[Plate A: OUTLINE MAP
 OF
 ALGERIA
 SHOWING MAIN FEATURES OF SURFACE DRAINAGE
 Compiled by G. Sykes, Esq. F. R. G. S.]

[Plate 1:

Illustration: Fig. 1. View of the Oasis of Laghouat

Illustration: Fig. 2. Wood market at Laghouat. Juniper, oak, pine,
 and other kinds of wood, each bunch a camel load, exposed for sale.

Illustration: Fig. 3. Wood market at Laghouat. Roots of Zizyphus,
 branches of juniper and pine, and pine bark. The latter is said to
 be used for staining.]





                          BOTANICAL FEATURES
                                OF THE
                            ALGERIAN SAHARA


                                  BY
                         WILLIAM AUSTIN CANNON


[Illustration: logo]


                           WASHINGTON, D.C.
          PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
                                 1913




                  CARNEGIE INSTITUTION OF WASHINGTON
                          PUBLICATION NO. 178


                   PRESS OF J.B. LIPPINCOTT COMPANY
                             PHILADELPHIA




                               CONTENTS.
                               * * * * *


                                                               PAGE
  Introduction                                                    1

  Geographical Characteristics of Algeria                         2

  Climate of Algeria                                              6

  Some Characteristics of the Vegetation of the Tell             15

  The Forests of Algeria                                         18

  The High Plateau                                               20

  Boghari to Laghouat                                            20

  Djelfa to Laghouat                                             22

  Laghouat                                                       23

    The Oasis of Laghouat                                        24

    The Plain                                                    25

    The Dunes near Laghouat                                      27

    The Mountains about Laghouat                                 28

    Effects of Grazing on Vegetation near Laghouat               29

  From Laghouat to Ghardaia                                      31

    Region of the Dayas                                          31

    Daya of Tilrempt                                             32

    The Chebka                                                   34

  Ghardaia                                                       36

    The Oasis of Ghardaia                                        38

    The Plain (Hamada) of Ghardaia                               39

    The Mountains about Ghardaia                                 41

    The Valley of the M’Zab                                      41

    Protected Areas near Ghardaia                                42

    Root-Habits in the Ghardaia Region                           44

    Leaf-Habits in the Ghardaia Region                           49

    Growth and Flowering Habits in the Ghardaia Region           49

  Ghardaia to Touggourt                                          50

  Ghardaia to Ouargla—Vegetation                                 53

  Ouargla to Touggourt—Vegetation                                56

  Touggourt to Biskra—Physical Features and Vegetation           58

  The Biskra Region                                              59

    Topography                                                   59

    Plant Habitats of the Biskra Region                          60

    Vegetation of the Biskra Region                              61

  General Summary and Conclusions                                66

    The Environmental Conditions of Plants in Arid Regions       66

    Environmental Features of the Flora of Algeria               69

    Some Effects of Temperature and Rainfall in Southern Algeria 71

    The Soil Relation in Southern Algeria                        73

    Root-Characters and Species Distribution in Southern Algeria 76

    The Biotic Factor                                            77

    Comparison of Some General Features of the Vegetation of
     Southern Algeria and of Southern Arizona                    79




                        LIST OF ILLUSTRATIONS.
                               * * * * *

                   Plate A. Outline map of Algeria, showing main
                     features of surface drainage.

  Pl.  1, fig.  1. View of the oasis of Laghouat.

                2. Wood market at Laghouat. Juniper, oak, pine, and
                     other kinds of wood, each bunch a camel load,
                     exposed for sale.

                3. Wood market at Laghouat. Roots of _Zizyphus_,
                     branches of juniper and pine, and pine bark.
                     The latter is said to be used for staining.

  Pl.  2, fig.  4. Shoot-habit of _Acanthyllis tragacanthoides_.
                     Laghouat.

                5. Shoot-habit of _Zollikoferia spinosa_. Laghouat.

                6. _Acanthyllis tragacanthoides_ on sandy plain.
                     Laghouat.

                7. _Zollikoferia spinosa_ in habitat, plain
                     (hamada). Laghouat.

  Pl.  3, fig.  8. Detail of north slope of Nomad Mountains, where
                     _Zollikoferia spinosa_ is the dominant species.
                     Laghouat.

                9. Vegetation of plain (hamada) at Tilrempt. The
                     conspicuous shrub is _Haloxylon articulatum_.

               10. The daya of Tilrempt from the plain, showing the
                     character of the depression.

  Pl.  4, fig. 11. Near view of the daya of Tilrempt. The fortified
                     stage station, bordj, and a nomad camp are to
                     be seen. The flattened, level, lower surface of
                     the trees is the effect of grazing, mainly by
                     camels.

               12. Jujube (_Zizyphus_) shrubs and betoum
                     (_Pistacia_) at the daya of Tilrempt.

  Pl.  5, fig. 13. A young specimen of betoum in the midst of a
                     protecting jujube at the daya of Tilrempt.

               14. Leaf and shoot habit of the jujube (_Zizyphus
                     lotus_). Daya of Tilrempt.

               15. Leaves of the betoum (_Pistacia atlantica_) from
                     the daya of Tilrempt.

  Pl.  6, fig. 16. South wall of the valley of the Oued M’Zab at
                     Ghardaia.

               17. Detail of an eroded bank of the Oued M’Zab at
                     Ghardaia. The overhanging stratum is hardpan
                     similar to the “caliche” of southwestern United
                     States.

  Pl.  7, fig. 18. Shoot-habit of _Haloxylon articulatum_, from the
                     plain near the daya of Tilrempt.

               19. Agriculture at Ghardaia. The fields are divided
                     into plots about 3 by 4 feet in size, for the
                     more economical use of water. Young barley is
                     shown growing (November).

  Pl.  8, fig. 20. Vegetation in upper part of a “draw” on plain
                     north of the M’Zab Valley, Ghardaia.

               21. Situation of square No. 2, on plain north of
                     valley at Ghardaia.

  Pl.  9, fig. 22. _Capparis spinosa_ at base of valley wall at
                     Ghardaia.

               23. Leaves of _Capparis spinosa_ from Ghardaia.

  Pl. 10, fig. 24. Leaf-habit of _Dæmia cordata_. Ghardaia.

               25. Shoot-habit of _Salvia ægyptica_. Ghardaia.

               26. Upper surface of rocks, showing small incrusting
                     lichens, on a low mountain about 5 km. north
                     of M’Zab Valley, Ghardaia.

  Pl. 11, fig. 27. Habitat of _Peganum harmala_, at Melika, Ghