains a reserve supply of water for the
time of extreme drouth. Xerophytic species of _Helianthus_ furnish
examples of transverse bundles of storage cells, while those of
_Mertensia_ illustrate the more frequent arrangement in which the water
tissue forms horizontal layers.

2. _The succulent form._ Many succulent leaves are normal in shape and
size, though always thicker than ordinary leaves. Usually, however, they
are reduced in size and are more or less cylindrical in form. The
necessary decrease in transpiration is effected by the reduction in
surface, the general storage of water, a waxy coating, and, often also,
by a very thick cuticle. _Agave_, _Mesembryanthemum_, _Sedum_, and
_Senecio_ furnish excellent examples of this type.

[Illustration: Fig. 34. Diplophyll of _Mertensia linearis_, showing
water cells (chresard, 3–9%, light, 1). × 130.]

3. _The dissected form._ The reduction in surface is brought about by
the division of the leaf blade into narrow linear or thread-like lobes
which are widely separated. The latter are themselves protected by a
hairy covering or a thick cuticle, which is often supplemented by many
rows of palisade, or by storage tissue. _Artemisia_, _Senecio_, and
_Gilia_ contain species which serve as good examples of this type.

4. _The grass form._ Xerophytic grasses and sedges have narrow
filamentous leaves with longitudinal furrows which serve to protect the
stomata. The furrows are sometimes filled with hairs which are an
additional protection, and the leaves often protect themselves further
by rolling up into a thread-like shape. The elongated subulate leaves of
_Juncus_ and certain _Cyperaceae_ are essentially of this type, although
they are usually not furrowed.

5. _The needle form._ This is the typical leaf of conifers, in which a
sweeping reduction of the leaf surface is an absolute necessity. The
relatively small water loss of the needle leaf is still further
decreased by a thick cuticle, and usually also by hypodermal layers of
sclerenchyma.

6. _The roll form._ Roll leaves are frequently small and linear. Their
characteristic feature is produced by the rolling in of the margin on
the under side, by which an almost completely closed chamber is formed
for the protection of the stomata which are regularly confined to the
lower surface of the leaf. The upper epidermis is heavily cutinized and
the lower one often protected by hairs. This type is found especially
among the genera of the _Ericales_, but it also occurs in a large number
of related families.

7. _The scale form._ Reduction of leaf surface for preventing excessive
water loss reaches its logical culmination in the scale leaf
characteristic of many trees and shrubs, e. g., _Cupressus_, _Tamarix_,
etc. Scale leaves are leathery in texture, short and broad, and closely
appressed to the stem, as well as often overlapping.

=169. Types of stem xerophytes.= In these types the leaves are deciduous
early in the growing period, reduced to functionless scales, or entirely
absent. The functions of the leaf have been assumed by the stem, which
exhibits many of the structural adaptations of the former. Warming[15]
has distinguished the following groups:

1. _The phyllode form._ The petiole is broadened and takes the place of
the leaf blade which is lacking. In other cases, the stem is flattened
or winged, and it replaces the entire leaf. This type occurs in
_Acacia_, _Baccharis_, _Genista_, etc.

2. _The virgate form._ The leaves either fall off early or they are
reduced to functionless scales. The stems are thin, erect, and rod-like,
and are often greatly branched. They are heavily cutinized and
palisaded, and the stomata are frequently in longitudinal furrows. This
type is characteristic of the _Genisteae_; it is also found in
_Ephedra_, many species of _Polygonum_, _Lygodesmia_, etc.

3. _The rush form._ In _Heleocharis_, many species of _Juncus_,
_Scirpus_, and other _Cyperaceae_, the stem, which is nearly or
completely leafless, is cylindrical and unbranched. It usually possesses
also a thick cuticle, and several rows of dense palisade tissue.

4. _The cladophyll form._ In _Asparagus_ the leaves are reduced to mere
functionless scales, and their function is assumed by the small
needle-shaped branches.

5. _The flattened form._ As in the preceding type, the place of the
scale-like leaves is taken by cladophylls, which are more or less
flattened and leaf-like. _Ruscus_ is a familiar illustration of this
form.

6. _The thorn form._ This is typical of many spiny desert shrubs, in
which the leaves are lost very early, or, when present, are mere
functionless scales. The stems have an extremely thick cuticle, and the
stomata are deeply sunken, as a rule. _Colletia_ and _Holacantha_ are
good examples of the type.

7. _The succulent form._ Plants with succulent stems such as the
_Cactaceae_, _Stapelia_, and _Euphorbia_ have not only decreased water
loss by extreme reduction or loss of the leaves, and the reduction of
stem surface, but they also offset transpiration by means of storage
tissues containing a mucilaginous sap. The cuticle is usually highly
developed and the stomata sunken. Thorns and spines are also more or
less characteristic features.

[Illustration: Fig. 35. _Polygonum bistortoides_, a stable type: 1,
mesophyll (chresard, 25%); 2, xerophyll (chresard, 3–5%). × 130.]

=170. Bog plants.= Many of the xerophytic types just described are found
in ponds, bogs, and swamps, where the water supply is excessive, and
hydrophytes would be expected. The explanation that “swamp xerophytes”
are due to the presence of humic acids which inhibit absorption and
aeration in the roots has been generally accepted. As Schimper has
expressed it, bogs and swamps are “physiologically dry”, i. e., the
available water is small in amount, in spite of the great total
water-content. Burgerstein (_l. c._, 142) has shown, however, that maize
plants transpire, i. e., absorb, three times as much water in a solution
of 0.5 per cent of oxalic acid as they do in distilled water, and that
branches of _Taxus_ in a solution containing 1 per cent of tartaric acid
absorb more than twice as much as in distilled water. Consequently, it
seems improbable that small quantities of humic acids should decrease
absorption to the extent necessary for the production of xerophytes in
ponds and bogs. Indeed, in many ponds and streams, where _Heleocharis_,
_Scirpus_, _Juncus_, etc., grow, not a trace of acid is discoverable.
Furthermore, plants with a characteristic hydrophytic structure
throughout, such as _Ranunculus_, _Caltha_, _Ludwigia_, _Sagittaria_,
etc., are regularly found growing alongside of apparent xerophytes. Many
of the latter, furthermore, show a striking contrast in size and vigor
of growth in places where they grow both upon dry gravel banks and in
the water, indicating that the available water-content is much greater
in the latter. Finally, many so-called “swamp xerophytes” possess
typically hydrophytic structures, such as air-passages, diaphragms, etc.
In spite of a growing feeling that the xerophytic features of certain
amphibious plants can not be ascribed to a low chresard in ponds and
swamps, a satisfactory explanation of them has been found but recently.
This explanation has come from the work of E. S. Clements already cited,
in which it was found that certain sun plants underwent no material
structural change when grown in the shade, and that the same was true
also of a few species which grew in two or more habitats of very
different water-content. In accordance with this, it is felt that the
xerophytic features found in amphibious plants are due to the
persistence of stable structures, which were developed when these
species were growing in xerophytic situations. When it is called to mind
that monocotyledons, and especially the grasses, sedges, and rushes, are
peculiarly stable, it may be readily understood how certain ancestral
characters have persisted in spite of a striking change of habitat. Such
a hypothesis can only be confirmed by the methods of experimental
evolution, and a critical study of this sort is now under way.

[Illustration: Fig. 36. _Hippuris vulgaris_: 1, submerged leaf; 2,
aerial leaf. × 130.]

=171. Hydrophytic types.= Hydrophytes permit a fairly sharp division
into three groups, based primarily upon the relation of the leaf surface
to the two media, air and water. In submerged plants, the leaves are
constantly below the water; in amphibious ones, they grow normally in
the air. Floating plants have leaves in which the upper surface is in
contact with the air, and the lower in contact with the water.
Transpiration is at a maximum in the amphibious plant; it is reduced by
half in the floating type, and is altogether absent in submerged plants.
Aeration reaches a high development in amphibious and floating forms,
but air-passages are normally absent from submerged forms except as
vestiges. Photosynthesis is marked in the former, but considerably
weakened in the latter. The vascular system, which attains a moderate
development in the amphibious type, is considerably reduced in floating
forms, and it is little more than vestigiate in submerged ones.

[Illustration: Fig. 37. Floating leaf of _Sparganium angustifolium_. ×
130.]

1. _The amphibious type._ Plants of this type grow in wet soil or in
shallow water. The leaves are usually large and entire, the stem well
developed, and the roots numerous and spreading. In the majority of
cases the leaves are constantly above the water, but in some species the
lower leaves are often covered, normally, or by a rise in level, and
they take the form or structure of submerged leaves. This is illustrated
by _Callitriche autumnalis_, _Hippuris vulgaris_, _Ranunculus
delphinifolius_, _Proserpinaca palustris_, _Roripa americana_, _etc._
The epidermis has a thin cuticle, or none at all, and is destitute of
hairs. The stomata are numerous and usually more abundant on the upper
than on the lower surface. The palisade tissue is represented by one or
more well-developed rows, but this portion of the leaf is regularly
thinner than that of the sponge part. The latter contains large
air-passages, or, in the majority of cases, numerous air-chambers,
usually provided with diaphragms. The stems are often palisaded, and are
characterized by longitudinal air-chambers crossed by frequent
diaphragms, which extend downward through the roots.

2. _The floating type._ With respect to form and the structure of the
upper part of the leaf, floating leaves are essentially similar to those
of amphibious plants. They are usually lacquered or coated with wax to
prevent the stoppage of the stomata by water. Stomata, except as
vestiges, are found only on the upper surface, and the palisade tissue
is much less developed than the sponge, which is uniformly characterized
by large air-chambers. The stems are elongated, the aerating system is
enormously developed, and the supportive tissues are reduced. In the
_Lemnaceae_, the leaf and the stem are represented by a mere frond or
thallus, and the roots are in the process of disappearance, e. g.,
_Spirodela_ has several, _Lemna_ one, and _Wolffia_ none.

3. _The submerged type._ Both stem and root have been greatly reduced in
submerged plants, owing to the generalization of absorption and the
density of the water. The leaves are greatly reduced in size and
thickness, chiefly, it would seem, for the purpose of insuring readier
aeration and great illumination. The leaf may be ribbon-like, linear,
cylindrical, or finely dissected. Stomata are sometimes present, but
they are functionless and vestigial. A distinction into palisade and
sponge tissues, when present, must also be regarded as a vestige; the
chlorenchym is essentially that of a shade leaf. The air-chambers are
much reduced, and sometimes lacking; they function doubtless as
reservoirs for air obtained from the water.


                              PHOTOHARMOSE


                              _ADJUSTMENT_

=172. Light as a stimulus.= In nature, light stimuli are determined by
intensity and not by quality. A single exception is afforded by those
aquatic habitats where the depth of water is great, and in consequence
of which certain rays disappear by absorption more quickly than others.
In forests and thickets, where the leaves transmit only the green and
yellow rays, it would appear that the light which reaches the herbaceous
layers is deficient in red and violet rays. The amount of light
transmitted by an ordinary sun leaf is so small, however, that it has no
appreciable effect upon the quality of the light beneath the facies,
which is diffuse white light that has passed between the leaves. Indeed,
it is only in the densest forests that distinct sunflecks do not appear.
Coniferous forests, with a light value less than .005, which suffices
only for mosses, lichens, and a few flowering plants, show frequent
sunflecks. This is convincing evidence that the light of such habitats
is normal in quality. It warrants the conclusion that in all habitats
with an intensity capable of supporting vascular plants the light, no
matter how diffuse, is white light. The direction of the light ray is of
slight importance in the field, apart from the difference in intensity
which may result from it. In habitats with diffuse light, the latter
comes normally and constantly from above. Likewise, in sunny situations,
direction can have little influence, since both the direction and the
angle of the incident rays change continually throughout the day, and
the position of the leaf itself is more or less constantly changed by
the wind. The influence of duration upon the character of light stimuli
is difficult to determine. There can be no question that the time during
which a stimulus acts has a profound bearing upon the response that is
made to it. In nature the problem is complicated by the fact that light
stimuli are both continuous and periodic. The duration of sunlight is
determined by the periodic return of night as well as by the irregular
occurrence of clouds. Since one is a regular, and the other at least a
normal happening, it is necessary to consider duration only with respect
to the time of actual sunlight on sunny days, except in the case of
formations belonging to regions widely different in the amount of normal
sunshine, i. e., the number of cloudy days. In consequence, duration is
really a question of the intensities which succeed each other during the
day. The differences between these have already been shown to fall
within the efficient difference for light, and for this reason the ratio
between the light intensity of a meadow and of a forest is essentially
the ratio between the sums of light intensity for the two habitats, i.
e., the duration. The latter is of importance only where there is a
daily alternation between sunshine and shadow, as at the edge of forest
and thicket, in open woodland, etc. In such places duration determines
the actual stimulus by virtue of the sum of preponderant intensities.
The periodicity of daylight is a stimulus to the guard cells of stomata,
but its relation to intensity in this connection is not clear.

The amount of change in light intensity necessary to constitute an
efficient stimulus seems to depend upon the existing intensity as well
as upon the plant concerned. Apparently, a certain relative decrease is
more efficient for sun plants than for shade plants. At least, many
species sooner or later reach a point where a difference larger than
that which has been efficient no longer produces a structural response.
This has been observed by E. S. Clements (_l. c._) in a number of shade
ecads. For example, a form of _Galium boreale_, which grew with
difficulty in a light value of .002, showed essentially the leaf
structure of the form growing in light of .03, while the form in full
sunlight showed a striking difference in the leaf structure. In
considering the light stimuli of habitats, it is unnecessary to discuss
the stimulus of total darkness upon chlorophyllous plants, although this
is of great importance in experimental evolution and in control
experiment. The normal extremes of light intensity, i. e., those within
which chlorenchym can function, are full sunshine represented by 1, and
a diffuseness of .002, though small flowering plants have once or twice
been found in an intensity of .001. The maximum light value, even on
high mountains, never exceeds 1 by more than an inconsiderable amount,
except for the temporary concentration due to drops of dew, rain, etc.
It seems improbable that the concentrating effect of epidermal papillae
can do much more than compensate for the reflection and absorption of
the epidermis. Experimental study has shown that the maximum intensity
in nature may be increased several, if not many times, without injurious
results and without an appreciable increase in the photosynthetic
response, thus indicating that the efficient difference increases toward
the maximum as well as toward the minimum.

=173. The reception of light stimuli.= Rays of light are received by the
epidermis, by which they are more or less modified. Part of the light is
reflected by the outer wall or by the cuticle, particularly when these
present a shining surface. Hairs diffract the light rays, and hairy
coverings consequently have a profound influence in determining stimuli.
The walls and contents of epidermal cells furthermore absorb some of the
light, especially when the cell sap is colored. In consequence of these
effects, the amount of light that reaches the chlorenchym is always less
than that incident upon the leaf, and in many plants, the difference is
very great. According to Haberlandt[16], the epidermal cells of some
shade plants show modifications designed to concentrate the light rays.
Of such devices, he distinguishes two types: one in which the outer
epidermal wall is arched, another in which the inner wall is deeply
concave. Although there can be no question of the effect of lens-shaped
epidermal cells, their occurrence does not altogether support
Haberlandt’s view. Arched and papillate epidermal cells are found in sun
plants where they are unnecessary for increasing illumination, to say
the least. A large number of shade plants show cells of this character,
but in many the outer wall is practically a plane. Shade forms of a
species usually have the outer wall more arched or papillate, but this
is not always true, and, in a few cases, it is the lower epidermis alone
that shows this feature. Finally, a localization of this function in
certain two-celled papillae, such as Haberlandt indicates for _Fittonia
verschaffelti_, does not appear to be plausible.

The epidermis merely receives the light; the perception of the stimulus
normally occurs in those cells that contain chloroplasts. The cytoplasm
of the epidermal cells, as well as that of the chlorenchym cells, is
sensitive to light, but the response produced by the latter is hardly
discernible in the absence of plastids, except in those plants which
possess streaming protoplasm. The daily opening and closing of the
stomata, which is due to light, is evidently connected with the presence
of chloroplasts in the guard cells. Naturally, the perception of light
and the corresponding response occur in the epidermis of many shade and
submerged plants which have chloroplasts in the epidermal cells. Such
cases merely serve to confirm the view that the perception of light
stimuli is localized in the chloroplast. In conformity with this view,
the initial response to such stimuli must be sought in the chloroplast,
and the explanation of all adaptations due to light must be found in the
adjustment shown by the chloroplasts.

=174. Response of the chloroplast.= The fundamental response of a
plastid to light is the manufacture of chlorophyll. In the presence of
carbon dioxide and water, leucoplasts invariably make chlorophyll, and
chloroplasts replace that lost by decomposition, in response to the
stimulus exerted by light. The latter is normally the efficient factor,
since water is always present in the living plant, and carbon dioxide
absent only locally at most. Sun plants which possess a distinct
cuticle, however, produce leucoplasts, not chloroplasts, in the
epidermal cells, although these are as strongly illuminated as the guard
cells, which contain numerous chloroplasts. This is evidently explained
by the lack of carbon dioxide in the epidermis. This gas is practically
unable to penetrate the compact cuticle, at least in the small quantity
present in the air. The supply obtained through the stomata is first
levied upon by the guard cells and then by the cells of the chlorenchym,
with the result that the carbon dioxide is all used before it can reach
the epidermal cells. This view is also supported by the presence of
chloroplasts along the sides and lower wall of palisade cells, where
there is normally a narrow air-passage, and their absence along the
upper wall when this is closely pressed against the epidermis, as is
usually the case. Furthermore, the leaves of some mesophytes when grown
in the sun develop a cuticle and contain leucoplasts. Under glass and in
the humid air of the greenhouse, the same plants develop epidermal
chloroplasts but no cuticle. This is in entire harmony with the
well-known fact that shade plants and submerged plants often possess
chloroplasts in the epidermis. Although growing in different media,
their leaves agree in the absence of a cuticle, and consequent
absorption of gases through the epidermis. The size, shape, number, and
position of the chloroplasts are largely determined by light, though a
number of factors enter in. No accurate studies of changes in size and
shape have yet been made, though casual measurements have indicated that
the chloroplasts in the shade form of certain species are nearly
hemispherical, while those of the sun form are plane. In the same
plants, the number of chloroplasts is strikingly smaller in the shade
form, but exact comparisons are yet to be made. The position and
movement of chloroplasts have been the subject of repeated study, but
the factors which control them are still to be conclusively indicated.
Light is clearly the principal cause, although there are many cases
where a marked change in the light intensity fails to call forth any
readjustment of the plastids. The position of air-spaces as reservoirs
of carbon dioxide and the movement of crude and elaborated materials
from cell to cell frequently have much to do with this problem. Finally,
it must be constantly kept in mind that the chloroplasts lie in the
cytoplasm, which is in constant contact with a cell wall. Hence, any
force that affects the shape of the cell will have a corresponding
influence upon the position of the chloroplasts. When it is considered
that in many leaves these four factors play some part in determining the
arrangement of the plastids, it is not difficult to understand that
anomalies frequently appear.

It may be laid down as a general principle that chloroplasts tend to
place themselves at right angles to rays of diffuse light and parallel
to rays of sunlight. This statement is borne out by an examination of
the leaves of typical sun and shade species, or of sun and shade forms
of the same species. Cells which receive diffuse light, i. e., sponge
cells, normally have their rows of plastids parallel with the leaf
surface, while those in full sunlight place the rows at right angles to
the surface. This disposition at once suggests the generally accepted
view that chloroplasts in diffuse light are placed in such a way as to
receive all the light possible, while those in sunlight are so arranged
as to be protected from the intense illumination. Many facts support
this statement with respect to shade leaves, but the need of protection
in the sun leaf is not clearly indicated. The regular occurrence of
normal chloroplasts in the guard cells seems conclusive proof that full
sunlight is not injurious to them. Although the upper wall of the outer
row of palisade cells is usually free from chloroplasts, yet it is not
at all uncommon to find it covered by them. These two conditions are
often found in cells side by side, indicating that the difference is due
to the presence of carbon dioxide and not to light. In certain species
of monocotyledons, the arrangement of the chloroplasts is the same in
both halves of the leaf, and there is no difference between the sun and
shade leaves of the same species. The experimental results obtained with
concentrated sunlight, though otherwise conflicting, seem to show
conclusively that full sunlight does not injure the chloroplasts of sun
plants, and that the position of plastids in palisade cells is not for
the purpose of protection. This arrangement, which is known as
_apostrophe_, is furthermore often found in shade forms of heliophytes.
In typical shade species, and in submerged plants, the disposition of
plastids on the wall parallel with the leaf surface, viz., _epistrophe_,
is more regular, but even here there are numerous exceptions to the
rule.

The absorption of the light stimulus by the green plastid results, under
normal conditions, in the immediate production of carbohydrates, which
in the vast majority of cases soon become visible as grains of starch.
The appearance of starch in the chloroplasts of flowering plants is such
a regular response to the action of light that it is regarded as the
normal indication of photosynthetic activity. The mere presence of
chlorophyll is not an indication of the latter, since chlorophyll
sometimes persists in light too diffuse for photosynthesis. The amount
of starch formed is directly connected with the light intensity, and in
consequence it affords a basis for the quantitative estimation of the
response to light. Two responses to light stimuli have a direct effect
upon the amount of transpiration. Of the light energy absorbed by the
chloroplast, only 2.5 per cent is used in photosynthesis, while 95–98
per cent is converted into heat, and brings about marked increase in
transpiration. Furthermore, in normal turgid plants, the direct action
of light, as is well known, opens the stomata in the morning and closes
them at night.

[Illustration: Fig. 38. Ecads of _Allionia linearis_, showing position
of chloroplasts. The palisade shows apostrophe, the sponge epistrophe:
1, sun leaf (chresard, 2–5%, light, 1); 2, shade leaf (chresard, 11%;
light, .012); 3, shade leaf (chresard, 11%; light, .003). × 250.]

=175. Aeration and translocation.= The movements of gases and of
solutions through the tissues of the leaf are intimately connected with
photosynthesis, and hence with responses to light stimuli. Aeration
depends primarily upon the periodic opening of the stomata, for, while
the carbon dioxide and oxygen of the air are able to pass through
epidermal walls not highly cutinized, the amount obtainable in this
manner is altogether inadequate, if not negligible. The development of
sponge tissue or aerenchym is intimately connected with the stomata. The
position and amount of aerenchym and the relative extent of sponge cells
and air-spaces are in part determined by the number and position of the
breathing pores. The disposition of air spaces has much to do with the
arrangement of chloroplasts in both palisade and sponge tissues. Starch
formation is also dependent upon the presence of air spaces, but,
contrary to what would be expected, it seems to be independent of their
size, since sun leaves, which assimilate much more actively than shade
leaves, have the smallest air spaces. From this fact, it appears that
the rapidity of aeration depends very largely upon the rapidity with
which the gases are used. Translocation likewise affects the arrangement
of the chloroplasts and the formation of starch. According to
Haberlandt, it also plays the principal part in determining the form and
arrangement of the palisade cells. Chloroplasts are regularly absent at
those points of contact where the transfer of materials is made from
cell to cell, though this is not invariably true. Since air passages are
necessarily absent where cell walls touch, it is possible that this
disposition of the plastids is likewise due to the lack of aeration.
Translocation is directly connected with the appearance of starch. As
long as all the sugar made by the chloroplasts is transferred, no starch
appears, but when assimilation begins to exceed translocation, the
increasing concentration of the sugar solution results in the production
of starch grains. The latter is normally the case in all flowering
plants, with the exception of those that form sugar or oil, but no
starch. The constant action of translocation is practically
indispensable to starch formation, since an over-accumulation of
carbohydrates decreases assimilation, and finally inhibits it
altogether. In consequence, translocation occurs throughout the day and
night, and by this means the accumulated carbohydrates of one day are
largely or entirely removed before the next.

[Illustration: Fig. 39. Position of chloroplasts in aerial leaf (1) and
submerged leaf (2) of _Callitriche bifida_. × 250.]

=176. The measurement of responses to light.= Responses, such as the
periodic opening and closing of stomata, which are practically the same
for all leaves, are naturally not susceptible of measurement. This is
also true of the transpiration produced by light, but the difficulty in
this case is due to the impossibility of distinguishing between the
water loss due to light and that caused by humidity and other factors.
If it were possible to determine the amount of chlorophyll or glucose
produced, these could be used as satisfactory measures of response. As
it is, they can only be determined approximately by counting the
chloroplasts or starch grains. The arrangement of the chloroplasts can
not furnish the measure sought, since it does not lend itself to
quantitative methods, and since the relation to light intensity is too
inconstant. Hesselmann (_l. c._, 400) has determined the amount of
carbon dioxide respired, by means of a eudiometer, and has based
comparisons of sun and shade plants upon the results. As he points out,
however, light has no direct connection with respiration. Although the
latter increases necessarily with increased nutrition, the relation
between them is so obscure, and so far from exact, that the amount of
respiration can in no wise serve as a measure of the response to light.
As a result of the foregoing, it is clear that no functional response is
able to furnish a satisfactory measure of adjustment to light, though
one or two have perhaps sufficient value to warrant their use. Indeed,
structural adaptations offer a much better basis for the quantitative
determination of the effects of light stimuli, as will be shown later.

In attempting to use the number of chloroplasts or starch grains as a
measure of response, the study should be confined to the sun and shade
forms of the same species, or, in some cases, to the forms of closely
related species. The margin of error is so great and the connection with
light sufficiently remote that comparisons between unrelated forms or
species are almost wholly without value. It has already been stated that
starch is merely the surplus carbohydrate not removed by translocation;
the amount of starch, even if accurately determined, can furnish no real
clue to the amount of glucose manufactured. In like manner, the number
of chloroplasts can furnish little more than an approximation of the
amount of chlorophyll, unless size and color are taken into account. In
sun and shade ecads of the same species, the general functional
relations are essentially the same, and whatever differences appear may
properly be ascribed to different light intensities for the two
habitats. The actual counting of chloroplasts and starch grains is a
simple task. Pieces of the leaves of the two or more forms to be
compared are killed and imbedded in paraffin in the usual way. To save
time, the staining is done _in toto_. Methyl green is used for the
chloroplasts and a strong solution of iodine for the starch grains. When
counts are to be made of both, the leaves are first treated with iodin
and then stained with the methyl green. The thickness of the microtome
sections should be less than that of the palisade cells in order that
the chloroplasts may appear in profile, thus facilitating the counting.
The count is made for a segment 100 μ in width across the entire leaf.
Two segments in different parts of the section are counted, and the
result multiplied by five to give the number for a segment 1 millimeter
in width. Although sun and shade leaves regularly differ in size and
thickness, no correction is necessary for these. Size and thickness
stand in reciprocal relation to each other in ecads, and thickness is
largely an expression of the absorption of light, and hence of its
intensity. In the gravel, forest, and thicket ecads of _Galium boreale_,
counts of the chloroplasts gave the following results. The gravel form
(light 1) showed 3,500 plastids in the 1–mm. segment, the forest form
(light .03) possessed 1,350, and the thicket form (light .002), 1,000.
In these no attention was paid to the size and form of the plastids in
the different leaves, since the differences were inappreciable. When
this is not the case, both factors should be taken into account. Starch
grains are counted in exactly the same way. Indeed, if care is taken to
collect leaves of forms to be compared, at approximately the same time
on sunshiny days, a count of the chloroplasts is equivalent to a count
of the starch grains in the vast majority of cases. Measurements of the
size of starch grains can be made with accuracy only when the leaves are
killed in the field at the same time, preferably in the afternoon.
Counts of chloroplasts alone can be used as measures of response in
plants that produce sugar or oil, while either chloroplasts or starch
grains or both may be made the basis in starch-forming leaves.

Hesselmann (_l. c._, 379) has employed Sachs’s iodine test as a measure
of photosynthesis. This has the advantage of permitting macroscopic
examination, but the comparison of the stained leaves can give only a
very general idea of the relative photosynthetic activity of two or more
ecads. The iodine test is made as follows:[17] fresh leaves are placed
for a few minutes in boiling water, and then in 95 per cent alcohol for
2–5 minutes, in order to remove the chlorophyll and other soluble
substances. The leaves are placed in the iodine solution for ½–3 hours,
or until no further change in color takes place. The strength of the
solution is not clearly indicated by Sachs, who says: “I used an
alcoholic solution of iodin which is best made by dissolving a large
quantity of iodin in strong alcohol and adding to this sufficient
distilled water to give the liquid the color of dark beer.” This
solution may be approximated by dissolving ⅓ gram of iodin in 100 grams
of 30 per cent alcohol. The stained leaves are put in a white porcelain
dish filled with distilled water, and the dish placed in the strong
diffuse light of a window. The colored leaf stands out sharply against
the porcelain, and the degree of coloration, and hence of starch
content, is determined by the following table:

    1. bright yellow or leather yellow (no starch in the chlorenchym)

    2. blackish (very little starch in the chlorenchym)

    3. dull black (starch abundant)

    4. coal black (starch very abundant)

    5. black, with metallic luster (maximum starch-content)


                              _ADAPTATION_

=177. Influence of chloroplasts upon form and structure.= The beginning
of all modifications produced by light stimuli must be sought in the
chloroplast as the sensitized unit of the protoplasm. Hence, it seems a
truism to say that the number and arrangement of the chloroplasts
determine the form of the cell, the tissue, and the leaf, although it
has not yet been possible to demonstrate this connection conclusively by
means of experiment. In spite of the lack of experimental proof, this
principle is by far the best guide through the subject of adaptations to
light, and in the discussion that follows, it is the fundamental
hypothesis upon which all others rest. The three propositions upon which
this main hypothesis is grounded are: (1) that the number of
chloroplasts increases with the intensity of the light; (2) that in
shaded habitats chloroplasts arrange themselves so as to increase the
surface for receiving light; (3) that chloroplasts in sunny habitats
place themselves in such fashion as to decrease the surface, and
consequently the transpiration due to light. In these, there can be
little doubt concerning the facts of number and arrangement, since they
have been repeatedly verified. The purpose of epistrophe and apostrophe,
however, can not yet be stated with complete certainty.

The stimulus of sunlight and of diffuse light is the same in one
respect, namely, the chloroplasts respond by arranging themselves in
rows or lines on the cell wall. The direct consequence of this is to
polarize the cell, and its form changes from globoid to oblong. This
effect is felt more or less equally by both palisade and sponge cells,
but the disturbing influence of aeration has caused the polarity of the
cells to be much less conspicuous in the sponge than in the palisade
tissue. While the cells of both are typically polarized, however, they
assume very different positions with reference to incident light. This
position is directly dependent upon the arrangement of the plastids as
determined by the light intensity. In consequence, palisade cells stand
at right angles to the surface and parallel with the impinging rays; the
sponge cells, conversely, are parallel with the epidermis and at right
angles to the light ray. Some plants, especially monocotyledons, exhibit
little or no polarity in the chlorenchym. As a result the leaf does not
show a differentiation into sponge and palisade, and the leaves of sun
and shade ecads are essentially alike in form and structure. The form of
the leaf is largely determined by the chloroplasts acting through the
cells that contain them. A preponderance of sponge tissue produces an
extension of leaf in the direction determined by the arrangement of the
plastids and the shape of the sponge cells, viz., at right angles to the
light. Shade leaves are in consequence broader and thinner, and
sometimes larger, than sun leaves of the same species. A preponderance
of palisade likewise results in the extension of the leaf in the line of
the plastids and the palisade cells, i. e., in a direction parallel with
the incident ray. In accordance, sun leaves are thicker, narrower, and
often smaller than shade leaves.

=178. Form of leaves and stems.= In outline, shade leaves are more
nearly entire than sun leaves. This statement is readily verified by the
comparison of sun and shade ecads, though the rule is by no means
without exceptions. In the leaf prints shown in figures 14 and 15, the
modification of form is well shown in _Bursa_ and _Thalictrum_; in
_Capnoides_ the change is less evident, while in _Achilleia_ and
_Machaeranthera_ lobing is more pronounced in the shade form, a fact
which is, however, readily explained when other factors are taken into
account. The leaf prints cited serve as more satisfactory examples of
the increase of size in consequence of an increase in the surface of the
shade leaf, although the leaves printed were selected solely with
reference to thickness and size or outline. In all comparisons of this
kind, however, the relative size and vigor of the two plants must be
taken into account. This precaution is likewise necessary in the case of
thickness, which should always be considered in connection with amount
of surface. The relation between surface and thickness is shown by the
following species, in all of which the size of the leaf is greater in
the shade than in the sun. In _Capnoides aureum_, the thickness of the
shade leaf is ½ (6 : 12) that of the sun leaf; in _Galium boreale_ the
ratio is 5 : 12, and in _Allionia linearis_ it is 3 : 12. The ratio in
_Thalictrum sparsiflorum_ is 9 : 12, and in _Machaeranthera aspera_ 11 :
12. The thickness of sun and shade leaves of _Bursa bursa-pastoris_ is
as 14 : 12, but this anomaly is readily explained by the size of the
plants; the shade form is ten times larger than the sun form. Certain
species, e. g., _Erigeron speciosus_, _Potentilla bipinnatifida_, etc.,
show no change in thickness and but little modification in size or
outline. They furnish additional evidence of a fundamental principle in
adaptation, namely that the amount of structural response is profoundly
affected by the stability of the ancestral type.

The effect of diffuse light in causing stems to elongate, though known
for a long time, is still unexplained. The old explanation that the
plant stretches up to obtain more light seems to be based upon nothing
more than the coincidence that the light comes from the direction toward
which the stem grows. Later researches have shown that the stretching of
the stem is due to the excessive elongation of the parenchyma cells, but
the cause of the latter is far from apparent. It is generally assumed to
be due to a lack of the tonic action of sunlight, which brings about a
retardation of growth in sun plants. The evidence in favor of this view
is not conclusive, and it seems probable at least that the elongation of
the parenchyma cells takes place under conditions which favor the
mechanical stretching of the cell wall, but inhibit the proper growth of
the wall by intussusception. It is hardly necessary to state that the
reduced photosynthetic activity of shade plants favors such an
explanation. Whatever the cause, the advantage that results from the
elongation of the internodes is apparent. Leaves interfere less with the
illumination of those below them, and the leaves of the branches are
carried away from the stem in such a way as to give the plant the best
possible exposure for its aggregate leaf surface.

[Illustration: Fig. 40. Isophotophyll of _Allionia linearis_, showing
diphotic ecads: 1, light 1; 2, light .012; 3, light .003. × 130.]

[Illustration: Fig. 41. Isophotophyll of _Helianthus pumilus_, showing
isophotic ecad: 1, sun leaf; 2, shade leaf (light .012). × 130.]

=179. Modification of the epidermis.= The development of epidermal
chloroplasts in diffuse light is the only change which is due to the
direct effect of light. This does not often occur in the shade ecads of
sun species, but chloroplasts are regularly present in the epidermis of
woodland ferns and of submerged plants. The slight development of hairs
in sciophilous plants is an advantage, but it must be referred to the
factors that determine water loss. The significance of epidermal
papillae in increasing the absorption of light by shade plants has
already been discussed. The questions as to what factor has called forth
these papillae and what purpose they serve must still be regarded as
unsettled. The increased size of the epidermal cells, which is a fairly
constant feature of shade ecads, seems to be for the purpose of
increasing translocation and transpiration, and to bear no relation to
light. The extreme development of the cells of the epidermis in
_Streptopus_ and _Limnorchis_, which grow at the edge of mountain
brooks, has been plausibly explained by E. S. Clements as a contrivance
to increase water loss. The presence of a waxy coating, such as that
found upon the leaves of _Impatiens aurea_ and _I. pallida_, is clearly
to prevent the wetting of the leaf and the consequent stoppage of the
stomata. In regard to the latter, different observers have noted that
the number of the stomata is greater in sun than in shade leaves. This
holds generally for sun and shade species, but it is most clearly
indicated by different ecads of the same species. In _Scutellaria
brittonii_, the sun form possesses 100 stomata per square millimeter,
but in the shade these are reduced to 40 per square millimeter; the sun
leaf of _Allionia linearis_ has 180 stomata to the square millimeter,
the shade leaf 90. In the stable leaf of _Erigeron speciosus_, however,
the number of stomata is the same, 180 per square millimeter, for
sunlight and for diffuse light. The presence of the larger number of
stomata in the plant exposed to greater loss, which at first thought
seems startling, is readily explained by the more intense photosynthetic
activity in the sun. Since the absorption of gases is the primary
function of the stomata, and transpiration merely secondary, it is
evident that sun plants must have more stomata than shade plants. This
is further explained by the fact that the small air passages of sun
leaves necessitate frequent inlets, which are less necessary in shade
leaves with their larger air spaces. In shade plants, moreover, the
decrease in the number is compensated in some measure by the ability of
the epidermal cells to absorb gases directly from the air.

[Illustration: Fig. 42. Diphotophylls of _Quercus novimexicana_: 1, sun
leaf; 2, shade leaf of the same tree (light .06). × 130.]

=180. The differentiation of the chlorenchym.= The division of the
chlorenchym into two tissues, sponge and palisade, is the normal
consequence of the unequal illumination of the leaf surfaces. Exceptions
to this rule occur only in certain monocotyledons, in which the leaf
tissue consists of sponge-like cells throughout, and in those stable
species that retain more or less palisade in spite of their change to
diffuse light. The difference in the illumination of the two surfaces is
determined by the position of the leaf. Leaves that are erect or nearly
so usually have both sides about equally illuminated, and they may be
termed isophotic. Leaves that stand more or less at right angles to the
stem receive much more light upon the upper surface than upon the lower,
and may accordingly be termed diphotic. Certain dorsiventral leaves,
however, absorb practically as much light on the lower side as upon the
upper. This is true of sun leaves with a dense hairy covering, which
screens out the greater part of the light incident upon the upper
surface. It occurs also in xerophytes which grow in light-colored sands
and gravels that serve to reflect the sun’s rays upon the lower surface.
In deep shade, moreover, there is no essential difference in the
intensity of the light received by the two surfaces, and shade leaves
are often isophotic in consequence. From these examples it is evident
that isophotic and diphotic leaves occur in both sun and shade, and that
the intensity of the light is secondary to direction, in so far as the
modification of the leaf is concerned.

The essential connection of sponge tissue with diffuse light is
conclusively shown by the behavior of shade ecads, but further evidence
of great value is furnished by diphotic leaves, and those with hairy
coverings. The sponge tissue, which in the shade leaf is due to the
diffuse light of the habitat, is produced in the hairy leaf as a
consequence of the absorption and diffraction of the light by the
covering. In ordinary diphotic leaves, the absorption of light in the
palisade reduces the intensity to such a degree that the cells of the
lower half of the leaf are in diffuse light, and are in consequence
modified to form sponge tissue. The sponge tissue of the diphotic leaf
is just as clearly an adaptation to diffuse light as it is in those
plants where the whole chlorenchym is in the shade of other plants or of
a covering of hairs. As is indicated later, all these relations permit
of ready confirmation by experiment, either by changing the position of
the leaf or by modifying the intensity or direction of the light.

[Illustration: Fig. 43. A plastic species, _Mertensia polyphylla_,
showing the effect of water upon the sponge: 1, chresard 25%; 2,
chresard 12%. × 130.]

The preceding discussion makes it fairly clear that sponge tissue is
developed primarily to increase the light-absorbing surface. Because of
its direct connection with photosynthesis, the sponge tissue is the
especial organ of aeration, also, and since it shows a high development
of air spaces for this purpose, it is inevitably concerned in
transpiration. It seems to be partly a coincidence, however, that the
sponge is found next to the lower surface upon which the stomata are
most numerous. This is indicated by artificial ecads of _Ranunculus
sceleratus_, in which sponge tissue is unusually developed, although the
stomata are much more numerous upon the upper surface. Palisade tissue
is apparently developed primarily as a protection against water loss,
particularly that due to the absorption of light by the chloroplast. The
small size of the intercellular passages between palisade cells likewise
aids in decreasing transpiration. The fact that leaves with much
palisade tissue transpire twice as much as shade leaves is hardly an
objection to this view, as Hesselmann (_l. c._, 442) would think. It is
readily explained by the intense photosynthesis of sun plants, which
makes necessary an increase, usually a doubling, in the number of
stomata, in consequence of which the transpiration is increased.

[Illustration: Fig. 44. A stable species, _Erigeron speciosus_: 1, sun
leaf; 2, shade leaf (light .03). × 130.]

[Illustration: Fig. 45. Spongophyll of _Gyrostachys stricta_ (light 1).
× 130.]

=181. Types of leaves.= Isophotic leaves are equally illuminated and
possess more or less uniform chlorenchym. Diphotic leaves are unequally
illuminated, and exhibit a differentiation into palisade and sponge
tissues. They may be distinguished as isophotophylls and diphotophylls
respectively.[18] Isophotic leaves fall into three types based upon the
intensity of the light. The staurophyll, or palisade leaf, is a sun type
in which the equal illumination is due to the upright position or to the
reflection from a light soil, and in which the chlorenchym consists
wholly of rows of palisade cells. The diplophyll is a special form of
this type in which the intense light does not penetrate to the middle of
the leaf, thus resulting in a central sponge tissue, or water-storage
tissue. The spongophyll, or sponge leaf, is regularly a shade type; the
chlorenchym consists of sponge cells alone. For the present at least it
is also necessary to refer to this group those monocotyledons which grow
in the sun but contain no palisade tissue. Diphotic leaves always
contain both palisade and sponge, though the ratio between them varies
considerably. Diphotophylls are characteristic of sunny mesophytic
habitats. They are frequent in xerophytic habitats as well as in
woodlands where the light is not too diffuse. In the case of stable
species, this type of structure sometimes persists in the diffuse light
of coniferous forests. Floating leaves, in which the light is almost
completely cut off from the lower surface, are also members of this
group. Submerged leaves, on the other hand, are spongophylls.

=182. Heliophytes and sciophytes.= The great majority of sun plants
possess diphotophylls. This type is represented by _Pedicularis procera_
(fig. 32). Plants with isophotophylls are found chiefly in xerophytic
places, though erect leaves of this type occur in most sunny habitats.
The staurophyll, in which the protection is due to the extreme
development of palisade tissue, is illustrated by _Allionia linearis_
(fig. 40) and _Bahia dissecta_ (fig. 33). The diplophyll, which is
characterized by a central band of sponge tissue or storage cells, is
found in _Mertensia linearis_ (fig. 34). The form of the spongophyll
that is found in certain monocotyledons is shown by _Gyrostachys
stricta_ (fig. 45). The spongophyll (fig. 38:3, 39:2) is frequent among
plants of deep shade, but as the leaf sections of _Allionia_ (figs. 38,
40) and _Quercus_ (fig. 42) show, the diphotophyll is the rule in shade
ecads.


                         EXPERIMENTAL EVOLUTION

=183. Scope.= The primary task of experimental evolution is the detailed
study, under measured conditions, of the origin of new forms in nature.
As a department of botanical research that is as yet unformed, it has
little concern with the host of hypotheses and theories which rest
merely upon general observation and conjecture. A few of these
constitute good working hypotheses or serve to indicate possible points
of attack, but the vast majority are worthless impedimenta which should
be thrown away at the start. It is the general practice to speak of
evolution as founded upon a solid basis of incontestible facts, but a
cursory examination of the evidence shows that it is drawn, almost
without exception, from observation alone, and has in consequence
suffered severely from interpretation. With the exception of De Vries’s
work on mutation, sustained and accurate investigation of the evolution
of plants has been lacking. As a result, botanical research has been
built high upon an insecure foundation, nearly every stone of which must
be carefully tested before it can be left permanently in place. In a
field so vast and important as evolution, experiment should far outrun
induction, and deduction should enter only when it can show the way to a
working hypothesis of real merit. The great value of De Vries’s study of
mutation as an example of the proper experimental study of evolution has
been seriously reduced by the fact that the “mutation theory” has
carried induction far beyond the warrant afforded by experiment. The
investigator who plans to make a serious study by experiment of the
origin of new plant forms should rest secure in the conviction that the
most rapid and certain progress can be made only by the accumulation of
a large number of unimpeachable facts, obtained by the most exact
methods of experimental study.

The general application of field experiment to evolution will render the
current methods of recognizing species quite useless. It will become
imperative to establish an experimental test for forms and species, and
to apply this test critically to every “new species.” Descriptive
botany, as practiced at present, will fall into disuse, as scientific
standards come to prevail, and in its place will appear a real science
of taxonomy. In the latter the criteria upon which species are based
will be obtained solely by experiment.

=184. Fundamental lines of inquiry.= There are two primary and sharply
defined fields of research in experimental evolution, namely, adaptation
in consequence of variation (and mutation), and hybridization. The
latter constitutes a particular field of inquiry, which is not
intimately connected with the problems of evolution in nature. In the
study of specific adaptation, two questions of profound importance
appear. One deals with the effects of ancestral fixity or plasticity in
determining the amount of modification produced by the habitat. These
are fundamental problems, and a solution of them can not be hoped for
until exact and trustworthy data have been provided by numerous
experimental researches. It thus becomes clear that the principal, if
not the sole task of experimental evolution for years to come is the
diligent prosecution of accurate and prolonged experiment in the
modification of plant forms. It seems inevitable that this will be
carried on along the lines that have already been indicated. Plants will
be grown in habitats of measured value, or in different intensities of
the same factor. The relation between stimulus and adjustment will form
the basis of careful quantitative study, and the final expression of
this relation in structural modifications will find an exact record in
drawings, photographs, exsiccati, and biometrical measures. The making
of an accurate and complete record of the whole course of each
experiment of this sort is an obligation that rests upon every
investigator. Studies in experimental evolution will prove
time-consuming beyond all other lines of botanical research, and the
work of one generation should appear in a record so perfect that it can
be used without doubt or hesitation as a basis for the studies of the
succeeding generation.

=185. Ancestral form and structure.= The significance of the fact that
some species have been found to remain unaltered structurally under
changes of habitats that produced striking modifications in others has
already been commented upon. It is hardly necessary to indicate the
important bearing which this has upon evolution. The very ability of a
plant to undergo modification, and hence to give rise to new forms,
depends upon the degree of fixity of the characters which it has
inherited. Stable plants are less susceptible of evolution than plastic
ones. The latter adapt themselves to new habitats with ease, and in each
produce a new form, which may serve as the starting point of a phylum.
There is at present no clue whatever as to what calls forth this
essential difference in behavior. This is not surprising in view of the
fact that there have been no comparative experimental studies of stable
and plastic species. Until these have been made, it is impossible to do
more than to formulate a working hypothesis as to the effect of
stability, and an explanation of the forces which cause or control it is
altogether out of the question.

=186. Variation and mutation.= New forms of plants are known to arise by
three methods, viz., variation, mutation, adaptation. The evidence in
support of these is almost wholly observational, and consequently more
or less inexact, but for each there exist a few accurate experiments
which are conclusive. Origin by variation and subsequent selection is
the essence of the Darwinian theory of the origin of species. According
to this the appearance of a new form is due to the accumulation, and
selection, through a long period, of minute differences which prove
advantageous to the plant in its competition with others in nature, or
are desirable under cultivation. Slight variations appear
indiscriminately in every species. Their cause is not known, but since
they are found even in the most uniform habitats, it is impossible to
find any direct connection between them and the physical factors. In the
case of origin by mutation, the new form appears suddenly, with definite
characteristics fully developed. Selection, in the usual sense of the
term, does not enter into mutation at all, though the persistence of the
new form is still to be determined by competition. Mutations are known
at present for only a few species, and their actual appearance has been
studied in a very few cases. Like variations, they are indiscriminate in
character. The chief difference between them is apparently one of
degree. Indeed, mutation lends itself readily to the hypothesis that it
is simply the sudden appearance of latent variations which have
accumulated within the plant. De Vries regards constancy as an essential
feature of mutation, but the evidence from the mutants of _Onagra_ is
not convincing. Indeed, while there can be no question of the occurrence
of mutation in plants, a fact known for many years, the facts so far
brought forward in support of the “mutation theory” fall far short of
proving “the lack of significance of individual variability, and the
high value of mutability for the origin of species.”[19] Mutations do
not show any direct connection with the habitat, but their sudden
appearance suggests that they may be latent or delayed responses to the
ordinary stimuli. Origin by adaptation is the immediate consequence of
the stimuli exerted by the physical factors of a habitat. This fact
distinguishes it from origin by variation, or by mutation. The new form
may appear suddenly, often in a single generation, or gradually, but in
either case it is the result of adaptation that is necessarily
advantageous, because it is the result of adjustment to controlling
physical factors. Origin by adaptation is perhaps only a special kind of
origin by variation, but this might be said with equal truth of
mutation. New forms resulting from adaptation are like those produced
from mutation, in that they appear suddenly as a rule and without the
agency of selection. They are essentially different, inasmuch as their
cause may be found at once in the habitat, and since a reversal of
stimuli produces, in many cases at least, a reversion in form and
structure to the ancestral type.

A valid distinction between forms or species upon the basis of constancy
is impracticable at the present time. It is doubtful that such a
distinction can ever be made in anything like an absolute sense, since
all degrees of fluctuation may be observed between constancy and
inconstancy. In all events, it is gratuitous to make constancy the
essential criterion in the present state of our knowledge. So little is
certainly known of it that it is equally unscientific to affirm or to
deny its value, and even a tentative statement can not be ventured until
a vast amount of evidence has been obtained from experiment.
Accordingly, there is absolutely no warrant, other than tradition, for
limiting the term species to a constant group. In the evolutionary
sense, a species is the aggregate ancestral group and the new forms
which have sprung from it by variation, mutation, or adaptation. It
should not be regarded as an isolated unit for purposes of descriptive
botany; indeed, its use in this connection is purely secondary. It is
properly the unit to be used in indicating the primary relationships
which are the result of evolution.

On the basis of their actual behavior in the production of new forms,
species may be distinguished as variable, mutable, or adaptable. The new
form which results from variation is a _variant_; the product of
mutation is a _mutant_, and that of adaptation, an _ecad_. The following
examples serve to illustrate these distinctions. _Machaeranthera
canescens_, judging from the numerous minute intergrades between its
many forms, is a variable species, i. e., one in which forms are arising
by the gradual selection of small variations. It apparently comprises a
large number of variants, _M. canescens aspera_, _superba_, _ramosa_,
_viscosa_, etc. _Onagra lamarckiana_ is a mutable species: it comprises
many mutants, e. g., _Onagra lamarckiana gigas_, _O. l. nanella_, _O. l.
lata_, etc. _Galium boreale_ is an adaptable species: it possesses one
distinct ecad, _Galium boreale hylocolum_, which is the shade form of
the species.

=187. Methods.= The best of all experiments in evolution are those that
are constantly being made in nature. Such experiments are readily
discovered and studied in the case of origin by adaptation; variants
present much greater difficulties, while mutants are very rare under
natural conditions. The method which makes use of these experiments may
be termed the _method of natural experiment_. The number of ecads which
appear naturally in vegetation is limited, however, and it is
consequently very desirable to produce them artificially, by the _method
of habitat culture_. This method, while involving more labor than the
preceding, yields results that are equally conclusive, and permits the
study of practically every species. The _method of control culture_,
which is carried on in the planthouse, naturally does not possess the
fundamental value of the field methods. It is an invaluable aid to the
latter, however, since it permits the physical factors to be readily
modified and controlled. All these methods are based on the
indispensable use of instruments for the measurement of physical
factors.


                      METHOD OF NATURAL EXPERIMENT

=188. Selection of species.= Species that are producing variants or
ecads are found everywhere in nature; those which give rise to mutants
seem, however, to be extremely rare. Consequently, mutants can not be
counted upon for experimental work, and their study scarcely needs to
be considered. When a mutant is discovered by some fortunate chance,
the mutable species from which it has sprung, and related species as
well, should be subjected to the most critical surveillance, in the
hope that new mutants will occur or the original one reappear. On
account of the suddenness with which they appear, mutants do not lend
themselves readily to natural experiment, and after they have once
been discovered, inquiry into the causes and course of mutation is
practicable only by means of habitat and control cultures. Among
variable species, those are most promising that show a wide range of
variation and are found in abundance over extensive areas. A species
which occurs in widely separated, or more or less isolated areas,
furnishes especially favorable material for investigation, since
distance or physical barriers partly eliminate the leveling due to
constant cross-fertilization. The individuals or groups which show
appreciable departure from the type are marked and observed critically
from year to year. The direction of the variation and the rapidity
with which small changes are accumulated can best be determined by
biometrical methods. Representative individuals of the species and
each of its variants should likewise be selected from year to year.
After being photographed, these are preserved as exsiccati, and with
the photographs constitute a complete graphic record of the course of
variation. When the latter is made evident in structural feature also,
histological slides are an invaluable part of the record.

Polydemic species are by far the best and most frequent of all natural
experiments. In addition to plants that are strictly polydemic, i. e.,
grow in two or more distinct habitats, there are a large number which
occur in physically different parts of the same habitat. The recognition
of polydemics is the simplest of tasks. As a rule, it requires merely a
careful examination of contiguous formations in order to ascertain the
species common to two or more of them. The latter are naturally most
abundant along the ecotones between the habitats, and, as a result,
transition areas and mixed formations are almost inexhaustible sources
of ecads. Many adaptable species are found throughout several
formations, however, and such are experiments of the greatest possible
value. Not infrequently species of the manuals are seen to be ecads, in
spite of their systematic treatment, and to constitute natural
experiments that can be readily followed. Finally, it must be kept in
mind that some polydemics are stable, and do not give rise to ecads by
structural adaptation. They not only constitute extremely interesting
experiments in themselves, but they should also be very carefully
followed year by year, since it seems probable that the responses are
merely latent, and that they will appear suddenly in the form of
mutants. In natural experiments it is sometimes difficult to distinguish
which form is the ecad and which the original form of the species. As a
rule, however, this point can be determined by the relative abundance
and the distribution, but in cases of serious doubt, it is necessary to
appeal to experimental cultures.

Although habitats differ more or less with respect to all their factors,
the study of polydemics needs to take into account only the direct
factors, water-content, humidity, and light. Humidity as a highly
variable factor plays a secondary part, and in consequence the search
for ecads may be entirely confined to those habitats that show efficient
differences in the amount of water-content or of light. Temperature,
wind, etc., do not produce ecads, and may be ignored, except in so far
as they affect the direct factors. Complexes of factors, such as
altitude, slope, and exposure, are likewise effective only through the
action of the component simple factors upon water and light. The
influence of biotic factors is so remote as to be negligible, especially
in view of the fact that ecads are necessarily favorable adaptations,
and are in consequence little subject to selective agencies. The
essential test of a habitat is the production of a distinguishable ecad,
but a knowledge of the water-content and light values of the habitats
under examination is a material aid, since a minute search of each
formation is necessary to reveal all the ecads. It is evident that
habitats or areas that do not show efficient differences of water or
light will contain no ecads of their common species, and also that
extreme differences in the amount of either of these two factors will
preclude origin by adaptation to a large degree, on account of the need
for profound readjustment. The general rule followed by most polydemics
is that sun species will give rise to shade forms, and vice versa, and
that xerophytes will produce forms of hydrophytic tendency, or the
converse, when the areas concerned are not too remote, and the water or
light differences are efficient, but not inhibitive. Some species are
capable of developing naturally two series of ecads, one in response to
light, the other to water-content, but they, unfortunately, have been
found to be rare. Greatly diversified regions, such as the Rocky
mountains, in which alternation is a peculiarly striking feature of the
vegetation, are especially favorable to the production of ecads, and
hence for the study of natural experiments in origin by adaptation.

=189. Determination of factors.= For the critical investigation of the
origin of new forms, an exact knowledge of the factors of the habitat,
both physical and biotic, is imperative. In the case of variable
species, these factors determine what variations are of advantage, and
thereby the direction in which the species can develop. They are the
agents of selection. With mutants, the factors of the habitat are
apparently neither causative nor selective, though it seems probable
that further study of mutants will show an essential connection between
mutant and factor. In any event, the persistence of a mutant in nature,
and its corresponding ability to initiate new lines of development, is
as much dependent upon the selection exerted by physical and biotic
factors as is the origin of variants. Physical factors are causative
agents in the production of ecads, as has been shown at length
elsewhere. The form and structure of the ecad are the ultimate responses
to the stimuli of light or water-content, and the quantitative
determination of the latter is accordingly of the most fundamental
importance. The measurement of factors has been treated so fully in the
preceding chapters that it is only necessary to point out that the
thorough investigation of habitats by instruments is as indispensable
for the study of experimental evolution as for that of the development
and structure of the formation. Furthermore, it is evident that a
knowledge of physical factors is as imperative for habitat and control
cultures as for the method of natural experiment. In the latter,
however, the biotic factors demand unusual attention, since pollination,
isolation, etc., are often decisive factors in origin by variation and
in the persistence of mutants.

Measurements of adjustment, i. e., functional response to the direct
factor concerned, are extremely valuable, but not altogether
indispensable to research in experimental evolution. This is due to the
fact that a knowledge of adjustment is important in tracing the origin
of new forms only when adjustment is followed by adaptation, and in all
such cases the ratio between the two processes seems to be more or less
constant. In the present rudimentary development of the subject,
however, it is very desirable to make use of all methods of measuring
functional responses to water and light that are practicable in the
field. Certain methods that are difficult of application in nature may
be used to advantage in control cultures, and the results thus secured
can be used to interpret those obtained from natural experiments and
field cultures.

=190. Method of record.= As suggested elsewhere, there are four
important kinds of records, which should be made for natural
experiments, and likewise for habitat and control cultures. These are
exsiccati, photographs, biometrical formulae and curves, and
histological sections. These serve not merely as records of what has
taken place, but they also make it possible to trace the course of
evolution through a long period with an accuracy otherwise impossible,
and even to foreshadow the changes which will occur in the future. The
possibility of doing this depends primarily upon the completeness of the
record, and for this reason the four methods indicated should be used
conjointly. In the case of ecads and mutants, exsiccati, photographs,
and sections are the most valuable, and in the majority of cases are
sufficient, since both ecads and mutants bear a more distinctive impress
than variants do. On the other hand, since variations are more minute,
the determination of the mean and extreme of variation by biometrical
methods is almost a prerequisite to the use of the other three methods,
which must necessarily be applied to representative individuals.

Exsiccati and photographs are made in the usual way for plants, but it
is an advantage to photograph each ancestral form alongside of its
proper ecads, mutants, or variants, in addition to making detail
pictures of each form and of the organs which show modification. In the
collection of material for histological sections, which deal primarily
with the leaf or with stems in the case of plants with reduced leaves, a
few simple precautions have been found necessary. Whenever possible,
material should be killed where it is collected, since in this way the
chloroplasts are fixed in their normal position. In case leaves that can
not be replaced easily have become wilted, an immersion of 5–6 hours in
water will make it possible to kill them without shrinkage. In selecting
leaves, great pains must be taken to collect only mature leaves. When
the plants have a basal rosette, or distinct radical leaves, mature
leaves are taken from both stem and base. In all cases where the two
surfaces of the leaf can not be readily distinguished, the upper one is
clearly marked.


                       METHOD OF HABITAT CULTURES

=191. Scope and advantages.= By means of experiments actually made in
the field, practically every species that is capable of modification can
be made to produce new forms, the origin of which can be traced in the
manner already indicated. Field experiments of this sort are especially
favorable to the production of ecads from adaptable species. No attempt
has yet been made to apply it to mutable or variable species, but its
ultimate application to these does not seem at all impossible. The chief
advantage of the method of habitat cultures is seen in the great range
of choice in selecting the plant for experiment, and the habitat or area
in which the experiment is carried out. A polydemic species which
already has one or more ecads can be extended to a number of different
habitats of known value, and a complete series of ecads obtained, based
either upon water-content, or light, or upon both. On the other hand, an
endemic species, or one brought from a remote flora, can be placed in as
many habitats as desired, and the appearance of ecads followed in each.
Frequently, results of much value are obtained in a diversified habitat
by growing its most plastic species in those areas which show the
greatest differences in water-content or light intensity. Habitat
cultures give results which are practically as perfect as those obtained
from natural experiments, since the course of adaptation in no wise
depends upon whether the agent by which the seed or propagule is carried
into the new habitat is natural or artificial. Cultures of this kind
further possess the distinct advantage of permitting more or less
modification of the physical factors themselves. However, when it is
desirable to have the factors under as complete control as possible, it
is necessary to use the method of control cultures in the planthouse.

=192. Methods.= All field experiments in evolution are based upon a
change of habitat. The latter is accomplished by the modification of the
habitat itself, or by the transfer of the species to one or more
different habitats, or to different areas of the same habitat. In both
cases the choice of habitats is made upon the basis of efficient
differences of water-content or light. Saline situations do not
constitute an exception, since the chresard is really the effective
stimulus. Cultures at different altitudes, which afford striking
results, appear to concern several factors, but in the final analysis,
water-content and humidity are alone found to be really formative.
Cultures may furthermore be distinguished as simple or reciprocal.
Simple cultures are those in which a species is transferred to one or
more habitats, or in which a habitat is modified in one or more ways.
Reciprocal cultures are possible only with polydemic species, or with
endemics after ecads have been produced by experiment. Modification or
transfer is made in the usual way, but reciprocally, i. e., the original
form is transferred to the habitat of the ecad, and the latter to the
habitat of the former; or the shade in which some individuals of the
ecad are growing may be destroyed, and at the same time individuals of
the type may be shaded. Both transfer and modification may be applied to
the same species, but since the same measured change of factor can be
obtained in either way, the use of both is undesirable, with the
exception of the rare cases where they serve as checks upon each other.
The transfer of a seed or plant is so much simpler and more convenient
that this method is the one regularly used. It sometimes happens,
however, that a change of water-content or light intensity is readily
and conveniently made, and is desirable for other reasons.

It is evident that both transfer and modification require that the
factor records of the various habitats or areas be as full as possible,
at least so far as water-content, humidity, and light are concerned. In
the case of the areas that are to be modified, these factors are
determined before the change is made. Afterward they are read from time
to time during the growing season, and are also checked by readings made
near at hand in the unmodified formation. The readings made in the
beginning should correspond closely to the check readings, but in case
of disagreement the latter are to be taken as conclusive.

=193. Transfer.= After the species to be used for experiment has been
chosen, the various habitats or areas selected, and the direct factors
measured by instruments, the actual transfer of the individuals is made
by means of seeds, preferably in autumn, though the results are
practically the same if seeds are kept over the winter and planted at
the opening of spring. The natural method is to scatter the seeds in the
place selected, as though they had been carried by the usual agents of
migration. The mortality is usually great in such case, however, and the
chances of success are increased by actually planting the seeds. This is
the method which has been used in making cultures of species of the
European Alps on the summit of Mount Garfield in the Rocky mountains.
The number of seeds used is recorded in order to obtain some estimate of
germination and competition. While the use of the seed or disseminule
possesses the great advantage of making the experiment essentially a
natural one, the transfer of rosettes, seedlings, or young plants makes
the results more certain, and consequently saves time, even though the
actual transfer is somewhat more difficult. It is hardly necessary to
point out that the removal of the plant should be made with the greatest
care. The best success is obtained by making the transfer on cloudy or
rainy days, and when shade plants are to be placed in sunny situations,
they should be transplanted late in the afternoon. When the task of
carrying them is not too great, it is a distinct advantage to move a
number of individuals in the same block of earth. The transfer of mature
plants is inadvisable, except for those perennials which can not readily
be secured in an early stage. This naturally does not apply to woody
plants, evergreen herbs, mosses and lichens; the last two may be
transferred at any time with satisfactory results. Each culture is
carefully marked with stakes, and definitely located by means of
landmarks.

[Illustration: Fig. 46. Series for producing hydrophytic forms under
control: 1, amphibious; 2, floating; 3, competition; 4, submerged.]

Reciprocal transfers may be made by means of seed or plant. Since the
experiment is a complex one, all the care possible should be taken to
make sure that the plants become established in the reciprocal
situations, and consequently, it is often advisable to transfer both
seeds and plants. Reciprocal transfer is of paramount value in solving
the problem which bog plants present. A slight modification of the
method makes it possible to obtain experimental evidence of the
polyphyletic origin of species in consequence of adaptation. In an
experiment mentioned elsewhere, the transfer of _Kuhnistera purpurea_ to
the area occupied by _K. candida_, and vice versa, is designed to show
whether one has been derived from the other. If the two species are
moved into an area which contains more water than that usually occupied
by _K. purpurea_, and less water than is found where _K. candida_
habitually grows, the resulting modifications will throw much light upon
the origin of polyphyletic species. In this connection, it hardly needs
to be pointed out that this simple transfer of a species to several
separated areas of a new habitat may often furnish complete proof that a
new form may arise at different times, and at different places.

[Illustration: Fig. 47. Control ecad of _Ranunculus sceleratus_, holard
10% (50 cc.).]

=194. Modification of the habitat.= Efficient changes in the habitat are
brought about by increasing or decreasing the water-content, or by
varying the light intensity between sunshine and the diffuse light of
deep forests. Humidity can not well be regulated except in so far as it
is connected with water-content. Since its effects merge with those of
the latter, its modification is unnecessary. An increase in
water-content is readily brought about by irrigation. A stream may be
dammed and its water allowed to spread over the area to be studied, or
the water may be carried to the proper place by deflecting the stream or
by digging a canal. The construction of earth reservoirs makes it
possible to obtain almost any per cent of soil water by varying the size
of the reservoir or the height of the wall or bank. Near a base station,
such as Minnehaha, where there is a simple system of water-works, the
experimental area may be watered whenever desirable by means of a hose.
Water-content may be readily decreased by drainage, or by the deflection
of a stream. When such means are not available, as in the case of
extensive marshes, hummocks may be used or constructed, and the soil
blocks containing plants placed upon them. By the use of sand or gravel,
the water-content of mesophytic areas can be reduced in a similar
manner, or by surrounding the plant _in situ_ with either of these soils
which hold little water. In meadows, especially, the addition of a large
quantity of alkaline salts decreases the amount of available water,
while the holard may be reduced by denuding the soil about the plants
concerned.

[Illustration: Fig. 48. Control ecad of _Ranunculus sceleratus_, holard
40% (200 cc.).]

In sunny habitats, the light intensity is most easily reduced by means
of cloth awnings, which can be put in place conveniently. It is not a
difficult matter to produce effective shade by using shrubs or small
trees for this purpose. This plan is especially advantageous in habitats
too remote to make frequent visits feasible. When a shrub or tree is
used, the experiment necessarily requires a longer time, though this
disadvantage is partly compensated by the fact that the shelter requires
practically no attention after the shrub is once established. Forest
plantations furnish excellent examples of this kind of experiment. On
the other hand, clearings afford the only examples of habitats modified
in such manner as to increase the light. In nature, the diffuse light in
which shade plants grow is due to the presence of tall plants, chiefly
shrubs and trees, and an increase in the light intensity is possible
only through the thinning-out or removal of the plant screen. This is a
task of considerable magnitude in forests, but it can be readily
accomplished in thickets and at the edges of woodlands. It is quite
practicable to establish a series of awnings or clearings of various
light values, but the labor required is hardly worth while when it is
recalled that the method of transfer makes it possible to take advantage
of the various intensities already found in nature.


                       METHOD OF CONTROL CULTURES

=195. Scope and procedure.= Control experiments are necessarily carried
on in the planthouse, since factors can be controlled in the field only
with great difficulty. Their greatest value is in connection with
experiments that are being carried on in the habitat, but they also
constitute an invaluable means of independent research, since it is not
at all difficult to approximate the conditions of a habitat, especially
with reference to water-content and light. The essential feature of the
method is that the less important factors are equalized as far as
possible, while the direct factors, water-content and light, are under
the complete control of the investigator. By the equalization of
humidity and temperature is meant experimentation in which all the
plants of each experiment are subjected to the same amounts of these
factors. It is a matter of no importance whatever whether the humidity
and temperature are constant or variable. In the case of soil, which is
not a variable, it naturally happens that the plants are placed once for
all in the same soil mixture. Batteries consisting of thermograph and
psychrograph have been kept in the different control houses, but
although used at first to give some idea of the hourly and daily
fluctuations of temperature and humidity, they have slight bearing upon
the evolution of new forms under control. For use in connection with
supplementary experiments in adjustment and adaptation, the batteries
have proved to be indispensable. Control experiments are regularly made
in series which are planned with reference to as many modifications as
the efficient difference of the factor and the plasticity of the species
concerned permit.

=196. Water-content series.= An account of the experiments which have
been carried on for four generations with _Ranunculus sceleratus_ will
serve to show the application of culture methods to the origin of new
forms in response to varying water-content. This species was chosen
because it grows readily in the planthouse, is plastic, and, since it is
naturally amphibious, permits of much modification in both directions.
The smallest amount of water per day under which the seedlings would
grow was found to be 25 cc. This was taken as one extreme for the
series, and deep water in which the plant could be submerged as the
other. An arbitrary series was tentatively made as follows: 25 cc., 50
cc., 100 c., 150 cc., 200 cc., mud, shallow water, and deep water.
Further study justified these divisions, since the first six gave
efficient differences in water-content, and the resulting forms all
showed differences of structure as well as of growth and form. Seedlings
of the same age, and as nearly alike as possible, were transplanted to
large pots of which there were four for each of the first six; they were
placed in half-barrels for mud and floating forms, and in a barrel for
submerged forms. After a few days, when they had become well
established, the plants in the pots were watered in the amounts
indicated, as often as was necessary to keep the most xerophytic form
alive; the soil for the mud form was kept covered with a thin film of
water; the leaves of the form in shallow water were kept floating on the
surface, and those of the last form submerged just below the surface.
The water in which the submerged form grew was aerated by means of a
spigot near the bottom of the barrel. From time to time water-content
determinations were made of the soil in the pots until it was definitely
ascertained that the holard was practically constant. The nine new forms
obtained by adaptation showed striking differences in vigor and growth,
as may be seen from the figures. In all cases, these were accompanied by
distinct and often striking differences in the number and position of
the stomata, the amount of sponge and palisade tissues, and the
development of air passages. Photographs were made of a typical plant of
each form, and the different leaf structures were preserved in permanent
mounts. The xerophytic and the submerged form were unable to produce
flowers, and it was necessary to develop them anew in each generation.
The other forms fruited abundantly, and the succeeding generations of
each form were produced from plants which had grown the year before in
the same conditions. In addition to the development of a series of new
water-content forms, this experiment was begun in the hope of
determining whether the modifications of a plastic species tend to
become fixed if each new form is grown constantly under the same
conditions. A period of four years is too short, however, to throw much
light upon this problem.

[Illustration: Fig. 49. Floating form of _Ranunculus sceleratus_ grown
under control.]

_Helianthus annuus_ has been used for other series of experiments, in
which alkaline salts or different soils are employed to vary the
water-content. These are more complex and hence are not as satisfactory
as the series described above, but they are valuable for the light they
throw upon the behavior of plants in similar conditions in nature. In
the case of soil, however, the adaptation may be referred to
water-content alone, if thoroughly leached sands and gravels are used,
so that the difference is solely one of water-retaining power.

=197. Light series.= Cloth tents have been found the most satisfactory
means of obtaining different light intensities in the planthouse. The
cloth permits the air to circulate to a considerable degree, and in
consequence the equalization of humidity and temperature is much more
complete than in the glass houses first employed. The cloth tents, or
shade tents as they are called, are cubical, each dimension being 1
meter. The series which has been most used consists of three tents: the
first is made of cheesecloth and has a light value of .1; the second is
of thin muslin, and has a value of .04, while the third is made of dark
cambric and the light is reduced to .01. A more desirable series is one
with five tents, which have approximately the following light
intensities: .1, .05, .01, .007, .003. Plants grown in shade tents
should be repotted as often as they will permit in order to increase the
aeration of the soil. The amount of water given them must also be
decreased as the shade increases. Mesophytic species give the best
results in shade tents, xerophytes thrive less well, and amphibious
plants do not grow at all except in the brightest light. Excellent
results have been obtained with _Helianthus_, _Taraxacum_, _Gaura_, and
_Onagra_, while _Ranunculus sceleratus_ is unable to produce flowers and
seeds in a light intensity of .01.

A number of important supplementary experiments have been made in
connection with light tents. These do not result in the production of
new forms, but they throw much light upon it. Plants have been placed in
the shade tents so that certain leaves would be in the sun and others in
the shade. Young leaves have been fixed at various angles with the stem,
and they have been revolved 90° or 180° in order to change the relation
of their surfaces. Soils of different colors, e. g., loam and sand, have
been used to determine the effect of light reflected from their
surfaces. Shade tents make it possible to illuminate plants from the
top, bottom, or side, and to carry on a large number of fundamental
experiments in adjustment and adaptation.




                    CHAPTER IV. THE PLANT FORMATION
                  METHODS OF INVESTIGATION AND RECORD


=198. The need of exact methods.= The use of instruments in the study of
the habitat has made it evident that the loose methods of descriptive
ecology were altogether inadequate to the accurate investigation of the
formation. This feeling has been heightened by the recognition of the
fact that vegetation exhibits both development and structure, and is, in
consequence, open to exact methods of inquiry. In the search for
feasible methods, it was quickly seen that the quadrat, first[20] used
for determining the abundance of species, furnished the key to the
problem. Accordingly, the principle underlying it, viz., that of
intimate detailed study and record, was developed and extended in such a
way as to give rise to a number of methods of precision. These have been
applied in the field for several years with signal success, and they are
here described in the conviction that they constitute a satisfactory
system, if not, indeed, the only one for the exact study of formations.

There has been a growing appreciation of the fact that the superficial
methods of descriptive ecology made it impossible to build upon such a
foundation, and they, indeed, were making actual progress in the field
of ecology more and more difficult. Ecologists have now begun to see
clearly that precise methods are as indispensable in the habitat as they
are to the study of the structure and modification of the plant. For
some reason, however, they have been slow to perceive that accuracy in
the investigation of the cause, the habitat, is a fruitless task unless
it be followed by corresponding exactness in the study of the effect,
the formation. After having urged the fundamental necessity of
instrumental methods, for six or seven years, both in season and out of
season, the writer does not feel called upon to further plead the cause
of the quadrat. The final acceptance of the instrument was inevitable if
progress were to be made in the habitat, and it is just as obvious that
the quadrat must be accepted if the study of the habitat is to bear
fruit in the interpretation of the formation. The use of the quadrat
does not mean that the general methods of descriptive ecology are all to
be discarded, whether they have value or not. The statement that quadrat
methods are indispensable signifies merely that they must be used for
research work in the development and structure of vegetation. They are
not necessary in reconnaissance, nor do they displace general methods of
real value. The use of the latter in even a supplementary way will
gradually be discontinued, however, as fields become smaller by reason
of increase in the number of workers, and as the need for precise
methods becomes more universally felt.

The quadrat constitutes the initial concept from which all the methods
have grown. In itself, it has given rise to a variety of quadrats
applicable to the most fundamental problems of vegetation. From it have
come, on the one hand, the migration circle, and on the other, the
transect. The latter in turn has yielded the ecotone chart, and the
layer chart. All of these are based upon direct and detailed contact
with vegetation itself, and permit accurate recording of all the results
obtained.


                                QUADRATS

=199. Uses.= In its simplest form, the quadrat, as the name implies, is
merely a square area of varying size marked off in a formation for the
purpose of obtaining accurate information as to the number and grouping
of the plants present. As indicated above, it was first used for
determining the abundance of the various species of a formation. This
made it possible to ascertain the relative rank of the species of layers
and formations, and enabled one for the first time to gain some idea of
the minute structure of a bit of vegetation. The results were at once
applied to the task of establishing a numerical basis for abundance, and
of working out a new system of abundance to correspond. The quadrat
method was also used to determine the character of seasonal aspects, and
to yield a knowledge of the exact differences in diverse areas of the
same formation. Incidentally, the determinations of abundance were made
the basis of an actual census of certain alpine formations. This, while
it was extremely interesting to find that a square mile of alpine meadow
contained approximately 1,500,000,000 plants, was confessedly destitute
of ecological value. The most important applications of the quadrat idea
were made by Clements[21] in the chart, the permanent and the denuded
quadrats. The development of these was due to the fact that zones or
formations permit of comparison upon floristic as well as physical
grounds, and that a detailed record of their structure is necessary for
this purpose. Similar comparisons are necessary for the consocies,
zones, and patches of the same formation, and the quadrat becomes an
indispensable means for studying alternation and zonation. For the
investigation of invasion year by year, and especially for succession,
the method of permanent quadrats is imperative, and the denuded quadrat
an invaluable aid. Changes, which would otherwise be incompletely
observed and imperfectly recorded, are followed in the minutest detail
and recorded with perfect accuracy.

=200. Possible objections.= The use of the quadrat has led to the
criticism that it is needlessly detailed and thorough, and that, after
all, the space covered is but a minute part of the entire formation. The
first objection is one that has also been urged against the use of
instruments of precision in the habitat. It is always brought forward by
those who have not used instruments, and as witnesses they are of
necessity incompetent. No one who is familiar with the instrument or the
quadrat by actual practice has felt that the methods based upon them
were too thorough. In no case has the writer ever listed or mapped a
quadrat without discovering some new fact or relation, or clearing up an
old question. It can not be denied that quadrat methods require both
time and patience, but this is true of any kind of research work that is
at all worth while. Every ecologist, moreover, that has the interests of
his field at heart and deprecates the present slipshod work, will
appreciate the necessity of methods which seem like drudgery to the mere
dabbler.

The second objection, that the quadrat is at best but a small bit of the
area under investigation, seems at first to be a valid one. It can not
be gainsaid that the actual space studied is insignificant as compared
with the whole formation; still, it must be obvious that even a single
quadrat can add at least some facts of value, which can never be
obtained by the best of general methods. Furthermore, if the formation
be an actual and not an imaginary one, a single quadrat will be in some
measure representative. In the more homogeneous ones, it will have much
the same value that a type specimen bears to the species established
upon it. In formations which are less uniform, its value is
correspondingly reduced, so that in formations which show marked zones,
consocies, or patches, it becomes necessary to locate a quadrat in each.
In the matter of representation alone, the graphic method of the quadrat
map with its close-focus detail photograph, is far superior to anything
that can be obtained by the ordinary description and photograph.
Finally, the scientific study and recording of succession, and
particularly of competition, is an impossibility without the aid of the
permanent and denuded quadrat. The stoutest champion of the practice of
walking through a formation, and jotting down impressions, can not avoid
their use if he would attack these problems, and, once familiar with the
quadrat, his objections to the drudgery of thoroughness will soon
vanish.


                   _Kinds of Quadrats and Their Use_

=201. Size and kinds.= The unit size of quadrat is the meter, and when
the term is used without qualification, it refers to the meter quadrat.
To make them strictly comparable, and exactly divisible, unit quadrats
are always grouped in squares; thus a major quadrat is a square of four
units, and a perquadrat one of sixteen units, or four meters square.
Quadrats of greater size are necessary in woodland and forest, where the
rule, however, is that the woody plants alone are recorded for the whole
quadrat, the herbaceous growth being listed or mapped for but one or two
representative units. For special purposes, quadrats of 3, 5, 6, etc.,
meters may be used, but they are much less convenient. Quadrats are
further distinguished with respect to their use. A list quadrat is one
in which the plants are merely listed and the number of individuals of
each species indicated. Chart quadrats are those in which the area
concerned is accurately mapped on plotting paper. Both list and chart
quadrats are rendered permanent by careful labeling, so that their
changes can be followed from year to year. The greater value of the
chart causes practically all permanent quadrats to be of this type, and
for the same reason only permanent chart quadrats are converted into
denuded ones.

=202. Tapes and stakes.= The lines for marking out quadrats are made of
strong white tape, ⅝ inches wide. This is doubled and sewed firmly at
both edges. Under moderate stretching, the tape is carefully marked off
into decimeters, and eyelets 5 mm. in diameter are set in at each end
and at the marks. This can readily be done by any shoemaker at slight
expense. The usual lengths are one and two meters, as these are most
frequently used, and they can also be easily combined to make larger
quadrats. The tapes are slightly longer than one meter in order that the
distance between the end eyelets may be exact. The tapes of the larger
forest quadrats should be divided into lengths of one meter, as these
permit ready plotting and also make it possible to interpolate a meter
quadrat for the study of the undergrowth at any point. The intervals of
the tape are numbered from left to right, as conspicuously and clearly
as possible. For this a waterproof ink or paint is very desirable. For
holding the tapes in position, hatpins, nails, and meat-skewers have
been used with more or less satisfaction. The ideal stake, however, is
one which holds the tape close to the ground, and can be readily moved.
It is merely a stout wire, 3 mm. in diameter and 8 inches long, looped
at the top, sharpened at the tip, and with a small ring of solder 3
inches from the tip.

=203. Locating quadrats.= In staking a quadrat, the end tapes are
invariably placed so that the numbers read from left to right, and the
side tapes so that they read down. In mapping, a fifth tape is stretched
parallel to the top, and as each decimeter strip is marked, the outer
tape is shifted to delimit the new strip. Indeed, the side tapes can be
placed alone, and the plotting tapes moved down one at a time as the
mapping proceeds, but it is usually more satisfactory to locate the
quadrat exactly and to square it first, a task most easily done by
enclosing the whole quadrat, and then using a fifth tape. In the case of
list quadrats in open vegetation, the measuring strip is unnecessary,
but as a rule it facilitates counting, as well as mapping.

[Illustration: Fig. 50. Mapping a major quadrat on Mount Garfield at
3,600 m.]


                           _The List Quadrat_

=204. Description.= This, as the simplest form of quadrat, is employed
primarily to ascertain the abundance of species in a formation or during
a particular aspect of it. Since this can be obtained readily from the
chart, the list quadrat has fallen more and more into disuse, except
where it is desired to determine abundance alone, or to aid in deciding
whether a chart is really representative. The size depends almost wholly
upon the nature of the vegetation. When the number of trees is to be
determined, a quadrat of 10 or 50 meters is necessary. In ordinary
herbaceous formations, the usual size is 2 meters, while the meter
quadrat is used when the plants are especially small and crowded, as in
alpine meadows. The location of the quadrat is based upon the general
rule, but since its especial task is the determination of the greatest
variable in vegetation, viz., number, it is necessary to use more
quadrats, and to place them in areas which show the greatest differences
in the mixture of species. For example, it was found that a half dozen
list quadrats, when carefully located in the prairie formation, gave
results almost identical with those obtainable from a larger number.
With a little experience, the various degrees of mixture can be picked
out superficially, and the corresponding number of quadrats established.
If a single list quadrat is to be made for a formation or station, such
a time should be selected as will make it possible to cover the greatest
number of plants. Fortunately, this usually falls near the middle of the
summer, when the remains of spring plants are still in evidence, and the
autumn ones are sufficiently developed to be recognizable. In taking the
census of different aspects, the quadrat should be made as near the
middle of the period as is possible.

=205. Manner of use.= In listing a quadrat, i. e., counting the
individuals of each species, the plan followed is to list the smaller,
less conspicuous plants first, since they are apt to be tramped down. As
a rule, the outside tapes and the taller species afford sufficient
landmarks. When this is not the case, the measure tape is used, and the
individuals of all species are checked as they are found, while in the
first method one species, rarely two, is taken at a time. In cases of
peculiar difficulty, it may be permissible to pull or break plants as
they are counted, but ordinarily this can and should be avoided.
Clusters, and bunches of stems from the same root are counted as single
plants, and the number of stems indicated by an exponent. In the case of
bunch grasses, each bunch counts as one plant.

=206. Table of abundance.= The species are arranged in the final list in
the order of their numerical importance, and are divided into groups
which correspond to the different degrees of abundance. The latter are
arranged in two series, based upon the fact that association is by
groups or by individuals. The table of abundance, based upon a 2–meter
quadrat rather than upon the 5–meter one, by means of which the earlier
results were obtained, is as follows:

             Social exclusive, no other species of
               vascular plants present

             social inclusive,  above 100

             gr^1 gregarious^1     100–50 copious^1  cop^1

             gr^2 gregarious^2      50–25 copious^2  cop^2

             gr^3 gregarious^3      25–10 copious^3  cop^3

             sg   subgregarious      10–5 subcopious sc

             vg   vixgregarious       5–1 sparse     sp

It is obvious that the above outline is faulty inasmuch as it takes no
account of the height and width of the individuals. This is a serious
defect, and it constitutes one of the many reasons why the list quadrat
should be replaced by the chart quadrat. The prairie formation affords
an unusually striking illustration of this. A single quadrat may be
filled by ten plants of _Psoralea floribunda_, and at the same time
contain 22,000 plants of _Festuca octoflora_. Yet the former is
conspicuous and controlling, the latter plays an altogether
insignificant role. This difference is readily shown by comparing a
plant of each. The one is 3 × 3 feet, the other 3 × ¼ inch. Such figures
furnish a valuable check upon mere number, but make the brief, graphic
designation of abundance difficult. An attempt has been made to solve
this problem by roughly determining the space occupied by the plant, by
means of the formula, height (π_R_^2) × abundance. This would give
_Psoralea_ a value of 210, and _Festuca_ one of 1.6, which much more
nearly represents their real importance in the formation. Abundance or
numerical value is a floristic concept entirely, and has little place in
ecology unless checked in the way indicated. The whole problem,
ecologically, depends upon an intimate knowledge of competition, and its
solution in consequence is at present impossible.


                          _The Chart Quadrat_

=207. Description and use.= The detailed labor required in mapping makes
it advisable to use the meter quadrat. An additional reason of much
importance is furnished by the desirability of securing a detail
photograph of the quadrat. This is impossible with field cameras, which
should not exceed 6½ x 8½ inches, and are indeed most serviceable in the
4 × 5 size, if the area be larger. In open formations, the major quadrat
of 2 meters can be used if necessary, but this is very rarely the case.
Forest quadrats of ten meters square are easily charted, but detail
photographs can not be made of them. Larger quadrats are impracticable;
they can be counted but not mapped to advantage. The location of the
chart quadrat must be decided by the structure to be studied. Its
greatest service is in connection with zones and societies of the same
formation, which can be easily compared in the chart form. In fact, the
chart quadrat may well be regarded as the fundamental method for inquiry
into zonation and alternation. It is an important aid in delimiting
areas from the contiguous formations, and in determining the
relationships of mixed formations. It is also used to record the
character of the different aspects, but this is done more satisfactorily
by the permanent quadrat.

=208. The chart= used is a decimeter square, and the scale is
consequently 10 : 1. It is outlined on centimeter plotting paper, and
the centimeter squares are numbered at the edges to correspond to the
intervals of the quadrat, i. e., the top and bottom lines are numbered
from left to right, and the side lines from top to bottom. These
outlines are ruled in quantity and used as needed, or the forms can be
furnished by the printer. In practice, a special quadrat book the size
of the chart has been used. The need of a second book may be avoided by
outlining two charts on the plotting sheet, and filing the latter in the
field record book. In the few cases where 2–meter quadrats are
desirable, four charts are used, care being taken to label them so that
they can be combined whenever necessary. Ten-meter quadrats are recorded
on the decimeter chart also, each meter interval corresponding to a
centimeter, i. e., the scale is 100 : 1.

[Illustration: Fig. 51. Permanent chart quadrat, _Andosacile_,
_Carex-Campanulacoryphium_.]

[Illustration: Fig. 52. Chart of the quadrat shown in figure 51. Legend:
a, _Androsace chamaejasme_; c, _Carex rupestris_; t, _Tetraneuris
lanata_; p, _Potentilla rubricaulis_; as, _Arenaria sajanensis_; ar,
_Artemisia scopulorum_; ag, _Agropyrum scribneri_; sa, _Silene acaulis_;
st, _Sieversia turbinata_; d, _Dasyphora fruticosa_; al, _Allium
reticulatum_; o, _Oreoxis alpina_.]

=209. Mapping= is invariably begun at the upper left-hand corner of the
chart, and is carried across the strip marked off by the plotting tape,
decimeter by decimeter. As soon as this strip is completed, a second one
is formed by moving the top tape to a position one decimeter below the
plotting tape, which then becomes the upper one. This is repeated until
the last strip is reached. Little difficulty is experienced in locating
each plant exactly, as the decimeter interval is small, and the
centimeter square which corresponds is divided into twenty-five tiny
squares. Each plant is put in whenever possible, but mats, turfs, and
mosses are merely outlined in mass if the individuals are not
distinguishable. This holds true of all large rosettes and mats, even
when they are single plants. Symbols were formerly used for indicating
the various species. They have the advantage of requiring little space
on the chart, and the disadvantage of necessitating constant reference
to the legend. They are at present replaced by initials. By this plan,
the decapitalized first letter of the generic name is used if no other
genus found in the quadrat begins with the same letter. If, however, two
or more genera begin with _a_, for example _Agropyrum_, _Anemone_, and
_Allium_, the most abundant one is indicated by _a_, and the others by
the first two letters, as _an_, _al_. In case two species of the same
genus are present, the species initial is used in connection with that
for the genus, as _ac_ and _ar_ for _Agropyrum caninum_ and _Agropyrum
richardsonii_ respectively. It is rarely necessary to exceed two letters
for any species. Plants which regularly have several stems from the same
root are indicated by the initial and an exponent as _a_^3. Seedlings
are represented by a line drawn through the letter. Usually the chart
sheet affords sufficient space below the chart for the legend. When the
list of species is long, the back of the sheet is used.

=210. Factors and photographs.= Each chart is numbered, and the
formation, station, and date indicated. The constant factors, altitude,
slope, and exposure are ascertained and recorded on the sheet. The
variable factors are read in each quadrat whenever possible, and in
addition to being preserved in the record book, are noted on the chart
sheet along with the base reading in the formation for the same time.
This facilitates the interpretation of the differences found when two or
more charts are compared. Chart quadrats are regularly photographed. For
this purpose a long focus 4 × 5 camera with a telephoto lens is used. At
the proper distance this will make a view of the same size as the chart,
thus making possible an exact comparison of the two. The chart and
photograph serve as mutual checks, as well as complements, since the
former shows number, position, and arrangement, and the latter, height,
form, position, and arrangement. The view is usually made by placing the
camera directly in front of the middle of the lower tape, at such a
distance that the side tapes fall just within the limits of the ground
glass. The swing is always used in order that the focus may be uniformly
sharp. Surface views of the quadrat can be taken by means of a device
which permits the camera to hang downward from the tripod, or by means
of a tripod with a swinging platform. Such views are especially valuable
for the study of competition, since they give a clear idea of the spread
and density of the various plants. They are difficult to make unless the
vegetation is low and nearly uniform in height. The usual photograph is
much more serviceable in regular quadrat work.


                        _The Permanent Quadrat_

=211. Description and uses.= As stated heretofore, either list or chart
quadrats may be rendered permanent in order that they may be followed
from season to season or from year to year. As a matter of fact,
however, an area which is to be studied repeatedly really demands
charting, and in practice chart quadrats alone are made permanent. This
is done simply by driving a labeled stake at one corner of the quadrat,
and locating the latter definitely in relation to a conspicuous
landmark. When one is in residence for several years, practically all
chart quadrats are converted into permanent ones, since the work already
done in the chart quadrat is so much accomplished towards the permanent
one. This is not necessary when one wishes merely to compare different
areas of stable formations. As a rule, however, some change is
constantly being wrought by invasion or competition, and the amount and
direction of this can only be revealed by the permanent quadrat. The
latter has a fundamental value for all kinds of invasion, but it is
absolutely indispensable in studying complete invasion or succession,
and in discovering and recording the gradual effects of competition. It
is in the detailed investigation of these dynamic phenomena that the
paramount importance of the quadrat is most evident. If the experience
of several years be taken as conclusive, no other method is capable of
revealing the minute changes as they are occurring.

[Illustration: Fig. 53. Permanent quadrat, _Polygonile_ (_Polygonum
bistortoides_) Ruxton Park; mapped and photographed July 22, denuded
September 8, 1903.]

The permanent quadrat is regularly 1 meter square, a size determined
both by the exigencies of charting and photographing. When ecograph
batteries are used, the quadrat is located as close to the latter as is
possible. Otherwise, the quadrat itself should constitute a station for
making factor observations. This connection is absolutely essential,
since the quadrat is used expressly to determine the structural changes,
which are produced by physical factors, and the reaction of vegetation
upon them. Permanent quadrats are established in different formations or
stages of a succession to trace the invasion of new species and the
dropping out of old ones in response to competition. They serve to
distinguish the proper formation, which represents a particular stage of
development, from the mixed formations which precede and follow, and
also to determine the exact course as well as the rapidity of the change
that follows each reaction. When applied to different examples of the
same stage, and to all the different stages of a succession, the whole
development of the latter may be minutely traced and definitely
recorded. The importance of following the changes from aspect to aspect
is much less, since these are periodical rather than dynamic. They are
an essential feature of structure, however, and it has been the practice
to make at least one series of aspect charts from each permanent
quadrat.

For tracing the invasion and competition of lichens and mosses, which
play a primary role in initial formations, a subquadrat is used. The
size varies, but it is usually smaller than the quadrat, although the
latter is entirely available in the case of the large foliose lichens.
For the crustose and smaller foliose forms, a subquadrat 2 decimeters
square is used, and for the larger forms and tufted mosses, one of 5
decimeters. In the case of ground forms, tapes are employed, and the
quadrat is permanently staked. On rocks and cliffs, where moss and
lichen stages are most common, tapes are impracticable, and the quadrat
is permanently outlined with paint. Charts of lichen quadrats are made
to the usual scale of 10 : 1.

=212. Manner of use.= Permanent quadrats are mapped and photographed in
exactly the same way as chart quadrats. As soon as this has been done, a
labeled stake is driven at the upper left-hand corner, so that its edge
indicates the exact position of the quadrat stake, and a smaller one is
placed at the opposite corner to facilitate the task of setting the
tapes accurately in later readings. The label stake bears merely the
number of the quadrat and the date when it was first established. It is
firmly fixed and allowed to project just enough to enable it to be
located readily. Its position requires careful landmarking when the
quadrat is to be visited year by year. In forest formations, this is
readily done by blazing, but in grassland it is necessary to have
recourse to compass and pacing, or to erect an artificial landmark.
After several charts have been made, a permanent quadrat attains a high
value, and every precaution must be taken to prevent losing its exact
location. At the second reading of a quadrat, whether in the succeeding
aspect or year, the tapes are placed with reference to the stakes, and a
chart and photograph are made in the usual manner. These are labeled and
dated like the original ones, but they are numbered to indicate both the
quadrat and the series, e. g., 15^2 indicates the second chart, and
photograph made of quadrat 15. The date indicates whether the readings
are by the aspect or the year, though this may be shown also in the name
of the series itself. It is clearly an advantage to have the two
successive charts of a quadrat upon the same sheet, and to file all the
charts and photographs of the same permanent quadrat together, and in
the proper order.

Since much of the value of a permanent quadrat depends upon its use as a
station for observing physical factors, it is unprofitable to establish
a large number. The results of invasion and competition can be
ascertained by the quadrat alone, but these should be merely preliminary
to seeking for their causes. Clearly, a quadrat should be established
for each battery of instruments, while additional ones should be located
only in so far as they can be visited often enough to give an insight
into the factors that control them. In view of the fact that the most
important factors, water-content and light, are less variable than
humidity, temperature, and wind, it will suffice if visits are made once
a week. This is especially true when it is possible to refer the more
variable factors to the continuous records of a base station. While all
the results determined for permanent quadrats are preserved in the field
record, a record of them is also kept on the reverse of the chart sheet
for convenience in interpreting the different charts.


                         _The Denuded Quadrat_

=213. Description.= This is primarily a permanent quadrat from which the
plant covering has been removed, after it has been charted and
photographed. What is practically the same thing is obtained by
establishing a permanent quadrat in a new soil, or in one recently laid
bare and not yet reclothed with plants. These, however, are merely
permanent quadrats, in which the first chart and photograph furnish a
record of the habitat alone. They are of great importance in succession,
and will be more fully discussed under experimental vegetation. The
denuded quadrat is of the usual size, 1 meter, though the smaller lichen
quadrats are also denuded. The location is subject to the conditions
already indicated, especially with reference to physical factors. The
denuded quadrat, however, is particularly adapted to the study of
invasion and the resulting competition. Consequently, when migration is
markedly from one direction, a series of denuded quadrats throws a flood
of light upon the actual steps in invasion. Denuding is a valuable aid
in succession, but it must be clearly recognized that, while permanent
quadrats register the exact course of the succession, denuded ones can
merely furnish facts as to the probable courses of stages not now in
evidence.

[Illustration: Fig. 54. Denuded quadrat; this is the quadrat shown in
figure 53; photographed September 7, 1904.]

=214. Methods of denuding and recording.= Permanent quadrats may be
denuded at any time during the time they are under observation. The best
results, however, are to be obtained by establishing the two side by
side, or at least close together. In this way, they are mutually
supplementary, and furnish the most evidence possible with regard to the
procedure of invasion and competition. Another advantage is found in
that the same observations of climatic factors will do for both, though
water-content and soil temperatures are necessarily different. A quadrat
which is to be denuded is first mapped, photographed, and labeled
exactly like a permanent quadrat. The vegetation is then destroyed. This
is usually done by removal, though it may also be burnt, destroyed by
flooding, or in some other manner. The method will depend upon the use
which the quadrat is to serve. If it is to throw light upon the
vegetation of an area in which denudation has affected the surface
alone, the aerial parts only are removed by paring the surface with a
spade. When the disturbance is to be more profound, the upper
seed-bearing layer is removed, and the underground parts dug up. In the
interpretation of a secondary succession, the denuding cause is made use
of in a fashion as nearly natural as possible. Ordinarily, the plants
are removed just below the top of the ground by a spade, leaving the
underground parts undisturbed. This method has yielded very interesting
results.

Quadrats have been denuded in the fall after the majority of the plants
have completed their growth. This is largely owing to the fact that
other field work is less pressing at this time. Denudation can be done
as well in the spring, though the invasion will be slower in this case,
since the seeds which have accumulated will be partly or entirely
removed. During the first season the denuded quadrat should be mapped
every month, and, if the invasion be rapid, photographed also. In open
formations, especially those of a xerophytic nature, a single chart and
photograph made at the end of the season are sufficient. In a few cases
of this sort, indeed, no invaders have appeared until the second year.
Beginning with the second season, a single record taken near the close
of the growing period will suffice. Denuded quadrats are labeled, dated,
and filed exactly as other permanent quadrats, but it should be noted
that the first member of the chart and photograph series is that which
records the original vegetation of the area denuded.

=215. Physical factors.= When denuded quadrats are single, their
physical factors must be observed in the usual way. If they are
associated with permanent ones, the ordinary readings are made for the
latter, and those factors which are affected by exposing the soil are
alone taken for the denuded area. These are the water-content, soil and
surface temperatures, and in some stations at least the humidity near
the surface. As everywhere, water-content is the most important, but the
temperature at or near the surface has a marked effect upon germination.
Because of its bearing upon the latter, the surface water-content is
usually determined also. This has been done by taking a surface sample 2
inches square and 1 inch deep. Denuded quadrats naturally show
considerable differences from year to year as the action of the invaders
becomes more pronounced. To this fact is due much of their value as aids
in interpreting succession.


                           _Aquatic Quadrats_

=216. Scope.= The preceding discussion of quadrat methods is based
wholly upon their use in terrestrial formations. Wet meadow and dry bog
are the wettest places in which quadrats have been used. It is clear,
however, that with certain necessary modifications, quadrats can be used
as successfully, though not as conveniently, in many water formations as
in land ones. The tapes need to be raised above the surface of the water
by longer stakes, and photographs often taken from a boat, but otherwise
the usual methods apply, at any rate for bogs and shallow bodies of
water. In lakes or streams the tapes might be attached to buoys or
floats. The determination of factors is made as usual. Permanent
quadrats are feasible in many cases at least, and denuded quadrats are
not altogether impossible.


                               TRANSECTS

=217. The transect= is essentially a cross section through the
vegetation of a station, a formation, or a series of formations. It is
designed primarily to show the order of arrangement of species in zones
and societies, but it also serves as a record of the heterogeneity of
any area. In the form of the layer transect, it furnishes a graphic
method of representing the spatial relations of the species in layered
formations, e. g., forests, ponds, and lakes. It is merely a logical
extension of the idea underlying the quadrat, and the transect is,
indeed, little more than an elongated quadrat. An important difference,
however, lies in the fact that the former normally traverses areas more
or less unlike, while the latter is always located in a homogeneous one.
Furthermore, the transect is plotted with especial reference to the
topography. With respect to dimension, transects are classified as line,
layer, and belt transects, and the latter may also be permanent or
denuded.


                          _The Line Transect_

=218. Description and method.= A simple transect is sometimes made by
establishing the points between which it is to be run, and then
recording the plants pace by pace along this line. This is satisfactory
where the striking changes in structure are desired. A more accurate
method is ordinarily used, since it gives detailed results, and at the
same time brings out the more general features. For this, use is made of
a tape of proper length which is divided into decimeters. Tapes of 10,
50, and 100 meters are used, and if they are furnished with eyelets,
transects of intermediate lengths may be run with them. When longer
transects are desired, as in the case of forest formations, tapes of 500
or 1,000 meters should be used with eyelets a meter apart. The transect
is located in the area to be studied by running the tape from one
landmark to another, fastening it here and there by means of quadrat
stakes. Previous to this, the shortest distance between landmarks is
ascertained when the transect runs through a depression or upon a level
surface. In the case of an elevation, the height is ascertained by a
barometer, the length and angle of the two slopes obtained, and the
length of the base line determined from these data. The field record of
the arrangement of the plants is made entirely without reference to the
surface line. The vertical lines on the centimeter sheet are taken to
correspond with the tape, and the individual which touches the latter on
either side is recorded to the right or left respectively and within the
proper square. The species are indicated as for quadrats. A single row
on either side may be taken alone, but the double series serves as a
desirable check. After the record is made, the topography of the
transect is drawn carefully to scale. This drawing is made upon the
scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for
those that are longer. The combination of this drawing with the line
series of plants can not be made advantageously in the field. For the
shorter transects, meter sizes of centimeter plotting paper can often be
used to advantage. In this event, the topographic line is drawn to the
scale of 10 : 1 and the series of plants transferred directly to it. In
the case of transects between 10 and 100 meters, the scale of the
drawing is increased from 1000 : 1 to 100 : 1, so that each decimeter of
the original series is compressed into a centimeter. For the longest
transects, corresponding reductions must be made, but in these it will
be remembered that the series is plotted by meter instead of decimeter.

[Illustration: Fig. 55. Line transect running east and west in the
_Picea-Pinus-hylium_, showing the relation of the herbaceous layer to
the _Carex-Catha-helium_, invading along the brook; ecotones at _e_.]

=219. The location and size= of line transects are determined by the
purpose for which they are designed. Short transects are valuable for
detail, but they can be used to advantage only where changes in
arrangement are taking place rapidly. They are especially adapted to the
study of minute alternations and to the zonation of small ponds,
streams, ditches, roads, blowouts, etc. Longer transects can not furnish
the same detail, on account of the amount of time necessary, but they
are invaluable for the zonation and alternation of larger areas, such as
the consocies, formation, and formation series. They are of particular
importance for the record of zonation, since they afford a clue to the
topographic symmetry of the area. The location of a transect depends
upon the area to be studied, though it should always run through a
portion as typical as possible. The general direction is ascertained by
means of the compass, and when there is a measurable difference in
elevation it is taken by the barometer or otherwise.

The points at which ecotones cross the transect are carefully indicated
upon the chart. They serve as stations for simultaneous readings of
physical factors, though in the majority of cases water-content readings
alone will determine the reason for the ecotone. Photographs of line
transects should be made while the tape is in position, in order that
the superficies of the series may be as evident as possible.


                          _The Belt Transect_

=220. Details.= This differs from the line transect in that it is wider,
and consequently affords a more accurate record of the arrangement of
plants. While both give the actual facts of distribution, the line
transect necessarily ignores the minor lateral deviations in position.
These are brought out in a strip of some width, and the belt transect
thus gives a more correct view of the variations which result from
competition in an area physically homogeneous. The width of such
transects depends upon the length, and the character of the vegetation.
The standard width is one decimeter in herbaceous formations, and one
meter in the long transects which are used in woodlands. In open
vegetation, especially in the initial stages of successions, the width
may often be increased to advantage, but ordinarily the amount of work
necessary to run a belt transect of some length limits the width to one
decimeter.

The location of a belt transect, the choice of landmarks, the
determination of direction and elevation are made exactly as for the
line transect. The topographic map is made in precisely the same way
also, the scale used depending upon the length. Two tapes, however, are
employed, and these are placed so that they mark off a strip just one
decimeter wide. Every few meters, or oftener if need be, they are
checked by a decimeter rule, and fixed firmly in place by quadrat
stakes. The arrangement of the plants is recorded as for the line
transect, except that the record covers a decimeter strip just as in
quadrat work. Accordingly, an interval of a centimeter is left on the
sheet between the successive portions of the strip, in order that the
latter may be put together without confusion when the topographic map
and the plant series are combined. The record should invariably start in
the upper left-hand corner and read down. The map and the centimeter
strip recording the plants of the transect are combined on a common
scale as already indicated for the line transect.

The ecotones of zones are shown on belt transects by single cross lines,
and those of consocies by parallel cross lines. In taking photographs of
the transect, it is desirable to use guidons to mark these points
clearly. The same device may also be used to indicate the course of the
transect, when the tapes are completely hidden by the plants. Physical
factor readings should always be taken, and, as before, they are best
made at the intersections of the ecotones.


                        _The Permanent Transect_

=221. Advantages.= Both line and belt transects, after they have been
recorded, should be rendered permanent, in order that they may serve to
indicate the changes of a heterogeneous area from year to year in the
same detailed fashion that the permanent quadrat does for homogeneous
ones. For historical as well as for physical reasons, the ecotones of
zones and of consocies are subject to change from year to year, and the
amount and direction of this change can only be ascertained from annual
records made in exactly the same spot. By means of the permanent
transect alone the very origin of such areas can be followed from one
stage to another of the succession. Moreover, the transect is equally
valuable with the quadrat in making it possible to follow every step of
the minute changes wrought by competition.

=222. Details.= The transect is made permanent by blazing the landmarks
at either end, if these already exist, or by erecting them when it is
necessary. A label stake is driven at each end, on which is painted the
number and date of the transect and its length. Each stake should also
indicate the exact direction in which the other lies. The position of
the ecotone is indicated by smaller stakes bearing the number of the
transect and the date when the ecotone was found at that point. These
are left in place, and in a few years show very graphically the change
in position of the zones. For the first season, permanent transects
afford results of great value when recorded for each aspect, but after
this an annual visit will suffice. The details of mapping, plotting,
etc., are identical with those indicated above, with the addition that
all charts and photographs must bear the number of the reading as well
as that of the transect. Physical factor observations are taken as often
as the charts are made, and the results noted on the back of the chart
sheet for purposes of ready comparison.


                         _The Denuded Transect_

=223.= The denuded transect bears exactly the same relation to a
permanent one as that which exists between the denuded and the permanent
quadrat. While the permanent transect records the actual mutations due
to changing physical factors or to competition, the denuded transect
throws needed light upon the mobility and ecesis of the various species,
and upon the nature of the competition between them. Denuded transects
may be established wherever it seems desirable, after the strip has been
properly charted and photographed. The most valuable results, however,
are secured by locating each one alongside of a permanent one. The best
plan is to locate and chart two permanent transects a meter apart. A
single view is then made of the two. One of them is denuded together
with a strip 2 decimeters on either side, resulting in a denuded
transect 5 decimeters wide. In charting this during succeeding years,
the entire width may well be plotted as long as the vegetation is open,
but after it has again become well established, it is necessary to save
time by confining one’s attention to the central decimeter strip.
Photographs can be made either of the permanent and denuded transects
singly, or of the two together. The latter method has certain obvious
advantages. Climatic factor readings can be made for both transects in
common, but all those factors which are affected by the exposure of the
soil surface must be observed in each.


                          _The Layer Transect_

=224.= This is a modification of the line transect, by means of which
the vertical relations of plants are also shown, especially the tendency
to form layers which is so regular a feature of forest formations. Owing
to the difficulty of charting in three planes, belt transects do not
lend themselves to this purpose. Because of the greater complexity,
layer transects can rarely exceed ten meters in length except in those
formations where layering is little or not at all developed. The
simplest method is to establish a line transect in the ordinary way, and
then to record the height of each plant as its position is noted. This
is done by means of a measuring stick ruled in decimeters, which can be
moved from interval to interval along the tape, or better, by two such
sticks connected by tapes a meter long at every five decimeters of the
sticks. These should be two meters high for woodland, and one meter for
grassland. Layer transects often run on even surfaces, but if this is
not the case, the usual data for a topographic map should be taken. The
final chart is constructed on the scale of 10:1, the height of each
plant being indicated by a vertical line equal to .1 of the observed
height. A photograph of a representative meter of the transect is taken
when the measuring sticks and rods indicated above are in position.
Physical factor readings, principally of light, but often also of
humidity, temperature, and wind are made at the height of the various
layers when these are present.


                             ECOTONE CHARTS

=225.= The contour lines of zones and consocies are of the utmost
importance in recording the structure of vegetation. They do not permit
such accuracy as do quadrats and transects, but this is hardly to be
considered a disadvantage in view of the fact that ecotones are rarely
sharply defined. In establishing the ecotones of zonation, the width and
the length of the base, i. e., the area of excess or deficiency, or as
much of it as is to be considered, are determined. This base may be
road, ditch, pool, lake, or stream, or the peak or crest of a hill,
ridge, or mountain. When the zonation is bilateral, meter tapes are run
at right angles to the base, at proper intervals, and the points and the
distances where the ecotones cross are noted. In the case of radial
zones, the tapes are run in the four cardinal directions, and if the
base be large, in the four intermediate ones also, the intersections
being likewise noted. From the data thus obtained, the zones may be
outlined with a fair degree of accuracy. If the series be an extensive
one, it is charted to the scale of 100:1; in cases of small areas,
however, the scale of 10:1 will give better results. Whenever the zones
show clearly enough to warrant, a photograph is also taken.
Water-content readings are of paramount importance in the interpretation
of zones. Samples should be taken at all intersections, and the
resulting values indicated at the corresponding points upon the chart.
When the zones are broken up into alternating patches in consequence of
asymmetry in the topography, the ecotones of the latter are traced in a
similar fashion from the center of each as a base, the absolute position
of which is ultimately determined with reference to the ecotone lines
already established.


                          THE MIGRATION CIRCLE

=226. Purpose.= The migration circle is designed to record the invasion
of species, since it operates outward from an individual or a group of
plants as a center. As migration takes place to a certain degree in all
directions, a circle is better adapted to the purpose than the quadrat.
From the very nature of invasion, migration circles should always he
permanent in order that the yearly advance may be accurately noted.
Circles of this character are important aids in the study of any
vegetation, except, perhaps, one that has practically become stabilized.
Their great value, however, is found in succession, where it is
necessary to trace the movement of new individuals away from the
original invaders as centers of colonization.

=227. Location and method.= The size of the migration circle is largely
controlled by the density of the vegetation, and in some degree by the
height of the species also, since this determines the trajectory of the
disseminule. In close formations, a circle of 1–, rarely of 5–meter
radius can best be used, but in the more open initial stages of
succession a radius of 5, 10, or, in exceptional cases such as open
woodland, even 25 meters, affords the best results. The location should
always be made with a plant or group of plants of the species to be
studied as a center. This migration circle differs from the quadrat in
that it is used to show the movement of one, rarely two or three
species, and not the position of all the plants within it. The center is
permanently fixed by driving a labeled stake with the number of the
circle and the data. Two tapes the length of the radius are used for
recording. These are provided with the usual eyelets, 5 decimeters
apart, and are fastened on a peg in the top of the central stake so that
they move readily. At the outer ends they are staked 5 decimeters apart
by a tape of this length when the radius is 1 meter, and 1 meter when
the radius is 5 or 10 meters. The record forms must be especially
prepared on blank sheets about 9 inches square. The scale is 10:1 for
circles of 1 meter, and 100:1 for those of 5– and 10–meter radius. In
the former, concentric circles are drawn about the center at intervals
of 5 decimeters, and radii are drawn to the circumference at the same
interval. In the larger circles, the intervals are 1 meter. Each segment
of the circle is read by means of the two tapes, and the position
indicated with reference to the concentric lines and radii. When but one
species is read, a tiny circle is used to denote the position of each
plant. If more than one is used, the symbols are those already indicated
for the quadrat. One tape is left in place and the other with the
segment tape is shifted to a new position, and the resulting segment is
read as before. The exact position of the base radius is fixed by a
label stake, in order that the segments of successive years may exactly
correspond. The record sheet is labeled, dated, and filed. By folding at
one edge, it may be filed in the regular field book.

=228. The denuded circle= is established in the same way as a permanent
one. The original position of the individuals of the species under
consideration may be recorded or not, depending upon the use to be made
of the results. The safest plan is first to read the circle in the usual
way, and then to denude it. The latter should be done in such a way as
to remove all the disseminules from the surface in so far as possible.
It is essential also that this be done before the seeds are mature and
begin to be scattered. The central plant or cluster is of course not
removed. In special cases, all the plants of the species are allowed to
remain to serve as centers of colonization. The successive yearly
readings of the denuded area are made exactly as for a permanent circle.
Permanent and denuded circles, like quadrats, should always be
established near each other so that they permit of ready comparison
under similar conditions.

=229. Photographs= of migration circles furnish the most detail when the
camera is placed just behind the central group in such a way as to show
its relation to the other individuals or clusters of the circle. In the
denuded circle, or when the plants stand out conspicuously from the bulk
of the vegetation, it is not necessary to use guidons, but in other
cases the latter greatly increase the value of the picture. Factor
readings are less important for migration circles than for quadrats and
transects. The factors of principal importance are those that deal with
migration and ecesis, i. e., wind, water-content, and soil temperatures.
The former may be determined for both circles in common, but the
conditions that affect ecesis must be observed separately for each.


                              CARTOGRAPHY

=230. Value of cartographic methods.= Chart, map, and photograph are
records indispensable to the systematic study of vegetation. They serve
not merely to preserve the facts ascertained, and to permit their ready
comparison, but they also put a premium upon accurate methods, and
consequently bring to light many points otherwise overlooked. For
ecology, they have the value which drawings possess in taxonomy, in that
they make clear at a glance what pages of description fail to indicate.
They are the fundamental material of comparative phytogeography, and in
all careful vegetational study their use is no longer optional but
obligatory. Hence it is obvious that cartographic methods should be
clear and simple, and that they should be uniform, so that charts and
maps of widely separated formations may be directly compared without
difficulty. It is not to be expected that uniform methods will come into
general use immediately, but a proper appreciation of the obligation
that rests upon every ecologist to make his results both easily
comprehensible and usable will serve to produce this very necessary
result. In the treatment that follows, as elsewhere, no attempt is made
to describe the general cartographic methods used by other ecologists,
notably Flahault. The methods employed by the author form a complete
system, which has proved valuable, and for various reasons it alone is
discussed here.

=231. Standard scale.= The question of the scale to which charts and
maps are to be made is of primary importance. The general principle is
that the ratio between area and drawing should be as small as possible.
Moreover, charts and maps of the same character should always be drawn
to the same scale, unless a good reason to the contrary exists. The
ideal scale is 1:1, which is manifestly an impossibility. This is
approached most nearly in the quadrat chart where the scale is 10:1.
Charts of definite areas are made on a scale as large as possible, while
maps of formations, regions, etc., are necessarily drawn upon a very
small scale. General maps designed to show the distribution of species
and formations, or the vegetation of continents, are usually not drawn
with reference to a scale at all. While it is manifestly impossible to
use the same scale for charts and maps, it is feasible and desirable
that they be constructed upon scales readily convertible into each
other. This is most satisfactorily accomplished by means of the decimal
system, and the various type scales are 10:1, 100:1, 1000:1, etc. The
first two or three scales are used for charts of quadrats, transects,
and circles; the remaining ones are employed in making maps of large
areas. No attempt has been made to draw an absolute line between charts
and maps, but an endeavor is made to restrict the term chart to the
record of the number and position of plants, while maps deal with the
arrangement and location of formational areas. It is hardly necessary to
point out the reasons why all charts and maps should be based upon the
decimal system of scales. Experience will furnish the very best of
arguments.

=232. Color scheme.= The first requisite for the graphic representation
of formations, regions, etc., is that each class of formations be
invariably indicated by the same color. It is also necessary that the
colors and shades be easily distinguishable, and it is at least
desirable that they be referred to the different classes in some
consistent sequence. Uniformity in all these points is greatly to be
desired at the hands of all ecologists. Here, as in the case of the
standard scale, uniformity will be found the more desirable the more
impossible it is made by ignoring it. In the use of color to represent
regions and provinces, on maps too small to indicate formations, the
color of each division is represented by the color of its dominant
formation; thus the prairie province is colored ochroleucus on account
of the color used to represent prairie formations, the boreal-subalpine
zone atrovirens on account of the typical coniferous forests, etc. No
endeavor has been made to take account of the various types of
formations, e. g., the different coniferous forests, as this is a
problem to be worked out for more local maps in various shades of dark
green, etc. The following color scheme which has been based upon the
points made above is proposed as a satisfactory solution of the problem.
The color standard used is that of Saccardo’s Chromotaxia.

    I. Hydrophytic Formations: _blue_

       1. Marine: _cyaneus_

       2. Brackish: _ardesiacus_

       3. Freshwater: _caeruleus_

       4. Swamps and marshes: _caesius_

   II. Mesophytic Formations

       A. Forest formations: _green_

             1. Coniferous forests: _atrovirens_

             2. Broadleaved evergreen forests: _viridis_

             3. Deciduous forests: _flavovirens_

       B. Grassland formations: _yellow_

             1. Meadows: _melleus_

             2. Prairies: _ochroleucus_

       C. Culture and waste formations: _red_

             1. Fields: _ruber_

             2. Groves and orchards: _atropurpureus_

             3. Wastes: _purpureus_

  III. Xerophytic Formations: _brown_

       1. Deserts: _isabellinus_

       2. Plains and steppes: _avellaneus_

       3. Saline formations: _umbrinus_

       4. Arctic-alpine formations: _testaceus_

=233. Formation and vegetation maps= are detailed maps of a single
formation or a series of them, showing the formational limits, and when
the scale is not too small, the ecotones of zones and consocies. In the
cases where the topography is level, as sometimes happens in mapping
single formations, the chain and pedometer must be used to ascertain the
size of the different areas. Indeed in all mapping of vegetation, the
methods of surveying are directly applicable. Over large areas, however,
it is not necessary that limits be drawn with mathematical accuracy, and
for the purposes of the ecologist, the plane table and camera are
satisfactory substitutes for the surveyor’s transit, at least in the
present aspect of the subject. When the formation or group of formations
is commanded by an elevation of some height, the latter is used as a
base. A plane table is established upon it and the topographical and
vegetational features are recorded in the usual way. This map is usually
supplemented by a series of views from the same base. Indeed it has come
to be recognized that a complete series of photographs of this kind give
a more valuable record than the plane table, and that the construction
of an accurate map from them is an easy matter. Since the camera saves
much time and energy also, it is used almost exclusively to furnish the
data for map making. In hilly, and especially in mountainous regions,
the photographic method is indispensable. Its application is extremely
simple. A central hill or mountain is selected, and from it a series of
views is taken so that the edge of one exactly meets the edge of the
other. This is an extremely important matter, and demands much nicety of
judgment. The camera is kept in the same spot, and after each exposure
it is turned as the operator looks through it until a landmark at one
edge just passes from view at the other. As soon as the new position is
determined, the tripod screw is turned to hold the box firmly in
position. In case of a slight jar, the exact position should again be
obtained. If the series is accurately made, the resulting prints will
give a complete panoramic view of the region, without overlap or
omission. For this purpose, a 6½ × 8½ camera is desirable, since the
topographic and vegetational features are larger and stand out more
distinctly. A large camera requires fewer changes of position, and hence
saves time and reduces the chance of error. A 4 × 5 camera serves the
purpose sufficiently well, though it requires a little more care in
operation on account of the greater number of exposures necessary. This
may be avoided in some degree by the use of a wide-angle lens if the
depth of the area is not too great. Whatever camera may be used, a
telephoto lens is a very desirable adjunct, since it enables one to
choose between three different sizes of the view without changing the
position of the camera. To avoid possible confusion, the exposures are
always made from right to left, and the plates are used in the numerical
order of their holders. For the same reason the landmarks are described
and numbered in their proper order. The prints obtained are mounted on a
card in sequence. The view map may be preserved in this form, or it may
be reduced or enlarged by making a copy to the size desired. Outline
maps of topography may be traced from the resulting negative, and the
formations filled in by means of the proper colors. The most
satisfactory method, however, is to have the original views or the copy
printed “light” and to color the formations just as they appear there,
with all the wealth of topographic and vegetational detail. If a
detailed topographic map alone is desired, this is traced directly from
the large copy.

=234. Continental maps.= A method of determining the general outlines of
regions, provinces, and vegetational zones as a preliminary to their
detailed study has been used successfully for several years.[22] This is
based upon provincial and continental maps on which are traced the
geographical areas of the species of genera typical of the various
formations. Detail topographic maps of the prairie province and the
North American continent have been used for this purpose. A number of
the facies of extensive and representative formations of the different
portions of the continent are selected and grouped according to genera.
One map is devoted to each genus, unless the number of species is large.
In this case a number of maps are used, since the limits are apt to
become confused. The range of each species is determined from all the
reliable sources, and a corresponding line is drawn upon the map to
delimit its geographical area. The limits of the area of each species
are drawn in a different color, and the name of the species printed in
the same color in the legend. Although this work has as yet been done
only for the trees of North America, and for the grasses and principal
species of the prairie province, it promises to constitute a final
method for the limitation of vegetational divisions. It is clear that if
the original data concerning ranges are accurate, the increasing study
of formations will do little more than rectify the detailed course of
the limiting line, since in most cases facies and formations coincide in
distribution. The limiting line or ecotone of a zone or province is a
composite obtained from the limits of certain representative facies and
principal species, and checked by the limits of species typical of the
contiguous vegetations. Thus, the boreal-subalpine zone is clearly
outlined by combining the limits of _Populus tremuloides_, _Larix
americana_, _Pinus banksiana_, _Abies balsamea_, _Picea mariana_, _Picea
canadensis_, and _Betula papyracea_, and checking the results by the
areal limits of the hardwoods and grasses to the southward.


                              PHOTOGRAPHY

=235.= The camera is an indispensable instrument for the ecologist.
Although it has too often been employed to give an air of thoroughness
to work of no ecological value, it is as important for recording the
structure of vegetation as the automatic instrument is for the study of
the habitat. No ecologist is equipped for systematic field investigation
until he is provided with a good camera and has become skilful in its
use. For this reason, it is felt that a few hints concerning
photographic methods and their application in ecology may not be out of
place. No written advice can take the place of experience, but certain
elementary suggestions and cautions will greatly shorten the
apprenticeship of one who does not have the good fortune to be taught by
a professional photographer. To the student of ecology, the camera is
not a toy. It must be understood and operated with as much thoroughness
as any other instrument, and when this is done, the results will be
equally certain and desirable.

[Illustration: Fig. 56. 4 × 5 long focus “Korona” camera (series V).]

[Illustration: Fig. 57. 5 × 7 long focus “Premo” camera.]

=236. The camera and its accessories.= Although two cameras are
desirable whenever it is possible to obtain them, a single one will meet
all the requirements of field work. This should be 4 × 5 inches in size,
since it is much more convenient and will do all the work that a larger
camera can. In the comparatively few cases in which larger views are
needed, the 4 × 5 negatives can be readily enlarged. The smaller
instrument is less expensive in operation because of the cheapness of
the plates, and it gives a negative of the proper size for lantern
slides and for reproduction. A 6½ × 8½ camera is valuable in special
cases, such as making a series of photographs for maps. In the writer’s
own experience, the 6½ × 8½ camera, although used exclusively at first,
has been almost completely supplanted by the 4 × 5. The best field
camera is of the folding type with a good stout box. It must be what is
known technically as a long focus instrument, which enables small
objects to be taken natural size and permits the use of a telephoto
lens. It should be provided with a swing and also a reversible back by
which the position of the plates can be changed instantly. The lens must
be of the telephoto pattern, which makes it possible to use the front or
back lens either alone or in combination. The chief advantage of this is
that the image, when distant, may be made of three different sizes
without changing the position of the camera. Generally speaking, the
high-priced rapid lenses are the best, since it is exceptional to get
the desired length of exposure in vegetation, on account of the ease
with which the plants move in the wind. Before buying such a lens it is
desirable to test its rapidity and depth of focus, since it is not
necessarily better than some of the lenses furnished with good cameras.
The lens should be provided with an iris diaphragm capable of being
stopped down to 128 or 256. The shutters furnished with the ordinary
lenses are satisfactory, since “snap-shots,” i. e., instantaneous
exposures, are practically never possible for plants. The automatic
shutter of the “Premo” camera is an especially convenient form. All
shutters should be carefully tested before using to determine the exact
time value of the exposures indicated. It is not uncommon for the
exposure at 1 second, or at other points, to have a value quite
different from the one indicated. When this is the case, it is evident
that it can not be known too soon. The camera should have at least a
half-dozen double plate-holders. These are numbered consecutively so
that the figure uppermost when the holder is in the camera will indicate
the number of the plate exposed. A carrying case is desirable on a long
trip when all the plate-holders must be taken, but ordinarily it is a
disadvantage, since the camera box will carry two or three holders. The
camera cloth should be as small and light as possible, and at the same
time opaque. The most satisfactory one for the field is the rubber
cloth. The tripod should be a happy combination of lightness and
stability, a condition more nearly reached by the aluminum tripod than
by any other. It should have not less than three joints in order to
facilitate the use of the long focus upon objects near the ground.

[Illustration: Fig. 58. 5 × 7 “Korona” Royal camera.]

=237. Choice of a camera.= There is not a great deal of choice between
the moderate-priced cameras of the various makers. A field camera is
restricted to certain special uses, and hence is more serviceable when
attachments useful only in portraiture or instantaneous work are absent.
Even the ray filter, which has some value in the indoor photography of
flowers, is useless in the field on account of the long exposure
required. From considerable experience, “Premo” and “Korona” cameras
have been found to be very satisfactory instruments. Doubtless the same
statement would be found true of all the standard makes, but they have
not been used by the writer. “Premo” cameras are made by the Rochester
Optical Co., Rochester, N. Y., and “Korona” cameras by the
Gundlach-Manhattan Optical Co., Rochester, N. Y. When two or more
cameras are used, the best results can be obtained if they are of the
same make, since the details of operation are then the same. The reduced
liability of making a blunder is often offset by the fact that a
different pattern will permit of a wider range of use. Any standard
brand of plates will produce good negatives when skilfully used; at
least, this has been proved in the case of the Cramer, Hammer, Seed, and
Stanley brands. Every professional photographer has his favorite brand
of plate, but the ecologist will do well to give the various kinds a
thorough trial, and then to invariably use the one which gives him the
best results. Thus, while it seems to be less popular with the
profession than the others mentioned, the writer has obtained at least
as satisfactory results with the Stanley plate as with the others, and
consequently now uses it exclusively, since it is cheaper. The one
important point is to make a final choice only after personal
experience, and then to always use plates of the same brand, and
preferably of the same rate of speed.

=238. The use of the camera.= To the ecologist, objects to be
photographed fall into two categories, viz., those that move, and those
that do not move. For practical purposes, areas sufficiently distant to
render the movement imperceptible belong to the latter, as well as
those, such as rock lichens, many fungi, etc., which can not be stirred
by ordinary winds. The treatment accorded the two is essentially
different. A fundamental rule of ecological photography is that detail
must receive the first emphasis. The ecological view should be a picture
as well as a map, however, but when one must be sacrificed, artistic
effect must yield to clearness, and accuracy, i. e., technically
speaking, contrast must give way to detail. Leaving apart the necessity
of securing a sharp focus, which holds for all work, detail or
definition depends directly upon the aperture of the diaphragm. Detail
is increased by decreasing the size of the aperture. This in turn
increases the length of time necessary for a proper exposure, and
consequently the danger that the plant will be moved in the midst of the
exposure. When the movement is negligible, the invariable rule should be
to reduce the aperture to its smallest size, and to expose for a
corresponding time. In all cases where the plants are close enough to
show even a slight blurring on account of the action of the wind, the
time of exposure must be reduced, in the hope that a short period of
quiet will suffice for it. This reduction in time must be compensated by
increasing the aperture of the diaphragm, and hence the amount of light
which strikes the plate. The proper balance between the two is a matter
of considerable nicety. It depends much upon the vagaries of the wind,
and can readily be determined only after considerable experience.
Although regions naturally differ somewhat in the nature of their winds,
much experience in prairie and mountain regions warrants the primary
rule that views of vegetation and plants subject to movement are not to
be attempted on windy or cloudy days when it can possibly be avoided.
Even on reconnaissance, a poor picture is no better than none at all,
while in resident work a time will come sooner or later which will
permit the making of a view satisfactory in all respects. There may be
occasional instances when one is rewarded for keeping the camera trained
on a particular spot for hours, and for wasting several plates in the
hope that still moments will prove to be of the requisite duration. As a
regular procedure, however, this has nothing to commend it.

Various methods have been tried to reduce or eliminate the trouble
caused by the wind. Canvas screens have been used for this purpose with
some benefit. When the picture is worth the trouble, a tent may be
erected to afford a very efficient protection. This is too prodigal of
time and energy, however, to be practicable under the usual conditions.
Flashlight exposures on still nights are sometimes feasible, but the
disadvantages connected with them are too great to bring them into
general use. The best procedure is to bide one’s time, and to take
quadrats, transects, and other detail areas, as well as many plant
groups, at a time that promises to be most favorable. Single plants can
often be moved in the field so that they are protected from the wind, or
so that they are more strongly lighted. Slender, or feathery plants are
usually very difficult to handle out of doors. The best plan is to
photograph them in a room that is well and evenly lighted, or, best of
all, in a stable, roomy tent.

=239. The sequence of details.= No photographer ever escapes blunders
entirely. At the outset of his work, the ecologist must fully realize
this, and accordingly plan a method of operating the camera which will
reduce the chance of mistake to a minimum. The usual blunders which
every one makes sooner or later, such as making two exposures on one
plate, drawing the slide before closing the shutter, allowing the light
to strike the plate through the slit in the holder, etc., can be all but
absolutely avoided by a fixed order of doing things. This order will
naturally not be the same for different persons; it is necessary merely
that each have his own invariable sequence. The following one will serve
as an illustration. As a preliminary, the plate-holders are filled,
after having been carefully dusted, and the slides are uniformly
replaced with the black edge inward. It is a wise precaution to again
see that all the slides are in this position before leaving the dark
room. This will ensure that a black edge outward always means that the
plate has been exposed. The tripod is first set up and placed in what
seems about the proper position. The camera is next attached to it, and
the front and back opened. The bellows is pulled out, a short distance
for views, and a longer one for detail pictures, and fastened. It is
necessary to move the diaphragm index to the largest aperture and to
open the shutter at “time.” The next steps are to orient the view or
object, and to bring it into sharp focus upon the ground glass. The
first is accomplished by moving the entire instrument, changing the
position of the tripod legs, swinging the camera upon the tripod, or by
raising or lowering the lens front. It is often desirable also to change
the position of the object on the plate by use of the reversible back.
In views with much distance, the foreground is brought into sharp focus.
In close views, especially of quadrats, the swing is used to increase
the distance for the foreground, and the focus is made upon the center.
After focusing, the shutter is closed, the indicator set at the time
desired, and the diaphragm “stopped down” as far as possible.
Plate-holder 1 is slipped into place, care being taken not to move the
camera by a sudden jar. The camera cloth is dropped above the holder and
allowed to hang down over the slide end. The slide is drawn and put on
top of the instrument, the black edge always up. The exposure is made
and the slide replaced _with the black edge outward_. This point should
receive the most critical attention, as a blunder here will often cause
the loss of two negatives. The plate-holder is returned to the
receptacle, or merely placed in the back of the camera, which is then
closed. The number of the plate, the name of the view or object, the
condition of the light, the length of exposure, and the aperture of the
diaphragm, as well as the date, are recorded in a notebook for this
purpose. The shutter is then opened at “time,” the diaphragm thrown wide
open, and the front of the camera closed. When distances are short, the
camera is often carried upon the tripod. As a rule, however, it is
usually removed, and the tripod folded. In making subsequent pictures,
the plates should always be used in their numerical order.

=240. The time of exposure= is obviously the most critical task in the
manipulation of a camera. The time necessary for a proper exposure
varies with the season, the hour, the condition of the sky, the light
intensity of the formation, the color and size of the area to be
photographed, and, finally, of course, with the aperture of the
diaphragm. Fortunately for the ecologist, the variation in light
intensity during the season, and even during the greater part of the
day, is not great, and can ordinarily be ignored. The beginner will make
the most progress by determining the exposure demanded by his instrument
for taking a general view in full sunlight and with the smallest stop of
the diaphragm. In standard cameras with lenses of ordinary rapidity,
this is usually about one second. This will serve as a basis from which
all other exposures may be reckoned until one has worked through a wide
range of conditions and can recall just what time each view requires. On
completely cloudy days the time required is five to ten times that
necessary on a clear day; filmy clouds and haze necessitate an exposure
of two or three seconds. The more open forest formations demand an
exposure of about five to ten seconds on a sunny day, while the deeper
ones require two or three times as long. A close view requires more time
than a distant one, since the light-reflecting surface is much smaller.
Quadrats require two or three seconds, and individual groups frequently
take a longer time. The color of the vegetation plays an important part
also: a dark green spruce forest requires twice as long an exposure as
the aspen forest, and a grassland quadrat takes more time than one
located in a gravel slide. In this connection, it is hardly necessary to
point out that the lighted side of objects should always be taken, never
the shaded one. The exposures indicated above are based upon the
smallest stop. The reasons for using this whenever possible have already
been given. When a larger stop is necessary, the exposure is decreased
to correspond; for example, a quadrat that takes three to four seconds
at 256 can be taken at 64 in one second. As a rule, the sun should not
be in front of the camera, but, when necessary, views can be made in
this position if the sun is prevented from shining directly into the
lens.

=241. Developing= is as important as exposing. Indeed, it may well be
considered more important, since a properly exposed plate may be spoiled
in developing, while an under-exposure or over-exposure may be saved.
Owing to the ease with which plants move in the wind, the ecologist is
obliged to reconcile himself to many under-exposures, which can be
converted into good negatives only by skilful developing. Every base
station should have a good dark room, equipped with running water when
possible, a good ruby lantern, and the proper trays and chemicals.
Prepared developing solutions are alluring because of their convenience,
but after an extended trial of several kinds, the writer has reached the
conviction that pyrogallic acid, or “pyro,” is by far the most
satisfactory in working with vegetation. Of almost innumerable formulae,
the following gives excellent satisfaction and is convenient to use.

                      I.                        II.

          500 cc. water              500 cc. water
           30 grams sodium sulphite    5 grams pyrogallic acid
           30 grams sodium carbonate

For developing, equal parts of I and II are mixed, and a few drops of a
10 per cent solution of potassium bromide added, unless there is reason
to suspect that the plate has been seriously underexposed. The fixing
bath is a concentrated solution of sodium hyposulphite, “hypo,” to which
a few drops of acetic acid are added. It should be replaced every week
or two, depending upon how much it is used. A tray of water is kept at
hand for bringing out the detail in underexposed negatives, and a second
tray is used for washing. The “pyro” and the bromide solution should
always be within reach, the former for accelerating, and the latter for
retarding the development of unsatisfactory plates.

The image will begin to show on a properly exposed plate within one to
three minutes after it has been put in the developer. If the image
appears almost instantly, and then recedes quickly, the plate is badly
overexposed, and should be thrown away. In case it “comes up” less
quickly, indicating that it is not greatly overexposed, it can be saved
by the addition of more bromide. When the image does not show till the
end of five to ten minutes, the plate has been underexposed. It is then
necessary to add more “pyro,” taking care not to pour it on the plate,
and, after the image appears with its striking contrast, to leave the
plate in water until as much detail as possible is brought out in the
shadows. In the case of a normal exposure, when greater detail is
desired, the negative is left for some time in water, and when contrast
is sought more “pyro” is used. Negatives with unusual detail lack
“snap”; they are “flat,” and fail to make artistic pictures. Contrast,
on the other hand, often obscures detail, and the best results can only
be obtained by a happy combination of the two. The most important maxim
in developing is that the process shall be continued until the image has
become indistinct. The universal tendency of the beginner is to remove
the negative the moment the outlines grow dimmer, and the result is a
thin, lifeless negative. It is almost impossible to develop too far, if
the image is not allowed to disappear. Negatives of this sort are
“thick,” and though they print more slowly, produce brilliant pictures.
A large quantity of the developing solution is used with single plates
in small trays, and is allowed to act without rocking the tray. Much
time is saved, however, by developing several plates together, and to
avoid using a large quantity of the solution, the tray is gently rocked
from time to time. This movement is particularly necessary at the
beginning, in order that the plates may be covered evenly, and at once.
Fifty cubic centimeters of the solution will develop three or four 6½ ×
8½ plates, and twice as many 4 × 5’s. After the developer has once been
used, it is kept for several days to restrain overexposed plates. As
soon as the plate is developed, it is rinsed in water, and placed in the
fixing fluid, until the white opaqueness is entirely removed. The “hypo”
is then washed out by immersing the negatives for one to two hours in
running water. If the latter can not be secured, the water in which they
are placed should be changed frequently. The negatives are then
air-dried within doors, in a place free from dust. Finally, they are
filed away in negative envelopes, each bearing the name and number of
the negative, and preferably also, the time and other exposure data.

=242. Finishing.= On account of the time demanded by other field tasks,
it has not been found desirable to make and finish prints in the field.
This, with the making of lantern slides, enlargements, etc., may well be
turned over to a professional photographer. It is the custom to make a
proof of each negative to meet the casual needs that arise in the field.
For this purpose, solio “seconds” are used, since they are both cheap
and satisfactory. When an urgent demand for a finished print does arise,
it is met by using “velox” paper, which can be exposed in the dark room,
and then developed and fixed exactly like a plate. Two standard papers
for views are “solio” and “platina.” The former gives brown tones, and
is used for contrast and brilliancy, hence it is especially good for
printing from negatives that have too much detail and too little
contrast. “Platina,” on the contrary, yields soft gray tones, and
softens contrasts.


                   FORMATION AND SUCCESSION HERBARIA

=243. Concept and purpose.= A formation herbarium is a collection of
exsiccati, in which the species are arranged with respect to their
position in the formation, instead of being grouped in genera and
families. Its primary purpose is to furnish a record of the constitution
and the structure of a formation or a series of formations. At the same
time, it affords the basal material for developing the subject of
comparative phytogeography. It is impossible for one ecologist to visit
many remote regions, to say nothing of spending a period sufficient for
obtaining even a fair knowledge of the vegetation. He can at the best
acquire an acquaintance with but few regions at first hand. In
consequence, a method that brings a vegetation to him, with its
structure carefully wrought out by years of study, is of the highest
value. Time, as well as distance, sets a narrow limit to the number of
formations which one man can investigate critically in a lifetime. It is
no longer possible for a botanist to explore vast regions, and to bring
back results which have anything more than a very general value. This
fact, far from restricting the comparative study of vegetation, will
serve to make it more accurate and systematic. The exact results of
numerous resident investigators, expressed in formation herbaria, with
the proper series of quadrat maps and photographs, will be worked over
by men who are themselves specially acquainted with a particular
vegetation. Comparisons will be founded upon a definite basis, and the
relationship of various vegetations can then be expressed in precise
rather than general terms. It is hardly too sweeping to assert that
accurate work in the field of comparative phytogeography can be done
only in this fashion. The value of formation herbaria in class work is
evident. On account of the limitations of time and distance, classes can
touch but few formations, and these at every time except the growing
period. For these reasons, an accurate and complete formational record
that can be consulted or studied at any time is almost indispensable to
class study in the development and structure of formations.

=244. Details of collecting.= Formational collections, unlike the
ordinary sets of exsiccati, can not be made upon the first visit to a
region, or by a single journey through it. The determination of
formation limits, and of developmental stages, of aspects, layers,
abundance, etc., must necessarily precede, a work which alone takes
several years. Moreover, collecting itself requires more than one year
in a region containing numerous formations. This is exemplified by the
_Herbaria Formationum Coloradensium_.[23] The preliminary study for this
was made from 1896–1899, the collecting was done chiefly in 1900 and
1901, while additional numbers were added in 1902–3. For the purposes of
the formation herbarium, specimens should be collected and pressed in
such fashion as to show all the ecological features possible. Plants
must be collected both in flower and in fruit, with the underground
parts as perfect as may be. Seedlings and rosettes should be included
whenever present. In pressing, one or two leaves should be arranged with
the lower side uppermost to admit of the ready comparison of both
surfaces. Opened flowers are valuable for flower biology, while seeds
and fruits are desirable for showing migration contrivances. The ferns,
mosses, and lichens of the formation should be fully represented,
together with the more important fungi and algae. The number of
photographs taken for each herbarium should be limited only by
considerations of time and expense. The ideal series consists of a
general view of each formation, showing its physiographic setting,
nearer views of each of its aspects, detail views of its consocies,
societies, and layers, and flower portraits of all the constituent
species. Such a series can only be obtained by residence through a long
term of years, and in most cases general and aspect views, with
portraits of the facies and a few of the striking principal species,
must suffice. Quadrat and transect charts, together with formational
maps, are extremely desirable, and, indeed, all but indispensable.

=245. Arrangement.= The arrangement of species within each formation
herbarium is based upon the structure of the vegetation. The primary
groupings are made with reference to time of appearance and abundance;
when definite zones, associations, or layers are present, they must
likewise be taken into account. In the Colorado collection, the first
division is into three aspects based upon the period of flowering
(_aspectus vernalis_, _aestivalis_, _autumnalis_). Within each aspect,
the species are arranged with respect to abundance in the groups,
facies, principal species, and secondary species. Each group is placed
in an ordinary manila cover, which bears a printed label indicating the
aspect and the group. The species labels give, in addition to the name,
date, and place of collection, the phyad or vegetation form, the
geographical area, the rank of the species, the aspect, and the
formation. To these may well be added data concerning migration
contrivances, seed production, pollination, period of flowering, etc.
The photographs are mounted on the usual herbarium sheets, and placed in
the proper order in the various groups, and a similar disposition is
made of quadrat and transect charts, and such physical factor summaries
as seem desirable.

=246. Succession herbaria.= The arrangement of formation herbaria may
follow the classification of formations with respect to character,
region, or development. The first is the most convenient for purposes of
instruction, and has distinct advantages in permitting a close
comparison of the vegetation of different habitats. The second basis,
which is the one used in the _Herbaria Formationum Coloradensium_, is
peculiarly adapted to mountain vegetation in which the zones are usually
very distinct. The arrangement of herbaria in a developmental series,
however, is the most logical and the most illuminating, since the
structure of the ultimate formations is not only made plain, but the
stages in their development are also laid bare. Such succession herbaria
are the natural outgrowth of formational ones. Indeed, the latter should
be made merely the starting point for these in all regions where the
causes which bring about successions are active. Where weathering is
still an important factor, as in mountains, the initial and intermediate
formations which lead to the final grassland or forest are often in
evidence. After a formation herbarium of each stage has been made in the
way indicated, a succession herbarium is obtained merely by arranging
the various herbaria in the sequence of the developmental stages. Thus,
in the Colorado collection, the subalpine formations are arranged
according to altitude in the following series: (1) the pine formation,
(2) the gravel slide formation, (3) the half gravel slide formation, (4)
the aspen formation, (5) the balsam-spruce formation, (6) the
spruce-pine formation, (7) the meadow thicket formation, (8) the brook
bank formation. Of these, five belong to the same succession, and it is
possible to indicate the development of the spruce-pine forest by
arranging these five formations in their proper order in a succession
herbarium, as follows: (1) the gravel slide formation, (2) the half
gravel slide formation, (3) the pine formation, (4) the balsam-spruce
formation, (5) the spruce-pine formation.


                       DEVELOPMENT AND STRUCTURE

=247. Vegetation an organism.= The plant formation is an organic unit.
It exhibits activities or changes which result in development,
structure, and reproduction. These changes are progressive, or periodic,
and, in some degree, rhythmic, and there can be no objection to
regarding them as functions of vegetation. According to this point of
view, the formation is a complex organism, which possesses functions and
structure, and passes through a cycle of development similar to that of
the plant. This concept may seem strange at first, owing to the fact
that the common understanding of function and structure is based upon
the individual plant alone. Since the formation, like the plant, is
subject to changes caused by the habitat, and since these changes are
recorded in its structure, it is evident that the terms, function and
structure, are as applicable to the one as to the other. It is merely
necessary to bear in mind that the functions of plants and of formations
are absolutely different activities, which have no more in common than
do the two structures, leaf and zone.

=248. Vegetation essentially dynamic.= As an organism, the formation is
undergoing constant change. Constructive or destructive forces are
necessarily at work; the former, as in the plant, predominate until
maturity, when the latter prevail. Consequently, it no longer seems
fruitful to classify the phenomena of vegetation as dynamic or static.
The emphasis which has been placed upon dynamic aspects of vegetation
has served a useful purpose by calling attention to the development of
the latter. Although it is a quarter of a century since Hult, and more
than a half century since Steenstrup, by far the greater number of
ecological studies still ignore the problem of development. This
condition, however, can be remedied more easily by insisting upon an
exact understanding of the nature of the formation than in any other
way. It is entirely superfluous to speak of dynamic and static effects
in the plant, and the use of these terms with reference to the formation
becomes equally unnecessary as soon as the latter is looked upon as an
organism. The proper investigation of a formation can no more overlook
development than structure, so closely are the two interwoven. Future
research must rest squarely upon this fact.

=249. Functions and structures.= The functions of a formation are
association, invasion, and succession: the second may be resolved into
migration and ecesis, and the third, perhaps, into reaction and
competition. Formational structures comprise zones, layers, consocies,
societies, etc., all of which may be referred to zonation, or to
alternation. The term association has been used in both an active and a
passive sense. In the former, it applies to the inevitable grouping
together of plants, by means of reproduction and immobility. Passively,
it refers to the actual groupings which result in this way, and in this
sense it is practically synonymous with vegetation. Invasion is the
function of movement, and of occupying or taking possession; with
association, it constitutes the two fundamental activities of
vegetation. It is the essential part of succession, but the latter is so
distinctive, because of the intimate relation of competition and
reaction, that clearness is gained by treating it as a separate function
which is especially concerned with development. Association, zonation,
and alternation are structural phenomena, which are in large part the
immediate product of habitat and function, and in a considerable degree,
also, the result of ancestral or historical facts. It is a difficult
matter to determine in what measure the last factor enters, but it is
one that must always be taken into account, particularly when the
physical factors of the habitat are inadequate to explain the structures
observed. Structurally, association regularly includes both zonation and
alternation. As there are certain typical instances in which it exhibits
neither, the treatment will be clearer if each is considered separately.


                              ASSOCIATION

=250. Concept.= The principle of association is the fundamental law of
vegetation. Indeed, association is vegetation, for the individual passes
into vegetation, strictly speaking, at the moment when other individuals
of the same kind or of different kinds become grouped with it. It is
then (and the same statement necessarily holds for vegetation) the
coming together and the staying together of individuals and, ultimately,
of species. A concrete instance will illustrate this fact. In the
development of the blowout formation of the Nebraska sand-hills
(_Redfieldia-Muhlenbergia-anemium_), association begins only when the
first plant of _Redfieldia flexuosa_ is joined by other plants that have
sprung from it, or have wandered in over the margin of the blowout.
Henceforth, whatever changes the blowout formation may undergo,
association is a settled characteristic of it until some new and
overwhelming physical catastrophe shall destroy the associated
individuals. It will readily be seen that association does not depend
upon particular individuals, for these pass and others take their place,
but that it does depend essentially upon number of individuals.

Association involves the idea of the relation of plants to the soil, as
well as that of plants to each other. It is synonymous with vegetation
only when the two relations are represented, since there may be
association such as that of a parasite with its host, which does not
constitute vegetation. But it will be seen that the relation of the
parasite to the host is practically identical with the relation of the
plant to the soil or stratum, and the two concepts mentioned above
become merged in such a case. From this it follows that association
results in vegetation only when the two ideas are distinct. The concept
of association contains a fact that is everywhere significant of
vegetation, namely, the likeness or unlikeness of the individuals which
are associated. In the case of parasite and host, this unlikeness is
marked; in vegetation, all degrees of similarity obtain. As will be
evident when the causes which lead to association are considered,
alternate similarity and dissimilarity of the constituent individuals or
species is subordinate as a feature of vegetation only to the primary
fact of association.

Since association contains two distinct, though related, ideas, it is of
necessity ambiguous. It is very desirable that this be avoided, in order
that each concept may be clearly delimited. For this reason, the act or
process of grouping individuals is termed _aggregation_, while the word
association is restricted to the condition or state of being grouped
together. In a word, aggregation is functional, association is
structural; the one is the result of the other. This distinction makes
clear the difference between association in the active and passive
sense, and falls in with the need of keeping function and structure in
the foreground.

=251. Causes.= In considering the causes which produce association, it
is necessary to call in evidence the primary facts of the process in
concrete examples of this principle. These facts are so bound up in the
nature of vegetal organisms that they are the veriest axioms.
Reproduction gives rise immediately to potential, and ultimately, in the
great majority of cases, to actual association. The degree and
permanence of the association are then determined by the immobility of
the individuals as expressed in terms of attachment to each other or to
the stratum, such as sheath, thallus, haustoria, holdfasts, rhizoids,
roots, etc. The range of immobility is very great. In terrestrial
plants, mobility is confined almost entirely to the period when the
individual lies dormant in the seed, spore, or propagative part, which
is alone mobile. In aquatic spermatophytes, the same is true of all
attached forms, while free floating plants such as _Lemna_ are mobile in
a high degree, especially during the vegetative period. Among the algae
and hydrophilous fungi, attached forms are mobile only in the spore or
propagative condition, while the motile forms of the plancton typify the
extreme development of mobility. The immediate result of reproduction in
an immobile species is to produce association of like individuals, while
in the case of a mobile species reproduction may or may not lead
immediately to association. We may lay down the general principle that
immobility tends to maintain the association of the individuals of the
same generation, i. e., the association of like forms, while mobility
tends to separate the similar individuals of one generation and to bring
unlike forms together. With the mobile algae, separation of the members
of each generation is the rule, unless the individuals come to be
associated in a thallus, or are grouped in contact with the substratum.
Flowering plants that are relatively immobile, especially in the seed
state, drop their seeds beneath and about the parent plants, and in
consequence dense association of the new plants is the rule. In very
many cases, however, this primitive tendency is largely or completely
negatived by the presence of special dissemination contrivances, which
are nearly, if not quite, as effective for many terrestrial plants as
the free floating habit is for algae. From this point, the whole
question of mobility belongs to migration, just as the adjustment
between the parent plants and their offspring, or between plants
established and the mobile plants to be established, belongs to
competition.

If association were determined by reproduction and immobility alone, it
would exhibit areas dissimilar in the mass of individuals, as well as
areas dissimilar in the kinds of individuals. Some areas would be
occupied by plants of a single species, others by plants of several or
many species. This tendency of association to show differences is,
however, greatly emphasized by the fact that vegetation is fundamentally
attached to and dependent upon a surface that exhibits the most extreme
physical differences. For this reason, new differences in association
appear, due not only to the morphological differentiation of vegetation
forms, but also to the changes in the degree and manner of association
produced directly by the different habitats. Association might then be
defined as a grouping together of plant individuals, of parents and
progeny, which is initiated by reproduction and immobility, and
determined by environment. It is a resultant of differences and
similarities. In consequence, association in its largest expression,
vegetation, is essentially heterogeneous, while in those areas which
possess physical or biological definiteness, habitats and vegetation
centers, it is relatively homogeneous. This fundamental peculiarity has
given us the concept of the formation, an area of vegetation, or a
particular association, which is homogeneous within itself, and at the
same time essentially different from contiguous areas, though falling
into a phylogenetic series with some and a biological series with
others. From its nature, the plant formation is to be considered the
logical unit of vegetation, though it is not, of course, the simplest
example of association.

=252. Aggregation.= As indicated under the causes of association, the
process by which groups of individuals are formed depends entirely upon
reproduction and migration. In short, aggregation is merely a corollary
of movement. The simplest example of this process occurs in forms like
_Gloeocapsa_, _Tetraspora_, and others, where the plants resulting from
fission are held together by means of a sheath. Though called a colony,
such a group of individuals is a family in the ordinary sense.
Practically the same grouping results in the case of terrestrial plants,
especially spermatophytes, when the seeds of a plant mature and fall to
the ground about it. The relation in both instances is essentially that
of parent and offspring, although the parent soon disappears in the case
of annuals, while among the algae its existence is regularly terminated
by fission. The size and the density of the family group are determined
by the number of seeds produced, and by their mobility. These are
further affected by the height and branching of the plant, and by the
position of the seeds upon it. The disseminules of immobile species fall
directly beneath the parent, and the resulting group is both uniform and
definite. A similar arrangement is caused likewise by offshoots. An
increase in mobility brings about a decrease of aggregation, since the
disseminules are carried away from the parent plant. Perfectly mobile
forms rarely produce family groups for this reason. It is evident,
however, that mobile perennials sometimes arrange themselves in similar
fashion in consequence of propagation by underground parts.
Consequently, it is possible to state the law of single aggregation,
viz., that immobility promotes the grouping of parent and offspring, and
mobility hinders it.

If all species were immobile, the family group would be characteristic
of vegetation. Since the great majority are more or less mobile,
aggregates of this sort are the exception rather than the rule. Mobility
not only decreases the number of offspring in the family group, but it
also spreads disseminules broadcast to enter dissimilar groups. It leads
directly to mixed aggregation, by which individuals of one or more
species invade the family group. Once established, the newcomers tend
also to produce simple groups, thus causing an arrangement corresponding
essentially to a community. Such collections of family groups are
extremely variable in size and definition. This arises in part from the
nature of simple aggregation, and in part from the varying mobility of
different species. Mobility alone often produces similar communities by
bringing together the disseminules of different plants, each of which
then becomes the center of a mixed group. In the case of permobile
species, several disseminules of each may be brought together. The
resulting area, though larger, is practically the same. At present, it
is difficult to formulate the law for this method of grouping. It may be
stated provisionally as follows: mixed aggregation is the direct result
of mobility, and the greater the mobility the more heterogeneous the
mixture.

The constitution of all the major areas of a formation is to be
explained upon the basis of aggregation by the two methods described.
The relative importance of family groups and communities differs for
every formation, and the exact procedure in each can be obtained only by
the detailed study of quadrats. The problem is further complicated by
competition and reaction, particularly in closed vegetation. For this
reason, aggregation can be studied most satisfactorily in a new or
denuded area, where these processes are not yet in evidence.


                         _Kinds of Association_

=253. Categories.= In the analysis of association, it must be kept
clearly in mind that the concrete examples from which all
generalizations must be drawn are often in very different stages of
development, and are of correspondingly different ages. For this reason
it has seemed best to consider the primary relations of association in
general in this place, leaving the treatment of the effects of invasion,
succession, alternation, and zonation to be taken up under these topics.

Various categories of association may be distinguished, according to the
dominant physical factor concerned or the point of view taken. These
will fall into two series, as we consider the relation of plant to plant
with reference to some object or characteristic, or the grouping of
plants together in response to some dominant factor. In the first series
may be placed association with reference to substratum, to the ground
(occupation), and to invasion; in the second belong light and
water-content association. It should be noted that these are all actual
associations in nature, and not concepts such as the vegetation form,
within which plants from widely different associations may be
classified. Naturally, it does not follow that it is not logical or
valuable to group together those plants, such as hydrophytes,
sciophytes, hysterophytes, etc., which have a common relation to some
factor, but belong to different formations.

=254. Stratum association.= Plants manifest independent or dependent
association with reference to the stratum to which they are attached and
from which they derive food or support. Independent association is
exhibited by those holophytic species of a formation which are entirely
independent of each other with respect to mechanical support or
nutrition. It is characteristic of the greater number of the constituent
species of formations. Dependent association is manifested in the
relation between host and parasite, stratum and epiphyte, support and
liane. Warming[24] has distinguished six kinds of associations:
parasitism, helotism, mutualism, epiphytism, lianism, and commensalism.
Commensalism corresponds to the primary principle of association which
has given rise to vegetation. Homogeneous commensalism is the term
applied to social exclusive plants, in which the patch is composed of a
single species. Such association is extremely rare in nature, and if the
most minute forms be considered, probably never occurs. On the other
hand, heterogeneous commensalism, in which individuals of more than one
species are present, is everywhere typical of vegetation. Warming
regards saprophytism merely as a specialized kind of parasitism, an
opinion that may well be defended. Helotism, however, is also a mere
modification of parasitism, if it is not indeed parasitism pure and
simple. Mutualism is an altogether vague concept, including parasites,
epiphytes, and endophytes of doubtful physiological relation. Pound and
Clements[25] treated lianes, parasites, and saprophytes as vegetation
forms, relating herbaceous creepers and twiners to the lianes, and
dividing the fungi and lichens into nine groups. Whatever the value of
these divisions may be from the standpoint of vegetation forms, they
represent the same relation between plant and nutritive stratum, and
with respect to association should be merged in one group. Schimper[26]
was the first to perceive the essential similarity of all such groups
from the standpoint of association. He terms these plant societies
(_Genossenschaften_), retaining the four groups already established,
lianae, epiphyta, saprophyta, and parasiticae. It is evident that
dependent association comprises extremely divergent forms, from the
slightly clinging herb, such as _Galium_, to the most intense parasite.
The distinction, however, is a clear one, if restricted to that relation
between plants in which one acts as a mechanical support or stratum or
as a nutritive host for the other.

=255. Ground association.= The first division of formations into open
and closed was made by Engler and Drude.[27] Open formations were
defined as those having incomplete stability and heterogeneous
composition, while closed formations have a more definite uniform stamp.
What is true of formations is equally true of vegetation, so that
association may be regarded as open or closed with reference to the
density and thoroughness with which the plants occupy the ground. In
open association, the ground is slightly or partially occupied, readily
permitting the entrance of new plants without the displacement of those
already present. Such an arrangement is characteristic of the early
stages of a formation, or of a succession of formations. It produces
unstable open formations, which arise, usually after denudation, in
sand-hills, blowouts, gravel slides, dunes, flood plains, burned areas,
etc. In closed association, occupation of the ground is complete, and
the invasion of new species can occur only through displacement. Closed
association results in stable, closed formations, such as forest,
thicket, meadow, and prairie. As open association characterizes the
early stages of a succession of formations, so closed association is
peculiar to the later or last stages of all such successions. In short,
open formations represent certain phases of the development of
vegetation, while closed formations correspond to the relatively final
structural conditions. It is a fundamental principle of association that
every succession from denudation, or from newly formed soils, begins
with open formations and ends with a closed formation. The causes
leading up to open and closed association are intimately connected with
development, and hence are considered under invasion and succession.

=256. Species guild association.= Drude has distinguished a kind
of association peculiar to invasion, in which there is a
successive or concomitant movement of certain species of a
formation into another formation or region, resulting in species
guilds (_Artengenossenschaften_). The association in this case is
largely one of community of origin or area, and of concomitant
migration. It is especially characteristic of areas adjacent to
formational and regional limits. Fundamentally, it is merely the
grouping of plants which are invading at the same time, and
consequently it differs only in degree from what occurs in every
invasion where more than a single individual is concerned.
Accordingly, this type of association has little more than
historical interest. This must not be construed to mean that it
does not occur, but that it differs in no essential from the
ordinary grouping of invaders.

=257. Light association.= The constituent species of formations show two
fundamentally different groupings with respect to light. In the one
case, the individuals are on the same level, or nearly so, in such a way
that each has direct access to sunlight. Such an arrangement is
characteristic of most grassland and herbaceous formations. In the case
of desert formations, there is often considerable difference in the
height of the plants, but the distance between them is so great as to
admit of direct illumination of all. This arrangement may be termed
coordinate association. In forests, thickets, and many herbaceous
wastes, the height and density of certain species enable them to
dominate the formation. In a dense forest, the trees receive practically
all the light incident upon the formation, and the shrubs, herbs, fungi,
and algae of lower habit and inferior position must adapt themselves to
the diffuse light which passes through or between the leaves. The same
is equally true of dense thickets and wastes, except that the vertical
distance is less, and the diffuseness of the light is correspondingly
modified. In these formations, the dominant trees, shrubs, or herbs, the
facies, constitute a primary or superior layer. The degree of
subordinate association, as a result of which inferior layers will
arise, is entirely determined by the density of the facies. In open
woodlands, which are really mixed formations of woodland and grassland,
the intervals, and usually the spaces beneath the trees also, are
covered with poophytes, showing an absence of subordination due to
light. This is the prevailing condition in the pine formation (_Pinus
ponderosa-xerohylium_) of the ridges and foot-hills of western Nebraska.
When, however, the trees stand sufficiently close that their shadows
meet or overlap throughout the day, the increasing diffuseness begins to
cause modification and rearrangement of the individuals. By photometric
methods, the light in a forest is found to be least diffuse just below
the facies, while the diffuseness increases markedly in passing to the
ground. The taller, stronger individuals are consequently in a position
to assimilate more vigorously, and to become still taller and stronger
as a result. Just as these have taken up a position inferior to that of
the facies, so the shorter or weaker species must come to occupy a still
more subordinate position. This results, not only because the light is
primarily weaker nearer the ground, but also because the taller plants
interpose as a second screen. The complete working out of this
arrangement with reference to light produces typical subordinate
association, which finds its characteristic expression in the layering
of forests and thickets. Layers tend to appear as soon as open woodland
or thicket begins to pass into denser conditions, and up to a certain
point, at which they disappear, they become the more numerous and the
more marked, the denser the forest.

In the Otowanie woods near Lincoln (_Quercus-Hicoria-hylium_), layering
usually begins at a light value of .1 (1 = normal sunshine in the open).
Thornber[28] has found the same value to obtain in the thickets of the
Missouri bluffs. In these, again, layers disappear at a value of .005,
the extreme diffuseness making assimilation impossible except for
occasional mosses and algae. A number of herbaceous plants are present
in the spring, but these are all prevernal or vernal bloomers, which are
safely past flowering before shade conditions become extreme. In the
_Fraxinus-Catalpa-alsium_, all inferior holophytic vegetation disappears
between the light value of .004 and that of .003. The spruce-pine
formation (_Picea-Pinus-hylium_) of the Rocky mountains, with a light
value of .01, usually contains but a few scattered herbs, mostly
evergreen; in some cases there are no subordinate plants other than
mosses and hysterophytes. The lodge-pole pine formation (_Pinus
murrayana-hylium_), with light values often less than .005, is nearly or
quite destitute of all but hysterophytic undergrowth. Such extremely
dense formations are examples of coordinate association merely, since
the formation is reduced to a single superior layer, in which the
individuals of the facies bear the same spatial relation to incident
light. In layered formations, in addition to the subordinate relation of
other species to the facies, there is, of course, a kind of coordinate
association manifested in each layer.

=258. Water-content association.= Schouw[29] was the first to give
definite expression to the value of the water-content of the soil for
the grouping of plants. He established four groups: (1) water plants,
(2) swamp plants, (3) plants of moist meadows, (4) plants of dry soils.
The first he termed hydrophytes, introducing the term halophytes to
include all saline plants. Thurmann[30] recognized the fundamental
influence of water-content upon association, and further perceived that
the amount of water present was determined primarily by the physical
nature of the soil. He distinguished plants which grow in soils that
retain water as _hygrophilous_, and those found upon soils that lose
water readily as _xerophilous_. Those which seemed to grow indifferently
upon either were termed _ubiquitous_. The latter correspond in some
measure to mesophytes, but they are really plants possessing a
considerable range of adaptability, and do not properly constitute a
natural group. Warming[31] proposed the term mesophytes to include all
the plants intermediate between hydrophytes and xerophytes. He
recognized the paramount value of water-content association as the basis
of ecology, and upon this made a logical and systematic treatise out of
the scattered results of many workers. Schimper[32] placed the study of
vegetation upon a new basis by drawing a distinction between physical
and physiological water-content, and by pointing out that the last alone
is to be taken into account in the study of plant life, and hence of
plant geography. Accepting the easily demonstrable fact that an excess
of salts in the soil water, as well as cold, tends greatly to diminish
the available water of the soil, i. e., the chresard, it is at once seen
why saline and arctic plants are as truly xerophytic as those that grow
on rocks or in desert sands. An anomalous case which, however, physical
factor records have explained fully, is presented by many plants growing
in alpine gravel slides, strands, blowouts, sandbars, etc., in which the
water-content is considerable, but the water loss excessive, on account
of extreme heat or reduced air pressure. The effect of these conditions
is to produce a plant xerophytic as to its aerial parts, and mesophytic
or even hydrophytic as to subterranean parts. Such plants may, from
their twofold nature, be termed _dissophytes_; they are especially
characteristic of dysgeogenous soils in alpine regions where
transpiration reaches a maximum, but are doubtless to be found in all
gravel and sand habitats with high water-content. With these
corrections, the concept of water-content association, which owes much
to both Warming and Schimper, but is largely to be credited to Thurmann,
becomes completely and fundamentally applicable to all vegetation.

Up to the present time, the general character of the habitat, together
with the gross appearance of the plant itself, has been thought
sufficient to determine the proper position of a plant or a formation in
the water-content classification. Such a method is adequate, however,
only for plants and formations which bear a distinct impress. For an
accurate classification into the three categories, hydrophytes,
mesophytes, and xerophytes, it is necessary to make exact determinations
of the normal holard and chresard of the habitat, and to supplement
this, in some degree at least, by histological studies. Except in the
case of saline, acid, and frozen soils, the holard alone will be a
fairly accurate index, especially in habitats of similar soil
composition. For an exact and comprehensive classification, however, and
particularly in comparative work, the chresard must constitute the sole
criterion. As the latter has been ascertained for very few formations,
and in Nebraska and Colorado alone, the present characterization of many
plants and formations as hydrophytic, mesophytic, or xerophytic must be
regarded as largely tentative, and the final classification will be
possible only after the thorough quantitative investigation of their
habitats.

The water-content groups, hydrophytia, mesophytia, and xerophytia,
include all formations found upon the globe. The exactness with which
this classification applies to vegetation is made somewhat more evident
by dividing mesophytia into forest and grassland. This is based
primarily upon light association, but it also reflects water-content
differences in a large degree. The groups thus constituted represent the
fundamental zonation of the vegetative covering with respect to
water-content. Ocean, forest, grassland, and desert correspond exactly
to hydrophytia, hylophytia, poophytia, and xerophytia. The difference is
merely one of terminology: the first series takes into account the
physiognomy of the vegetation itself, while the other emphasizes the
causative factors.


                    THE DEVELOPMENT OF THE FORMATION

=259.= A strict account of development should trace the results of the
various activities of vegetation in their proper sequence. This is
aggregation, migration, ecesis, reaction, and competition. These
functions are so intimately and often so inextricably associated that it
is hardly feasible to discuss development by treating each one
separately. In consequence, the two fundamental phenomena, invasion and
succession, which they produce, are taken as the basis of the
discussion. These, moreover, are different only in degree; succession is
merely complete, periodic invasion. Nevertheless, the subject gains in
clearness by a separate treatment of each.


                                INVASION

=260.= By invasion is understood the movement of plants from an area of
a certain character into one of a different character, and their
colonization in the latter. This movement may concern an individual, a
species, or a group of species. From the nature of invasion, which
contains the double idea of going into and taking possession of, it
usually operates between contiguous formations, but it also takes place
between formational zones and patches. More rarely and less noticeably,
there may be invasion into a remote vegetation, as a result of long
carriage by wind, water, birds, railroads, or vessels. Movement or
migration, however, represents but one of the two ideas involved in
invasion. Migration merely carries the spore, seed, or propagule into
the area to be invaded. In ecesis, the spores or seeds germinate and
grow, after more or less adjustment, and in case the latter becomes
sufficiently complete, the new plants reproduce and finally become
established. With all terrestrial plants, invasion is possible only when
migration is followed by ecesis, because of the inherent differences of
formations or of areas of the same formation. In the case of surface
floating forms, such as _Lemnaceae_, and of the plancton, ecesis is of
much less importance, on account of the uniformity of the medium and the
lack of attachment, and migration is often practically synonymous with
invasion.


                              _MIGRATION_

=261.= Migration has been sometimes used loosely as a synonym for
invasion, but it is here employed in its proper sense of removal or
departure, i. e., movement, and is contrasted with ecesis, the making of
a home, the two ideas being combined in invasion, which is a moving into
and a taking possession of. An analysis of migration reveals the
presence of four factors, mobility, agency, proximity, and topography.
Not all of these are present in every instance of migration, as for
example in the simple elongation of a rootstalk, but in the great
majority of cases each plays its proper part. Mobility represents the
inherent capacity of a plant for migration, and in its highest
expression, motility, is in itself productive of movement. As a general
rule, however, modifications for securing mobility are ineffective in
the absence of proper agents, and the effective operation of the two
will be profoundly influenced by distance and topography.

=262. Mobility= denotes potentiality of migration as represented by
modifications for this purpose. It corresponds, in a sense, to
dissemination, though seed production also enters into it. Its most
perfect expression is found in those plants which are themselves motile,
_Bacteriaceae_, _Oscillatoria_, _Volvocaceae_, and _Bacillariaceae_, or
possess motile propagules, such as most _Phycophyta_. On the other hand,
it is entirely undeveloped in many plants with heavy unspecialized seeds
and fruits. Between these two extremes lie by far the greater number of
plants, exhibiting the most various degrees of mobility, from the motile
though almost immobile offshoots of many _Liliaceae_ to the immotile but
very mobile spores of fungi. It is thus seen that motility plays a
relatively small part in migration, being practically absent in
terrestrial forms, and that it bears a very uncertain relation to
mobility. In analyzing the latter, contrivances for dissemination are
seen to determine primarily the degree of mobility, while the number of
seeds produced will have an important effect in increasing or decreasing
it. A third factor of considerable importance is also involved, namely,
position with reference to the distributive agent, but any exact
knowledge of its importance must await systematic experiment somewhat
after the methods of Dingler, but with air-currents, etc., of known
velocity and direction. The time is not distant when by such methods it
will be possible to establish a coefficient of mobility, derived from
terms of position, weight, resistant surface, and trajectory for
definite wind velocities or for particular propulsive mechanisms.

=263. Organs for dissemination.= Plants exhibit considerable diversity
with reference to the part or organ modified, or at least utilized, for
dissemination. This modification, though usually affecting the
particular product of reproduction, may, in fact, operate on any part of
the plant, and in certain cases upon the entire plant itself. In the
majority of plants characterized by alternation of generations, the same
individual may be disseminated in one generation by a reproductive body,
and in the other by a propagative one, as is the case in the oogones and
conidia of _Peronospora_, the spores and gemmae of _Marchantia_, the
fruits and runners of _Fragaria_, etc. Special modifications have, as a
rule, been developed in direct connection with spores and seeds, and
mobility reaches its highest expression in these. It is, on the other
hand, greatly restricted in offshoots and plant bodies, at least in
terrestrial forms, though it will now and then attain a marked
development in these, as shown by the rosettes of _Sempervivum_ and the
tumbling plants of _Cycloloma_. For the sake of convenience, in
analyzing migration, all plants may be arranged in the following groups
with reference to the organ or part distributed.

1. Spore-distributed, sporostrotes. This includes all plants possessing
structures which go by the name of spore, such as the acinetes of Nostoc
and _Protococcus_, the zoogonidia of _Ulothrix_, _Ectocarpus_, etc., the
conidia, ascospores, and basidiospores of fungi, the tetraspores of red
seaweeds, and the gemmae and spores proper of liverworts, mosses, and
ferns. These are almost always without especial contrivances for
dissemination, but their extreme minuteness results in great mobility.

2. Seed-distributed, spermatostrotes. This group comprises all flowering
plants in which the seed is the part modified or at least disseminated.
The mobility of seeds is relatively small, except in the case of minute,
winged or comate seeds.

3. Fruit-distributed, carpostrotes. The modifications of the fruit for
distribution exceed in number and variety all other modifications of
this sort. All achenes, perigynia, utricles, etc., properly belong here.

4. Offshoot-distributed, thallostrotes. To this class are referred those
plants, almost exclusively cormophytes, which produce lateral,
branch-like propagules, such as root-sprouts, rhizomes, runners,
stolons, rosettes, etc. Migration with such plants is extremely slow,
but correspondingly effective, since it is almost invariably followed by
ecesis.

5. Plant-distributed, phytostrotes. This group includes all plancton and
surface forms, whether motile or non-motile, and those terrestrial
plants in which the whole plant, or at least the aerial part, is
distributed, as in tumbleweeds and in many grasses.

=264. Contrivances for dissemination.= Any investigation of migration to
be exact must confine itself to fixed forms. For these the degree of
perfection shown by dissemination contrivances corresponds almost
exactly to the degree of mobility. Because of the difficulty of
ascertaining the effect of ecesis, it is impossible to determine the
actual effectiveness in nature of different modifications, and the best
that can be done at present is to regard mobility, together with the
occurrence and forcefulness of distributive agents, as an approximate
measure of migration. The general accuracy of such a measure will be
more or less evident from the following. Of 118 species common to the
foot-hill and sand-hill regions of Nebraska, regions which are
sufficiently diverse to indicate that these common species must have
entered either one by migration from the other, 83 exhibit modifications
for dissemination, while 8 others, though without special contrivances,
are readily distributed by water, and 4 more are mobile because of
minuteness of spore or seed. Some degree of mobility is present in 73
per cent of the species common to these regions, while of the total
number of species in which the mode of migration is evident, viz., 95,
66 per cent are wind-distributed, 20 per cent animal-distributed, and 14
per cent are water-distributed. It need hardly be noted that this
accords fully with the prevalence and forcefulness of winds in these
regions. Of the species peculiar to the foot-hill region, many are
doubtless indigenous, though a majority have come from the montane
regions to the westward. The number of mobile species is 121, or 60 per
cent of the entire number, while the number of wind-distributed ones is
85, or 70 per cent of those that are mobile. Among the 25 species found
in the widely separated wooded bluff and foot-hill regions, 2 only,
_Amorpha nana_ and _Roripa nasturtium_, are relatively immobile, but the
minute seeds of the latter, however, are readily distributed, and the
former is altogether infrequent.

The following groups of plants may be distinguished according to the
character of the contrivance by which dissemination is secured:

1. Saccate, saccospores. Here are to be placed a variety of fruits, all
of which agree, however, in having a membranous envelope or an
impervious, air-containing pericarp. In _Ostrya_, _Physalis_,
_Staphylea_, the modification is for wind-distribution, while in
_Carex_, _Nymphaea_, etc., it is for water-transport.

2. Winged, pterospores. This group includes all winged, margined, and
flattened fruits and seeds, such as are found in _Acer_, _Betula_,
_Rumex_, many _Umbelliferae_, _Graminaceae_, etc.

3. Comate, comospores. To this group belong those fruits and seeds with
long silky hairs, _Gossypium_, _Anemone_, _Asclepias_, etc., and those
with straight capillary hairs or bristles not confined to one end,
_Typha_, _Salix_, etc.

4. Parachute, petasospores. The highly developed members of this group,
_Taraxacum_, _Lactuca_, and other _Liguliflorae_ are connected through
Senecio and _Eriophorum_ with the preceding. These represent the highest
development of mobility attained by special modification.

5. Chaffy-pappose, carphospores. In this group are placed those achenes
with a more or less scaly or chaffy pappus with slight mobility, as in
_Rudbeckia_, _Brauneria_, _Helianthus_, etc.

6. Plumed, lophospores. In the fruits of this class, the style is the
part usually modified into a long plumose organ, possessing a high
degree of mobility, as in _Pulsatilla_, _Sieversia_, and _Clematis_.

7. Awned, ascospores. These are almost exclusively grasses, in which the
awns serve for distribution by wind, water, or animals, and even,
according to Kerner, by hygroscopic creeping movements. The mobility in
many cases is great.

8. Spiny, centrospores. This group contains a few representatives which
possess a moderate degree of mobility by attachment, as in _Tribulus_
and _Cenchrus_.

9. Hooked, oncospores. The members of this group are extremely numerous,
and the degree of mobility as a rule is very high. All exhibit in common
the development of hooks or barbs, by which they are disseminated in
consequence of attachment, though the number, size, and disposition of
the hooks vary exceedingly.

10. Viscid, gloeospores. In these, the inflorescence is more or less
covered with a viscid substance, as in species of _Silene_, or the fruit
is beset with glandular hairs, as in _Cerastium_, _Salvia_, etc.

11. Fleshy, sarcospores. These are intended for dissemination by
deglutition, largely by birds; the effectiveness of the modification
depends in a large degree upon the resistance of the seed envelope to
digestion. The mobility varies greatly, but the area over which
migration may be effected is large.

12. Nut-fruited, creatospores. This group includes those plants with nut
fruits which are carried away and secreted by animals for food.

13. Flagellate, mastigospores. These are plants with ciliate or
flagellate propagative cells, i. e., zoogonidia, as in _Protococcus_,
_Ulothrix_, _Oedogonium_, _Ectocarpus_, etc., or with plant bodies
similarly motile, _Bacteriaceae_ and _Volvocaceae_.

=265. Position of disseminule.= The position on the plant of the organ
to be disseminated, i. e., its exposure to the distributing agent, plays
a considerable part in determining the degree of mobility. In the
majority of plants, the position of the inflorescence itself results in
maximum exposure, but in a large number of forms special modifications
have been developed for placing the spores or seeds in a more favorable
position. In both cases, there are often present also devices for
bringing about the abscission of the seed or fruit. It is, moreover,
self-evident that the height of the inflorescence above ground or above
the surrounding vegetation is likewise of considerable importance in
increasing the trajectory. It is yet too early to make a complete
classification of contrivances for placing disseminules in the most
favorable exposure, but the following will serve as a basis for future
arrangements.

1. In all operculate _Discomycetes_, and especially in the
_Ascobolaceae_, where the asci project above the hymenium, the spores
are raised above the surface by tensions within the apothecium. This
might be regarded as dissemination by expulsion, if it were not for the
fact that the spores fall back into the cup, unless carried away by the
wind.

2. In _Gasteromycetes_ and in certain _Hepaticae_, the spores are not
only elevated slightly above the sporophore by the expanding capillitium
or by the mass of elaters, but they are also held apart in such a way
that the wind blows them out much more readily.

3. In _Bryophyta_, the sporophore regularly dehisces by a slit, or is
provided with a peristome. Both structures are for the purpose of
sifting the spores out into the wind; by reason of their hygroscopicity,
they also insure that the spores will not be shaken out in wet weather.

4. In a few grasses, such as _Stipa_ and _Aristida_, the twisting and
intertwining of the awns lift the floret out of the glumes, and at the
same time constitute a contrivance readily blown away by the wind or
carried by attachment.

5. In certain _Compositae_, the involucral scales are reflexed at
maturity, and at the same time the disk becomes more or less convex,
serving to loosen the achenes. This result is also secured in certain
species by the drying and spreading of the pappus hairs.

6. The scapose _Liguliflorae_, _Taraxacum_, _Agoseris_, etc., are
characterized by the elongation of the scape after anthesis, with the
result that the head is raised to a considerable height by the time the
achenes are mature.

7. Carpotropic movements, though primarily for another purpose, often
serve to bring seeds and fruits into a better position for
dissemination.

=266. Seed production.= The relation of spore or seed-production to
mobility is obvious in the case of mobile species; in the case of
immobile ones, it is just as evident that it has no effect, though it
may still have considerable influence in increasing migration. In the
case of two species with equally effective dissemination contrivances,
the one with the largest seed-production will be the more mobile. On the
basis of the relation of seeds to flower, two groups of plants may be
distinguished, one, _Polyanthae_, in which the flowers are many and the
seeds few or single, as in _Compositae_, and the other, _Polyspermatae_,
_Portulaca_, _Yucca_, etc., in which the number of seeds to each flower
is large. So far as the actual number of seeds produced is concerned,
polyanthous plants may not differ from polyspermatous ones, but, as a
rule, they are much more highly specialized for dissemination and are
more mobile. The number of fertile seeds is also much greater, a fact
which is of great importance in ecesis, and which, taken in connection
with mobility, partially explains the supremacy of the composites. Among
the fungi and algae, the amount of spore-production in a large degree
determines the mobility, since these forms are intrinsically permobile.

=267. Agents of migration.= In the last analysis, however, the
possibility of migration depends upon the action of distributive agents;
in the absence of these, even the most perfect contrivance is valueless,
while their presence brings about the distribution of the most immobile
form. In short, migration depends much more upon such agents than upon
mobility, however perfect the latter may be. It is, moreover, evident
that the amount and extent of migration will be determined primarily by
the permanence and forcefulness of the agent, as indicated by its
ability to bring about transportation. Finally, as will be shown later,
the direction and rapidity of migration depend directly upon the
direction and intensity of the agent.

Migration results when spores, seeds, fruits, offshoots, or plants are
moved out of their home by water, wind, animals, man, gravity, glaciers,
growth, or mechanical propulsion. Corresponding to these agents, there
may be recognized the following groups:

1. Water, hydrochores. These comprise all plants distributed exclusively
by water, whether the latter acts as ocean currents, tides, streams, or
surface run-off. In the case of streams and run-off, especially,
mobility plays little part, provided the disseminules are impervious or
little subject to injury by water. Motile plants, or those with motile
cells, which belong entirely to this group, may be distinguished as
autochores, which correspond closely to mastigospores.

2. Wind, anemochores. This group includes the majority of all permobile
terrestrial plants, i. e., those in which modifications for increasing
surface have been carried to the extreme, or those which are already
permobile by reason of the minuteness of the spore or seed. Saccate,
winged, comate, parachute, pappose, plumed, and, to a certain extent,
awned seeds and fruits represent the various types of modifications for
wind-distribution.

3. Animals, zoochores. Among terrestrial plants, dissemination by
attachment represents essentially the same degree of specialization as
is found in wind-distributed plants. The three types of contrivances for
this purpose are found in spinose, hooked, and glandular fruits.
Dissemination by deglutition and by carriage, either intentional or
unintentional, though of less value, play a striking part on account of
the great distance to which the seeds may be carried. Dissemination by
deglutition is characteristic of sarcospores, and distribution by
carriage of creatospores.

4. Man, brotochores. Dissemination by man has practically no connection
with mobility. It operates through great distances and over immense
areas as well as near at hand. It may be intentional, as in the case of
cultivated species, or unintentional, as in thousands of native or
exotic species. No other disseminating agent is comparable with man in
respect to universal and obvious migration.

5. Gravity, clitochores. The members of this group are exclusively
colline, montane, and alpine plants, growing on rocks, cliffs, and
gravel slides (talus), etc., in which the seeds reach lower positions
merely by falling, or more frequently by the breaking away and rolling
down of rock or soil masses and particles. Dissemination by this method
is relatively insignificant, though it plays an important part in the
rock fields and gravel slides of mountain regions, particularly in the
case of immobile species.

6. Glaciers, crystallochores. At the present time, transport by glaciers
is of slight importance, because of the restriction of the latter to
alpine and polar regions, where the flora is poorly developed. In the
consideration of migrations during the glacial epoch, however, it plays
an important point.

7. Growth, blastochores. The mobility of species disseminated by
offshoots is extremely slight, and the annual movement relatively
insignificant. The certainty of migration and of ecesis, is, however, so
great, and the presence of offshoots so generally the rule in
terrestrial plants that growth plays an important part in migration,
especially within formations.

8. Propulsion, bolochores. Like growth, dissemination by mechanical
propulsion, though operating through insignificant distances, exerts an
important effect in consequence of its cumulative action. The number of
plants, however, with contrivances for propulsion is very much smaller
than the number of blastochores. All bolochorous species agree in having
modifications by means of which a tension is established. At maturity,
this tension suddenly overcomes the resistance of sporangium or fruit,
and throws the enclosed spores or seeds to some distance from the parent
plant. In accordance with the manner in which the tension is produced,
sling-fruits may be classified as follows:

(_a_) Hygroscopicity, pladoboles. These include the ferns with annulate
sporangia, in which the expansion of the annulus by the absorption of
moisture bursts the sporangium more or less suddenly, though the actual
propulsion of the spores seems to come later as a result of dessication.

(_b_) Turgescence, edoboles. Dissemination by turgescence is highly
developed in _Pilobolus_ and in _Discomycetes_, though in the latter
turgescence results rather in placing the spores in a position to be
readily carried by the wind. _Impatiens_ and _Oxalis_ furnish familiar
examples of fruits which dehisce in consequence of increased turgidity.

(_c_) Dessication, xerioboles. The number of fruits which dehisce upon
drying is very large, but only a small portion of these expel their
seeds forcibly. _Geranium_, _Viola_, _Erysimum_, and _Lotus_ illustrate
the different ways in which dessication effects the sudden splitting of
fruits.

(_d_) Resilience, tonoboles. In some plants, especially composites,
labiates, and borages, the achenes or nutlets are so placed in the
persistent calyx or involucre that the latter serves as a sort of mortar
for projection, when the stem of the plant is bent to one side by any
force, such as the wind or an animal. It will be noticed that two
separate agents are actually concerned in dissemination of this sort.

Frequently, two or more agents will act upon the same disseminule,
usually in succession. The possibility of such combinations in nature is
large, but actual cases seem to be infrequent, except where the
activities of man enter into the question. Some parts, moreover, such as
awned inflorescences, are carried almost equally well by wind or
animals, and may often be disseminated by the cooperation of these two
agents. The wind also often blows seeds and fruits into streams by which
they are carried away, but here again, parts adapted to
wind-dissemination are injured as a rule by immersion in water, and the
number of plants capable of being scattered by the successive action of
wind and water is small.

In the present state of our knowledge of migration, it is impossible to
establish any definite correspondence between dissemination-contrivance,
agent, and habitat. As a general rule, plants growing in or near the
water, in so far as they are modified for this purpose at all, are
adapted to water-carriage. Species which grow in exposed grassy or
barren habitats are for the most part anemochores, while those that are
found in the shelter of forests and thickets are usually zoochorous,
though the taller trees and shrubs, being exposed to the upper air
currents, are generally wind-distributed. There is then a fair degree of
correspondence, inasmuch as most hydrophytes are hydrochorous, most
hylophytes, zoochorous, and the majority of poophytes and xerophytes,
anemochorous. Definite conclusions can be reached, however, only by the
statistical study of representative formations.

With respect to their activity, agents may be distinguished as constant,
as in the case of currents, streams, winds, slope, growth, and
propulsion, or intermittent, animals and man. In the former, the
direction is more or less determinate, and migration takes place year by
year, i. e., it is continuous, while in the latter dissemination is
largely an accidental affair, indeterminate in direction, and recurring
only at indefinite intervals. The effective conversion of migration into
invasion is greatest when the movement is continuous, and least when it
is discontinuous, since, in the latter, species are usually carried not
only out of their particular habitat but even far beyond their
geographical area, and the migration, instead of being an annual one
with the possibility of gradual adjustment, may not recur for several
years, or may, indeed, never take place again. The rapidity of migration
is greatest in the case of intermittent agents, while the distance of
migration is variable, being great chiefly in the case of man, ocean
currents, and wind, and slight when the movement is due to slope,
growth, or propulsion. Disregarding the great distances over which
artificial transport may operate, seeds may be carried half way across
the continent in a week by strong-flying birds, while the possibilities
of migration by growth or expulsion are limited to a few inches, or at
most to a few feet per year. This slowness, however, is more than
counterbalanced by the enormously greater number of disseminules, and
their much greater chance of becoming established.

=268. The direction of migration= is determinate, except in the case of
those distributive agents which act constantly in the same direction.
The general tendency is, of course, forward, the lines of movement
radiating in all directions from the parent area. This is well
illustrated by the operation of winds which blow from any quarter. In
the case of the constant winds, migration takes a more or less definite
direction, the latter being determined to a large degree by the fruiting
period of any particular species. In this connection, it must be kept
clearly in mind that the position of new areas with reference to the
original home of a species does not necessarily indicate the direction
of migration, as the disseminules may have been carried to numerous
other places in which ecesis was impossible. The local distribution of
zoochorous species is of necessity indeterminate, though distant
migration follows the pathways of migratory birds and animals. In so far
as dissemination by man takes place along great commercial routes, or
along highways, it is determinate. In ponds, lakes, and other bodies of
standing water, migration may occur in all directions, but in ocean
currents, streams, etc., the movement is determinate, except in the case
of motile species. The dissemination of plants by slopes, glaciers,
etc., is local and definite, while propulsion is in the highest degree
indeterminate. Migration by growth is equally indefinite, with the
exception that hydrotropism and chemotropism result in a radiate
movement away from the mass, while propulsion throws seeds indifferently
into or away from the species-mass. From the above it will be seen that
distant migration may take place by means of water, wind, animals or
man, and, since all these agents act in a more or less definite
direction over great distances, that it will be in some degree
determinate. On the other hand, local migration will as regularly be
indeterminate, except in the case of streams and slopes. The direction
of migration, then, is controlled by these distributive agents, and the
limit of migration is determined by the intensity and duration of the
agent, as well as by the character of the space through which the latter
operates.


                                _ECESIS_

=269. Concept.= By the term ecesis is designated the series of phenomena
exhibited by an invading disseminule from the time it enters a new
formation until it becomes thoroughly established there. In a word,
ecesis is the adjustment of a plant to a new habitat. It comprises the
whole process covered more or less incompletely by acclimatization,
naturalization, accommodation, etc. It is the decisive factor in
invasion, inasmuch as migration is entirely ineffective without it, and
is of great value in indicating the presence and direction of migration
in a great number of species where the disseminule is too minute to be
detected or too little specialized to be recognizable.

The relation of migration to ecesis is a most intimate one: the latter
depends in a large measure upon the time, direction, rapidity, distance,
and amount of migration. In addition, there is an essential alternation
between the two, inasmuch as migration is followed by ecesis, and the
latter then establishes a new center from which further migration is
possible, and so on. The time of year in which fruits mature and
distributive agents act has a marked influence upon the establishment of
a species. Disseminules designed to pass through a resting period are
often brought into conditions where they germinate at once, and in which
they perish because of unfavorable physical factors, or because
competing species are too far advanced. On the other hand, spores and
propagules designed for immediate germination may be scattered abroad at
a time when conditions make growth impossible. The direction of movement
is decisive in that the seed or spore is carried into a habitat
sufficiently like that of the parent to secure establishment, or into
one so dissimilar that germination is impossible, or at least is not
followed by growth and reproduction. The rapidity and distance of
migration have little influence, except upon the less resistant
disseminules, conidia, gemmae, etc. Finally, the amount of migration, i.
e., the number of migrants, is of the very greatest importance,
affecting directly the chances that vigorous disseminules will be
carried into places where ecesis is possible.

Normally, ecesis consists of three essential processes, germination,
growth, and reproduction. This is the rule among terrestrial plants, in
which migration regularly takes place by means of a resting part. In
free aquatic forms, however, the growing plant or part is usually
disseminated, and ecesis consists merely in being able to continue
growth and to insure reproduction. Here establishment is practically
certain, on account of the slight differences in aquatic habitats,
excepting of course the extremes, fresh water and salt water. The ease
indeed with which migration and ecesis are effected in the water often
makes it impossible to speak properly of invasion in this connection,
since aquatics are to such a large extent cosmopolitan. In dissemination
by offshoots, the conditions are somewhat similar. Here, also, ecesis
comprises the sequence of growth and reproduction, and invasion, in the
sense of passing from one habitat to another, is of rare occurrence, as
the offshoot grows regularly under the same conditions as the parent
plant. The adjustment of growing plants and parts is so slight, and
their establishment so certain on account of their inability to migrate
into very remote or different habitats, that they may be ignored in the
following discussion.

In accordance with the above, it would be possible to distinguish three
groups of terrestrial plants: (1) those migrants which germinate and
disappear, (2) those which germinate and grow but never reproduce, (3)
those which reproduce, either by propagation or generation, or both.
Such a classification has little value, however, since the same species
may behave in all three fashions, depending upon the habitat to which it
has migrated, and since invasion does not occur unless the plant
actually takes possession, i. e., reproduces. From the latter statement,
it follows that invasion occurs only when a species migrates to a new
place, in which it germinates, matures, and reproduces. Maintenance by
annual invasion simply, in which the plants of each year disappear
completely, can not then be regarded as invasion proper. On the other
hand, though such instances are rare, it is not necessary that the
invaders produce fruit, provided they are able to maintain themselves,
or to increase by propagation. Furthermore, if a plant germinate, grow,
and reproduce, it is relatively immaterial whether it persist for a few
years or for many, since, as we shall see under Succession, the plants
of one invasion are displaced by those of the next, the interval between
invasions increasing with the stabilization.

=270. Germination of the seed.= The germination of seed or spore is
determined by its viability and by the nature of the habitat. Viability
depends upon the structural characters of fruit, seed-coat, and
endosperm, and to a degree upon the nature of the protoplasm or embryo.
The first three affect the last directly, by protecting the embryo
against dryness, against injury due to carriage by water, or by
deglutition, and probably in some cases against excessive heat or cold.
Marloth[33] has investigated the structure of seed coats, establishing
the following groups, which are summarized here somewhat fully because
of their bearing upon ecesis: (1) seed coats without protective
elements, endosperm absent or rudimentary, _Epilobium_, _Impatiens_,
_Parnassia_, _Sagittaria_, etc.; (2) protective elements lacking or few,
endosperm highly developed with thick-walled cells, _Liliaceae_,
_Primulaceae_, _Rubiaceae_, etc.; (3) protective cells present in the
seed coats, endosperm little or none, _Boraginaceae_, _Crassulaceae_,
_Cruciferae_, _Labiatae_, _Papilionaceae_, etc.; (4) protective elements
present, _Asclepias_, _Campanula_, _Gentiana_, _Silene_, _Saxifraga_,
etc.; (5) protective cells present, endosperm thick-walled, _Euonymus_,
_Helianthemum_, _Ribes_. The protective cells are of various kinds: (1)
epidermal cells strongly cuticularized, _Caryophyllaceae_,
_Crassulaceae_, _Fumariaceae_, _Saxifragaceae_; (2) parenchyma
thick-walled, several-layered, _Aesculus_, _Castanea_, _Fagus_; (3)
parenchyma cells with the inner or radial walls thickened, _Campanula_,
_Erythraea_, _Gentiana_; (4) epidermal cells cup-shaped, thick-walled,
_Cruciferae_, _Ribes_, _Vaccinium_; (5) parenchyma with thickened,
cellulose walls, _Geranium_, _Viburnum_; (6) a single row of
stone-cells, _Labiatae_; (7) tissue of stone-cells, _Hippuris_, _Naias_,
_Potamogeton_; (8) elongate stone-cells, _Coniferae_, _Cupuliferae_,
_Euphorbia_, _Linum_, _Malva_, _Viola_; (9) short, columnar,
thick-walled branched cells, _Cucurbitaceae_, _Datura_, _Hypericum_;
(10) prosenchyma with cellulose walls, _Clematis_; (11) prosenchyma with
lignified walls, _Fraxinus_, _Rhamnus_, _Ranunculus_. The seed coats
have a certain influence in determining germination at the proper time,
inasmuch as they make it difficult for the seed to germinate under the
stimulus of a quantity of warmth and moisture insufficient to support
the seedling. The effect of the endosperm, as well as that of other food
supply in the seed, upon germination and the establishment of the
seedling is obvious.

The behavior of seed or spore with respect to germination depends in a
large degree upon the character of the protoplasm or embryo, though in
just what way is at present a matter of conjecture. It is evident that
many seeds are not viable because fertilization has not been effected,
and in consequence no embryo has developed. This is the usual
explanation of the low germinating power of the seeds of some species,
especially polyspermatous ones. But even in viable seeds the behavior is
always more or less irregular. The seeds of some species will grow
immediately after ripening, while others germinate only after a resting
period of uncertain duration. The same is true of spores. Even in the
case of seeds from the same parent, under apparently similar conditions,
while the majority will germinate the first year, some will lie dormant
for one or more years. The precise reason why many seeds and spores
germinate more readily after being frozen is equally obscure. The period
of time for which disseminules may remain viable is extremely diverse,
though, as would be expected, it is much longer as a rule for seeds than
for spores. The greater vitality of seeds in the case of ruderal plants
suggests that this diversity may be due simply to variation in the vigor
of the embryos. It would seem that under proper conditions seeds may
retain their viability for an indefinite period.

The influence of habitat upon germination is of primary importance,
though the manner in which its influence is exerted is by no means as
evident as might be supposed. In the case of seeds sown in the
planthouse, it is almost universally the case that germination is less
than in nature, notwithstanding the fact that temperature and moisture
appear to be optimum. In nature, the seeds of the species may be carried
into a number of different formations, any one or all of which may
present conditions unfavorable to germination. With respect to
probability of germination, habitats are of two sorts: those which are
denuded and those which bear vegetation. It is impossible to lay down
general propositions with respect to either group, since germination
will vary with the character of the invading species, the annual
distribution of heat and moisture in the habitat, etc. In a general way,
however, it may be stated that the chances for germination are greater
in vegetation than in denuded areas, chiefly because the latter are
usually xerophytic. On the other hand, the lack of competition in the
denuded area tends to make ultimate establishment much more certain.
Here, as elsewhere when exact statistical results are desired, the use
of the quadrat, and especially of the permanent quadrat, is necessary to
determine the comparative germination of the invading species in
relation to denudation and vegetation.

=271. Adjustment to the habitat.= The seedling once established by
germination, the probability of its growing and maturing will depend
upon its habitat form, plasticity, and vegetation form. Even though it
may germinate under opposite conditions, a typical hylophyte, such as
Impatiens for example, will not thrive in an open meadow, nor will
characteristic poophytes, such as most grasses, grow in deep shade. In
the same way, xerophytes do not adapt themselves to hydrophytic
habitats, nor hydrophytes to xerophytic conditions. Many mesophytes,
however, possess to a certain degree the ability to adjust themselves to
somewhat xerophytic or hydrophytic situations, while woodland plants
often invade either forest or meadow. This capability for adjustment, i.
e., plasticity, is greatest in intermediate species, those that grow in
habitats not characterized by great excess or deficiency of some factor,
and it is least in forms highly specialized in respect to water-content,
shade, etc. It may then be established as a fundamental rule that ecesis
is determined very largely by the essential physical similarity of the
old and the new habitat, except in the case of plastic forms, which
admit of a wider range of accommodation. The plasticity of a plant is
not necessarily indicated by structural modification, though such
adjustment is usually typical of plastic species, but it may sometimes
arise from a functional adaptation, which for some reason does not
produce concomitant structural changes. The former explains such various
habitat forms of the same species as are found in _Galium boreale_,
_Gentiana acuta_, etc., and the latter the morphological constancy of
plants like _Chamaenerium_, which grow in very diverse habitats.

The vegetation form of the invading species is often of the greatest
importance in determining whether it will become established. The
vegetation form represents those modifications which, produced in the
original home by competition, i. e., the struggle for existence, are
primarily of value in securing and maintaining a foothold. These
comprise all structures by means of which the plant occupies a definite
space in the air, through which the necessary light and heat reach it,
and in the soil, from which it draws its food supply. These structures
are all organs of duration or of perennation, such as root, rootstalk,
bulb, tuber, woody stem, etc., which find their greatest development
among trees and shrubs, and their least among annual herbs. But while
the invaders are aided in securing possession by the proper vegetation
form, the occupation of the plant already in possession is increased by
the same means, and the outcome is then largely determined by other
factors. To avoid repetition, the bearing of occupation upon invasion
will be considered under succession.


                               _BARRIERS_

=272. Concept.= DeCandolle[34] seems to have been the first to use the
term barrier and to distinguish the various kinds, though Hedenberg[35]
clearly saw that stations of one kind were insurmountable obstacles to
plants belonging to a very different type. De Candolle pointed out that
the natural barriers to continuous invasion (“transport de proche en
proche”) are: (1) seas, which decrease invasion almost in inverse
proportion to their extent; (2) deserts; (3) mountain ranges, which are
less absolute on account of passes, valleys, etc.; (4) vegetation,
marshes being barriers to dry land plants, forests to those that fear
the shade, etc. Grisebach[36], in discussing the effect of barriers upon
the constitution of vegetation, laid down the fundamental rule that:
“The supreme law which serves as the basis of the permanent
establishment of natural floras is to be recognized in the barriers
which have hindered or completely prevented invasion.”

Any feature of the topography, whether physical or biological, that
restricts or prevents invasion, is a barrier. Such features are usually
permanent and produce permanent barriers, though the latter may often be
temporary, existing for a few years only, or even for a single season.
In this last case, however, they are as a rule recurrent. Barriers may
furthermore be distinguished as complete or incomplete with respect to
the thoroughness with which they limit invasion. Finally, the
consideration of this subject gains clearness if it be recognized that
there are barriers to migration as well as to ecesis, and if we
distinguish barriers as physical or biological with reference to the
character of the feature concerned.

=273. Physical barriers= are those in which limitation is produced by
some marked physiographic feature, such as the ocean or some other large
body of water, large rivers, mountain ranges and deserts (including ice
and snow fields). All of these are effective by virtue of their dominant
physical factors; hence they are barriers to the ecesis of species
coming from very different habitats, but they act as conductors for
species from similar vegetation, especially in the case of water
currents. A body of water, representing maximum water-content, is a
barrier to mesophytic and xerophytic species, but a conductor for
hydrophytic ones; deserts set a limit to the spread of mesophytic and
hydrophytic plants, while they offer conditions favorable to the
invasion of xerophytes; and a high mountain range, because of the
reduction of temperature, restricts the extension of macrothermal and
mesothermal plants. A mountain range, unlike other physical barriers, is
also an obstacle to migration, inasmuch as natural distributive agents
rarely act through it or over it.

=274. Biological barriers= include vegetation, man and animals, and
plant parasites. The limiting effect of vegetation is exhibited in two
ways. In the first place, a formation acts as a barrier to the ecesis of
species invading it from the formations of another type, on account of
the physical differences of the habitats. Whether such a barrier be
complete or partial will depend upon the degree of dissimilarity
existing between the formations. Hylophytes are unable to invade a
prairie, though open thicket plants may do so to a certain degree. In
the same way, a forest formation on account of its diffuse light is a
barrier to poophytes; and a swamp, because of the amount and character
of the water-content, sets a limit to both hylophytes and poophytes.
Formations, such as forests, thickets, etc., sometimes act also as
direct obstacles to migration, as in the case of tumbleweeds and other
anemochores, clitochores, etc. A marked effect of vegetation in
decreasing invasion arises from the closed association typical of stable
formations and of social exclusive species. In these, the occupation is
so thorough and the struggle for existence so intense that the invaders,
though fitted to grow under the physical factors present, are unable to
compete with the species in possession for the requisite amount of some
necessary factor. Closed associations usually act as complete barriers,
while open ones restrict invasion in direct proportion to the degree of
occupation. To this fact may be traced a fundamental law of succession,
viz., the number of stages in a succession is determined largely by the
increasing difficulty of invasion as the habitat becomes stabilized. Man
and animals affect migration directly, though not obviously, by the
destruction of disseminules. They operate as a pronounced barrier to
ecesis wherever they alter conditions in such a way as to make them
unfavorable to invading species, or when, by direct action upon the
latter, such as grazing, tramping, parasitism, etc., they turn the scale
in the struggle for existence. The absence of insects adapted to insure
fertilization is sometimes a serious barrier to the establishment of
adventitious or introduced plants. The presence of parasitic fungi, in
so far as they destroy the seeds of plants, acts as an obstacle to
migration, and restricts or prevents ecesis in so far as the fungi
destroy the invaders, or place them at a disadvantage in the struggle
for existence.

=275. Influence of barriers.= Physical barriers are typically permanent
in character, while biological ones are either permanent or temporary,
depending upon the permanence of the formation and the constancy of the
physical factors which determine it. A stable formation, such as a
forest or meadow, which acts as a decided barrier to invasion from
adjacent vegetation, may disappear completely, as a result of a
landslide, flood, or burn, or through the activity of man, and may leave
an area into which invaders crowd from every point. Often, without
undergoing marked change, a formation which has presented conditions
unfavorable to the ecesis of species of mesophytic character may, by
reason of a temporary change in climate, become sufficiently modified to
permit the invasion of mesophytes. On the other hand, a meadow ceases to
be a barrier to prairie xerophytes during a period of unusually dry
years. A peculiar example of the modification of a barrier is afforded
by the defoliation of aspen forests in the mountains as a result of
which poophytes have been enabled to invade them. Nearly all xerophytic
stretches of sand and gravel, dunes, blowouts, gravel slides, etc., and
even prairies to a certain degree, exhibit a recurrent seasonal change
in spring, as a result of which the hot, dry surface becomes
sufficiently moist to permit the germination and growth of invaders,
which are entirely barred out during the remainder of the year. In an
absolute sense, no barrier is complete, since the coldest as well as the
driest portions of the earth’s surface are capable, at times at least,
of supporting the lowest types of vegetation. Relatively, however, in
connection with the natural spread of terrestrial plants, it is possible
to distinguish partial barriers from complete ones. Such a distinction
is of importance in the consideration of invasions from a definite
region, as it is only in this restricted sense that complete barriers
have produced endemism.

Distance, though hardly to be considered a barrier in the strict sense
of the word, unquestionably plays an important part in determining the
amount of invasion. The effect of distance is best seen in the case of
migration, as it influences ecesis only in those rare cases where
viability is affected. The importance of distance, or take the converse,
of proximity, is readily ascertained by the study of any succession from
denudation. It has been established that the contiguous vegetation
furnishes 75–90 per cent of the constituent species of the initial
formation, and in mountainous regions, where ruderal plants are
extremely rare, the percentage is even higher. The reason for this is to
be found not only in the fact that the adjacent species have a much
shorter distance to go, and hence will be carried in much greater
quantity, but also in that the species of the formations beyond must
pass through or over the adjacent ones. In the latter case, the number
of disseminules is relatively small on account of the distance, while
invasion through the intermediate vegetation, if not entirely
impossible, is extremely slow, so that plants coming in by this route
reach the denuded area only to find it already occupied. It is as yet
impossible to give a definite numerical value to proximity in the
various invasions that mark any particular succession. This will not be
feasible until a satisfactory method has been found for determining a
coefficient of mobility, but, this once done, it will be a relatively
simple matter, not merely to trace the exact evolution of any succession
of formations, but actually to ascertain from the adjacent vegetation
the probable constitution of a particular future stage.

From what has been said, it follows that the primary effect of barriers
upon vegetation is obstruction. Where the barrier is in the pathway of
migration, however, it causes deflection of the migrant as a rule, and
sets up migration in a new direction. This is often the case when the
strong winds of the plains carry disseminules towards the mountains and,
being unable to cross the range, drop them at the base, or, being
deflected, carry them away at right angles to the original direction.
The same thing happens when resistant fruits and seeds borne by the wind
fall into streams of water or into ocean currents. The direction of
migration is changed, and what is normally a barrier serves as an agent
of dissemination.


                               _ENDEMISM_

=276. Concept.= Since its first use by DeCandolle, the term endemic has
been employed quite consistently by phytogeographers with the meaning of
“peculiar to a certain region.” Some confusion, however, has arisen from
the fact that a few authors have made it more or less synonymous with
indigenous and autochthonous, while others have regarded it as an
antonym of exotic. In its proper sense, endemic refers to distribution,
and not to origin. Its exact opposite will be found then in Fenzl’s term
polydemic, dwelling in several regions. Indigenous (autochthonous) and
exotic, on the contrary, denote origin, and are antonyms, indigenous
signifying native, and exotic foreign. As Drude has shown, endemic
plants may be either indigenous, as in the case of those species that
have never moved out of the original habitat, or exotic, as in the much
rarer instances where a polydemic species has disappeared from its
original home and from all regions into which it has migrated except
one. It is understood that not all indigenous or exotic species are
endemic. The proportion of endemic to polydemic species is a variable
and somewhat artificial one, depending upon the size of the divisions
employed.

=277. Causes.= The primary causes of endemism are two, lack of migration
and presence of barriers. Since distributive agents are practically
universal, lack of migration corresponds essentially to immobility, a
fact which decreases the difficulty of ascertaining the immediate causes
of endemism in any particular species. Either immobility or a barrier
may produce endemism; extremely immobile plants, for example, liliaceous
species propagating almost wholly by underground parts, are as a rule
endemic, while alpine plants and those of oceanic islands are endemic in
the highest degree, regardless of their mobility. When the two
conditions act concomitantly upon a species, endemism is almost
inevitable. It can not be supposed, however, that immobility or natural
barriers alone, or the concomitance of the two, must invariably give
rise to endemic species; the most immobile plant may be carried into
another region by unusual or accidental agencies, or the most formidable
barrier to migration may be overcome by the intensity of an agent or
through the action of man. Endemism is also brought about by the
modification of species; new or nascent species are as a rule endemic.
Whether they will remain endemic or not will depend upon the perfection
of their contrivances for dissemination and upon the presence of
barriers to migration or ecesis. Finally, as Drude was the first to
point out, the disappearance of a polydemic species in all regions but
one, owing to the struggle for existence or to changed physical
conditions, will result in endemism.

=278. Significance.= Endemism is readily recognized by methods of
distributional statistics, applied to areas limited by natural barriers
to migration or ecesis. For political areas, it has no significance
whatever, unless the boundaries of these coincide with barriers. It
determines in the first degree the validity of regions, though the
latter are often recognized also by the presence of barriers and by the
character of the vegetation. Endemism may occur in areas of vegetation
of any rank from a formation to a zone. When the term is not qualified,
however, it should be used of species with reference to formations
alone. Comparisons to be of value, however, can be instituted only
between areas of the same order, i. e., between two or more formations,
two or more regions, provinces, etc. In the same way, taxonomic groups
of the same rank should be used in such comparisons, i. e., species
should be contrasted with species, genera with genera, and families with
families, except when it is desired to obtain some measure of the age of
the vegetation by the differentiation of the endemic phyla within it.
There will be seen to exist a fundamental correspondence between the
rank of the floral division and the taxonomic group, though the apparent
exceptions to this are still too numerous to warrant its expression in a
general law. As a rule, however, formations most frequently show endemic
habitat forms and species, more rarely endemic genera; regions and
provinces commonly exhibit endemic species and genera, rarely endemic
families; while zones and hemispheres contain endemic orders as well as
families. This correspondence is readily seen to depend primarily upon
the fact that increased differentiation in the taxonomic sense is a
concomitant of the increased invasion of endemic species, measured in
terms of distance and difference in habitat.

It is too early to decide satisfactorily whether it is proper to speak
of formations as endemic. At first thought it would seem that all
formations, with the exception of ruderal ones, were endemic, but a
study of almost any transition area between regions would seem to point
to the opposite conclusion, viz., that no formations are properly
endemic. It is equally impossible at present to distinguish different
types of endemics, such as relictae, etc., as any such classification
must await the elaboration of a method for determining the phylogeny of
a natural group of species by an investigation of their comparative
differentiation in connection with their migration in all directions
from the vegetation center into new habitats. In short, it will not be
possible to make a thorough study of endemism and to postulate its laws
until modern methods of research have been extended to a much larger
portion of the vegetation of the globe. The final task of phytogeography
is the division of the earth’s vegetation into natural areas. It will be
at once evident that most plants can not properly be called endemic
until the natural regions in which they are found have been accurately
defined, a work which has barely begun. In the much simpler matter of
distribution, upon which the accuracy of statistical methods depends
directly, there are few regions sufficiently well known at the present
time to yield anything like permanent results.


                     _POLYPHYLESIS AND POLYGENESIS_

=279. Concept.= The idea of polyphylesis, as advanced by Engler,
contains two distinct concepts: (1) that a species may arise in two
different places or at two different times from the same species, and
(2) that a genus or higher group may arise at different places or times
by the convergence of two or more lines of origin. It is here proposed
to restrict polyphylesis, as its meaning would indicate, to the second
concept, and to employ for the first the term polygenesis,[37] first
suggested by Huxley in the sense of polyphylesis. The term polyphylesis
is extended, however, to cover the origin of those species which arise
at different places or times from the convergence of two or more
different species, a logical extension of the idea underlying
polyphyletic genera, though it may seem at first thought to be absurd.
Polygenesis may be formally defined as the origin of one species from
another species at two or more distinct places on the earth’s surface,
at the same time or at different times, or its origin in the same place
at different times. Polyphylesis, on the contrary, is the origin of one
species from two or more different species at different places, at the
same time or at different times. It is evident that what is true of
species in this connection will hold equally well of genera and higher
groups. Opposed to polygenesis is monogenesis, in which a species arises
but once from another species; with polyphylesis is to be contrasted
monophylesis, in which the species arises from a single other species.
It will be noticed at once that these two concepts are closely related.
The following diagrams will serve to make the above distinctions more
evident:

[Illustration: I. Polygenesis II. Polyphylesis III. Monogenesis
(Monophylesis)]

In I, a species A, becomes scattered over a large area in a series of
places, _m_ ... _m^n_, with the same physical factors, in any or all of
which may arise the new species _a_. In II, a species with xerophytic
tendency, _A_, and one with mesophytic tendency, _B_, in the course of
migration find themselves respectively in a more mesophytic habitat,
_m_, and a more xerophytic one, _x_, in which either may give rise to
the new form, _c_, which is more or less intermediate between _A_ and
_B_. In III, the method of origin is of the simplest type, in which a
species is modified directly into another one, or is split up into
several.

=280. Proofs of polygenesis.= In affirming the probability of a
polygenetic origin of species, there is no intention of asserting that
all species originate in this way. It seems evident that a very large
number of species of restricted range are certainly monogenetic, at
least as far as origin in space is concerned. It is possible that any
species may arise at two or more distinct times. Polygenesis can occur
readily only in species of more or less extensive area, in which recur
instances of the same or similar habitat. The relative frequence and
importance of the two methods can hardly be conjectured as yet, but
origin by monogenesis would seem to be the rule.

The arguments adduced by Engler in support of polygenesis are in
themselves conclusive, but the investigations of the past decade have
brought to light additional proofs, especially from the experimental
side. In determining the physical factors of prairie and mountain
formations, and especially by methods of experimental ecology, the
author has found that habitats are much less complex than they are
ordinarily thought to be, since water-content and humidity, and to a
less degree light, constitute the only factors which produce direct
modification. In addition, it has been ascertained that the minimum
difference of water-content, humidity, or light, necessary to produce a
distinguishable morphological adjustment is much greater than the unit
differences recorded by the instruments. In short, the differences of
habitats, as ascertained by thermograph, psychrometer and photometer,
are much greater than their efficient differences, and, with respect to
their ability to produce modification, habitats fall into relatively few
categories. A striking illustration of this is seen in the superficially
very different habitats, desert, strand, alkali plain, alpine moor, and
arctic tundra, all of which are capable of producing the same type of
xerophyte. It follows from this that many more or less plastic species
of extensive geographical area will find themselves in similar or
identical situations, measured in terms of efficient differences, and
will be modified in the same way in two or more of these. In mountain
regions, where interruption of the surface and consequent alternation
are great, the mutual invasion of contiguous formations is of frequent
occurrence, often resulting in habitat forms. The spots in which these
nascent species, such as _Galium boreale hylocolum_, _Aster levis
lochmocolus_, etc., are found, are often so related to the area of the
parent species as to demonstrate conclusively that these forms are the
result of polygenesis and not of migration. Naturally, what is true of a
small area will hold equally well of a large region, and the recurrence
of the same habitat form may be accepted as conclusive proof of
polygenesis. The most convincing evidences of multiple origin, however,
are to be found in what De Vries has called “mutations.” It makes little
difference whether we accept mutations in the exact sense of this
author, or regard them as forms characterized by latent variability. The
evidence is conclusive that the same form may arise in nature or in
cultivation, in Holland or in America, not merely once, but several or
many times. In the presence of such confirmation, it is unnecessary to
accumulate proofs. Polygenesis throws a new light upon many difficult
problems of invasion and distribution, and, as a working principle,
admits of repeated tests in the field. It obviates, moreover, the almost
insuperable difficulties in the way of explaining the distribution of
many polygenetic species on the basis of migration alone.

=281. Origin by polyphylesis.= In 1898, the author first advanced a
tentative hypothesis to the effect that a species homogeneous
morphologically may arise from two distinct though related species.
During subsequent years of formational study, the conviction has grown
in regard to the probability of such a method of origin. Since the
appearance of Engler’s work, a polyphyletic origin for certain genera
has been very generally accepted by botanists, but all have ignored the
fact that the polyphylesis of genera carries with it the admission of
such origin for species, since the former are merely groups of the
latter. I can not, however, agree with Engler, that polyphyletic genera,
and hence species also, are necessarily unnatural. If the convergence of
the lines of polyphylesis has been great, resulting in essential
morphological harmony, the genus is a natural one, even though the
ancestral phyla may be recognizable. If, on the other hand, the
convergence is more or less imperfect, resulting in subgroups of species
more nearly related within the groups than between them, the genus can
hardly be termed natural. This condition may, however, prevail in a
monophyletic genus with manifest divergence and still not be an
indication that it is artificial.

Darwin[38], in speaking of convergence, has said: “If two species,
belonging to two distinct though allied genera, had both produced a
large number of new and divergent forms, it is conceivable that these
might approach each other so closely that they would have all to be
classified under the same genus; and thus the descendants of two
distinct genera would converge into one.” The application of this
statement to species would at once show the possibility of polyphylesis
in the latter, and a further examination of the matter will demonstrate
its probability. It is perfectly evident that a species may be split
into two or more forms by varying the conditions, let us say of
water-content, and that the descendants of these forms may again be
changed into the parent type by reversing the process. This has, in
fact, been done experimentally. Since it is admittedly impossible to
draw any absolute line between forms, varieties, and species, it is at
once clear that two distinct though related species, especially if they
are plastic, may be caused to converge in such a way that the variants
may constitute a new and homogeneous species. This may be illustrated by
a concrete case at present under investigation. _Kuhnistera purpurea_
differs from _K. candida_ in being smaller, in having fewer, smaller,
and more narrow leaflets, and a globoid spike of purple flowers in place
of an elongated one of white flowers; in a word, it is more xerophytic.
This conclusion is completely corroborated by its occurrence. On dozens
of slopes examined, _Kuhnistera purpurea_ has never been found mingling
with _K. candida_ on lower slopes, except where an accident of the
surface has resulted in a local decrease of water-content. The
experiment as conducted is a simple one, consisting merely in sowing
seed of each in the zone of the other, and in growing _K. purpurea_
under controlled mesophytic conditions, and _K. candida_ under similarly
measured xerophytic conditions in the planthouse.

While the polyphyletic origin of species is in a fair way to be decided
by experiment, it receives support from several well-known phenomena.
The striking similarity in the plant body of families taxonomically so
distinct as the _Cactaceae_, _Stapeliaceae_, and _Euphorbiaceae_, or
_Cyperaceae_ and _Juncaceae_, indicates that a vegetation form may be
polyphyletic. On the other hand, the local appearance of zygomorphy, of
symphysis, and of aphanisis in the floral types of phylogeneticallv
distinct families is a proof of the operation of convergence in
reproductive characters. To be sure, the convergence is never so great
as to produce more than superficial similarity, but this is because the
groups are markedly different in so many fundamental characters. The
same tendency in closely related species would easily result in
identity. As in the case of polygenesis, the relatively small number of
typically distinct habitats makes it clear that two different species of
wide distribution, bearing to each other the relations of xerophyte to
mesophyte, of hydrophyte to mesophyte, or of poophyte to hylophyte,
might often find themselves in reciprocal situations, with the result
that they would give rise to the same new form. The final proof of the
polyphylesis of species is afforded by the experiments of De Vries in
mutation. De Vries found that _Oenothera nanella_ arose from _O.
Lamarckiana_, _O. laevifolia_, and _O. scintillans_; _Oenothera
scintillans_ arose from _O. lata_ and _O. Lamarckiana_; _Oenothera
rubrinervis_ from _O. Lamarckiana_, _O. laevifolia_, _O. lata_, _O.
oblonga_, _O. nanella_, and _O. scintillans_, etc. Whatever may be the
rank assigned to these mutations, whether form, variety, or species,
there can be no question of their polyphyletic origin, nor, in
consequence of the connection of mutations with variations through such
inconstant forms as _O. scintillans_, _O. elliptica_, and _O.
sublinearis_, of the possibility of polyphylesis in any two distinct
though related species or genera.


                          _KINDS OF INVASION_

=282. Continuous and intermittent invasion.= With respect to the
frequency of migration, we may distinguish invasion as _continuous_, or
_intermittent_. Continuous invasion, which is indeed usually mutual,
occurs between contiguous formations of more or less similar character,
in which there is an annual movement from one into the other, and at the
same time a forward movement through each, resulting from the invaders
established the preceding year. By far the greater amount of invasion is
of this sort, as may readily be seen from the fact that migration varies
inversely as the distance, and ecesis may decrease even more rapidly
than the distance increases. The significant feature of continuous
invasion is that an outpost may be reinforced every year, thus making
probable the establishment of new outposts from this as a center, and
the ultimate extension of the species over a wide area. The
comparatively short distance and the regular alternation of migration
and ecesis render invasion of this sort very effective. An excellent
illustration of this is seen in transition areas and regions, which are
due directly to continuous and usually to mutual invasion. Intermittent
invasion results commonly from distant carriage, though it may occur
very rarely between dissimilar adjacent formations, when a temporary
swing in the physical factors makes ecesis possible for a time. It is
characterized by the fact that the succession of factors which have
brought about the invasion is more or less accidental and may never
recur. Intermittent invasion is relatively rare, and from the small
number of disseminules affected, it is of little importance in modifying
vegetation quantitatively. On the other hand, since a species may often
be carried far from its geographical area, it is frequently of great
significance in distribution.

=283. Complete and partial invasion.= When the movement of invaders into
a formation is so great that the original occupants are finally driven
out, the invasion may be termed _complete_. Such invasion is found
regularly in the case of many ruderal formations, and is typical of the
later stages of many successions. It is ordinarily the result of
continuous invasion. If the number of invaders is sufficiently small
that they may be adopted into the formation without radically changing
the latter, the invasion is _partial_. This is doubtless true of the
greater number of invasions, though these are regularly much less
striking and important than instances of complete invasion.

[Illustration: Fig. 59. Continuous invasion into a new area; mats of
_Arenaria sajanensis_. _Silene acaulis_ and _Sieversia turbinata_
invading an alpine gravel slide.]

=284. Permanent and temporary invasion.= The permanence of invasion
depends upon the success attending ecesis, and upon the stability of the
formation. It has already been noticed that under certain conditions
plants may germinate and grow, and if they are perennials, even become
established, and still ecesis be so imperfect that reproduction is
impossible. Others may find the conditions sufficiently favorable for
propagation, but unfavorable for the formation of flowers and fruits.
Finally there are plants which seem to be perfectly established for a
few years, only to disappear completely. The latter are examples of
_temporary invasion_. It is necessary to draw clearly the line between
complete and partial invasion in this connection. The former is
temporary in the initial or intermediate stages of nearly all
successions, as compared with the ultimate stages, though it is in a
large degree permanent in comparison with the partial invasion of
species which are able to maintain themselves for a few years. In a
sense, there is a real distinction between the two, inasmuch as a
particular stage of succession is permanent as long as the habitat
remains essentially the same. A critical study of the species of such
stages shows, however, that they manifest very different degrees of
permanence. Species which invade stable vegetation temporarily have been
termed _adventive_ by A. DeCandolle. _Permanent invasion_ occurs when a
species becomes permanently established in a more or less stable
formation. It is characteristic of the great majority of invaders found
in the grassland and forest stages of successions.

Plants which have arisen within a formation or have been a constituent
part of it since its origin are _indigenous_. Contrasted with these are
the species which have invaded the formation since it received its
distinctive impress: these are _derived_. The determination of the
indigenous and derived species of a formation or larger division is of
the utmost importance, as it enables us to retrace the steps by which
the formation has reached its present structure, and to reconstruct
formations long since disappeared. To render it less difficult, it is
necessary to scrutinize the derived elements closely, first, because it
is easiest to recognize the indigenous species by eliminating the
derived, and second, because this analysis will show that not all
derived species have entered the formation at the same time and from the
same sources. Derived species may be termed _vicine_, when they are
fully established invaders from adjacent formations or regions, and
_adventitious_, when they have come from distant formations and have
succeeded in establishing themselves. Finally, those derived species
which are unable to establish themselves permanently are _adventive_.


                          _MANNER OF INVASION_

=285. Entrance into the habitat.= Since the ecesis of invaders depends
in large measure upon the occupation of the plants in possession, the
method and degree of invasion will be determined by the presence or
absence of vegetation. Areas without vegetation are either originally
_naked or denuded_, while vegetation with respect to the degree of
occupation is open (_sporadophytia_), or closed (_pycnophytia_). Each
type of area presents different conditions to invaders, largely with
respect to the factors determining ecesis. Naked habitats, rocks, talus,
gravel slides, and dunes, while they offer ample opportunity for
invasion on account of the lack of occupation, are really invaded with
the greatest difficulty, not only because they contain originally few or
no disseminules, but also because of their xerophytic character and the
difficulty of obtaining a foothold, on account of the extreme density or
instability of the soil. Denuded habitats, blowouts, sand draws, ponds,
flood plains, wastes, fields, and burns, usually afford maximum
opportunity for invasion. They invariably contain a large number of
disseminules ready to spring up as soon as the original vegetation is
destroyed. The surface, moreover, is usually such as to catch
disseminules and to offer them optimum conditions of moisture and
nutrition. Open formations are readily invaded, though the increased
occupation renders entrance more difficult than it is in denuded areas.
Closed formations, on the other hand, are characterized by a minimum of
invasion, partly because invaders from different formations find
unfavorable conditions in them, but chiefly because the occupation of
the inhabitants is so complete that invaders are unable to establish
themselves.

Invasion takes place by the penetration of single individuals or groups
of individuals. This will depend in the first place upon the character
of the disseminule. It is evident that, no matter how numerous the
achenes may be, the invasion of those anemochorous species with comate
or winged seeds or one-seeded fruits will be of the first type, while
all species in which the disseminule is a several or many-seeded fruit
or plant, as in hooked fruits, tumbleweeds, etc., will tend to produce a
group of invaders. Occasionally of course, the accidents of migration
will bring together a few one-seeded disseminules into a group, or will
scatter the seeds of a many-seeded fruit, but these constitute
relatively rare exceptions. This distinction in the matter of invasion
is of value in studying the relative rapidity of the latter, and the
establishment of new centers, but it is of greatest importance in
explaining the historical arrangement of species in a formation, and
hence has a direct bearing upon alternation. It is entirely independent
of the number of invaders, which, as we have seen, depends upon
seed-production, mobility, distance, occupation, etc., but is based
solely upon mode of arrangement, and will be found to underlie the
primary types of abundance, copious, and gregarious. In this connection,
it should also be noted that the contingencies of migration, especially
the concomitant action in the same direction of two or more distributive
agencies, often results in the penetration of a group of individuals
belonging to two or more species. This may well be termed _mass
invasion_; it is characteristic of transition areas or regions, and
along valleys or other natural routes for migration it gives rise to
species guilds. The movement of species guilds constitutes one of the
most complex and interesting problems in the whole field of invasion,
the solution of which can be attempted only after the thorough analysis
of the simpler invasions between formations. A better understanding of
the meaning of invasion by species guilds is imperative for the natural
limitation of regions, as at present such groups constitute alien
associations in many regions otherwise homogeneous.

=286. Influence of levels.= The invasion of a formation may occur at
three different levels: (1) at the level of the facies, (2) below the
facies, (3) above the facies, depending directly upon the relative
height of invaders and occupants. The invasion level is an extremely
simple matter to determine, except in the case of woody plants, such as
shrubs and trees, which attain their average height only after many
years. Its importance is fundamental. The level at which invasion occurs
not only determines the immediate constitution of the formation, whether
its impress shall still be given by the occupants or by the invaders or
by both together, but it also decides the whole future of the formation,
i. e., whether the invaders or occupants shall persist unmodified or
modified. The problem is an extremely complex one, but the careful
analysis of invasion at each level throws a flood of light upon it. The
entrance of invaders of the same general height as the facies of a
formation results regularly in mixed formations. This is well
illustrated by the structure of the transition areas between two
formations of the same category, i. e., forests, meadows, etc. It is
seldom, however, that the facies and invaders are so equally matched in
height and other qualities that they remain in equilibrium for a long
period. One or the other has a slight advantage in height, or the one
suffers shading or crowding better than the other, is longer-lived or
faster-growing, with the result that invader yields to occupant, or
occupant to invader. It is a well-known fact that many mixed formations
represent intermediate stages of development.

Invasion at a level different from that of the facies is inevitably
followed by modification. If the invasion takes place below the facies,
the invaders will be exterminated gradually, or slowly assimilated. In
either case, there is little structural change in the formation, and its
stability is affected slightly or not at all. If the invaders overtop
the facies in any considerable number, the entire formation undergoes
partial or complete modification, or in extreme cases it disappears, as
is typically the case in succession. A peculiar variation of invasion at
a level above the facies is seen where woody plants invade grassland,
when the trees or shrubs become more or less uniformly scattered in an
open woodland or open thicket. Here the grassland takes on an altogether
different appearance superficially, though it is usually unchanged,
except beneath and about the invaders, where either adaptation or
extermination results. Finally, it should be borne in mind that the
invasion of a particular formation, especially in the case of layered
thickets and forests, often takes place at two levels, at the height of
the facies and below the facies.


                      _INVESTIGATION OF INVASION_

=287.= The methods to be used in the study of invasion are those already
described elsewhere. The migration circle is of the first importance
because it makes it possible to secure an accurate record of actual
movement. Quadrat and transect are valuable, but from their nature they
are more serviceable for ecesis than for migration. All of these should
be of the permanent type, in order that the fate of invaders may be
followed for several years at least. Permanent areas furnish evidence of
the changes wrought in the actual vegetation, while denuded ones can
serve only to show the potential migration and ecesis of the constituent
species. Transition zones and areas are special seats of invasion; they
are best studied by means of the belt transect and the ecotone chart.
The movement of a line of invaders or of scattered outposts is traced by
the use of labeled stakes at the points concerned. It is clear that this
method will yield conclusive data in regard to the great invasions
between regions, such as the movement of species guilds, the advance of
the forest frontier, etc. When invasion is scattered, factor instruments
can not be used to advantage, but where the invading line is well
marked, or where extra-formational areas occur, a knowledge of the
physical factors is a great aid.

An invasion that has been completed can not be studied in the manner
indicated. A method of comparison must be used, in order to determine
the original home of the invaders. For this an exact knowledge of the
contiguous formations and of the abundance of the species common to all
is a prerequisite. With this as a basis, it is usually a simple task to
refer all the species of the formation concerned to their proper place
in the groups, indigenous, derived, and adventitious.


                               SUCCESSION

=288. Concept.= Succession is the phenomenon in which a series of
invasions occurs in the same spot. It is important, however, to
distinguish clearly between succession and invasion, for, while the one
is the direct result of the other, not all invasion produces succession.
The number of invaders must be large enough, or their effect must be
sufficiently modifying or controlling to bring about the gradual
decrease or disappearance of the original occupants, or a succession
will not be established. Partial or temporary invasion can never
initiate a succession unless the reaction of the invaders upon the
habitat is very great. Complete and permanent invasion, on the other
hand, regularly produces successions, except in the rare cases where a
stable formation entirely replaces a less stable one without the
intervention of other stages. Succession depends in the first degree
upon invasion in such quantity and of such character that the reaction
of the invaders upon the habitat will prepare the way for further
invasion. The characteristic presence of stages in a succession, which
normally correspond to formations, is due to the peculiar operation of
invasion with reaction. In the case of a denuded habitat, for example,
migration from adjacent formations is constantly taking place, but only
a small number of migrants, especially adapted to somewhat extreme
conditions, are able to become established in it. These reach a maximum
development in size or number, and in so doing react upon the habitat in
such a way that more and more of the dormant disseminules present, as
well as those constantly coming into it, find the conditions favorable
for germination and growth. The latter, as they in turn attain their
maximum, cause the gradual disappearance of the species of the first
stage, and at the same time prepare the way for the individuals of the
succeeding formation. It is at present impossible to determine to what
degree this substitution is due to the struggle for existence between
the individuals of each species and between the somewhat similar species
of each stage, and to what degree it arises out of the physical
reaction.

It is evident that geological succession is but a larger expression of
the same phenomenon, dealing with infinitely greater periods of time,
and produced by physical changes of such intensity as to give each
geological stage its peculiar stamp. If, however, the geological record
were sufficiently complete, we should find unquestionably that these
great successions merely represent the stable termini of many series of
smaller changes, such as are found everywhere in recent or existing
vegetation.

=289. Kinds of succession.= The fundamental causes of succession are
invasion and reaction, but the initial causes of a particular succession
are to be sought in the physical or biological disturbances of a habitat
or formation. With reference to the initial cause, we may distinguish
_normal succession_, which begins with nudation, and ends in
stabilization, and _anomalous succession_, in which the facies of an
ultimate stage of a normal succession are replaced by other species, or
in which the direction of movement is radically changed. The former is
of universal occurrence and recurrence; the latter operates upon
relatively few ultimate formations. In the origin of normal successions,
nudation may be brought about by the production of new soils or
habitats, or by the destruction of the formation which already occupies
a habitat. In a few cases, the way in which the habitat arises or
becomes denuded is not decisive as to the vegetation that is developed
upon it, but as a rule the cause of nudation plays as important a part
in the development of a succession as does the reaction exerted by the
invaders. The importance of this fact has been insisted upon under
invasion. New soils present extreme conditions for ecesis, possess few
or no dormant disseminules, and in consequence their successions take
place slowly and exhibit many stages. Denuded soils as a rule offer
optimum conditions for ecesis as a result of the action of the previous
succession, dormant seeds and propagules are abundant, and the
revegetation of such habitats takes place rapidly and shows few stages.
The former may be termed _primary succession_, the latter _secondary
succession_.


                         _PRIMARY SUCCESSIONS_

=290.= These arise on newly formed soils, or upon surfaces exposed for
the first time, which have in consequence never borne vegetation before.
In general they are characteristic of mountain regions, where weathering
is the rule, and of lowlands and shores, where sedimentation or
elevation constantly occur. The principal physical phenomena which bring
about the formation of new soils are: (1) elevation, (2) volcanic
action, (3) weathering, with or without transport.

=291. Succession through elevation.= Elevation was of very frequent
occurrence during the earlier, more plastic conditions of the earth, and
the successions arising as a result of it must have been important
features of the vegetation of geological periods. To-day, elevation is
of much less importance in changing physiography, and its operation is
confined to volcanic islands, coral reefs, and islets, and to rare
movements or displacements in seacoasts, lake beds, shore lines, etc.
There has been no investigation of the development of vegetation on
islands that are rising, or have recently been elevated, probably
because of the slow growth of coral reefs and the rare appearance of
volcanic islands. On coral reefs, the first vegetation is invariably
marine, but as the reef rises higher above the surf line and the tide,
the vegetation passes into a xerophytic terrestrial type adapted to an
impervious rock soil, and ultimately becomes mesophytic. In volcanic
islands, unless they are mere rocks over which the waves rush, the
succession must always begin with a xerophytic rock formation. The best
known example of a rising coast line is found in Norway and Sweden,
where the southeastern coast is rising at the rate of five or six feet a
century. There can be little question that such changes of level will
produce marked changes in vegetation, but the modification will be so
gradual as to be scarcely perceptible in a single generation. It is
probable that the forests of the Atlantic coastal plains are the
ultimate stages of successions initiated at the time of the final
elevation of the sea bottom along the coast line.

[Illustration: Fig. 60. A lichen formation (_Lecanora-Physcia-petrium_),
the first stage of the typical primary succession
(_Lecanora-Picea-sphyrium_) of the Colorado mountains.]

=292. Succession through volcanic action.= The deposition of volcanic
ashes and flows of lava are relatively infrequent at present,
occurring only in the immediate vicinity of active volcanoes, chiefly
in or near the tropics. Successions of this sort are in consequence
not only rare, but they are also relatively inaccessible to
investigators. They have been studied in a few cases, for example,
those of Krakatoa by Treub, but this study has been confined to the
general features of revegetation. Ash fields and lava beds are widely
different in compactness, but they agree in having a low water- and
nutrition-content. The pioneer plants in both will be intense
xerophytes, but the soil differences will determine that these shall
be sand-binders in the former, and rock-weathering plants in the
latter.

=293. Weathering.= Practically all primary successions start on soils
produced by weathering. This is also true of coral or volcanic islets
and of lava beds, for no terrestrial vegetation can secure a foothold
upon them until the surface of the rock has been to some extent
decomposed or disintegrated. Weathering, as is well known, consists of
two processes, disintegration and decomposition, which usually operate
successively, though they are sometimes concomitant. Disintegration
usually precedes, especially in rock masses, and unless it is soon
followed by decomposition, results in dysgeogenous soils. Decomposition
often goes hand in hand with disintegration, or it takes place so
rapidly and perfectly that it alone seems to be present. In either case,
the resulting soil is eugeogenous. The relation of decomposition to
disintegration determines the size and compactness of the soil
particles, and upon the latter depend the porosity, capillarity, and
hygroscopicity of the soil. These control in large degree the character
of the first vegetation to appear on the soil.

Another point of fundamental value in determining revegetation is the
disposition of the weathered rock. If it remains _in situ_, it will
evidently differ in respect to compactness, homogeneity,
nutrition-content, water-content, disseminules, etc., from weathered
material which has been transported. An essential difference also arises
from the fact that a rock may be weathered a long distance from the
place where the decomposed particles are finally deposited, and in the
midst of a vegetation very different from that found in the region of
deposit. The disposition of the weathered material affords in
consequence a satisfactory basis for the arrangement of primary
successions. The following classification is proposed, based upon the
soil groups established by Merrill.[39]

=294. Succession in residuary soils.= Residuary soils are always
sedentary, i. e., they are formed _in situ_. They show certain
differences dependent upon the rock from which they originate, which may
be mixed crystalline shale, sandstone, or limestone, but the
thoroughness of decomposition causes these differences to be
comparatively small. Residuary soils are typically eugeogenous; their
successions in consequence usually begin with mesophytes, and consist of
a few stages. The soluble salt-content is comparatively low, since all
soluble matters are readily leached out. Successions in these soils are
especially characteristic of shale, sandstone, and limestone ledges or
banks. Cumulose deposits, like residuary ones, are sedentary in
character, but as they are produced by the accumulation of organic
matter, they will be considered under reactions of vegetation upon
habitat.

[Illustration: Fig. 61. Talus arising from the disintegration of a
granitic cliff; the rocks are covered with crustose lichens.]

=295. Succession in colluvial soils.= Colluvial deposits owe their
aggregation solely or chiefly to the action of gravity. They are the
immediate result of the disintegration of cliffs, ledges, and mountain
sides, decomposition appearing later as a secondary factor. The masses
and particles arising from disintegration are extremely variable in
size, but they agree as a rule in their angular shape. The typical
example of the colluvial deposit is the talus, which may originate from
any kind of rock, and contains pieces of all sizes. Gravel slides differ
from ordinary talus in being composed of more uniform particles, which
are worn round by slipping down the slope in response to gravity and
surface wash. Boulder fields are to be regarded as talus produced by
weathering under the influence of joints, resulting in huge boulders
which become more and more rounded under the action of water and
gravity. This statement applies to those fields which are in connection
with some cliff that is weathering in this fashion; otherwise, boulder
fields are of aqueous or glacial origin. The character of the
successions in talus will depend upon the kind of rock in the latter. If
the rock is igneous or metamorphic, decomposition will be slow, and the
soil will be dysgeogenous. Successions on such talus consist of many
stages, and the formations are for a long time open and xerophytic. In
talus formed from sedimentary rocks, especially shales, limestones, and
calcareous sandstones, decomposition is much more rapid, and the
successions are simpler and more mesophytic.

=296. Succession in alluvial soils.= Alluvial soils are fluvial when
laid down by streams and rivers, and litoral when washed up by the waves
or tides. They are formed when any obstacle retards the movement of the
water, decreasing its carrying power, and causing the deposit of part or
all of its load. They consist of more or less rounded, finely comminuted
particles, mingled with organic matter and detritus. Alluvial deposits
are especially frequent at the mouth of streams and rivers, on their
terraces and flood plains, and along silting banks as compared with the
erosion banks of meanders. The filling of ponds by the erosion due to
surface drainage, and of lakes by the deposition of the loads of streams
that enter them, results in the formation of new alluvium. A similar
phenomenon occurs along coasts, where bays and inlets are slowly
converted into marshes in consequence of being shallowed by the material
washed in by the waves and tides. Such paludal deposits are invariably
salt water or brackish. Contrasted with these, which are uniformly black
in consequence of the large amount of organic matter present, are the
sandbars and beaches, which, though due to the same agents, are light
grey or white in color, because of the constant leaching by the waves.
Two kinds of alluvial deposits may accordingly be distinguished: (1)
those black with organic matter, and little disturbed by water, and (2)
those of a light color, which are constantly swept by the waves. The
successions corresponding to these are radically different. In the
first, the pioneer vegetation is hydrophytic, consisting largely of
amphibious plants. The pioneer stages retard the movement of the water
more and more, and correspondingly hasten the deposition of its load.
The marsh bed slowly rises in consequence, and finally the marsh begins
to dry out, passing first into a wet meadow, and then into a meadow of
the normal type. A notable exception to this sequence occurs when the
swamp contains organic matter or salts in excess, in which case the
vegetation consists indefinitely of swamp xerophytes, or halophytes. The
first vegetation on fresh water sandbars is xerophytic, or, properly,
dissophytic, unless they remain water-swept, and the ultimate stages of
their successions are mesophytic woodlands composed of water-loving
genera, _Populus_, _Salix_, etc. It seems certain, however, that these
will finally give way to longer-lived hardwoods. Maritime sandbars and
beaches are always saline, and their successions run their short course
of development entirely within the group of halophytes, unless the
retreat of the sea or freshwater floods change the character of the
soil. The chemical action of underground waters also produces new soils,
which might be classed as alluvial. These soils are essentially rock
deposits, travertine, silicious sinter, etc., made by iron and lime
springs and by geysers, and they must be changed by decomposition into
soils proper to be comparable with alluvial soils.

[Illustration: Fig. 62. Talus arising from the decomposition of granite;
the gravel is covered with a formation of foliose lichens
(_Parmelia-chalicium_), the second stage of the primary talus
succession; the herbs are pioneers of the next stage.]

=297. Succession in aeolian soils.= The only wind-borne soils of
geological importance at the present time are those which form dunes,
both inland and coastal. Aeolian deposits consist largely of rounded
sand particles, which are of almost uniform size in any particular dune,
but vary greatly in dunes of different ages. The reaction of the
pioneers on dunes plays an important part in building the latter, but
the immense dunes of inland deserts, which are entirely destitute of
vegetation, seem to indicate that its value has been overestimated. The
first stages in dune successions are dissophytic, i. e., the plants grow
in a soil of medium or high water-content, but in an atmosphere that is
extremely xerophytic. The ultimate stages vary widely in accordance with
the region in which they occur; they may be xerophytic heaths or
mesophytic meadows and forests. Because of their striking character and
economic significance, dunes have received much attention, with the
result that their successions are the most thoroughly known of all.
Prairie and steppe formations are probably to be regarded as the
ultimate stages of successions established on wind-borne loess, and it
is possible that the same is true of sand-hill vegetation in the prairie
province.

=298. Succession in glacial soils.= The formation of glacial deposits is
at present confined to alpine and arctic regions. Recent successions in
such soils are localized in these regions, and are in consequence
relatively unimportant. There can be little question, however, that the
thorough investigation of succession in and near the moraines of
existing glaciers will throw much light upon the successions of the
glacial period. Moraines, drumlins, eskars, and alluvial cones represent
the various kinds of glacial deposits. They agree in being heterogeneous
in composition, and are covered to-day with ultimate stages of
vegetation, except in the immediate vicinity of glaciers.


                        _SECONDARY SUCCESSIONS_

=299.= Generally speaking, all successions on denuded soils are
secondary. When vegetation is completely removed by excessive erosion,
it is an open question whether the resulting habitat is to be regarded
as new or denuded. Erosion is rarely so extreme and so rapid, however,
as to produce such a condition, even when it results from cultivation or
deforestation. It is, moreover, especially characteristic of newly
formed soils, and in studying succession in eroded habitats, it is
fundamentally important to determine whether erosion has produced
denudation, or has operated upon a new soil. The great majority of
secondary successions owe their origin to floods, animals, or the
activities of man, and they agree in occurring upon decomposed soils of
medium water-content, which contain considerable organic matter, and a
large number of dormant migrants. These successions consist of
relatively few stages, and are rarely of extreme character.

=300. Succession in eroded soils.= Eroded soils show considerable
differences, as they arise in consequence of erosion by water or by
wind, though the initial stages of revegetation derive their character
more from the aggregation of the soil than from the nature of the
erosive agent. Eroded soils are as a rule xerophytic. In the case of
erosion by water, dysgeogenous soils are readily worn away in
consequence of their lack of cohesion, as in sand draws, etc., while
eugeogenous soils are easily eroded only on slopes, as in the case of
ravines, hillsides, etc. In the former, the extreme porosity and slight
capillarity of the sand and gravel result in a low water-content. In the
finer soils, the water-content is also low, on account of the excessive
run-off, due to compactness of the particles and to the slope. The
erosive action of winds upon soils bearing vegetation is not very
general; it is found to some extent in more or less established dunes,
and exists in a marked degree in buttes, mushroom rocks, and blowouts.
The first two are regularly xerophytic, the last as a rule, dissophytic.
The early stages of successions in eroded soils are composed of
xerophytes. In loose soils, these are forms capable of binding the soil
particles together, thus preventing wash, and increasing the
accumulation of fine particles, especially of organic matter. In compact
soils, the effect is much the same; the pioneers not only decrease
erosion, but at the same time also increase the water-content by
retarding the movement of the run-off.

=301. Succession in flooded soils.= The universal response of vegetation
to floods is found in the amphibious plant, which is a plastic form
capable of adjustment to very different water-contents. Floods are
confined largely to river basins and coasts. In hilly and mountainous
regions, where the slope is great, any considerable accumulation of
flood waters is now impossible, although of frequent occurrence when
land forms were more plastic.

In all streams that have become graded, the fall is insufficient to
carry off the surplus water in the spring when snows are melting
rapidly, or at times of unusual precipitation. These waters accumulate,
and, overflowing the banks, spread out over the lowlands, resulting in
the formation of a well-defined flood plain. This is a periodical
occurrence with mature streams, and it occurs more or less regularly
with all that are not torrent-like in character. The effect of the
overflow is to destroy or to place at a disadvantage those plants of the
flood plain that are not hydrophytes. At the same time, a thin layer of
fresh silt is deposited upon the valley floor of sand or alluvium.
Flooding is most frequent and of longest duration near the banks of the
stream. It extends more or less uniformly over the flood plain, and
disappears gradually or abruptly as the latter rises into the bench
above. Floods destroy vegetation and make a place for secondary
successions by drowning out mesophytic species, by washing away the
aquatic forms of ponds and pools, and by the erosion of banks and
sandbars. They affect the amphibious vegetation of swamp and shore to a
certain extent, but, unless the period of flooding is long, they tend to
emphasize such formations rather than to destroy them. The still-water
formations of many cutoff and oxbow lakes owe their origin to a river
which cuts across a meander in time of flood. This result is more often
attained by the alternate silting and erosion of a meandering river by
which it cuts across a bend in its channel. The usual successions in
flooded lands are short as a rule; amphibious algae, liverworts, and
mosses soon give way to ruderal plants, and these in turn to the
original mesophytes of meadows, or dissophytes of sandbars. In the case
of ponds and pools, the process of washing-out or silting up merely
removes or destroys the vegetation, without effectively modifying the
habitat, and the secondary successions that follow are extremely short.

=302. Succession by subsidence.= Subsidence is a factor of the most
profound importance in changing vegetation. It operates over vast areas
through immense periods of time. For these reasons, the changes are so
slow as to be almost imperceptible, and the resulting successions can be
studied only in the geological record. Extensive subsidence is confined
to-day to coastal plains, as in Greenland, the south Atlantic coast, and
the region of the Mississippi delta, where its effects are merged with
the paludation of tidal rivers, and the wave and tide erosion of the sea
shore. Such successions are unique, inasmuch as the denuding force
operates very slowly instead of quickly, and the first pioneers of the
new vegetation appear before the original formation has been destroyed.
In all cases, the succession is from mesophytic or halophytic formations
to paludose, and, finally, marine vegetation. In small areas of
subsidence, such as shore slips along lakes and streams, sink holes, and
sunken bogs, the succession is usually both short and simple, mesophytes
giving place to amphibious and ultimately to aquatic forms.

=303. Successions in landslips.= Landslips occur only in montane and
hilly regions, and here they are merely of local importance. In many
respects, they are not unlike talus; they show essential differences,
however, in that they are not sorted by gravity, and in that they
destroy vegetation almost instantly. The succession arises as a rule,
not upon the original soil, but upon that of the landslip, and, as
pointed out elsewhere, might well be regarded as primary.

=304. Succession in drained, or dried soils.= In geological times, the
subsidence of barriers must often have produced drainage and drying-out,
just as elevation frequently resulted in flooding and lake formation. At
the present time, the drying-out of lakes and ponds is the result of
artificial drainage, or of climatic changes. The former will be
considered under successions brought about by the agency of man.
Climatic changes when general operate so slowly that the stages of such
successions are perceptible only when recorded in strata. More locally,
climate swings back and forth through a period of years, with the result
that in dry years the swamps and ponds of wetter seasons are dried out,
and the vegetation destroyed or changed. If the process be gradual, the
succession passes from hydrophytic through amphibious to mesophytic,
and, in dry regions, xerophytic conditions. When the process of
drying-out occurs rapidly, as in a single summer, the original formation
is destroyed, and the new vegetation consists largely of ruderal plants.
A peculiar effect of climate occurs in regions with poor drainage, where
the result of intense evaporation is to produce alkaline basins and salt
lakes, in which the succession becomes more and more open, and is
finally represented by a few stabilized halophytes, or disappears
completely.

[Illustration: Fig. 63. A typical gravel slide (talus) of the Rocky
mountains, before invasion.]

=305. Succession by animal agency.= Successions of this class are
altogether of secondary importance, the instances in which animals
produce denudation being relatively few. Such are the heaps of dirt
thrown up by prairie dogs and other burrowing animals, upon which
ruderal plants are first established, to be finally crowded out by the
species of the original formation. Buffalo wallows furnish examples of
similar successions in which the initial stages are subruderal, while
overstocking and overgrazing frequently produce the same result with
ruderal plants.

=306. Succession by human agency.= The activities of man in changing the
surface of the earth are so diverse that it is impossible to fit the
resulting successions in a natural system. While man does not exactly
make new soils, he exposes soils in various operations: mining,
irrigation, railroad building, etc. He destroys vegetation by fires,
lumbering, cultivation, and drainage, and if he can not control climate,
he at least modifies its natural effects by irrigation and the
conservation of moisture. The operations of man extend from seacoasts
and swampy lowlands through mesophytic forests and prairies to the
driest uplands and inlands. Since the adjacent formations determine in
large degree the course and constitution of a succession, it will be
seen that the effects of any particular activity upon vegetation will
differ greatly in different regions. For convenience, all classes of
successions arising from the presence and activity of man will be
considered in this place, though, as indicated above, some might well be
regarded as producing primary successions, while others produce
anomalous ones.

=307. Succession in burned areas.= It will suffice merely to point out
that “burns” may arise naturally through lightning, volcanic cinders,
lava flows, etc., but the chances are so slight that these causes may be
ignored. The causes of fires are legion, and as they have little or no
effect upon results, they need not be considered. From their nature,
fires are of little significance in open vegetation, deserts, polar
barrens, alpine fields, etc., since the area of the burn can never be
large. In closed formations, the extent of fires is limited only by the
area of the vegetation, and the effect of wind, rain, and other forces.
Forest fires usually occur during the resting period, except in the case
of coniferous forests. In grassland, the living parts are underground
during autumn and winter, when prairie fires commonly occur. As a
consequence, the repeated annual burning of meadow or prairie does not
result in denudation and subsequent succession. On the contrary, it acts
in part as a stabilizing agent, inasmuch as it injures the typical
vegetation forms of grassland much less than it does the woody invaders.
All formations with perennial parts above ground, viz., thicket, open
woodland, and forest, are seriously injured by fire. A severe general
fire destroys the vegetation completely; a local fire destroys the
formation in restricted areas; while a slight or superficial burn
removes the undergrowth and hastens the disappearance of the weaker
trees. In the latter case, while the primary layer of the forest remains
the same, succession takes place in the herbaceous and shrubby layers.
These successions are peculiar in that they are composed almost wholly
of the proper species of the forest, and that they are very short,
showing only a few poorly defined stages. A local fire initiates a
succession in which the pioneers are derived largely from the original
formation, particularly when the latter encloses the burned area more or
less completely. The constitution of the intermediate and ultimate
stages will depend in a larger degree still upon the size and position
of the burn. When a particular formation is destroyed wholly or in large
part, the first stages of the new vegetation are made up by invaders
from the adjacent formations. In the most perfect types of succession,
this dissimilarity between the new and the old vegetation continues to
the last stage, in which the reappearance of the facies precedes that of
the subordinate layers. In many forest successions, however, the general
physical similarity of the ultimate stages permits the early
reappearance of the herbaceous and shrubby species, and the final stages
affect the facies alone. Successions in burned areas operate usually
within the water-content groups. The reconstruction of a mesophytic
forest takes place by means of mesophytes; of the rarer xerophytic and
hydrophytic forests, through xerophytes and hydrophytes respectively.
This is due to the fact that the alteration of the soil is slight,
except where the burning of the vegetation permits the entrance of
erosion, as on mountain slopes.

[Illustration: Fig. 64. Gravel slide formation
(_Pseudocymopterus-Mentzelia-chalicium_), stage III of the talus
succession.]

=308. Succession in lumbered areas.= Commercial lumbering, especially
where practiced for wood-pulp as well as for timber, results in complete
or nearly complete destruction of the vegetation by removal and the
change from diffuse light to sunlight, or by the action of erosion upon
the exposed surface. In the first place, short mesophytic successions
will result; in the second, the successions will be long and complex,
passing through decreasingly xerophytic conditions to a stable
mesophytic forest. Where a forest is cut over for certain species alone,
the undisturbed trees soon take full possession, though the causes
effective in the beginning will ultimately restore the original facies
in many instances. Such successions are anomalous, and will be treated
under that head.

=309. Succession by cultivation.= The clearing of forests and the
“breaking” of grassland for cultivation destroy the original vegetation;
the temporary or permanent abandonment of cultivated fields then permits
the entrance of ruderal species, which are the pioneers of new
successions. This phenomenon takes place annually in fields after
harvest, resulting in the secondary formations of Warming, in which
practically the same species reappear year after year. In fields that
lie fallow for several years, or are permanently abandoned, the first
ruderal plants are displaced by newcomers, or certain of them become
dominant at the expense of others. In a few years, these are crowded out
by invaders from the adjacent formations, and the field is ultimately
reclaimed by the original vegetation, unless this has entirely
disappeared from the region. The number of stages depends chiefly upon
whether the final formation is to be grassland or woodland. Other
activities of man, such as the construction of buildings, roads,
railways, canals, etc., remove the native vegetation, and make room for
the rapid development of ruderal formations. In and about cities, where
the original formations have entirely disappeared, the chance for
succession is remote, and the initial ruderal stages become more or less
stabilized. Elsewhere the usual successions are established, and the
ruderal formation finally gives way to the dominant type. In mountain
and desert regions, where ruderal plants are rare or lacking, their
place is taken by subruderal forms, species of the native vegetation
capable of rapid movement in them. These, like ruderal plants, are
gradually replaced by other native species of less mobility, but of
greater persistence, resulting in a short succession operating often
within a single formation. From the nature of cultivated plants,
succession after cultivation generally operates within the mesophytic
series.

=310. Succession by drainage.= Successions of this kind show much the
same stages as are found in those due to flooding. They proceed from
aquatic or swamp formations to mesophytic termini, either grassland or
woodland. When drainage takes place rapidly and completely, the pioneer
stages are usually xerophytic; cases of this sort, however, are
infrequent.

=311. Succession by irrigation.= Irrigation produces short successions
of peculiar stamp along the courses of irrigating canals and ditches,
and in the vicinity of reservoirs. These are recent, as a rule, and are
usually found in the midst of cultivated lands, so that their complete
history is still a matter of conjecture. The original xerophytes are
forced out not only by the disturbance of the soil, but also by its
increased water-content. A few of them often thrive under the new
conditions, and, together with the usual ruderal plants and a large
number of lowland mesophytes and amphibious forms derived from the banks
of the parent stream, constitute a heterogeneous association. This is
doubtless to be regarded as an initial stage of a succession, but it is
an open question whether the succession will early be stabilized as a
new formation, or whether the original vegetation will sooner or later
be reestablished under somewhat mesophytic conditions. From the number
of mesophytes and from the behavior of valleys, it seems certain that
the banks of such canals will ultimately be occupied by a formation more
mesophytic than hydrophytic, into which some of the surrounding
xerophytes of plastic nature have been adopted.

=312. Anomalous successions= are those in which the physical change in
the habitat is relatively slight, resulting in a displacement of the
ultimate stage, or the disturbance of the usual sequence, merely,
instead of the destruction and reconstruction of a formation, or the
gradual development of a new series of stages on new soil. In nature,
the ultimate grass or forest stage of a normal succession is often
replaced by a similar formation, especially if the facies be few or
single. It is evident that certain trees naturally replace others in the
last stages of a forest succession, without making the latter anomalous.
The last occurs only when a normal stage is replaced by one belonging
properly to an entirely different succession, as when a coniferous
forest replaces a deciduous one in a hardwood region. The presence and
development of such successions can be determined only after the normal
types are known. The interpolation of a foreign stage in a natural
succession, or a change of direction, by which a succession that is
mesotropic again becomes hydrophytic, is easily explained when it is the
result of artificial agents, as is often the case. In nature, anomalous
successions are commonly the result of a slow backward and forward swing
of climatic conditions.

=313. Perfect and imperfect successions.= A normal succession will
regularly be perfect; it passes in the usual sequence from initial to
ultimate conditions without interruption or omission. Imperfect
succession results when one or more of the ordinary stages is omitted
anywhere in the course, and a later stage appears before its turn. It
will occur at any time when a new or denuded habitat becomes so
surrounded by other vegetation that the formations which usually furnish
the next invaders are unable to do so, or when the abundance and
mobility of certain species enable them to take possession before their
proper turn, and to the exclusion of the regular stage. Incomplete
successions are of great significance, inasmuch as they indicate that
the stages of a succession are often due more to biological than to
physical causes, the proximity and mobility of the adjacent species
being more determinative than the physical factors. Subalpine gravel
slides regularly pass through the rosette, mat, turf, thicket, woodland,
and forest stages; occasionally, however, they pass immediately from the
rosette, or mat condition, to an aspen thicket which represents the next
to the last stage. Such successions are by no means infrequent in hilly
and montane regions; in regions physiographically more mature or stable,
perfect successions are almost invariably the rule.

[Illustration: Fig. 65. Half gravel slide formation
(_Elymus-Muhlenbergia-chalicium_), stage IV of the talus succession.]

=314. Stabilization.= It may be stated as a general principle that
vegetation moves constantly and gradually toward stabilization. Each
successive stage modifies the physical factors, and dominates the
habitat more and more, in such a way that the latter seems to respond to
the formation rather than this to the habitat. The more advanced the
succession, i. e., the degree of stabilization, the greater the climatic
or physiographic change necessary to disturb it, with the result that
such disturbances are much more frequent in the earlier stages than in
the later development. Constant, gradual movement toward a stable
formation is characteristic of continuous succession. Contrasted with
this is intermittent succession, in which the succession swings for a
time in one direction, from xerophytic to mesophytic for example, and
then moves in the opposite direction, often passing through the same
stages. This phenomenon usually is characteristic only of the less
stable stages, and is generally produced by a climatic swing, in which a
series of hot or dry years is followed by one of cold or wet years, or
the reverse. The same effect upon a vast scale is produced by alternate
elevation and subsidence, but these operate through such great periods
of time that one can not trace, but can only conjecture their effects. A
normal continuous succession frequently changes its direction of
movement, or its type, in transition regions or in areas where the
outposts of a new flora are rapidly advancing, as in wide mesophytic
valleys that run down into or traverse plains. Here the change is often
sudden, and grass and desert formations are replaced by thickets and
forests, resulting in abrupt succession. Species guilds are typical
examples of this. More rarely, a stage foreign to the succession will be
interpolated, replacing a normal stage, or slipping in between two such,
though finally disappearing before the next regular formation. This may
be distinguished as interpolated succession.

The apparent terminus of all stabilization is the forest, on account of
the thoroughness with which it controls the habitat. A close examination
of vegetation, however, will show that its stable terms are dependent in
the first degree upon the character of the region in which the formation
is indigenous. It is obviously impossible that successions in desert
lands, in polar barrens, or upon alpine stretches should terminate in
forest stages. In these, grassland must be the ultimate condition,
except in those extreme habitats, alpine and polar, where mosses and
lichens represent the highest type of existing vegetation. Forests are
ultimate for all successions in habitats belonging to a region generally
wooded, while grassland represents the terminus of prairie and plains
successions as well as of many arctic-alpine ones.


                         _CAUSES AND REACTIONS_

=315.= The initial cause of a succession must be sought in a physical
change in the habitat; its continuance depends upon the reaction which
each stage of vegetation exerts upon the physical factors which
constitute the habitat. A single exception to this is found in anomalous
successions, where the change of formation often hinges upon the
appearance of remote or foreign disseminules. The causes which initiate
successions have already been considered; they may be summarized as
follows: (1) weathering, (2) erosion, (3) elevation, (4) subsidence, (5)
climatic changes, (6) artificial changes. The effect of succeeding
stages of vegetation upon a new or denuded habitat usually finds
expression in a change of the habitat with respect to a particular
factor, and in a definite direction. Often, there is a primary reaction,
and one or more secondary ones, which are corollaries of it. Rarely,
there are two or more coordinate reactions. The general ways in which
vegetation reacts upon the habitat are the following: (1) by preventing
weathering, (2) by binding aeolian soils, (3) by reducing run-off and
preventing erosion, (4) by filling with silt and plant remains, (5) by
enriching the soil, (6) by exhausting the soil, (7) by accumulating
humus, (8) by modifying atmospheric factors. The direction of the
movement of a succession is the immediate result of its reaction. From
the fundamental nature of vegetation, it must be expressed in terms of
water-content. The reaction is often so great that the habitat undergoes
a profound change in the course of the succession, changing from
hydrophytic to mesophytic or xerophytic, or the reverse. This is
characteristic of newly formed or exposed soils. Such successions are
_xerotropic_, _mesotropic_, or _hydrotropic_, according to the ultimate
condition of the habitat. When the reaction is less marked, the type of
habitat does not change materially, and the successions are
_xerostatic_, _mesostatic_, or _hydrostatic_, depending upon the
water-content. Such conditions obtain for the most part only in denuded
habitats.

=316. Succession by preventing weathering.= Reactions of this nature
occur especially in alpine and boreal regions, in the earlier stages of
lichen-moss successions. They are typical of igneous and metamorphic
rocks in which disintegration regularly precedes decomposition. The
influence of the vegetation is best seen in the lichen stages, where the
crustose forms make a compact layer, which diminishes the effect of the
atmospheric factors producing disintegration. In alpine regions
especially, this protection is so perfect that the crustose lichens may
almost be regarded as the last stage of a succession. There are no
recorded observations which bear upon this point, but it seems certain
that the pioneer rock lichens, _Lecanora_, _Lecidea_, _Biatora_,
_Buellia_, and _Acarospora_, cover alpine rocks for decades, if not for
centuries. Ultimately, however, the slow decomposition of the rock
surface beneath the thallus has its effect. Tiny furrows and pockets are
formed, in which water accumulates to carry on its ceaseless work, and
the compact crustose covering is finally ruptured, permitting the
entrance of foliose forms. The latter, like the mosses, doubtless
protect rock surfaces, especially those of the softer rocks, in a slight
degree against the influence of weathering, but this is more than offset
by their activity in hastening decomposition, and thus preparing a field
for invasion. Rocks and boulders (_petria_, _petrodia_, _phellia_)
furnish the best examples of this reaction; cliffs (_cremnia_) usually
have a lichen covering on their faces, while the forces which produce
disintegration operate from above or below.

[Illustration: Fig. 66. Thicket formation
(_Quercus-Holodiscus-driodium_), stage V of the talus succession.]

=317. Succession by binding aeolian soils.= Dunes (_thinia_) are classic
examples of the reaction of pioneer vegetation upon habitats of
wind-borne sand. The initial formations in such places consist
exclusively of sand-binders, plants with masses of fibrous roots, and
usually also with strong rootstalks, long, erect leaves, and a vigorous
apical growth. They are almost exclusively perennial grasses and sedges,
possessing the unique property of pushing up rapidly through a covering
of sand. They react by fixing the sand with their roots, thus preventing
its blowing about, and also by catching the shifting particles among
their culms and leaves, forming a tiny area of stabilization, in which
the next generation can establish a foothold. The gradual accumulation
of vegetable detritus serves also to enrich the soil, and makes possible
the advent of species requiring better nourishment. Blowouts (_anemia_)
are almost exact duplicates of dunes in so far as the steps of
revegetation are concerned; while one is a hollow, and the other a hill,
in both the reaction operates upon a wind-swept slope. Sand-hills
(_amathia_) and deserts (_eremia_) show similar though less marked
reactions, except where they exhibit typical inland dunes. Sand-binders,
while usually classed as xerophytic or halophytic, are in reality
dissophytes. Their roots grow more or less superficially in moist sand,
and are morphologically mesophytic while their leaves bear the stamp of
xerophytes. The direction of movement in successions of this kind is
normally from xerophytes to mesophytes, i. e., it is mesotropic. In
sand-hills and deserts, the succession operates wholly within the
xerophytic (dissophytic) series. Along seacoasts, the mesophytic
terminus is regularly forest, except where forests are remote, when it
is grassland.

=318. Succession by reducing run-off and erosion.= All bare or denuded
habitats that have an appreciable slope are subject to erosion by
surface water. The rapidity and degree of erosion depend upon the amount
of rainfall, the inclination of the slope, and the structure of the
surface soil. Regions of excessive rainfall, even where the slope is
slight, show great, though somewhat uniform erosion; hill and mountain
are deeply eroded even when the rainfall is small. Slopes consisting of
compact eugeogenous soils, notwithstanding the marked adhesion of the
particles, are much eroded where the rainfall is great, on account of
the excessive run-off. Porous dysgeogenous soils, on the contrary,
absorb most of the rainfall; the run-off is small and erosion slight,
except where the slope is great, a rare condition on account of the
imperfect cohesion of the particles. In compact soils, the plants of the
initial formations not merely break the impact of the raindrops, but,
what is much more important, they delay the downward movement of the
water, and produce numberless tiny streams. The delayed water is largely
absorbed by the soil, and the reduction of the run-off prevents the
formation of rills of sufficient size to cause erosion. As in dunes,
such plants are usually perennial grasses, though composites are
frequent; the root system is, however, more deeply seated, and a main or
tap root is often present. On sand and gravel slopes, the loose texture
of the soil results generally in the production of sand-binders with
fibrous roots. Unlike dunes, such slopes exhibit a large number of mats
and rosettes with tap-roots, which are effective in preventing the
slipping or washing of the sand, and run little danger of being covered,
as is the case with duneformers. In both instances, each pioneer plant
serves as a center of comparative stabilization for the establishment of
its own offspring, and of such invaders as find their way in. From the
nature of these, slopes almost invariably pass through grassland stages
before finding their termini in thickets or forests. Bad lands (_tiria_)
furnish the most striking examples of eroded habitats. The rainfall in
the bad lands of Nebraska and South Dakota is small (300 mm.); yet the
steepness of the slope and the compactness of the soil render erosion so
extreme that it is all but impossible for plants to obtain a foothold.
Their reaction is practically negligible, and the vegetation passes the
pioneer stages only in the relatively stable valleys. Mountain slopes
(_ancia_), and ridges and hills (_lophia_) are readily eroded in new or
denuded areas. This is especially true of hill and mountain regions
which have been stripped of their forest or thicket cover by fires,
lumbering, cultivation, or grazing. Where the erosion is slight, the
resulting succession may show initial xerophytic stages, or it may be
completely mesostatic. Excessively eroded habitats are xerostatic, as in
the case of bad lands, or, more frequently, they are mesotropic, passing
first through a long series of xerophytic formations. Sandbars
(_cheradia_, _syrtidia_) should be considered here, though they are
eroded by currents and waves, and not by run-off. They are fixed and
built up by sand-binding grasses and sedges, usually of a hydrophytic
nature, and pass ultimately into mesophytic forest.

=319. Succession by filling with silt and plant remains.= All aquatic
habitats into which silt, wash, or other detritus is borne by streams,
currents, floods, waves, or tides are slowly shallowed by the action of
the water plants present. These not only check the movement of the
water, thus greatly decreasing its carrying power, and causing the
deposition of a part or all of its load, but they also retain and fix
the particles deposited. In accordance with the rule, each plant becomes
the center of a stabilizing area, which rises faster than the rest of
the floor, producing the well-known hummocks of lagoons and swamps. All
aquatics produce this reaction. It is more pronounced in submerged and
amphibious forms than in floating ones, and it takes place more rapidly
with greatly branched or dissected plants than with others. In pools
(_tiphia_) and lakes (_limnia_), debouching streams and surface waters
deposit their loads in consequence of the check exerted by the still
water and the marginal vegetation, and delta-like marshes are quickly
built up by filling. Springs (_crenia_) likewise form marshes where they
gush forth in sands, the removal of which is impeded by vegetation. The
flood plains and deltas of rivers show a similar reaction. The heavily
laden flood waters are checked by the vegetation of meadows and marshes,
and deposit most of their load. The banks of streams (_ochthia_) and of
ditches (_taphria_) are often built up in the same fashion by the action
of the marginal vegetation upon the current. The presence of marginal
vegetation often determines the checking or deflecting of the current in
such a way as to initiate meanders, while natural levees owe their
origin to it, in part at least. Along low seacoasts, waves and tides
hasten the deposit of river-borne detritus, causing the water to spread
over the lowlands and form swamps. They often throw back also the
sediment that has been deposited in the sea, the marsh vegetation acting
as a filter in both cases. Successions of the kind indicated above are
regularly mesotropic. Where the soil is sandy, and the filling-up
process sufficiently great, or where salts or humus occur in excess,
xerophytic formations result. In certain cases, these successions appear
to be permanently hydrostatic, changing merely from floating or
submerged to amphibious conditions, but this is probably due to the
slowness of the reaction. As a rule, the accumulation of plant remains
is relatively slight, and plays an unimportant part in the reaction. In
peat bogs and other extensive swamps, the amount of organic matter is
excessive, and plays an important role in the building up of the swamp
bed.

[Illustration: Fig. 67. Pine forest formation (_Pinus-xerohylium_),
stage VI of the talus succession.]

=320. Succession by enriching the soil.= This reaction occurs to some
degree in the great majority of all successions. The relatively
insignificant lichens and mosses produce this result upon the most
barren rocks, while the higher forms of later stages, grasses, herbs,
shrubs, and trees, exhibit it in marked progression. The reaction
consists chiefly in the incorporation of the decomposed remains of each
generation and each stage in the soil. A very important part is played
by the mechanical and chemical action of the roots in breaking up the
soil particles, and in changing them into soluble substances.
Mycorrhizae, bacterial nodules, and especially soil bacteria play a
large part in increasing the nutrition-content of the soil, but the
extent to which they are effective in succession is completely unknown.
The changes in the color, texture, and food value of the soil in passing
from the initial to ultimate stages of a normal succession are well
known, and have led many to think them the efficient reactions of such
successions. It seems almost certain, however, that this is merely a
concomitant, and that, even in anomalous successions where facies
replace each other without obvious reasons, the reactions are concerned
more with water-content, light, and humidity than with the food-content
of the soil.

=321. Succession by exhausting the soil.= This is a reaction not at all
understood as yet in nature. A number of phenomena, such as the “fairy
rings” of mushrooms and other fungi, the peripheral growth and central
decay of lichens, _Lecanora_, _Placodium_, _Parmelia_, and of matforming
grasses, such as _Muhlenbergia_, and the circular advance of the
rootstalk plants, indicate that certain plants at least withdraw much of
the available supply of some essential soil element, and are forced to
move away from the exhausted area. It is probable that the constant
shifting of the individuals of a formation year after year, a phenomenon
to be discussed under alternation, has some connection with this. It
will be impossible to establish such a relation, however, until the
facts are exactly determined by the method of quadrat statistics. So far
as native formations are concerned, there can not be the slightest
question that prairies and forests have existed over the same area for
centuries without impoverishing the soil in the least degree, a
conclusion which is even more certain for the open vegetation of deserts
and plains. With culture formations, the case is quite different. The
exhaustion of the soil by continuous or intensive cultivation is a
matter of common experience in all lands settled for a long period.
Calcium, phosphorus, and nitrogen compounds especially are used up by
crops, and must be supplied artificially. The reason for this difference
in reaction between native and culture formations seems evident. In
harvesting, not merely the grain, but the stems and leaves, and in
gardening often the root also, are removed, so that the plant makes
little or no return to the soil. In nature, annual plants return to the
ground every year all the solid matter of roots, stems, leaves, and
fruits, with the exception of the relatively small number of seeds that
germinate. Perennial herbs return everything but the persistent
underground parts. Shrubs and trees replace annually an immense amount
of material used in leaves and fruits, and sooner or later, by the
gradual decay of the individuals or by the destruction of the whole
formation, they restore all that they have taken from the soil. This
balance is further maintained to an important degree by the activity of
the roots, which take from the deep-seated layers of the soil the crude
materials necessary for the formation of leaves and fruits. Upon the
fall and decay of these, their materials are incorporated with the upper
layers of the formation floor, from which they may be absorbed by the
undergrowth, or find their way again into the layers permeated by the
tree roots. From the universal occurrence of weeds in cultivated
regions, the pioneers in impoverished or exhausted fields are uniformly
ruderal plants. As is well known, the seed production and ecesis of
these forms are such that they take possession quickly and completely,
while their demands upon the soil are of such a nature that the most
sterile field can rapidly be covered by a vigorous growth of weeds. As
indicated elsewhere, ruderal formations ultimately yield to the native
vegetation, though in regions so completely given over to culture that
native formations are lacking or remote, it is probable that successions
reach their final stage within the group of ruderal plants.

[Illustration: Fig. 68. Spruce forest formation
(_Picea-Pseudotsuga-hylium_), stage VII, the ultimate stage of the talus
succession.]

=322. Succession by the accumulation of humus.= This is the
characteristic reaction of peat bogs and cypress swamps (_oxodia_), in
which the accumulation of vegetable matter is enormous. The plant
remains decompose slowly and incompletely under the water, giving rise
to the various humic acids. These possess remarkable antiseptic
qualities, and have an injurious effect upon protoplasm. They affect the
absorption of water by the root-hairs, though this is also influenced by
poor aeration. The same acids are found in practically all inland
marshes and swamps, but the quantity of decomposing vegetation in many
is not great enough to produce an efficient reaction. Formations of this
type usually start as freshwater swamps. The succession is apparently
hydrostatic, but no thorough study of its stages has as yet been made.

=323. Succession by modifying atmospheric factors.= All layered
formations, forests, thickets, many meadows and wastes, etc., show
reactions of this nature, and are in fact largely or exclusively
determined by them. The reaction is a complex one, though it is clear
that light is the most efficient of the modified factors, and that
humidity, temperature, and wind, while strongly affected, play
subordinate parts. In normal successions, the effect of shade, i. e.,
diffuse light, enters with the appearance of bushes or shrubs, and
becomes more and more pronounced in the ultimate forest stages. The
reaction is exerted chiefly by the facies, but the effect of this is to
cause increasing diffuseness in each successively lower layer, in direct
ratio with the increased branching and leaf expansion of the plants in
the layer just above. In the ultimate stage of many forests, especially
where the facies are reduced to one, the reaction of the primary layer
is so intense as to preclude all undergrowth. Anomalous successions
often owe their origin to the fact that certain trees react in such a
way as to cause conditions in which they produce seedlings with
increasing difficulty, and thus offer a field favorable to the ecesis of
those species capable of enduring the dense shade. Successions of this
kind are almost invariably mesostatic, as it is altogether exceptional
that layered formations are either xerophytic or hydrophytic.


                          _LAWS OF SUCCESSION_

=324.= The investigation of succession has so far been neither
sufficiently thorough nor systematic to permit the postulation of
definite laws. Enough has been done, however, to warrant the formulation
of a number of rules, which apply to the successions studied, and afford
a convenient method for the critical investigation of all successions
upon the basis of initial causes, and reactions. Warming has already
brought together a few such rules, and an attempt is here made to reduce
the phenomena of succession, including its causes and effects, to a
tentative system. At present it is difficult to make a thoroughly
satisfactory classification of such rules, and they are here arranged in
general conformity with the procedure in succession.

       I. Causation. The initial cause of a succession is the formation
         or appearance of a new habitat, or the efficient change of an
         existing one.

       II. Reaction. Each stage reacts upon the habitat in such a way as
         to produce physical conditions more or less unfavorable to its
         permanence, but advantageous to the invaders of the next stage.

       III. Proximity and mobility.

       (1) The pioneers of a succession are those species nearest at
         hand that are the most mobile.

       (2) The number of migrants from any formation into a habitat
         varies inversely as the square of the distance.

       (3) The pioneer species are regularly derived from different
         formations, as the latter nearly always contain permobile
         species capable of effective ecesis.

       (4) The plants of the initial stages are normally algae and
         fungi, with minute spores, composites, and grasses, which
         possess permobile fruits, or ruderal plants, on account of
         their great seed production.

       IV. Ecesis.

       (1) All the migrants into a new, denuded, or greatly modified
         habitat are sorted by ecesis into three groups: (1) those that
         are unable to germinate or grow, and soon die; (2) those that
         grow normally under the conditions present; (3) those that pass
         through one or more of the earlier stages in a dormant state to
         appear at a later stage of the succession.

       (2) Wherever ruderal vegetation is present, it contributes a
         large number of the pioneer species of each succession, on
         account of the thorough ecesis. In other regions this part is
         played by subruderal native species.

       (3) Annuals and biennials are characteristic of the early stages
         of secondary successions, on account of their great seed
         production and ready ecesis.

       (4) In layered formations, heliophytes appear before sciophytes;
         they ultimately yield to the latter, except where they are able
         to maintain a position in the primary layer.

       (5) Excessive seed production and slight mobility lead to the
         imperfect ecesis of individuals in dense stands, and in
         consequence usually produce great instability.

       (6) Each pioneer produces about itself a tiny area of ecesis and
         stabilization for its own offspring, for the disseminules of
         its fellows, or of invaders.

       (7) Species propagating by offshoots, or producing relatively
         immobile disseminules in small number, usually show effective
         ecesis, as the offspring appear within the area of the reaction
         of the parent forms.

       V. Stabilization.

       (1) Stabilization is the universal tendency of vegetation.

       (2) The ultimate stage of a succession is determined by the
         dominant vegetation of the region. Lichen formations are often
         ultimate in polar and niveal zones; grassland is the final
         vegetation for plains and alpine stretches, and for much
         prairie, while forest is the last stage for mesophytic midlands
         and lowlands, as well as for subalpine regions.

       (3) Grassland or forest is the usual terminus of a succession;
         they predominate in lands physiographically mature.

       (4) The limit of a succession is determined in large part by the
         progressive increase in occupation, which makes the entrance of
         invaders more and more difficult.

       (5) Stabilization proceeds radiately from the pioneer plants or
         masses. The movement of offshoots is away from the parent mass,
         and the chances of ecesis are greatest near its edges, in a
         narrow area in which the reaction is still felt, and the
         occupation is not exclusive.

       VI. General laws.

       (1) The stages, or formations, of a succession are distinguished
         as initial (_prodophytia_), intermediate (_ptenophytia_), and
         ultimate (_aiphytia_).

       (2) Initial formations are open, ultimate formations are closed.

       (3) The number of species is small in the initial stages; it
         attains a maximum in intermediate stages; and again decreases
         in the ultimate formation, on account of the dominance of a few
         species.

       (4) The normal sequence of vegetation forms in succession is: (1)
         algae, fungi, mosses; (2) annuals and biennials; (3) perennial
         herbs; (4) bushes and shrubs; (5) trees.

       (5) The number of species and of individuals in each stage
         increases constantly up to a maximum, after which it gradually
         decreases before the forms of the next stage. The interval
         between two maxima is occupied by a mixed formation.

       (6) A secondary succession does not begin with the initial stage
         of the primary one which it replaces, but usually at a much
         later stage.

       (7) At present, successions are generally mesotropic, grassland
         and forest being the ultimate stages, though many are
         xerostatic or hydrostatic. If erosion continue until the sea
         level is reached, the ultimate vegetation of the globe will be
         hydrophytic. Should the heat of the sun decrease greatly before
         this time, the last vegetation will be xerophytic, i. e.,
         crymophytic.

       (8) The operation of succession was essentially the same during
         the geological past as it is to-day. From the nature of their
         vegetation forms, the record deals largely with the ultimate
         stages of such successions.


                   _CLASSIFICATION AND NOMENCLATURE_

=325. Basis.= New or denuded habitats arise the world over by the
operation of the same or similar causes, and they are revegetated in
consequence of the same reactions. Similar habitats produce similar
successions. The vegetation forms and their sequence are usually
identical, and the genera are frequently the same, or corresponding in
regions not entirely unrelated. The species are derived from the
adjacent vegetation, and, except in alpine and coast regions, are
normally different. The primary groups of successions are determined by
essential identity of habitat or cause, e. g., aeolian successions,
erosion successions, burn successions, etc. When they have been more
generally investigated, it will be possible to distinguish subordinate
groups of successions, in which the degree of relationship is indicated
by the similarity of vegetation forms, the number of common genera, etc.
For example, burn successions in the Ural and in the Rocky mountains
show almost complete similarity in the matter of vegetation forms and
their sequence, and have the majority of their genera in common. A
natural classification of successions will divide them first of all into
normal and anomalous. The former fall into two classes, primary and
secondary, and these are subdivided into a number of groups, based upon
the cause which initiates the succession.

[Illustration: Fig. 69. Aspen forest formation (_Populus-hylium_), the
typical stage of burn successions in the Rocky mountains; it is
sometimes an anomalous stage in primary successions, interpolated in
place of the thicket formation.]

=326. Nomenclature.= The need of short distinctive names of
international value for plant formations is obvious; it has become
imperative that successions also should be distinguished critically and
designated clearly. From the very nature of the case, it is impossible
to designate each formation or succession by a single Greek or Latin
term, as habitats of the same character will show in different parts of
the world a vegetation taxonomically very different. It may some day be
possible to use a binomial or trinomial for this purpose, somewhat after
the fashion of taxonomy, in which the habitat name will represent the
generic idea as applied to formations, and a term drawn from the
floristic impress the specific idea. Such an attempt would be futile or
valueless at the present time; it could not possibly meet with success
until there is more uniformity in the concept of the formation, and
until there has been much accurate and thorough investigation of actual
formations, a task as yet barely begun. At present, it seems most
feasible as well as scientific to designate all formations occupying
similar habitats by a name drawn from the character of the latter, such
as a meadow formation, _poium_, a forest formation, _hylium_, a desert
formation, _eremium_, etc. A particular formation is best designated by
using the generic name of one or two of its most important species in
conjunction with its habitat term, as _Spartina-Elymus-poium_,
_Picea-Pinus-hylium_, _Cereus-Yucca-eremium_, etc. Apparently a somewhat
similar nomenclature is adapted to successions. The cause which produces
a new habitat may well furnish the basis for the name of the general
groups of successions, as _pyrium_ (literally, a place or a habitat
burned over), a burn succession, _tribium_, an erosion succession, etc.
A burn succession consists of a sequence of certain formations in one
part of the world, and of a series of quite different ones,
floristically, in another. A particular burn succession should be
designated by using the names of a characteristic facies of the initial
and ultimate stages in connection with the general term, e. g.,
_Bryum-Picea-pyrium_, etc. A trinomial constructed in this way
represents the desirable mean between definition and brevity. Greater
definiteness is possible only at the expense of brevity, while to
shorten the name would entirely destroy its precision. The following
classification of successions is proposed, based upon the plan outlined
above. The termination _-ium_ (εῖον) has been used throughout in the
construction of names for successions, largely for reasons of euphony.
If it should become desirable to distinguish the names of formations and
successions by the termination, the locative suffix _-on_ (-ών) should
be used for the latter. The terms given below would then be _hypson_,
_rhyson_, _hedon_, _sphyron_, _prochoson_, _pnoon_, _pagon_, _tribon_,
_clyson_, _repon_, _olisthon_, _xerasion_, _theron_, _broton_, _pyron_,
_ecballon_, _camnon_, _ocheton_, _ardon_.

    I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ,
         succession)

       _a._ Primary successions: protodochae (πρῶτος, first, primary)

    1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον, place)

    2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially of
         fire)

    3. In residuary soils: hedium (ἔδος, τό, a sitting base)

    4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus)

    5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition of mud)

    6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast)

    7. In glacial soils: pagium (πάγος, ὁ, that which becomes solid, i.
         e., a glacier)

       _b._ Secondary successions: hepodochae (ἕπω, to follow)

    8. In eroded soils: tribium (τρίβω, wear or rub away)

    9. In flooded soils: clysium (κλύσις, ὁ, a drenching, flooding)

   10. By subsidence: repium (ῥέπω, incline downwards, sink)

   11. In landslips: olisthium (ὄλισθος, ὁ, slip)

   12. In drained and dried out soils: xerasium (ξηρασία, ἡ, drought)

   13. By animal agencies: therium (θήρ, ὁ, wild animal)

   14. By human agency: brotium (βροτός, ὁ, a mortal)

       _a._ Burns: pyrium (πῦρ, τό, fire)

       _b._ Lumbering: ecballium (ἐκβάλλω, cut down forests)

       _c._ Cultivation: camnium (κάμνω, cultivate)

       _d._ Drainage: ochetium (ὀχετός, ὁ, drain)

       _e._ Irrigation: ardium (ἄρδω, irrigate)

   II. Anomalous successions: xenodochae (ξένος, strange, unusual)

=327. Illustrations.= The following series will illustrate the
application of this system of nomenclature to particular successions,
and their stages, or formations.

 Thlaspi-Picea-sphyrium: pennycress-spruce talus succession
   Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide
      formation
   Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation
   Quercus-Holodiscus-driodium: oak-fringewood dry thicket formation
   Pinus-xerohylium: pine dry forest formation
   Picea-Pseudotsuga-hylium: spruce-balsam forest formation

 Bryum-Picea-pyrium: moss-spruce burn succession
   Bryum-telmatium: moss meadow formation
   Aster-Chamaenerium-poium: aster-fireweed meadow formation
   Deschampsia-Carex-poium: hairgrass-sedge meadow formation
   Salix-Betula-helodrium: willow-birch meadow thicket formation
   Populus-hylium: aspen forest formation
   Picea-hylium: spruce forest formation

 Lecanora-Carex-hedium: lichen-carex residuary succession
   Lecanora-Gyrophora-petrium: crustose lichen rock formation
   Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation
   Paronychia-Silene-chalicium: nailwort-campion gravel slide formation
   Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation

 Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage
    succession
   Eragrostis-Polygonum-telmatium: eragrostis-heartsease wet meadow
      formation
   Helianthus-Ambrosia-chledium: sunflower-ragweed waste formation


                     _INVESTIGATION OF SUCCESSION_

=328. General rules.= The study of succession must proceed along two
fundamental lines of inquiry: it is necessary to investigate
quantitatively the physical factors of the initial stages and the
reactions produced by the subsequent stages. This should be done by
automatic instruments for humidity, light, temperature, and wind, in
order that a continuous record may be obtained. Water-content is taken
daily or even less frequently, while soil properties, and physiographic
factors, altitude, slope, surface, and exposure are determined once for
all. It is equally needful to determine the development and structure of
each stage with particular reference to the adjacent formations, to the
stage that has just preceded, and the one that is to follow. For this,
the use of the permanent quadrat is imperative, as the sequence and
structure of the stages can be understood only by a minute study of the
shifting and rearrangement of the individuals. Permanent migration
circles are indispensable for tracing movement away from the pioneer
areas by which each stage reaches its maximum. Denuded quadrats are a
material aid in that they furnish important evidence with respect to
migration and ecesis, By means of them, it is possible to determine the
probable development of stages which reach back a decade or more into
the past. In the examination of successions, since cause and effect are
so intimately connected in each reaction, it is especially important
that general and superficial observations upon structure and sequence be
replaced by precise records, and that vague conjectures as to causes and
reactions be supplanted by the accurate determination of the physical
factors which underlie them.

[Illustration: Fig. 70. Alternating gravel slides on Mounts Cameron and
Palsgrove, from the comparison of which the initial development of the
talus succession has been reconstructed.]

=329. Method of alternating stages.= The period of time through which a
primary succession operates is usually too great to make a complete
study possible within a single lifetime. Secondary successions run their
course much more quickly, and a decade will sometimes suffice for
stabilization, though even here the period is normally longer. The
longest and most complex succession, however, may be accurately studied
in a region, where several examples of the same succession occur in
different stages of development. In the same region, the physical
factors of one example of a particular succession are essentially
identical with those of another example in the same stage. If one is in
an initial stage, and the other in an intermediate condition, the
development of the former makes it possible to reestablish more or less
completely the life history of the latter. The same connection may be
made between intermediate and ultimate stages, and it is thus possible
to determine with considerable accuracy and within a few years the
sequence of stages in a succession that requires a century or more for
its complete development. In the Rocky mountains, gravel slides (talus
slopes) are remarkably frequent. They occur in all stages of
development, and the alternating slides of different ages furnish an
almost perfect record of this succession. This method lacks the absolute
finality which can be obtained by following a succession in one spot
from its inception to final stabilization, but it is alone feasible for
long successions, i. e., those extending over a score or more of years.
When it comes to be universally recognized as a plain duty for each
investigator to leave an exact and complete record in quadrat maps and
quadrat photographs of the stages studied by him, it will be a simple
task for the botanists of one generation to finish the investigations of
succession begun by their predecessors.

=330. The relict method= of studying succession is next in importance to
the method of alternating areas. The two in fact are supplementary, and
should be used together whenever relicts are present. This method is
based upon the law of successive maxima, viz., the number of species and
of individuals in each stage constantly increases up to a certain
maximum, after which it gradually decreases before the forms of the next
stage. In accordance with this, secondary species usually disappear
first, principal species next, and facies last of all. There are notable
exceptions to this, however, and the safest plan is to use the relict
method only when principal species or facies are left as evidence. An
additional reason for this is that secondary species are more likely to
be common to two or more formations. In the majority of cases, the
relict is not modified, and is readily recognized as belonging properly
to a previous stage. This is true of herbs in all the stages of
grassland, and in the initial ones of forest succession. The herbs and
shrubs of earlier stages, which persist in the final forest stages, are
necessarily modified, often in such a degree as to become distinct
ecads, or species. The facies of the stages which precede the ultimate
forest are rarely modified. The application of the relict method,
together with the modification just described, is nicely illustrated by
the balsam-spruce formation at Minnehaha. Of the initial gravel slide
stage, the relicts are _Vagnera stellata_ and _Galium boreale_, the one
modified into _Vagnera leptopetala_, and the other into _G. boreale
hylocolum_. The thicket stage is represented by _Holodiscus dumosa_,
greatly changed in form and branching, and in the shape and structure of
the leaf. The most striking relict of the aspen formation is the facies
itself, _Populus tremuloides_. The tall slender trunks of dead aspens
are found in practically every balsam-spruce forest. In many places,
living trees are still found, with small, straggling crowns, which are
vainly trying to outgrow the surrounding conifers. Of the aspen
undergrowth, _Rosa sayii_, _Helianthella parryi_, _Frasera speciosa_,
_Zygadenus elegans_, _Castilleia confusa_, _Gentiana acuta_, and
_Solidago orophila_ remain more or less modified by the diffuse light.
It is still a question whether the aspen stage passes directly into the
balsam-spruce forest, or whether a pine forest intervenes. The presence
of both _Pinus ponderosa_ and _P. flexilis_, which are scattered more or
less uniformly through the formation, furnishes strong evidence for the
latter view.

[Illustration: Fig. 71. Relict spruces and aspens, showing the character
of the succession immediately preceding the burn succession now
developing.]

The lifetime of forest and thicket stages of successions is ascertained
by counting the annual rings of the stumps of facies. This is a
perfectly feasible method for many woodland formations where stumps
already abound or where a fire has occurred, and it is but rarely
necessary to cut down trees for this purpose. When trees or shrubs are
present as relicts, the same method is used to determine the length of
time taken by the development of the corresponding stages.


                     THE STRUCTURE OF THE FORMATION

=331.= Since all the structures exhibited by formations, such as zones,
layers, consocies, etc., are to be referred to zonation or alternation,
these principles are first considered in detail. This, then, constitutes
the basis for a consideration of the structure of a normal formation,
with special reference to the different parts that compose it. The
investigation of formational structure, since the latter is the result
of aggregation, invasion, and succession, is accomplished by
instruments, quadrats, etc., in the manner already indicated under
development, and no further discussion of it is necessary here.


                                ZONATION

=332. Concept.= The recognition of vegetation zones dates from
Tournefort[40], who found that, while the plants of Armenia occupied the
foot of Mount Ararat, the vegetation of the slopes above contained many
species of southern Europe. Still higher appeared a flora similar to
that of Sweden, and on the summit grew arctic plants, such as those of
Lapland.

As the historical summary shows, the concept of zonation is the oldest
in phytogeography. Notwithstanding this, it has never been clearly
defined, nor has there been any detailed investigation of the phenomenon
itself, or of the causes which produce it. Zones are so common, and
often so clearly marked, that they invite study, but no serious attempt
has heretofore been made to analyze zonation, or to formulate a definite
method of investigating it. Zonation is the practically universal
response of plants to the quantitative distribution of physical factors
in nature. In almost all habitats, one or more of the physical factors
present decreases gradually in passing away from the point of greatest
intensity. The result is that the plants of the habitat arrange
themselves in belts about this point, their position being determined by
their relation to the factor concerned. Close investigation will show
that there is hardly a formation that is entirely without zonation,
though in many cases the zones are incomplete or obscure for various
reasons. Zonation is as characteristic of vegetation as a whole as it is
of its unit, the formation, a fact long ago recognized in temperature
zones. A continental climate, however, often results in the interruption
of these, with the consequence that these belts of vegetation are not
always continuous.


                          _CAUSES OF ZONATION_

=333. Growth.= The causes that produce zones are either biological or
physical: the first have to do with some characteristic of the plant,
the second with the physical features of the habitat. Biological causes
arise from the method of growth, from the manner of dissemination, or
from the reaction of the species upon the habitat. The formation of
circles as a result of radial growth is a well-known occurrence with
certain plants, but it is much more common than is supposed. In the case
of agarics, this phenomenon has long been known under the name of
“fairy-rings.” It is found in a large number of moulds, and is
characteristic of early stages of the mycelium of the powdery mildews.
It occurs in nearly all maculicole fungi, and is exhibited by certain
xylogenous fungi, such as _Hysterographium_. Among the foliose lichens,
it is a common occurrence with the rock forms of _Parmelia_,
_Placodium_, _Physcia_, and _Lecanora_, and with the earth forms of
_Parmelia_ and _Peltigera_. The thalloid liverworts show a similar
radial growth. The flowering plants, and many mosses also, furnish good
examples of this sort of growth in those species which simulate the form
of the mycelium or thallus. These are the species that form mats, turfs,
or carpets. Alpine mat formers, such as _Silene acaulis_, _Paronychia
pulvinata_, _Arenaria sajanesis_, etc., are typical examples.
Xerophytic, turf-forming species of _Muhlenbergia_, _Sporobolus_,
_Bouteloua_, _Festuca_, _Poa_, and other grasses form striking ring-like
mats, while creeping species of _Euphorbia_, _Portulaca_, _Amarantus_,
etc., produce circular areas. Rosettes, bunch-grasses, and many ordinary
rootstalk plants spread rapidly by runners and rhizomes. The direction
of growth is often indeterminate in these also, and is in consequence
more or less bilateral or unilateral. Growth results in zonation only
when the older central portions of the individual or mass die away,
leaving an ever-widening belt of younger plants or parts. This
phenomenon is doubtless due in part to the greater age of the central
portion, but seems to arise chiefly from the demands made by the young
and actively growing parts upon the water of the soil. There may
possibly be an exhaustion of nutritive content, as in the case of the
fungi, but this seems improbable for the reason that young plants of the
same and other species thrive in these areas. It must not be inferred
that these miniature growth zones increase in size until they pass into
zones of formations. Growth contributes its share to the production of
these, but there is no genetic connection between a tiny plant zone and
a zone of vegetation.

Radial and bilateral growth play an important part in formational zones
in so far as they are related to migration. The growth of the runner or
rhizome itself is a very effective means of dissemination, while the
seeding of the plants thus carried away from the central mass is most
effective at the edge of the newly occupied area. This holds with equal
force for plants with a mycelium or a thallus. The circular area becomes
larger year by year. Sooner or later, the younger, more vigorous, and
more completely occupied circumference passes into a more or less
complete zone. This will result from the reaction of the central
individuals upon the habitat, so that they are readily displaced by
invaders, or from their increasing senility and dying out, or from the
invasion of forms which seed more abundantly and successfully. This
result will only be the more marked if the radiating migrants reach a
belt of ground especially favorable to their ecesis. In this connection
it must be carefully noted that vegetation pressure, before which weaker
plants are generally supposed to flee, or by which they are thought to
be forced out into less desirable situations, is little more than a
fanciful term for radial growth and migration. It has been shown under
invasion that disseminules move into vegetation masses, as well as away
from them, the outward movement alone being conspicuous, because it is
only at the margin and beyond that they find the necessary water and
light for growth.

=334. Reactions.= Certain reactions of plants upon habitats produce
zonation. The zones of fungi are doubtless caused by the exhaustion of
the organic matter present, while in lichens and mosses the decrease in
nutritive content has something to do with the disappearance of the
central mass. In the mats of flowering plants, the connection is much
less certain. The reaction of a forest or thicket, or even of a tall
herbaceous layer, is an extremely important factor in the production of
zonation. The factor chiefly concerned here is light. Its intensity is
greatest at the edge of the formation and just below the primary layer;
the light becomes increasingly diffuse toward the center of the forest,
and toward the ground. In response to this, both lateral and vertical
zones appear. The former are more or less incomplete, and are only in
part due to differences in illumination. The vertical zones or layers
are characteristic of forest and thickets, and are caused directly by
differences in light intensity.

[Illustration: Fig. 72. Zones of _Cyperus erythrorrhizus_ produced by
the recession of the shore-line.]

=335. Physical factors.= The physical causes of zonation are by far the
most important. They arise from differences in temperature, water, and
light. In the large, temperature differences are the most important,
producing the great zones of vegetation. In a particular region or
habitat, variations of water-content and humidity are controlling, while
light, as shown above, is important in the reactions of forest and
thicket. Physical factors produce zonation in a habitat or a series of
habitats, when there is either a gradual and cumulative, or an abrupt
change in their intensity. Gradual, slight changes are typical of single
habitats; abrupt, marked changes of a series of habitats. This
modification of a decisive factor tends to operate in all directions
from the place of greatest intensity, producing a characteristic
symmetry of the habitat with reference to the factor concerned. If the
area of greatest amount is linear, the shading-out will take place in
two directions, and the symmetry will be bilateral, a condition well
illustrated by rivers. On the other hand, a central intense area will
shade out in all directions, giving rise to radial symmetry, as in
ponds, lakes, etc. The essential connection between these is evident
where a stream broadens into a lake, or the latter is the source of a
stream, where a mountain ridge breaks up into isolated peaks, or where a
peninsula or landspit is cut into islands. The line that connects the
points of accumulated or abrupt change in the symmetry is a stress line
or _ecotone_. Ecotones are well-marked between formations, particularly
where the medium changes; they are less distinct within formations. It
is obvious that an ecotone separates two different series of zones in
the one case, and merely two distinct zones in the other.

[Illustration: Fig. 73. Regional zones on a spur of Pike’s Peak (3,800
m.); the forest consists of _Picea engelmannii_ and _Pinus aristata_,
the forewold is _Salix pseudolapponum_, and the grassland, alpine meadow
(_Carex-Campanula-coryphium_).]

=336. Physiographic symmetry.= The physical symmetry of a habitat
depends upon the distribution of water in it, and this is profoundly
affected by the soil and the physiography. The influence of
precipitation is slight or lacking, as it is nearly uniform throughout
the habitat; the effects of wind and humidity are more localized.
Differences of soil rarely obtain within a single habitat, though often
occurring in a zoned series. The strikingly zonal structure or
arrangement of habitats is nearly always due to differences in
water-content produced by physiographic factors, slope, exposure,
surface, and altitude. The effect of these upon water-content and
humidity is obvious. Wherever appreciable physiographic differences
occur, there will be central areas of excess and deficiency in
water-content, between which there is a symmetrical modification of this
factor. Peaks are typical examples of areas of deficiency, lakes and
oceans of areas of excess. When these areas are extreme and close to
each other, the resulting zonation will be marked; when they are
moderate, particularly if they are widely separated, the zones produced
are obscure. Asymmetry of a habitat or a region practically does not
exist. Central areas of excess and deficiency may be very large and in
consequence fail to seem symmetrical, or the space between them so great
that the symmetry is not conspicuous, but they are everywhere present,
acting as foci for the intervening areas.

The response of vegetation to habitat is so intimate that physiographic
symmetry everywhere produces vegetational symmetry, which finds its
ready expression in plant zones. The reaction of vegetation upon habitat
causes biological symmetry, typical of growth zones and light zones.
From these facts it is clear that zonation will be regularly
characteristic of the vegetative covering. The zonal arrangement of
formations is usually very evident; the zones of a formation are often
obscured, or, where the latter occupies a uniform central area of excess
or deficiency, they are rudimentary or lacking, as in shallow ponds.
Zones are frequently imperfect, though rarely entirely absent in new
soils, such as talus. They are rendered obscure in several ways. In the
initial stages of a succession, as well as in the transitions between
the various stages, the plant population is so scattered, so transient,
or so dense as to respond not at all to a degree of symmetry which
produces marked zonation in later formations. The alternation of
conspicuous species not only causes great interruption of zones, but
often also completely conceals the zonation of other species, such as
the grasses, which, though of more importance in the formation, have a
lower habit of growth. Furthermore, the ecotones of one factor may run
at right angles to those of another, and the resulting series of zones
mutually obscure each other. Finally, such a physiographic feature as a
hill may have its symmetry interrupted by ridges or ravines, which
deflect the zones downward or upward, or cause them to disappear
altogether, while the shallows or depths of a pond or lake may have the
same effect. An entire absence of zones, i. e., azonation, is
exceptional in vegetation. Almost all cases that seem to exhibit it may
be shown by careful examination to arise in one of the several ways
indicated above.


                          _KINDS OF ZONATION_

=337.= Two kinds of zonation are distinguished with reference to the
direction in which the controlling factor changes. When this is
horizontal, as with water-content and temperature, zonation will be
lateral; when it is vertical, as in the case of light, the zonation is
vertical. There exists an intimate connection between the two in
forests, where the secondary layer of small trees and shrubs is
continuous with a belt of trees and shrubs around the central nucleus,
and the lower layers of bushes and herbaceous plants with similar zones
still further out. This connection doubtless arises from the fact that
conditions are unfavorable to the facies, outside of the nucleus as well
as beneath it. Floristically, each layer and its corresponding zone are
distinct, as the one consists of shade, the other of sun species.
Lateral zonation is radial when the habitat or physiographic feature is
more or less circular in form, and it is bilateral, when the latter is
elongated or linear. Vertical zonation is unilateral.

=338. Radial zonation= is regularly characteristic of elevations and
depressions. From the form of the earth, it reaches its larger
expression in the girdles of vegetation corresponding to the zones of
temperature. The zones of mountain peaks are likewise due largely to
temperature, though humidity is a very important factor also. Mountain
zones are normally quite perfect. The zonation of islands, hills, etc.,
is due to water-content. In the former, the zones are usually quite
regular and complete; in the latter, they are often incomplete or
obscured. Prairies and steppes are not zoned as units, but are complexes
of more or less zonal hills and ridges. Ponds, lakes, and seas regularly
exhibit complete zones, except in those shallow ponds where the depth is
so slight that what is ordinarily a marginal zone is able to extend over
the entire bottom. The line between an elevation and a depression, i.
e., the edge of the water level, is the most sharply defined of all
ecotones. It separates two series of zones, each of which constitutes a
formation. One of these is regularly hydrophytic, the other is usually
mesophytic. The line between the two can rarely be drawn at the water’s
edge, as this is not a constant, owing to waves, tides, or periodical
rise and fall. There is in consequence a more or less variable
transition zone of amphibious plants, which are, however, to be referred
to the hydrophytic formation. Nearly all forest formations serve as a
center about which are arranged several somewhat complete zones. As a
rule, these merge into a single heterogeneous zone of thickets.

=339. Bilateral zonation= differs from radial only in as much as it
deals with linear elevations and depressions instead of circular ones.
With this difference, the zones of ranges and ridges correspond exactly
to those of peaks and hills, while the same relation is evident between
the zones of streams, and of lakes and ponds. The ecotones are identical
except as to form; they are linear in the one and circular in the other.
Incompleteness is more frequently found in bilateral zonation, though
this is a question of distance or extent, rather than one of symmetry.

=340. Vertical zonation= is peculiar in that there is no primary ecotone
present, on either side of which zones arrange themselves with reference
to the factor concerned. This arises from the fact that the controlling
factor is light, which impinges upon the habitat in such manner as to
shade out in but one direction, i. e., downward. Vertical zones appear
in bodies of water, on account of the absorption of light by the water.
In a general way, it is possible to distinguish bottom, plancton, and
surface zones, consisting almost wholly of algae. There is little
question that minor zones exist, especially in lakes and seas, but these
await further investigation. The most characteristic vertical zones
occur in forests, where the primary layer of trees acts as a screen. The
density of this screen determines the number of zones found beneath it.
In extreme cases the foliage is so dense that the light beneath is
insufficient even for mosses and lichens. As a rule, however, there will
be one or more zones present. In an ordinary deciduous forest, the
layers below the facies are five or six in number: (1) a secondary layer
of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper
herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a
lower herbaceous layer, (6) a ground layer of mosses, lichens, other
fungi, and algae. The upper layers are often discontinuous, the lower
ones are more and more continuous. As a forest becomes denser, its
layers disappear from the upper downward, the ground layer always being
the last to disappear because of its ability to grow in very diffuse
light. A vertically zoned formation shows a complex series of reactions.
The primary layer determines the amount of heat, light, water, wind,
etc., for the subordinate layers in general. Each of these layers then
further determines the amount for those below it, the ground layer being
subject in some degree to the control of every layer above it. This
accounts probably for the definiteness and permanence of this layer. The
degree to which the lower layers influence the upper by reacting upon
the habitat is not known. It is evident that this influence must be
considerable by virtue of their control of the water supply in the upper
soil strata, by virtue of their transpiration, their decomposition, etc.

The ecotone between two formations is never a sharp line, but it is an
area of varying width. The edge of this area which is contiguous to one
formation marks the limit for species of the other. Both formations
disappear in this transition zone, but in opposite directions. The
overlapping which produces such zones arises from the fact that the
physical factors tend to approach each other at the line of contact
between formations, and that many species are more or less adjustable to
conditions not too dissimilar.

=341. Vegetation zones.= As a fundamental expression of progressive
change in the amount of heat and water, zonation is the most important
feature of vegetation. It constitutes the sole basis for the division of
continental as well as insular vegetation. The continent of North
America furnishes striking proof of the truth of this. Conforming to the
gradual decrease of temperature and water-content northward, three
primary belts of vegetation stretch across the continent from east to
west. These are forest, grassland, and polar desert. The first is
further divided into the secondary zones of broad-leaved evergreen,
deciduous, and needle-leaved forests. At right angles to this
temperature-water symmetry lies a symmetry due to water alone, in
accordance with which forest belts touch the oceans, but give way in the
interior to grasslands, and these to deserts. It is at once evident that
the mutual interruption of these two series of zones has produced the
primary features of North America vegetation, i. e., tropical forests
where heat and water are excessive, deserts where either is unusually
deficient, grassland when one is low, the other moderate, and deciduous
and coniferous forests, where the water-content is as least moderate and
the temperature not too low. Such a simple yet fundamental division has
been modified, however, by the disturbing effect which three continental
mountain systems have had upon humidity and upon temperature symmetry.
The two are intimately interwoven. The lowering of temperature due to
altitude produces the precipitation of the wind-borne moisture upon
those slopes which look toward the quarter from which the prevailing
winds blow. A mountain range thus makes an abrupt change in the
symmetry, and renders impossible the gradual change from forest to
grassland and desert. The Appalachian system is not sufficiently high to
produce a pronounced effect, and forests extend far beyond it into the
interior before passing into prairies and plains. On the other hand, the
influence of the Rocky mountains and the Sierra Nevada is very marked.
The latter rise to a great height relatively near the coast, and
condense upon their western slopes nearly all of the moisture brought
from the Pacific. The Rocky mountains have the same effect upon the much
drier winds that blow from the east, and the two systems in consequence
enclose a parched desert. This series of major zones thus becomes,
starting at the east, forest, grassland, desert, and forest, instead of
the more symmetrical series, forest, grassland, desert, grassland,
forest, which would prevail were it not for these barriers. This actual
series of major zones undergoes further interruption by the action of
these mountain systems in deflecting northern isotherms far to the
south. This action is greatest in the high ranges, the Rocky mountains
and the Sierras, and least in the lower Appalachians. Its result is to
carry the polar deserts of the north far southward along the crests of
the mountains, and to extend the boreal coniferous forests much further
south along their slopes. In the Appalachians, this means no more than
the extension of a long tongue of conifers into the mass of deciduous
forests, and the occasional appearance of an isolated peak. In the
western ranges, it produces two symmetrical series of minor mountain
zones, forest, alpine grassland or desert, and forest, to say nothing of
the foot-hill and timber-line zones of thicket.

There seems to be no good reason for distinguishing the zones of
mountains as regions. The term itself is inapplicable, as it has no
reference to zonation, and is used much more frequently as a term of
general application. Its use tends to obscure also the essential
identity of the so-called vertical zones of mountains with the major
continental zones, an identity which can not be insisted upon too
strongly. For the sake of clearness, it is important to distinguish all
belts of vegetation as zones, though it is evident that these are not
all of the same rank. The following division of the vegetation of North
America is based upon the fundamental principles of continental symmetry
and the community of continental and mountain zones.

    I. Polar-niveal zone—zona polari-nivalis

   II. Arctic-alpine zone—zona arctici-alpina

       Arctic province—provincia arctica

       Alpine province—provincia alpina

  III. Boreal-subalpine zone—zona boreali-subalpina

       Alaska province—provincia alaskana

       Cordilleran province—provincia cordillerana

       Ontario province—provincia ontariensis

   IV. Temperate zone—zona temperata

       Atlantic province—provincia atlantica

       Appalachian province—provincia appalachiana

       Nebraska province—provincia nebraskensis

       Utah province—provincia utahensis

       Coast province—provincia litoralis

       Pacific province—provincia pacifica

    V. Subtropical zone—zona subtropicalis

       Florida province—provincia floridana

       Mexican province—provincia mexicana

   VI. Tropical zone—zona tropicalis

       Antilles province—provincia antilleana

       Andean province—provincia andeana


                              ALTERNATION

=342. Concept.= The term alternation is used to designate that
phenomenon of vegetation, in which a formation recurs at different
places in a region, or a species at separate points in a formation.
Although it is a fundamental feature of vegetation, it has been
recognized but recently.[41]

Alternation is the response of vegetation to the heterogeneity of the
surface of the earth. It is in sharp contrast to zonation, inasmuch as
it is directly caused by asymmetry in the topography. In consequence, it
deals with the subdivisions of zones, arising from physical differences
within the symmetrical area. It deals with vegetation areas of every
rank below that of major zone, with the habitat and geographical areas
of species, and, in a certain way, with the correspondence of vicarious
genera. The breaking up of vegetation into formations is a striking
example of alternation. The same phenomenon occurs in every formation,
producing consocies and minor plant groups, and everywhere giving
variation to its surface and structure. The essential idea involved in
this principle is the recurrence of like formations, consocies, or
groups, which are more or less separated by formations, consocies, or
groups differing from them. It is an exact expression of the primary law
of association that heterogeneity of structure varies directly as the
extent and complexity of the habitat, or the series of habitats.
Vegetation is made up of what are superficially homogeneous formations,
but upon analysis these are seen to contain consocies. The latter,
though more uniform than formations, break up into groups, each of which
still shows a characteristic heterogeneity arising from the varying
number and arrangement of its constituent species.

=343. Causes.= The primary cause of alternation is physical asymmetry,
which is everywhere present within the symmetrical areas which produce
zones. This is influenced so strongly, however, by migration and plant
competition (_phyteris_) that the consideration of this subject will
gain in clearness if these are treated as separate causes. The essential
relation between them must not be lost sight of, however. Migration
carries disseminules into all, or only some of the different areas of a
formation, or into different formations, with little respect to the
physical nature of these. The physical character of these asymmetrical
areas determines that some of these plants shall be established in one
series of places, and some in another, while the competition between the
individuals in the various areas determines the numerical value of each
species as well as its persistence. These three causes are invariably
present in the production of alternating areas, and originally, i. e.,
in new or denuded soils, the sequence is constant, viz., migration,
ecesis in asymmetrical areas, and competition.

With respect to the different portions of an asymmetrical area,
migration will have one of three effects: (1) it will carry disseminules
into both favorable and unfavorable areas, (2) into favorable ones only,
or (3) into unfavorable ones alone. From the radial nature of migration,
the first case is far the most frequent; it is typical of sporostrotes,
and the highly specialized spermatostrotes and carpostrotes. The effect
of migration is uniform here, and alternation arises in consequence of
the selective power of ecesis. It is evident that migration does not
have an even indirect effect, when the disseminules are carried into
none but unfavorable situations. Where the movement is into favorable
places alone, alternation is the immediate result. The intermittent
operation of migration and the presence of barriers are responsible for
the absence of plants in situations favorable to them, and in
consequence bring about a certain alternation between corresponding
species.

The selective operation of physical factors upon the disseminules
carried into the different parts of an asymmetrical area is the usual
cause of alternation. Asymmetry alone is universal within the more
conspicuous structures termed zones, down to the smallest areas which a
group of plants can occupy. The difference between contiguous areas,
particularly within the same habitat, is often small. It sometimes seems
inefficient in the initial stages of a succession when a single species
is present, but even in extreme cases its effect will be recognizable in
the size and density of the individuals. Asymmetry is clearly evident in
vegetation where two symmetrical series cross each other, or when a
symmetry is interrupted by barrier-like elevations or depressions.
Within formations, it arises from differences, often very slight, in
slope, exposure, elevation, from irregularities of surface, differences
in soil structure, or composition, in the amount of cover, and in the
reactions of the living plants. At the last point, it is in direct
connection with plant competition.

=344. Competition.= Much uncertainty, as well as diversity of opinion,
seems still to exist in regard to the precise nature of the competition
between plants that occupy the same area. It has long been admitted that
the phrase, “struggle for existence,” is true of this relation only in
the most figurative sense, but the feeling still prevails that, since
plants live in associations, there must be something mysterious and
vitalistic in their relation. No one has been able to discover anything
of this nature, but nevertheless the impression remains. Such a direct
relation exists only between parasites, epiphytes, and lianes, and the
plants which serve to nourish or support them. In the case of plants
growing on the same stratum, actual competition between plant and plant
does not occur. One individual can affect another only in as much as it
changes the physical factors that influence the latter. Competition is a
question of the reaction of a plant upon the physical factors which
encompass it, and of the effect of these modified factors upon the
adjacent plants. In the exact sense, two plants do not compete with each
other as long as the water-content and nutrition, the heat and light are
in excess of the needs of both. The moment, however, that the roots of
one enter the area from which the other draws its water supply, or the
foliage of one begins to overshade the leaves of the other, the reaction
of the former modifies unfavorably the factors controlling the latter,
and competition is at once initiated. The same relation exists
throughout the process; the stronger, taller, the more branched, or the
better rooted plant reacts upon the habitat, and the latter immediately
exerts an unfavorable effect upon the weaker, shorter, less branched, or
more poorly rooted plant. This action of plant upon habitat and of
habitat upon plant is cumulative, however. An increase in the leaf
surface of a plant not merely reduces the amount of light and heat
available for the plant near it or beneath it, but it also renders
necessary the absorption of more water and other nutritive material, and
correspondingly decreases the amount available. The inevitable result is
that the successful individual prospers more and more, while the less
successful one loses ground in the same degree. As a consequence, the
latter disappears entirely, or it is handicapped to such an extent that
it fails to produce seeds, or these are reduced in number or vitality.

Competition in vegetation furnishes few instances as simple as the
above, but this will serve to make clear the simplest case of ordinary
competition, i. e., that in which the individuals belong to a single
species. The various individuals of one species which grow together in a
patch show relatively slight differences, in height, width, leaf
expanse, or root surface. Still, some will have the largest surfaces for
the impact of water, heat, and light, while others will have the
smallest; the majority, perhaps, will occupy different places between
the extremes. The former will receive more than their share of one or
more factors. The reaction thus produced will operate upon the plants
subject to it inversely as the amount of surface impinged upon. The
usual expression of such competition is seen in the great variation in
height, branching, etc., of the different individuals, and in the
inability of many to produce flowers. This is particularly true of
annuals, and of perennials of the same generation. In the competition
between parents and offspring of the same perennial species, the former
usually have so much the advantage that the younger plants are often
unable to thrive or even germinate, and disappear, leaving a free space
beneath and about the stronger parents. This illustrates the primary law
of competition, viz., that this is closest when the individuals are most
similar. Similar individuals make nearly the same demands upon the
habitat, and adjust themselves least readily to their mutual reactions.
The more unlike plants are, the greater the difference in their needs,
and some are able to adjust themselves to the reactions of others with
little or no disadvantage.

In accordance with the above principles, the competition is closer
between species of like form than between those that are dissimilar.
This similarity must be one of vegetation or habitat form, not one of
systematic position. The latter is in fact of no significance, except
where there is a certain correspondence between the two. Leaf, stem, and
root characters determine the outcome, and those species most alike in
these features will be in close competition, regardless of their
taxonomic similarity or dissimilarity. This is as conclusive of the
competition between the species of the same genus as it is between those
belonging to genera of widely separated families. From this may be
deduced a second principle of competition, viz., the closeness of the
competition between the individuals of different species varies directly
with their similarity in vegetation or habitat form. This principle is
of primary importance in the competition which arises between occupants
and invaders in the different stages of succession. Those invading
species that show the greatest resemblance to occupants in leaf, stem,
and root form experience the greatest difficulty in establishing
themselves. The species, on the contrary, which are so unlike the
occupants that they come in at a clear advantage or disadvantage,
establish themselves readily, in the one case as a result of the
reaction, in the other by taking a subordinate position. This principle
lies at the base of the changes in succession which give a peculiar
stamp to each stage. A reaction sufficient to bring about the
disappearance of one stage can be produced only by the entrance of
invaders so different in form as to materially or entirely change the
impress of the formation. Stabilization results when the entrance of
invaders of such form as to exert an efficient reaction is no longer
possible. In forests, while many vegetation forms can still enter, none
of these produce a reaction sufficient to place the trees at a
disadvantage, and the ultimate forest stage, though it may change in
composition, can not be displaced by another.

It is obvious that the vegetation forms and habitat forms of associated
species are of fundamental importance in determining the course and
result of competition. Identity of vegetation form regularly produces
close competition, and the consequent numerical reduction or
disappearance of one or more species. Dissimilarity, on the other hand,
tends to eliminate competition, and to preserve the advantage of the
superior form. Species of trees compete sharply with each other when
found together; the same is true of shrubs, or rosettes, etc. The
relation of the shrubs to the trees, or of the rosettes to the shrubs of
a formation is one of subordination rather than of competition. The
matter of height and width often enters here also to such a degree that
the tallest herbs compete with the bushes and shrubs, and rosettes with
mats or grasses. The amount and disposition of the leaf surface are
decisive factors in the competition between species of the same
vegetation form, in so far as this is governed by light. In those plants
in which the leaves are usually erect, notably the grasses and sedges,
the competition between the aerial parts is relatively slight, and the
result is determined by the reactions of the underground stems and
roots.

The position of the competing individuals is of the greatest importance.
The distance between the plants affects directly the degree of
competition, while their arrangement, whether in groups according to
species or singly, exerts a marked influence by determining that the
contest shall be between like forms, or unlike forms. Position is
controlled primarily by the relation existing between seed-production
and dissemination. It is of course influenced in large measure by the
initial position taken by the invaders into a nudate area, but this is
itself a result of the same phenomena. The individuals of species with
great seed-production and little or no mobility usually occur in dense
stands. In these, the competition is fierce, for the two reasons of
similarity and density, and the result is that the plants fall far below
the normal in height and width. This is an extreme example of the group
arrangement. When the seed-production is small, the mobility may be
great or little without seriously affecting the result. The individuals
of a species of this kind will be scattered among those of other
species, and the closeness of competition will depend largely upon the
similarity existing between the two. The arrangement in such cases is
sparse. A species with great seed-production and great mobility usually
shows both kinds of arrangement, the position of the individuals and the
competition between them varying accordingly. This is due to the
intermittent action of distributing agents, making it possible for the
seeds to fall directly to the ground during the times that winds, etc.,
are absent. The three types of arrangement indicated above are termed
gregarious, copious, and gregario-copious. They furnish the basis for
the investigation of abundance which deals essentially with the number
and arrangement of the individuals of competing species. The effect of
distance, i. e., the interval between individuals, upon competition is
fundamental. The competition increases as the interval diminishes, and
the reverse.

The view here advanced, i. e., that competition is purely physical in
nature, renders untenable the current conceptions of vegetation
pressure, occupation, etc. Masses of vegetation are thought to force the
weaker species toward the edge, thus initiating an outward or forward
pressure. As has been shown above, no such phenomenon occurs in
vegetation. This movement is nothing but simple migration, followed by
ecesis, and has no connection with “weaker” species, or the development
of a vital pressure. The direction taken by the migrating disseminules
is essentially indeterminate. Migration seems to be outward, or away
from the mass, merely because the ecesis is greater at the edge, where
the increased dissimilarity between plant forms diminishes the
competition. The actual movement is outward, but it takes place through
the normal operation of competition. In this connection, it should be
pointed out that the common view that plants require room is inexact, if
not erroneous. This is difficult of proof, as it is impossible to
distinguish room as such from the factors normally present, light, heat,
water, and nutrient salts, but it seems obvious that the available
amounts of these will determine the space occupied by a plant,
irrespective of the room adjacent plants may allow it. The explanation
of competition upon physical grounds likewise invalidates the view that
plants possess spheres of influence other than the areas within which
they exert a demonstrable reaction upon the physical factors present.

Competition plays a very important role in alternation. It produces
minor examples of alternation in the physical units of an asymmetrical
series. Its greatest influence, however, is exerted in modifying the
effects of asymmetry. The reaction of occupants emphasizes or reduces
the effect of asymmetry, and has a corresponding action upon
alternation. This result of competition is typical of succession, in
which the sequence of stages arises from the interaction of occupant and
invader.

=345. Kinds of alternation.= Alternation involves two ideas, viz., the
alternation of different species or formations with each other, and the
alternation of the same species or formation in similar but separate
situations. This is the evident result of asymmetry, in response to
which contiguous areas are dissimilar and remote ones often similar.
Individuals of the same species or examples of the same formation may be
said to alternate between two or more similar situations, while
different species or formations are said to alternate _with_ each other,
occurring usually in situations different in character. From the nature
of alternation, the two phenomena are invariably found together.

It is possible to distinguish three kinds of alternation: (1) of a
formation, consocies, layer, facies, or species in similar situations;
(2) of similar or corresponding formations, species, etc., in similar
situations; (3) of facies and other species with respect to number. The
last two are merely variations of the first, arising out of slight
differences in the physical factors of the alternating areas, the
adjacent flora, or the course of competition. The alternation of
different examples of the same formation is a significant feature of
greatly diversified areas, such as mountains. It is naturally much less
characteristic of lands physiographically more uniform. A xerophytic
formation will alternate from ridge to ridge, a mesophytic formation
between the intermediate valleys; aquatic vegetation will alternate from
pond to pond, or stream to stream. The appearance of new or denuded
soils upon which successions establish themselves is the most important
cause of the alternation of formations. The weathering of rocks in
different areas of the same region produces in each a sequence of
similar or identical formations. The same statement is true in general
of other causes of succession, such as erosion, flooding, burning,
cultivation, etc., wherever they operate upon areas physically similar
and surrounded by the same type of vegetation. The areas of more or less
heterogeneous formations characterized by major physical differences are
occupied by consocies. In an extensive formation, the same consocies
alternates from one to another of these areas that are similar. When the
formation is interrupted and occurs here and there in separate examples,
a consocies often alternates from one to another of these. A consocies
regularly derives its character from the fact that one or more of the
facies of the formation is more intimately connected with certain areas
of the latter than with others. This explains why the alternations of
consocies and facies are usually identical. Layers sometimes alternate
between different examples of the same forest or thicket formation, when
they are suppressed in some by the diffuseness of the light.

The alternation of species is a typical feature of formations; it is
absent only in those rare cases where the latter consist of a single
species. The areas of a habitat which show minor physical or historical
(i. e., competitive) differences are occupied by groups of individuals
belonging to one or more species responsive to these differences. Each
of these groups will recur in all areas essentially similar, the
intervals being occupied of course by slightly different groups. Such
groups are constituted by gregarious or copious species of restricted
adjustability. Sparse plants likewise alternate, but they necessarily
play a much less conspicuous part. In habitats not too heterogeneous, a
large number of species are sufficiently adjustable to the slight
differences so that they occur throughout the formation. Often, to be
sure, they show a characteristic response, expressed in the size or
number. This is illustrated by the facies and many of the principal
species of the prairie formation. _Festuca_, _Koelera_, _Panicum_, and
_Andropogon_ occur throughout, except in the moist ravines which are
practically meadows. _Astragalus_, _Psoralea_, _Erigeron_, and _Aster_
grow everywhere on slopes and crests, but they are much more abundant in
certain situations. Other plants, _Lomatium_, _Meriolix_, _Anemone_,
_Pentstemon_, etc., recur in similar or identical situations upon
different hills. _Lomatium_ alternates between sandy or sandstone
crests, _Meriolix_ and _Pentstemon_ occur together upon dry upper
slopes, while _Anemone_ alternates between dry slopes and crests.

Owing to the accidents of migration and competition, similar areas
within a habitat are not occupied by the same species, or group of
species. A species found in one area will be replaced in another by a
different one of the same or a different genus. The controlling factors
of the area render imperative an essential identity of vegetation and
habitat form, though in systematic position the plants may be very
diverse. Such genera and species may be termed _corresponding_. The
relation between such plants is essentially alternation; it should,
perhaps, be distinguished from alternation proper as _corresponsive_.
The prairie formation furnishes a good example of this on exposed sandy
crests, upon which _Lomatium_, _Comandra_, and _Pentstemon_ alternate.
Formations exhibit a similar correspondence.

[Illustration: Fig. 74. Numerical alternation of _Pinus_ and
_Pseudotsuga_ upon east and west slopes.]

All species that alternate show a variation in abundance from one area
to another. Frequently, the difference is slight, and may be ignored,
except in determining abundance. Very often, however, the variation is
so great that a facies may be reduced, numerically, to the rank of a
principal species, or one of the latter to a secondary species. This
phenomenon is distinguished as _numerical_ alternation. It arises from
the fact that the similar areas are sufficiently different to affect the
abundance, without producing complete suppression. It is probable that
this result is due almost entirely to competition. _Astragalus
crassicarpus_ grows on all the slopes of the prairie formation, but on
some it has the abundance of a facies, while on others it is represented
by a few scattered individuals. This difference is much more striking in
separate examples of the same formation, particularly when a normal
facies is reduced to the numerical value of a secondary species. This is
a matter of great importance in the study of formations, for it has
doubtless often resulted in mistaking a consocies for a formation.

Alternation furnishes the logical basis for what may be called
comparative phytogeography. The latter is of much broader scope than the
old subject of geographical distribution, for it treats not only of the
distribution of formations and associations as well as of species, but
it also seeks to explain this by means of principles drawn from the
relation between habitat and vegetation. When the latter come to be
fully based upon physical factor investigations, and upon the effects of
migration and competition as shown in alternation, the comparative study
of formations will represent the highest type of phytogeographical
activity.


                        THE FORMATION IN DETAIL

=346. The rank of the formation.= There have been as many different
opinions in regard to the application of the term formation as there are
concerning the group which is to be called a species. In taxonomy,
however, the concept of the species is purely arbitrary, and agreement
can not be hoped for. In vegetation, on the contrary, the connection
between formation and habitat is so close that any application of the
term to a division greater or smaller than the habitat is both illogical
and unfortunate. As effect and cause, it is inevitable that the unit of
the vegetative covering, the formation, should correspond to the unit of
the earth’s surface, the habitat. This places the formation upon a basis
which can be accurately determined. It is imperative, however, to have a
clear understanding of what constitutes the difference between habitats.
A society is in entire correspondence with the physical factors of its
area, and the same is true of the vegetation of a province.
Nevertheless, many societies usually occur in a single habitat, and a
province contains many habitats. The final test of a habitat is an
efficient difference in one or more of the direct factors,
water-content, humidity, and light, by virtue of which the plant
covering differs in structure and in species from the areas contiguous
to it. A balsam-spruce forest shows within itself certain differences of
physical factors and of structure. The water-content will range from
20–25 per cent, and the light from .02–.003. One portion may consist
chiefly of _Pseudotsuga mucronata_, another of _Picea engelmannii_, and
a third of _Picea parryana_, or these species may be intermingled. If,
however, this forest is compared with the gravel slide, which touches it
on one side, and the meadow thicket, which meets it on another, the
physical factors and the species both demonstrate that it is the forest,
and not its parts, which corresponds to a distinct physical entity, the
habitat. This test of a formation is superfluous in a great many cases,
where the physiognomy of the contiguous areas is conclusive evidence of
their difference. It is evident also that remote regions which are
floristically distinct, such as the prairies and the steppes, may
possess areas physically almost identical and yet be covered by
different formations. This point is further discussed under
classification.

The existing confusion in the matter of formations is due to two causes.
The first arises from the fact that much ecological work has been hasty.
Little or no attention has been given to development, and in consequence
rudimentary and transitory stages of succession have often been
described as formations. Mixed areas in particular have caused trouble.
In the second place, there has been a marked tendency to minimize the
need of thoroughness and training by calling every slightly different
area a formation. A failure to recognize the primary value of
alternation has also contributed materially to this. Alternating facies,
and principal species, when separated from each other, have often been
mistaken for formations. This is a danger that must be fully appreciated
and guarded against. In practically all regions, the same formation is
represented by numerous scattered areas, all showing greater or less
differences arising from alternation. This is especially true of thickly
populated regions where virgin areas are rare. The fact that twenty-five
miles intervene to-day between two small stretches of primitive prairie
is permitted to unduly emphasize their differences. It requires the
study of a number of such examples to counteract this tendency, and to
cause one to see clearly that they must have been at one time merely so
many bits of the prairie formation.

In this connection, the lichen and moss groups which are found on rocks
constitute an interesting problem. It is clear that _Peltigera_ and
_Cladonia_, which grow on the forest floor, and _Evernia_, _Ramalina_,
and _Physcia_, which are found on the trees, are merely constituent
species of the forest formation. The same is true of _Cladonia_,
_Urceolaria_, and _Parmelia_, which are found among the sedges and
grasses of alpine meadows. The physical conditions are essentially those
of the formation, and the lichens themselves are more or less peculiar
to it. This is particularly true of the forest, in which the two strata,
bark and moist shaded soil, are present because of the trees. In the
case of granitic rocks, the circumstances are very different. The
species of lichens found on the rocks are not peculiar to the formation,
but they also occur elsewhere. In the forest, _Parmelia_, _Placodium_,
_Physcia_, _Rinodina_, _Urceolaria_, _Lecanora_, _Lecidea_, etc., occur
on the rocks. In the alpine meadows, the rock groups are composed of
_Parmelia_, _Gyrophora_, _Cetraria_, _Acarospora_, _Lecanora_,
_Lecidea_, _Buellia_, etc. The stratum itself is physically very
different and constitutes a distinct habitat. These groups are really
small formations, which are quite distinct from the surrounding forest
or meadow. This is proven conclusively in many places in the mountains
where areas of the characteristic lichen formations of cliffs are
carried by the fall of rock fragments into forest and meadow, where they
persist without modification. This also shows clearly that the groups on
scattered rocks in the same area are to be regarded as examples of the
same cliff formation, except where the differences are evidently to be
ascribed to development and not to alternation. Where these rock
formations can not be traced to cliffs or magmata with certainty, they
must be considered as antedating the vegetation in which they occur.
Often, indeed, especially in igneous areas, they are relicts of the
initial stage of a primary succession. Finally, they prove their
independence of the forest or meadow formation by initiating a distinct
succession within these. Crustaceous groups or formations yield to
foliose ones, and these in turn give way to formations of mosses,
particularly in the forest where the effect of the diffuse light is
felt. From the above, the following rule of formational limitation is
obtained: any area, which shows an essential difference in physical
character, composition, or development from the surrounding formation is
a distinct formation.

[Illustration: Fig. 75. Relict lichen formation in a spruce forest,
invaded by rock mosses.]

=347. The parts of a formation.= All the parts which make up the
structure of a formation are directly referable to zonation and
alternation, alone or together, or to the interaction of the two. The
principles which underlie this have already been discussed under the
phenomena concerned. It is necessary to point out further that the
structure may be produced in several ways: (1) by zonation alone, (2) by
alternation alone, (3) by zonation as primary and alternation as
secondary, (4) by primary alternation and secondary zonation, (5) by the
interaction of the two, as in layered formations. Though all these
methods occur, the first two are relatively rare, and the resulting
structure comparatively imperfect. The typical structure of formations
can best be made clear by the consideration of a prairie which belongs
to the fourth group, and a forest which represents the last.

[Illustration: Fig. 76. Early (prior) aspect of the alpine meadow
formation (_Carex-Campanula-coryphium_), characterized by _Rydbergia
grandiflora_.]

The major divisions of prairie and forest formations are regularly due
to alternation. There is an inherent tendency to the segregation of
facies, arising out of physical or historical reasons, or from a
combination of both. Not all formations show this, but it is
characteristic of the great majority of them. The primary areas which
thus arise have been called associations: they are naturally subordinate
to the formation. To avoid the confusion which inevitably results from
using the word association in two different senses, it is proposed to
term this primary division of the formation, a _consociation_, or
better, a _consocies_. This term is applied only to an area
characterized by a facies, or less frequently, by two or more facies
uniformly commingled. The consocies of grassland are determined by
grasses, those of forests by trees, etc. From the different position of
the facies in these two types of vegetation such areas are readily seen
at all times in the forest, but they are often concealed in grassland by
the tall-growing principal species of the various aspects. When definite
consocies are present, they are often found to mingle where they touch,
producing miniature transition areas, and, very rarely, they sometimes
leave gaps in which no facies appears.

[Illustration: Fig. 77. Late (serotinal) aspect of the alpine meadow,
characterized by _Campanula petiolata_, _Rydbergia_ in fruit.]

The seasonal changes of a formation, which are called aspects, are
indicated by changes in composition or structure, which ordinarily
correspond to the three seasons, spring, summer, and autumn. The latter
affect the facies relatively little, especially those of woody
vegetation, but they influence the principal species profoundly, causing
a grouping typical of each aspect. For these areas controlled by
principal species, but changing from aspect to aspect, the term
_society_ is proposed. They are prominent features of the majority of
herbaceous formations, where they are often more striking than the
facies. In forests, they occur in the shrubby and herbaceous layers, and
are consequently much less conspicuous than the facies. A close
inspection of the societies formed by principal species shows that they
are far from uniform. Since they usually fail to exhibit distinct parts,
it becomes necessary to approach the question of their structure from a
new standpoint. Such is afforded by aggregation, which yields the
simplest group in vegetation, i. e., that of parent and offspring. This
is so exactly a family in the ordinary sense that there seems to be
ample warrant for violating a canon of terminology by using the word for
this group, in spite of its very different application in taxonomy. It
has already been shown that aggregation further produces a grouping of
families, which may properly be called a _community_. As they are used
here, _family_ and _community_ become equally applicable to the
association of plants, animals, or man. Both families and communities
occur regularly in each society of the formation, and they represent its
two structures. In some cases, all the families are grouped in
communities, two or more of which then form the society. Very
frequently, however, families occur singly, without reference to a
community, and the two then constitute independent parts of the same
area. This is typically the case wherever gregarious species are
present, since these are merely family groups produced by aggregation.

[Illustration: Fig. 78. _Calthetum_ (_Caltha leptosepala_), a consocies
of the alpine bog formation.]

[Illustration: Fig. 79. _Iridile_ (_Iris missouriensis_), a society of
the aspen formation.]

Objection may be made that this analysis of formational structure has
been carried too far, and that some of the structures recognized are
mere interpretations, and not actual facts. Such a criticism will not
come from one who has got beyond the superficial study of formations,
for he will at once recognize that certain probable features of
structure have not been considered. On the other hand, the ecologist or
the botanist who has not made a careful investigation from the
standpoints of development and structure will naturally refrain from
expressing an opinion, until he has obtained an acquaintance at first
hand with the facts. Over-refinement is the usual penalty of intensive
work. The unbiased investigator, however, will not be misled by the
suddenness with which new concepts appear. It seems plausible that the
structure of a formation, if not as definite, is at least nearly as
complex as that of an individual plant. Few botanists will insist that
the refinement of tissues and tissue systems has been carried further
than the differentiation of the plant warrants. Yet, if these had been
defined within a period of a few years rather than slowly recognized
during more than a century, they would have been called seriously in
question. As a matter of fact, the consocies, under the term
association, and the society, under various names, have been recognized
by ecologists for several years. They are definite phenomena of
alternation which can be found anywhere. The family and the community,
though the latter is less distinct in outline, are equally valid
structures, the proof of which anyone can obtain by thorough methods of
study.

=348. Nomenclature of the divisions.= The suffix _-etum_ is used to
designate a consocies of a formation, e. g., _Picetum_, _Caricetum_,
etc. When two or more species characterize the area, the most
important, or more rarely, the two are used. The termination used to
designate a society is _-ile_, as _Asterile_, _Sedile_, _Rosile_. The
suffix which denotes the community is _-are_, and for the family, it
is _-on_, viz., _Giliare_, _Bromare_, _Bidenton_, _Helianthon_, etc.
Layers are indicated by the affix _-anum_, as _Opulasteranum_,
_Verbesina-Rudbeckianum_, etc. It is evident that these suffixes, like
the terms to which they refer, must be used always for the proper
divisions if they are to have any value at all. There has been a
marked tendency, for example, to use _-etum_ in connection with the
names of groups of very different rank. It is hardly necessary to
point out that such a practice does not promote clearness. The
following tabular statement will illustrate the application of both
terms and suffixes:

 _Picea-Pseudotsuga-hylium_ formation (_-ium_)  _Paronychia-Silene-chalicium_
     _Picetum_              consocies (_-etum_)     _Paronychietum_
 _Opulaster-Ribesanum_      layer (_-anum_)
     _Opulasterile_         society (_-ile_)        _Androsacile_
     _Thalictrare_          community (_-are_)      _Festucare_
     _Pirolon_              family (_-on_)          _Arenarion_

=349. The investigation of a particular formation.= A comprehensive and
thorough study of a formation should be based upon as many examples of
it as are accessible. The example which is at once the most typical and
the most accessible is made the base area. This plan saves time and
energy, reduces the number of instruments that are absolutely necessary,
and establishes a common basis for comparison. The inquiry should be
made along four lines, all fundamental to a proper knowledge of the
formation. These lines are: (1) the determination of the factors of the
habitat, (2) a quadrat and a transect study of the structure of the
formation, (3) a similar investigation of development, (4) a floristic
study of the contiguous formation, with special reference to migration.
The sequence indicated has proven to be the most satisfactory, and is to
be regarded as all but absolutely essential. Naturally, this applies
only to the order in which the various lines are to be taken up, as they
are carried on together when the work is fully under way. Since
instrument and quadrat methods have already been given in detail, it is
unnecessary that they be repeated. Similarly, the questions which
pertain to structure and development and to the surrounding vegetation
are considered in detail in the pages which precede.

[Illustration: Fig. 80. _Eritrichiare_ (_Eritrichium aretioides_), a
community of the alpine meadow formation.]


                    CLASSIFICATION AND RELATIONSHIP

=350. Bases.= Formations may be grouped with reference to habitat or
kind, development or position. Classification upon the basis of habitat
places together formations which are similar in physiognomy and
structure. Developmental classification is based upon the fact that the
stages of a particular succession are organically connected or related,
though they are normally different in both physiognomy and structure.
Grouping with respect to position is made solely upon occurrence in the
same division of vegetation. The formations thus brought together
usually possess neither similarity of kind or structure, nor do they
have any necessary developmental connection. Habitat and developmental
classification are of fundamental value; regional arrangement is more
superficial in character. All serve, however, to emphasize different
relations, and, while the developmental system expresses the most, they
should all be used to exhibit the vegetation of a region, province, or
zone.

[Illustration: Fig. 81. _Pachylophon_ (_Pachylophus caespitosus_), a
family of the gravel slide formation.]

=351. Habitat classification.= In arranging formations with reference to
habitats, the direct factors, water and light, can alone be used to
advantage. Such a system is fundamental, because it is founded upon
similarity of habitat and of structure. Proposed groupings based upon
nutrition-content, or upon the division of factors into climatic and
edaphic, have elsewhere[42] been shown to be altogether of secondary
importance, if not actually erroneous. The basis of the habitat grouping
is water-content, which is supplemented by light whenever the factor is
decisive. The primary divisions thus obtained are water, forest,
grassland, and desert, which are characterized respectively by
associations of hydrophytes, mesophytes, hylophytes, poophytes, and
xerophytes respectively. Within these, formations are arranged according
to the type of habitat, i. e., pond, meadow, forest, dune, etc. These
divisions comprise all formations which belong to the type by virtue of
their physiognomy and structure. Such formations differ from each other
very considerably or completely in the matter of floristic, i. e.,
component species, but they still belong to the same type. A dune
formation in the interior and one on the coast may not have a single
species in common, and yet they are essentially alike in habitat,
development, and structure.

=352. Nomenclature.= The names of formations are taken from the habitats
which they occupy. Each formation should have a vernacular and a
scientific name. The latter is especially important since it ensures
brevity and uniformity, and obviates the obscurity and confusion that
arise from vernacular terms in many tongues. Scientific names have been
made uniformly from Greek words of proper meaning by the addition of the
suffix _-ium_ (εῖον), which denotes place.[43] The following list gives
the English and the scientific name of the various habitats, and their
corresponding formations, and indicates the primary divisions into which
these fall.

    I. Hydrophytia: water plant formations

         1. ocean: oceanium: oceanad,[44] oceanophilous, etc.

         2. sea: thalassium

            surface of the sea: pelagium

            deep sea: pontium

         3. lake: limnium, limnad

         4. pond, pool, tiphium, tiphad

         5. stagnant water: stasium: stasad

         6. salt marsh: limnodium, limnodad

         7. fresh marsh: helium

         8. wet meadow: telmatium

         9. river: potamium

        10. creek: rhoium

        11. brook: namatium

        12. torrent: rhyacium

        13. spring: crenium

        14. warm spring: thermium

        15. ditch: taphrium

        16. sewer: laurium

        17. swamp forest: helohylium

        18. swamp open woodland: helodium

        19. meadow thicket: helodrium

        20. bank: ochthium

            rock bank: petrochthium

            sand bank: ammochthium

            mud bank: pelochthium

        21. rocky seashore: actium

        22. sandy seashore: agium

        23. sandbar: cheradium

        24. tank: phretium

   II. Mesophytia: middle plant formations

       _a._ Sciophytia: shade plant formations

            26. forest: hylium

            27. grove: alsium

            28. orchard: dendrium

            29. canyon: ancium

            30. open woodland: orgadium

            31. thicket: lochmium

       _b._ Heliophytia: sun plant formations

            32. meadow: poium

            33. pasture: nomium

            34. culture land: agrium

            35. waste place: chledium

  III. Xerophytia: dry plant formations

        36. desert: eremium

        37. sand-hills, sandy plain: amathium

        38. prairie, plains: psilium

        39. dry, open woodland: hylodium

        40. dry thicket: driodium

        41. dry forest: xerohylium

        42. gravel slide: chalicium

        43. sandbar: syrtidium

        44. sand draw: enaulium

        45. blowout: anemium

        46. strand: psamathium

        47. dune: thinium

        48. badlands: tirium

        49. hill, ridge: lophium

        50. cliff: cremnium

        51. rock field: phellium

        52. boulder field: petrodium

        53. rock, stone: petrium

        54. humus marsh: oxodium

        55. alkali area: drimium

        56. heath, dry meadow: xeropoium

        57. moor: sterrhium

        58. alpine meadow: coryphium

        59. polar barrens: crymium

        60. snow: chionium

        61. wastes: chersium

Particular formations are indicated by means of floristic
distinctions. Thus, _Populus-hylium_ is the aspen forest as
distinguished from the _Picea-Pseudotsuga-hylium_, or the
balsam-spruce forest; and the _Bulbilis-psilium_, or buffalo-grass
prairie, from the _Bouteloua-Andropogon-psilium_, or grama-bluestem
prairie. Similarly, the aspen formation of the Old World and of the
New may be distinguished as _Populus-tremula-hylium_ and
_Populus-tremuloides-hylium_, respectively. In all formational names,
the facies alone should be used. Frequently, a single facies will
suffice for clearness. As a rule, however, the two most important
facies should be employed; in rare cases only is it necessary to use
the names of three. When it is desirable to refer to two or more
examples of the same formation, a geographical term is added, e. g.,
(1) _Populus-hylium_ (_Crystal Park_), (2) _Populus-hylium_ (_Cabin
Canyon_).

=353. Developmental classification.= This is based upon succession as
the record of development. Upon the basis of development, all the
formations which belong to the same succession are classed together.
They are arranged within each group in the sequence found in the
particular succession. From its nature, developmental classification is
of primary importance in exhibiting the history of vegetational changes.
It has less value than the habitat system for summarizing the essential
structure of a vegetation, inasmuch as it places the emphasis upon
historical rather than structural features. It is evident that both deal
with the same formations, and that the difference is merely one of
viewpoint. The habitat classification is simpler in that it considers
only those formations actually on the ground, while development has
regularly to take into account stages which have disappeared. The groups
of the developmental system, and the arrangement of formations within
them have already been indicated under the nomenclature of succession
(sections 326 and 327).

=354. Regional classification.= The grouping of formations with respect
to the divisions of vegetations is chiefly of geographical value. It
indicates a certain general relationship, but its principal use is to
summarize the structure of the vegetative covering of a region. The
arrangement of formations in the various divisions is made with
reference to the outline of North American vegetation (section 341).
This is naturally based upon the identity of altitude and latitude
zones. In the study of mountain countries, it is often desirable to
group formations with reference to altitude alone. In this case, the
grouping is based upon the following divisions: (1) _bathyphytia_,
lowland plant formations; (2) _mesiophytia_, midland formations; (3)
_pediophytia_, upland formations; (4) _pagophytia_, foot-hill
formations; (5) _orophytia_, subalpine formations; (6) _acrophytia_,
alpine formations; (7) _chionophytia_, niveal formations.

=355. Mixed formations.= These are mixtures of two, rarely more,
adjacent formations, or of two consecutive stages of the same
succession. Mixed formations are really transitions in space or in time
between two distinct formations. Theoretically, they are to be referred
to one or the other, according to the preponderance of species.
Actually, however, they often persist in an intermediate condition for
many years, and it becomes necessary to devote considerable attention to
them. In some cases, there is good reason to think that the species of
two contiguous formations have become permanently associated, and thus
constitute a new formation. This is often apparently true in succession,
when the change from one stage to the next requires a long term of
years, but it is really true only of the very rare cases in which a
succession becomes stabilized in a transition stage. When the mixture is
due to development, the formations concerned are often quite dissimilar,
e. g., grassland and thicket, thicket and forest. If it is the result of
position, the formations are usually similar, i. e., both are grassland,
thicket, or forest, since the plants of the lower level are regularly
assimilated or destroyed, when invasion occurs at two levels. The term
_mictium_ (μικτόν, mixture) is here proposed for the designation of all
mixed formations, whether they arise from succession or from
juxtaposition. Thus, the _Mentzelia-Elymus-mictium_ is the transition
between the _Mentzelia-Pseudocymopterus-chalicium_ and the
_Elymus-Muhlenbergia-chalicium_. Similarly, the _Populus-Picea-mictium_
and the _Pinus-Pseudotsuga-mictium_ are transition stages in the
development of the _Picea-hylium_. On the other hand, the
_Andropogon-Bulbilis-mictium_ is a mixture produced by the mingling of
two contiguous prairie formations. In the future development of this
subject, it will probably become desirable to name mixed formations on
the basis of origin, but at present this is unnecessary. Both in
classification and in description they should be considered between the
formations which give rise to them, and this will at once indicate their
origin.

[Illustration: Fig. 82. A mixed formation of aspens and spruces
(_Populus-Picea-mictium_), preceding the final spruce forest of a burn
succession.]

Puzzling cases of mixture resulting from position occur toward the
limits of facies which occupy extensive areas. _Bouteloua oligostachya_,
and _Andropogon scoparius_ extend from the prairies through the
sand-hills and plains, and into the foot-hills of the Rocky mountains.
Their abundance at once raises a question as to the validity of the
prairie, sand-hill, plain, and foot-hill formations. If these two
grasses were controlling, and equally characteristic throughout, then
the entire stretch would have to be regarded as a single formation.
Since they are often absent, or mixed with other facies of greater
importance, they can not be considered the sole tests of the formation.
This view is reinforced by the fact that prairie, sand-hill, plains, and
foot-hill all have their characteristic principal and secondary species,
in addition to facies that are more or less typical. In certain
formations, doubtless, _Bouteloua_ and _Andropogon_ are relicts, in
others invaders, while in the formations actually constituted by them
they are dominant. The final solution of such problems is quite
impossible, however, until the comparative study of large areas can be
based upon the accurate detailed investigation of the component
formations.


                        EXPERIMENTAL VEGETATION

=356. Scope and methods.= The experimental study of the formation as a
complex organism rests upon methods essentially similar to those
discussed under experimental evolution. The scope of the two fields is
practically the same, moreover, in that both deal with the experimental
development of an organism and the structures that result. The actual
problems are naturally very different, since the formation is a complex
of individual plants, but the fundamental basis of habitat, function,
and structure is common to both. However, the functions now to be
considered are aggregation, invasion, competition, etc., and the
structures, zones, consocies, societies, communities, and families. The
latter may properly be regarded as adaptations called forth by the
adjustment, i. e., aggregation, migration, ecesis, etc., of the
formation to the physical factors of the habitat. As consequences of
measured factors, formational adjustment and adaptation must themselves
be carefully measured and recorded. For these purposes, the methods of
quadrat and transect, of chart, photograph, and formation herbarium are
used. Invaluable as they are for any scientific inquiry into vegetation,
such methods form the very foundation of experimental study in which
accuracy is the first desideratum.

It has already been shown that nature’s own experiments in the
production of new forms furnish the best material for experimental
evolution. This statement is equally true of experimental vegetation.
The formation of new habitats by weathering and transport, and the
denuding of old ones, yield experimental plots of the greatest value.
This is likewise the case in the great majority of formations, where
invasion or competition is active. These are the phenomena that must be
considered in any careful study of vegetation, but in taking them up
from the experimental standpoint, greater attention must be paid to
detail, and the changes must be followed closely for a longer time. The
method that makes use of existing changes in vegetation is designated
the _method of natural habitats_. In contrast with this is the _method
of artificial habitats_, in which the habitat itself is definitely
modified, or a group of species actually transferred to a different
habitat. Many problems of vegetation can be attacked with greater
success under control than in the field. This is particularly true of
competition, in which results can be obtained most readily by means of
the _method of control habitats_, as carried on in the plant house.


                       METHOD OF NATURAL HABITATS

=357. Natural experiments.= Every family as well as every community
constitutes an experiment in competition; the same statement necessarily
holds for the larger groups, society, consocies, and formation, which
are composed of families and communities. The last also make it possible
to study competition in two typical instances, viz., in the family,
where the individuals are of one kind, and in the community, where they
belong to two or more different species. The community, moreover, is a
product of invasion, and it furnishes material for the study of this
function, as well as for that of aggregation and competition.
Practically every formation shows some invasion, but as a rule stable
formations contain so few invaders that they are relatively unimportant
in this connection. Invasion is most active in transition areas and in
mixed formations, whether produced by juxtaposition or by succession,
and its study in these places yields by far the largest number of
valuable results.

As typical complete invasion, a succession is the best of all natural
experiments in aggregation, migration, ecesis, and competition. This is
especially true of the initial stages in which changes in the number and
position are most readily followed. The methods used in studying
successions have been given elsewhere. In addition, it should be pointed
out that one of the first tasks in taking up the ecological
investigation of a region is to make a careful search for all new and
denuded areas, as well as for those in which succession is taking place.
The phenomena in these areas can not be explained until the habitats and
formations have been worked over critically, but the facts must be
collected at the earliest possible moment, since the stages of the
succession are constantly changing, while the stable formations are not.


                     METHOD OF ARTIFICIAL HABITATS

=358. Modification of habitat.= As the final factors in ecesis and
competition, water, light, and temperature control the grouping of
plants into vegetation. An efficient change in one of these, or in all
of them, brings about a visible adjustment in the structure of the plant
group concerned. Modifications of water-content and light are readily
produced in the field by drainage, irrigation, shading, clearing, etc.
In fact, all the changes of habitat indicated under experimental
evolution serve equally well to initiate experiments in experimental
vegetation; indeed, the same experiment covers both fields. It is
impracticable, however, to modify the temperature of a habitat without
changing its water-content or light, and consequently the influence of
temperature can not be determined through experiment by modification.
The extent of the area modified should be as large as convenience will
permit, in order that the number of individuals may be large enough to
indicate clearly the resulting adjustment in position and arrangement.
The best results can be obtained where a small separate area of a
formation can be modified, e. g., where a small swamp can be drained, or
a depression flooded. In the case of light, however, it is usually
impossible to clear or to shade a large area, and the study must be
restricted to a relatively small group of plants. In regions where
lumbering is actively carried on, the consequent clearing initiates
invaluable experiments over large areas, and this is likewise true of
forest plantations. Modification of a large area has decided advantages
in bringing out the changes in the more prominent structural features,
but the causes and the details of the adjustment can be worked out much
more satisfactorily in a small area.

=359. Denuding.= The modification of the habitat by denuding is the sole
method of initiating succession by experiment. It is consequently of the
most fundamental importance in investigating aggregation, ecesis, and
competition, as well as the reactions exerted by the invaders of the
different stages. The possibilities of denuding an entire habitat or an
extensive area are not great, and the investigator must content himself
with denuded quadrats, transects, and migration circles, which are small
enough to permit a critical study of all the factors in succession. It
is of course unnecessary that the denuding be done by the ecologist
himself, provided he is able to follow the succession from the very
beginning. Accordingly, it becomes possible for him to make the very
best use of all those changes wrought by man in which the vegetation is
destroyed over considerable areas. These are essentially natural
experiments, and at this point the methods of natural and artificial
habitats merge.

The manner of denuding depends in a degree upon the nature of
vegetation, but, when time, convenience, and safety are all taken into
account, the actual removal of the vegetation as indicated under the
denuded quadrat is by far the most satisfactory. Under certain
conditions, flooding or burning can be used to advantage, but cases of
this kind are infrequent. The purpose of the experiment determines the
kind of area to be denuded. Quadrat, transact, and migration circle are
equally valuable for ecesis and competition. The quadrat is best adapted
to work in a homogeneous area, while the transect is suited to a
heterogeneous one characterized by zones, societies, or communities. It
is an advantage to replace the denuded transect by a series of denuded
quadrats, one for each zone or society, when the transect would be too
long for convenience. The denuded migration circle is invaluable for
aggregation and ecesis, since it makes possible the study of migration
as a distinct function. A series of denuded quadrats, consisting of one
or more in the different stages of a succession, furnishes important
evidence concerning the development of each stage. By far the best
method, however, for making a comparative study of the stages of a
succession is the quadrat sequence. A quadrat is denuded each year, thus
yielding a complete sequence of miniature stages through the whole
course of succession. This method is especially valuable when a
succession is represented by a single example, and there is no
opportunity of reconstructing it by the comparison of various stages. A
quadrat sequence is naturally of the greatest value if begun at the time
when the first invaders appear.

=360. Modification of the formation by transfer.= The study of partial
and intermittent invasion into an established vegetation is made through
the transfer of a species or group of species by means of seeding or
planting. The process differs in no way from that described for
experimental evolution, except in so far that an endeavor is made to
establish a family or a community, and not merely a few individuals.
Transfer makes possible the critical investigation of ecesis under
conditions of intense competition, as well as the study of aggregation
and the origin of plant groups under these conditions. Perhaps its
greatest value is in the experimental study of alternation and zonation,
especially the former. It is practically impossible to determine whether
alternation, especially when corresponsive, is due to physical or
historical causes, i. e., migration and competition, except by means of
the reciprocal transfer of the species concerned.

Field cultures for the careful study of ecesis and competition are made
by transferring seeds or plants to new or denuded soils. This is
practically a combination of the methods of modification and transfer.
It has a unique value in making it possible to initiate artificial
successions of almost any character that is desired, and to carry them
out with the reactions more or less under control. This opens up an
extremely important field of experimental inquiry, which promises to put
the study of succession upon a much more exact basis. Competition
cultures in the field are not essentially different from those under
control, and they will be considered under the next method.


                       METHOD OF CONTROL HABITATS

[Illustration: Fig. 83. Simple culture of floating ecads of _Ranunculus
sceleratus_.]

=361. Competition cultures.= Although it is quite possible to carry on
experiments in invasion and succession in the planthouse, the limited
space usually available makes this undesirable, except in a few problems
where control is necessary. Competition cultures, on the other hand,
yield better results in the planthouse than in the field, since the
physical factors and the appearance of unwelcome migrants are much more
easily controlled. The possibilities of the culture method in the study
of competition seem inexhaustible, and the author has found it necessary
to confine his own investigations to a few of the fundamental problems.
In this work, he has distinguished several kinds of cultures, based
chiefly upon the species concerned and the arrangement of the
individuals. _Simple_ cultures are those in which a single species is
used. The resulting group is a family, and the competition is between
like individuals. In such cultures, the problem of the factors in
competition is reduced to its simplest terms. _Mixed_ cultures are based
upon two or more species, and the problem is correspondingly
complicated. As a rule, all the seeds have been sown at the same time in
both simple and mixed cultures, but it has been found desirable to make
some _heterochronous_ cultures, in which seeds are also sown after the
plants have appeared. Mixed cultures are distinguished as _layered_
cultures, when the species are of very different height. Thus, rosettes
have been grown with stemmed plants, tall slender forms with low
branching ones, erect plants with twining and climbing plants, etc.
Further evidence as to the nature of competition has been sought by
means of _ecad_ cultures, and _factor_ cultures. In the former, plants
of different response to water and light are grown together under the
same conditions, in order to evaluate the part played by the nature of
the plant. In a factor culture, the area is divided into two or more
parts which are given different amounts of water or of light, in order
to determine the influence of slight variations upon the same
competitors. In somewhat similar fashion, an attempt has been made to
ascertain the bearing of biotic factors upon competition. Cultures are
easily made in which _Cuscuta_ or parasitic fungi are used to place
certain species at a disadvantage. _Permanent_ cultures are obtained by
allowing the plants to ripen and drop their seeds for several
generations, just as in nature. They are indispensable for determining
the final outcome of the competition between different species.

[Illustration: Fig. 84. Mixed culture of _Solidago rigida_ and _Onagra
biennis_.]

=362. Details of culture methods.= All competition cultures have been
made 1 meter square. In other words, they are quadrats, and they are
treated exactly as denuded quadrats in the field with respect to factor
readings, charts, and photographs. In the writer’s studies, germination
tests were made of a large number of species, and those selected which
showed a high per cent of germinability. Since this was the first
experimental study of competition, this test was deemed necessary, but
it is quite evident that no such selection is made in nature.
Consequently, when the seeds used are known to be fresh, a germination
test is usually superfluous. Considerable care was taken also to select
species known to be vigorous growers, with the result that practically
all the species used for experiment were ruderal or subruderal. The
species employed, and the kinds of cultures in which they were grouped
were as follows:

[Illustration: Fig. 85. Heterochronous culture of _Helianthus annuus_
and _Datura stramonium_. Family culture of _Datura_, _Verbascum_, etc.,
in the foreground.]


  1. _Simple culture of Helianthus annuus._ The culture plot was divided
  into four equal parts; 12 seeds were planted in one, 25 in another, 50
  in the third, and 100 in the fourth.

  2. _Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus
  canadensis._ Twenty-five seeds each of _Helianthus_ and _Panicum_ were
  planted alternately at equal distances in one-half of the plot, while
  the other half was planted similarly with _Helianthus_ and _Elymus_.

  3. _Mixed culture of Solidago rigida and Onagra biennis._ Over
  one-half of the plot were scattered 50 seeds of _Solidago_ and 100 of
  _Onagra_; over the other, 100 and 200 seeds respectively.

  4. _Layered culture of Laciniaria punctata, Bidens frondosa, Salvia
  pitcheri, Cassia chamaecrista and Kuhnia glutinosa._ Fifty seeds of
  each species were scattered more or less uniformly over the entire
  plot.

  5. _Layered culture of Silphium laciniatum, Datura stramonium and
  Lactuca ludoviciana._ Fifty seeds of _Datura_ and _Lactuca_, and 25 of
  _Silphium_ were sown uniformly in one-half of the plot. In the other
  half, 25 holes were made at equal intervals, and one seed of each of
  the three planted in each hole.

  6. _Ecad culture of Oenothera rhombipetala (xerophytic), Verbascum
  thapsus (mesophytic), and Penthorum sedoides (hydrophytic)._ One
  hundred seeds of _Oenothera_ and 200 each of _Verbascum_ and
  _Penthorum_ were scattered over the plot.

  7. _Heterochronous culture of Helianthus annuus and Datura
  stramonium._ One hundred seeds of Helianthus were scattered over one
  half, and the same number of Datura seeds over the other half of the
  plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25
  seeds in a second circle at some distance. A month later, 100 seeds of
  _Helianthus_ were sown in the _Datura_ plot, and _vice versa_.

  8. _Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra,
  Datura, Penthorum, Solidago and Verbascum._ The plot was divided into
  9 squares and in each were sown 50 seeds of one of these plants.

  9. _Community culture._ The sowing was made exactly as for the family
  culture, except that 20 seeds of each plant were used. In the middle
  of each square, 5 seeds of a different species were planted. For the
  _Helianthus_, _Kuhnia_, and _Panicum_ groups, _Onagra_ was used; for
  _Bidens_, _Onagra_, and _Datura_, _Helianthus_ was used, and for
  _Penthorum_, _Solidago_, and _Verbascum_, _Panicum_.


At the time the cultures were started, check plants were sown in pots.
The most vigorous seedlings were transplanted singly to large pots, and
grown under conditions of water, light, and soil as similar as possible
to those of the competition plots. Photographs of check plants and plots
were made at the proper intervals, and the plots were charted in
quadrats to show the course of competition. The factors which control
competition were sought in a critical study of water-content and light
values, which is still in process. This work has gone far enough to
indicate the correctness of the view[45] that competition is purely
physical in character. It has, moreover, been demonstrated that “room”
in competition is merely a loose expression for the relation between the
number of individuals in a given space, and the amount of water, light,
and temperature available in the same space.




                                GLOSSARY


NOTE: Last terms frequent in compounds are found in their proper place
alphabetically. The accent is indicated only in those words accented on
the penult; all others are accented on the antepenult, or recessively.

  =abundance=, the total number of individuals in an area.

  =acospore= (ἀκή, point), a plant with awned disseminules.

  =acrophyti´um= (ἄκρον, peak), an alpine plant formation.

  =acti´um= (ἀκτή, rocky coast), a rocky seashore formation; =actad=,
    plant of a rocky seashore.

  =-ad= (-αδης, patronymic suffix), suffix for denoting an ecad.

  =adaptable=, able to originate ecads; =adaptation=, the structural
    response to stimuli.

  =adjustment=, the functional response to stimuli.

  =adventicious= (_adventicius_, foreign), invading from distant
    formations.

  =adventive= (_adventivus_, accidental), established temporarily.

  =aggregation=, the coming together of plants into groups.

  =agi´um= (ἀγή, beach), a beach formation; =agad=, a beach plant.

  =agri´um= (ἀγρός, field), a culture formation; =agrad=, a cultivated
    plant.

  =aiphyti´um= (ἀεί, permanent), an ultimate formation.

  =alsi´um= (ἄλσος, grove), a grove formation; =alsad=, a grove plant.

  =alternation=, the heterogeneous arrangement of plant groups and
    formations universally present in vegetation.

  =amathi´um= (ἄμαθος, sand of the plain), a sand-hill or sandplain
    formation; =amathad=, a sand-hill plant.

  =ammochthi´um= (ἄμμος, sand, ὄχθη, bank), a sand bank formation;
    =ammochthad=, a sand bank plant.

  =anci´um= (ἄγκος, mountain glen), a canyon formation; =ancad=, a
    canyon plant.

  =anemi´um= (ἄνεμος, wind), a blowout formation; =anemad=, a blowout
    plant; =anemochore=, a plant distributed by wind.

  =-anum= (locative suffix), a suffix denoting a layer.

  =apostrophe= (ἀπό, away from, στροφή, a turning), the arrangement of
    the row of chloroplasts parallel to the rays of light.

  =apparent noon=, the time when the sun crosses the meridian, i. e.,
    sun noon as distinguished from noon, standard time.

  =-ard= (ἄρδον, water of the land), combining term for water-content;
    =ardium=, a succession due to irrigation.

  =ardesiacus=, slate colored.

  =-are= (locative suffix), suffix denoting a community.

  =aspect= (_aspectus_, appearance), the seasonal impress of a
    formation, e.g., the spring aspect.

  =association=, the arrangement of individuals in vegetation.

  =atmometer= (ἀτμός, vapor), an instrument for measuring evaporation.

  =atropurpureus=, dark purple.

  =atrovirens=, dark green.

  =autochore= (αὐτός, self), motile plants, or those with motile spores;
    =autochthonous= (χθών, ground), native.

  =avellaneus=, drab.

  =barrier=, a physical or biological obstacle to migration or ecesis.

  =bathyphyti´um= (βαθύς, low), a lowland plant formation.

  =blastochore= (βλάστη, growth), a plant distributed by offshoots.

  =-bole= (βολή, a throw), combining term for propulsion; =bolochore=, a
    plant distributed by propulsion.

  =broti´um= (βροτός, mortal), a succession caused by man; =brotochore=,
    a plant distributed by man.

  =caeruleus=, pale blue.

  =caesius=, eye-blue.

  =camni´um= (κάμνω, cultivate), a succession due to cultivation.

  =carphospore= (κάρφος, scale), a plant with disseminules possessing a
    scaly or chaffy pappus.

  =carpostrote= (καρπός, fruit), a plant migrating by means of fruits.

  =centrospore= (κέντρον, spur), a plant with spiny disseminules.

  =chalici´um= (χάλιξ, gravel), a gravel slide formation; =chalicad=, a
    gravel slide plant.

  =cheradi´um= (χέραδος, a sandbar), a wet sandbar formation;
    =cheradad=, a wet sandbar plant.

  =chersi´um= (χέρσος, dry barren waste), a dry waste formation;
    =chersad=, plant of a dry waste.

  =chioni´um= (χιών, όνος, snow), a snow formation; =chionad=, a snow
    plant; =chionophyti´um=, a niveal plant formation.

  =chledi´um= (χλῆδος, rubbish), a ruderal formation; =chledad=, a
    ruderal plant.

  =chlorenchym= (χλωρός, greenish yellow, ἐνχύμα, infusion), the
    chlorophyll tissue of the leaf.

  =-chore= (χωρέω, to spread abroad), combining term to denote agent of
    migration.

  =chresard= (χρῆσις, use), the available water of the soil, the
    physiological water-content.

  =clitochore= (κλίτος, slope), a plant distributed by gravity.

  =clysi´um= (κλύσις, a flooding), a succession in a flooded soil.

  =-colus= (κόλος, dwelling in), combining term for habitat forms.

  =community=, a mixture of the individuals of two or more species, a
    group of families.

  =comospore= (κόμη, hair) a plant with hairy or silky disseminules.

  =competition=, the relation between plants occupying the same area,
    and dependent upon the same supply of physical factors.

  =consocies=, that subdivision of a formation controlled by a facies.

  =copious=, used of species in which the individuals are arranged
    closely but uniformly.

  =coryphi´um= (κορυφή, peak), an alpine meadow formation; =coryphad=,
    an alpine meadow plant.

  =creatospore= (κρέας, ατος, meat), a plant with nut fruits.

  =cremni´um= (κρημνός, crag, cliff), a cliff formation; =cremnad=, a
    cliff plant.

  =creni´um= (κρήνη, spring), a spring formation; =crenad=, a spring
    plant.

  =crymi´um= (κρυμός, frost), a polar barren formation; =crymad=, a
    polar plant; =crymophytic=, pertaining to polar plants.

  =crystallochore= (κρύσταλλος, ice), a plant distributed by glaciers.

  =cyaneus=, azure.

  =cyriodoche= (κύριος, regular), a normal succession.

  =dendri´um= (δένδρα, fruit trees), an orchard formation; =dendrad=, an
    orchard plant.

  =derived=, coming from other formations or regions, not native.

  =diphotic= (δι-, two), the two surfaces unequally lighted;
    =diphotophyll=, a leaf differentiated into palisade and sponge
    tissues owing to unequal illumination.

  =diplophyll= (διπλόος, twofold), an isophotic leaf with water-storage
    cells in the middle.

  =disseminule= (semen, seed), a seed fruit modified for migration.

  =dissophyte= (δισσός, double), a plant with xerophytic leaves and
    stems, and mesophytic roots.

  =-doche= (δοχή, succession), succession.

  =drimi´um= (δριμύς, biting, pungent), an alkaline habitat, and the
    corresponding formation; =drimad=, a plant of such a formation.

  =driodi´um= (δρίος, thicket), a dry thicket formation; =driodad=,
    plant of a dry thicket.

  =dysgeogenous= (δυς-, bad, γῆ, soil), weathering with difficulty to
    form soil.

  =ecad= (οἶκος, home), a habitat form due to origin by adaptation;
    =ece´sis= (οἰκῆσις, act of coming to be at home), the germination
    and establishment of invaders; =ecograph=, an instrument for
    measuring a physical factor of a habitat; =ecotone= (τόνος,
    tension), the tension line between two zones, formations, consocies,
    etc.

  =ecballi´um=, (ἐκβάλλω, cut down forests), a succession due to
    lumbering.

  =echard= (ἔχω, to withhold), the non-available water of the soil.

  =edobole= (οἶδος, swelling), a plant whose seeds are scattered by
    propulsion through turgescence.

  =efficient difference=, the amount of a physical factor necessary to
    produce a change in the response.

  =enauli´um= (ἔναυλος, hollow channel), a sanddraw formation;
    =enaulad=, a sanddraw plant.

  =ende´mic= (ἐν, within, δῆμος, district), occurring in a single
    formation, or natural region; =ende´mism=, the condition of growing
    in but one natural area.

  =epistrophe= (ἐπί, towards, στροφή, a turning), the arrangement of the
    row of chloroplasts at right angles to the incident light.

  =eremi´um= (ἔρημος, desert), a desert formation; =eremad=, a desert
    plant.

  =estival=, pertaining to summer.

  =-etum= (locative suffix), suffix used to denote a consocies.

  =eugeogenous= (εὖ-, well, γῆ, soil), weathering readily to form soil.

  =facies=, a dominant species of a formation: a distinct area
    controlled by it is a consocies.

  =family=, a group of individuals belonging to one species.

  =fixity=, the condition characterized by little or no response to
    stimuli.

  =flavovirens=, yellow green.

  =forewold=, equivalent to the German “vorwald,” the thicket zone
    bordering a forest.

  =-genous= (γένω, to produce), producing.

  =geotome= (γῆ, earth, τομή, edge), an instrument for obtaining soil
    samples.

  =gloeospore= (γλοιός, sticky stuff), a plant with viscid disseminules.

  =-graph= (γραφή, a writing), combining term for a recording
    instrument.

  =gregarious= (gregarius, grouped in herds), used of species in which
    the individuals occur in groups.

  =habitat=, a definite physical area characterized by a formation;
    =habitat form=, the impress given the plant by the habitat.

  =harmosis= (ἅρμοσις, an adapting), response to stimuli, comprising
    both adjustment and adaptation.

  =hedi´um= (ἕδος, a sitting, base), a succession in a residuary soil.

  =heliad= (ἥλιος, sun), a heliophyte; =heliophyll=, the leaf of a sun
    plant; =heliophyte=, a sun plant; =heliophyti´um=, a sun plant
    formation; =heliophilous=, sun-loving.

  =heli´um= (ἕλος, marsh), a marsh formation; =helad=, a marsh plant;
    =helodi´um= (ἑλώδης, marshy), a swampy open woodland formation;
    =helodad=, a marsh plant; =helodrium= (δρίος, thicket), a thicket
    formation; =helodrad=, a plant of a marshy thicket; =helohyli´um=
    (ὕλη, forest) a marsh forest formation; =helohylad=, a marsh forest
    plant.

  =hepodoche= (ἕπω, follow), a secondary succession.

  =hizometer= (ἵζω, to sink), an instrument for measuring gravitation
    water.

  =holard= (ὅλος, whole), the total water-content of the soil.

  =hydrad= (ὑδρο-, water), a hydrophyte; =hydrochore=, a plant
    distributed by water; =hydroharmose=, response to water stimuli;
    =hydrophyll=, the leaf of a hydrophyte; =hydrophyte=, a water plant;
    =hydrophyti´um=, a water plant formation; =hydrophilous=,
    water-loving; =hydrosta´tic= (στατικός, standing), completing the
    succession under hydrophytic conditions; =hydrotropic= (τροπικός,
    turning), applied to successions which become mesophytic.

  =hygrome´tric= (ὑγρίς, wet), measuring or absorbing water;
    =hygroscopic= (σκοπέω, look), measurable only by a hygroscope; able
    to absorb moisture.

  =hyli´um= (ὕλη, forest), a forest formation; =hylad=, a forest plant;
    =hylocolum=, dwelling in a forest; =hylodi´um= (ὑλώδης, wooded), a
    dry open woodland formation; =hylodad=, a plant of this formation;
    =hylophyte=, a forest plant.

  =hypsi´um= (ὕψος, elevation), a succession caused by elevation.

  =-ile= (locative affix), suffix denoting a society.

  =immobile=, without effective devices for migration.

  =indigenous= (_indigena_, sprung from the land), native.

  =insolation=, exposure to intense heat and light.

  =isabellinus=, leather-colored.

  =isolation=, separation by barriers.

  =isopho´tic= (ἴσος, equal), equally illuminated; =isophotophyll=, a
    leaf in which both halves of the chlorenchym are alike, due to equal
    illumination.

  =-ium= (-εῖον, locative affix), suffix denoting a formation.

  =labile=, plastic, easily modified.

  =lauri´um= (λαύρα, drain), a drain formation; =laurad=, a drain plant.

  =limni´um= (λίμνη, lake), a lake formation; =limnad=, a lake plant;
    =limnodium= (λιμνῶδες, marshy ground), a salt marsh formation;
    =limnodad=, a plant of a salt marsh.

  =lochmi´um= (λόχμη, thicket), a thicket formation; =lochmad=, a
    thicket plant.

  =lophi´um= (λόφος, crest, hill), a hill formation; =lophad=, a hill
    plant; =lophospore=, a plant with plumose disseminules.

  =mastigospore= (μάστιξ, ιγος, lash), a plant with ciliate or
    flagellate disseminules.

  =melleus=, honey-colored.

  =meridian=, used chiefly as a synonym for apparent noon; also an
    imaginary line of longitude.

  =mesad= (μέσος, middle), a mesophyte; =mesophilous=, growing in moist
    soils; =mesophyll=, the leaf of a mesophyte; =mesophyte=, a plant of
    moist soils; =mesophyti´um=, a mesophytic formation; =mesosta´tic=
    (στατικίς, standing), completing the succession under mesophytic
    conditions; =mesotro´pic= (τροπικός, turning), applied to
    successions which become mesophytic.

  =-meter= (μέτρον, measure), combining term for instrument.

  =micti´um= (μικτόν, mixture), a mixed formation.

  =migrant=, a plant that is migrating or invading.

  =migration= (_migratio_, removal), the movement of plants into new
    areas; =migration circle=, a circle employed to measure migration.

  =mobile=, able to be moved, i. e., modified for migration.

  =monochronic= (μόνος, single, χρόνος, time), arising but once;
    =monogenesis= (γένεσις, origin), the origin of a new form at a
    single place or time; =monophyle´sis= (φῦλον, race), origin from a
    single ancestral type; =monoto´pic= (τόπος, place), arising at one
    place only.

  =motile=, able to move by growth, by means of cilia, etc.

  =mutable=, able to produce mutants; =mutant=, a form arising by
    mutation; =mutation=, the sudden appearance of new forms.

  =namati´um= (νάμα, ατος, brook), a brook formation; =namatad=, a brook
    plant.

  =nomi´um= (νομός, pasture), a pasture formation; =nomad=, a pasture
    plant.

  =occupation=, possession of the ground by plants.

  =oceani´um= (ὠκεανός, ocean), an ocean formation; =oceanad=, an ocean
    plant; =oceanophyte=, an ocean plant; =oceanophilous=,
    ocean-dwelling.

  =ocheti´um= (ὀχετός, drain), a succession due to artificial drainage.

  =ochroleucus=, yellowish white.

  =ochthi´um= (ὄχθη, bank), a bank formation; =ochthad=, a bank plant.

  =oligope´lic= (ὀλίγος, little, πηλός clay), containing little clay;
    =oligopsam´mic= (ψάμμος, sand), containing little sand.

  =olisthi´um= (ὄλισθος, slip), a succession in a landslip.

  =ombrometer= (ὄμβρος, a rainstorm), a rain gauge.

  =-on= (-ών, locative suffix), suffix used to denote a family.

  =oncospore= (ὄγκος, hook), a plant with hooked disseminules.

  =orgadi´um= (ὀργάς, άδος, meadowland partially wooded), an open
    woodland formation; =orgadad=, an open woodland plant.

  =orophyti´um= (ὄρος, mountain), a subalpine plant formation.

  =oxodi´um= (ὀξώδης, sour), a humus marsh formation; =oxodad=, a plant
    of a humus marsh.

  =pagi´um= (πάγος, rocky hill, glacier), a succession in a glacial
    soil; =pagophyti´um=, a foot-hill plant formation.

  =pediophyti´um= (πεδίον, plain), an upland plant formation.

  =pelagi´um= (πέλαγος, surface of the sea), a surface sea formation;
    =pelagad=, a plant of the sea surface.

  =pelochthi´um= (πηλός, mud, ὄχθη, bank), a mud bank form;
    =pelogenous=, producing clay; =pelopsammic= (ψάμμος, sand), composed
    of mixed clay and sand; =pelopsammogenous=, producing clay and sand.

  =permobile=, extremely mobile.

  =perquadrat=, a quadrat of 16 square meters or more.

  =petasospore= (πέτασος, sunshade), a plant with parachute-like
    disseminules.

  =petri´um= (πέτρα, rock, stone), a rock formation; =petrad=, a rock
    plant; =petrochthi´um= (ὄχθη, bank), a rock bank formation.

  =petrodi´um= (πετρώδης, abounding in boulders), a boulder field
    formation; =petrodad=, a plant of a boulder field.

  =phelli´um= (φελλεύς, stony ground), a rock field formation;
    =phellad=, a rock field plant.

  =-philous= (φίλος), loving, dwelling in.

  =-photic= (φῶς, φωτός, light), pertaining to light; =photoharmose=,
    response to light stimuli; =photometer=, an instrument for measuring
    light.

  =phreti´um= (φρητός, tank), a tank formation; =phretad=, a tank plant.

  =phyad= (φυή, form of growth), a vegetation form, e. g., tree, shrub,
    etc.

  =-phyll= (φύλλον, leaf), combining term for leaf.

  =-phyte= (φυτόν, plant), combining term denoting plant; =phyteris=
    (ἔρις, strife), plant competition; =-phyti´um= (φυτεῖον, place
    covered with plants), combining term for formation; =phytostrote=, a
    species migrating by means of the plant body.

  =pladobole= (πλάδος, moisture), a plant whose seeds are scattered by
    propulsion due to moisture.

  =plasticity=, the condition characterized by ready response to
    stimuli.

  =pnoi´um= (πνοή, blast), a succession in an aeolian soil.

  =poi´um= (πόα, meadow), meadow formation; =poad=, a meadow plant;
    =poophyte=, a meadow plant.

  =polyan´thous= (πολύς, many, ἄνθος, flower), producing many flowers;
    =polychro´nic= (χρόνος, time), arising at two or more times;
    =polyde´mic= (δῆμος, district), occurring in two or more formations
    or natural regions; =polygenesis= (γένεσις, origin), the origin of a
    new form at two or more places or times; =polyphyle´sis= (φῦλον,
    race), the origin of a form, species, or genus from two or more
    ancestral types; =polyspermatous= (σπέρμα, seed), producing many
    seeds in each flower; =polyto´pic= (τόπος, place), arising at two or
    more distinct places.

  =ponti´um= (πόντος, deep sea), a deep sea formation.

  =potami´um= (ποταμός, river), a river formation; =potamad=, a river
    plant.

  =potometer= (ποτόν, drink), an instrument for measuring absorption.

  =prevernal=, pertaining to early spring.

  =prior=, earlier, used of alpine aspects.

  =prochosi´um= (πρόχωσις, a deposition of mud), a succession in an
    alluvial soil.

  =prodophyti´um= (πρόοδος, pioneer), an initial formation.

  =protodoche= (πρῶτος, first), a primary succession.

  =proximity= (proximitas, nearness), nearness to the area invaded.

  =psamathi´um= (ψάμαθος, sand of the seashore), a strand formation;
    =psamathad=, a strand plant; =psammogenous= (ψάμμος, sand),
    producing a sandy soil.

  =psili´um= (ψιλά, land without trees), a prairie formation; =psilad=,
    a prairie plant.

  =psychrometer= (ψυχρός, chill), an instrument that measures humidity
    by means of a fall in temperature; =psychrograph=, a psychrometer
    that records automatically.

  =ptenophyti´um= (πτηνός, passing), an intermediate formation.

  =pterospore= (πτερόν, wing), a plant with winged disseminules.

  =purpureus=, purple.

  =pycnophyti´um= (πυκνός, thick), a closed formation.

  =pyri´um= (πῦρ, fire), a burn succession.

  =quadrat= (_quadratum_, a square), a square meter of vegetation marked
    off for counting, mapping, etc.; =major=, a quadrat of 2–14 square
    meters.

  =reaction=, the effect of the formation upon the habitat.

  =relict= (_relictus_, left), a species belonging properly to an
    earlier type of succession than the one in which it is found.

  =repi´um= (ῥέπω, sink), a succession due to subsidence.

  =rhoi´um= (ῥόος, stream), a creek formation; =rhoad=, a creek plant.

  =rhoptometer= (ῥοπτόν, something absorbed), an instrument to measure
    absorption of water by the soil.

  =rhyaci´um= (ῥύαξ, ακος, mountain torrent), a torrent formation;
    =rhyacad=, a torrent plant.

  =rhysi´um= (ῥυσίς, a flowing of fire), a succession due to volcanic
    action.

  =ruber=, red.

  =saccospore= (σάκκος, sack), a plant with sack-like disseminules.

  =sarcospore= (σάρξ, σαρκός, flesh), a plant with fleshy disseminules.

  =sciad= (σκιά, shade), a sciophyte; =sciophyll=, the leaf of a shade
    plant; =sciophyte=, a shade plant; =sciophyti´um=, a shade plant
    formation; =sciophilous=, shade-loving.

  =selagraph= (σέλας, light), an instrument for recording light values
    automatically.

  =serotinal=, late, pertaining to autumn.

  =social=, used of plants in which the individuals are compactly
    grouped; =exclusive=, excluding individuals of other species;
    =inclusive=, permitting the entrance of individuals of other
    species.

  =society=, a subdivision of the formation, characterized by a
    principal species.

  =sparse=, scattered singly.

  =spermatostrote= (σπέρμα, ατος, seed), a plant migrating by means of
    seeds.

  =sphyri´um= (σφύρον, ankle, talus), a succession in a talus soil.

  =spongophyll= (σπόγγος, a sponge), a leaf consisting of sponge tissue.

  =sporadophyti´um= (σποράς, άδος, scattered), an open formation.

  =-spore= (σπορά, seed, fruit), combining term for migration
    contrivance; =sporostrote=, a plant migrating by means of spores.

  =stability=, the condition in which the plant makes little or no
    response.

  =stabilization=, the tendency typical of succession, in which the
    successive stages become more stable.

  =stasi´um= (στάσις, a standing), a stagnant pool formation; =stasad=,
    a plant of stagnant water.

  =staurophyll= (σταυρός, a pale), a leaf consisting of palisade tissue.

  =sterrhi´um= (στερρός, barren), a moor formation; =sterrhad=, a moor
    plant.

  =-strote= (στρώτος, strewn), combining term for means of migration.

  =subcopious=, scattered somewhat loosely.

  =subgregarious=, arranged in loose groups.

  =subquadrat=, a quadrat of 1–8 decimeters.

  =succession=, complete and continuous or repeated invasion, in
    consequence of which formations succeed each other.

  =symmetry=, used of topography when it shows uniform changes;
    =radial=, a condition in which the different areas are concentric;
    =bilateral=, where the areas occur in two similar rows.

  =syrtidi´um= (σύρτις, ιδος, sandbar), a dry sandbar formation;
    =syrtidad=, a plant of a dry sandbar.

  =taphri´um= (τάφρος, ditch), a ditch formation; =taphrad=, a ditch
    plant.

  =telmati´um= (τέλμα, ατος, water meads), a wet meadow formation;
    =telmatad=, a wet meadow plant.

  =testaceus=, pale brick colored.

  =thalassi´um= (θάλασσα, sea), a sea formation; =thalassad=, a sea
    plant.

  =thallostrote= (θαλλός, shoot), a species migrating by means of
    offshoots.

  =theri´um= (θήρ, wild animal), a succession due to animals.

  =thermi´um= (θέρμη, hot spring), a hot spring formation; =thermad=, a
    hot spring plant.

  =thini´um= (θίς, θινός, a dune), a dune formation; =thinad=, a dune
    plant.

  =tiphi´um= (τῖφος, pool), a pool formation; =tiphad=, a pond plant.

  =tiri´um= (τείρω, rub away), a bad land formation; =tirad=, a bad land
    plant.

  =tonobole= (τόνος, tension), a plant whose seeds are scattered by
    projection from calyx or involucre.

  =transect= (_transectus_, cut through), a cross section of vegetation.

  =trechometer= (τρέχω, to run off), an instrument for measuring
    run-off.

  =tribi´um= (τρίβω, wear or rub away), a succession in an eroded soil.

  =umbrinus=, umber.

  =variable=, able to produce variants; =variant=, a form arising from
    origin by variation; =variation=, the origin of new forms by the
    action of selection upon minute differences.

  =vegetation form=, a characteristic plant form, e. g., tree, rosette,
    etc.

  =vernal=, pertaining to spring.

  =vicine= (_vicinus_, neighboring), invading from adjacent formations.

  =viridis=, green.

  =vixgregarious=, arranged in small or indistinct groups.

  =water-content=, the water of the soil or habitat; =physiological=,
    the available soil water; =physical=, the total amount of soil
    water.

  =xenodoche= (ξένος, strange), an anomalous succession.

  =xerad= (ξηρός, dry), a xerophyte; =xerasi´um= (ξηρασία, drought), a
    succession due to drainage or drought; =xeriobole= (ξηρία, dryness),
    a plant whose seeds are scattered by dehiscence due to dryness;
    =xerohyli´um= (ὕλη, forest), a dry forest formation; =xerohylad=, a
    dry forest plant; =xerophyll=, the leaf of a xerophyte; =xerophyte=,
    a dry soil plant; =xerophyti´um=, a xerophytic formation;
    =xerophilous=, dwelling in a dry habitat; =xeropoi´um=, a heath
    formation; =xeropoad=, a heath plant; =xerosta´tic= (στατικός,
    standing), used of successions which are completed under xerophytic
    conditions; =xerotro´pic= (τροπικός, turning), applied to
    successions which become xerophytic.

  =zonation=, that condition in which plant groups or formations appear
    in belts or zones.

  =zone=, a belt of more or less uniform vegetation.

  =zoochore= (ζῶον, animal), a plant distributed by animals.




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-----

Footnote 1:

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  BONNIER, G., ET CH. FLAHAULT

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Footnote 2:

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Footnote 3:

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  Certain Plants, Principally Mesophytes. Rep. Bot. Surv. Nebr., 6:48.
  1902.

Footnote 4:

  BUNSEN, R., AND ROSCOE, H. Photometrische Untersuchungen.
  Poggendorff’s Annalen., 117:529. 1862.

  WIESNER, J.

  Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete.
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  Untersuchungen über das photochemische Klima von Wien, Cairo, und
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  Untersuchungen über den Lichtgenuss der Pflanzen im arktischen
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Footnote 5:

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Footnote 6:

  MEYEN, F. J. F. Grundriss der Pflanzengeographie, 12. 1836.

Footnote 7:

  Instructions to Field Parties and Descriptions of Soil Types, 35.
  1903.

Footnote 8:

  PFEFFER-EWART. Physiology of Plants, 1:13. 1900.

Footnote 9:

  The Relation of Leaf Structure to Physical Factors. 1905.

Footnote 10:

  JEVONS, W. A. The Principles of Science, 2:137. 1874.

Footnote 11:

  Die Transpiration der Pflanzen, 14. 1904.

Footnote 12:

  Recherches sur le Rôle Physiologique de l’Eau dans la Végétation. Ann.
  Nat. Sci., 7:20:65. 1895.

Footnote 13:

  Observations on Stomata by a New Method. Proc. Camb. Phil. Soc.,
  9:303. 1897.

Footnote 14:

  Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot.
  Cent., 18:311. 1904.

Footnote 15:

  Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902.

Footnote 16:

  Physiologische Pflanzenanatomie. 3d ed., 537. 1904.

Footnote 17:

  SACHS, J. Ein Beitrag zur Kenntniss der Ernährungsthätigkeit der
  Blätter. Gesammelte Abhandlungen über Pflanzenphysiologie. 1:355.
  1892.

Footnote 18:

  CLEMENTS, E. S. The Relation of Leaf Structure to Physical Factors.
  1905.

Footnote 19:

  DE VRIES, H. Die Mutationstheorie, 1:6. 1901.

Footnote 20:

  POUND AND CLEMENTS. A Method of Determining the Abundance of Secondary
  Species. Minn. Bot. Studies, 2:19. 1898.

Footnote 21:

  The Development and Structure of Vegetation, 84. 1904.

  THORNBER, J. J. The Prairiegrass Formation in Region I. Rep. Bot.
  Surv. Neb., 5:29. 1901.

Footnote 22:

  Pound and Clements. The Vegetation Regions of the Prairie Province.
  Bot. Gaz., 25:381. 1898.

Footnote 23:

  Clements, F. E. and E. S. Herbaria Formationum Coloradensium. 1902.

Footnote 24:

  Lehrbuch der Ökologischen Pflanzengeographie, 97. 1896.

Footnote 25:

  Phytogeography of Nebraska, 1st ed., 101. 1898.

Footnote 26:

  Pflanzengeographie auf physiologischer Grundlage, 208. 1898.

Footnote 27:

  Die Vegetation der Erde. Engler Bot. Jahrb., 17:b55. 1893.

Footnote 28:

  Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot.
  Surv. Nebr., 5:36, 46. 1901.

Footnote 29:

  Grundzüge einer allgemeinen Pflanzengeographie, 157. 1823.

Footnote 30:

  Essai de phytostatique, etc. 1849.

Footnote 31:

  _l. c._, 116. 1896.

Footnote 32:

  _l. c._, 3. 1898.

Footnote 33:

  Uber mechanische Shutzmittel der Samen gegen schädliche Einflüsse von
  aussen. Engler Bot. Jahrb., 5:56. 1883.

Footnote 34:

  Essai Elémentaire de Géographie Botanique, 45. 1820.

Footnote 35:

  Stationes Plantarum Amoen. Acad., 4:64. 1754.

Footnote 36:

  Die Vegetation der Erde, 4. 1872.

Footnote 37:

  When this word was first proposed, the author did not know that
  Briquet had already applied the term _polytopism_ to this concept
  (Ann. Conserv. Bot. Gen., 5:73. 1901). Since polygenesis expresses the
  idea of origin, and applies to multiple origin in time as well as in
  space, it is retained as the name of this concept. Polytopic and
  monotopic are adopted for multiple and single origin in space
  respectively, and _polychronic_ and _monochronic_ are proposed for
  similar origin in time.

Footnote 38:

  The Origin of Species, 186. 1859.

Footnote 39:

  Rocks, Rock-weathering, and Soils, 300. 1897.

Footnote 40:

  Relation d’un Voyage du Levant. 1717.

Footnote 41:

  Clements, F. E. The Development and Structure of Vegetation. Rep. Bot.
  Surv. Nebr., 7:163. 1904.

Footnote 42:

  Clements, F. E. The Development and Structure of Vegetation, 24, 27.
  1904.

Footnote 43:

  Clements, F. E. A System of Nomenclature for Phytogeography. Engler
  Jahrb., 31:b70:1. 1902.

Footnote 44:

  The terms, _oceanad_, _hylad_, _poad_, _eremad_, etc., are proposed in
  place of _oceanophyte_, _hylophyte_, etc. They are much shorter and
  make consistent groups under the general term, _ecad._, i. e., habitat
  form.

Footnote 45:

  CLEMENTS, F. E. The Development and Structure of Vegetation, 166.
  1904.

------------------------------------------------------------------------




                          TRANSCRIBER’S NOTES


 Page Changed from                     Changed to

   84 144 square inches of water, with 144 cubic inches of water, with
      an opening ¼ × 12 inches at the  an opening ¼ × 12 inches at the
      base                             base

  234 as many readily be seen from the as may readily be seen from the
      fact that migration varies       fact that migration varies
      inversely as the                 inversely as the

  319 ochthi´um (ὄχφη, bank), a bank   ochthi´um (ὄχθη, bank), a bank
      formation; ochthad, a bank plant formation; ochthad, a bank plant

 1. Typos fixed; non-standard spelling and dialect retained.
 2. Enclosed italics font in _underscores_.
 3. Enclosed bold or blackletter font in =equals=.
 4. The caret (^) serves as a superscript indicator, applicable to
      individual characters (like 2^d) and even entire phrases (like
      1^{st}).
 5. Subscripts are shown using an underscore (_) with curly braces { },
      as in H_{2}O.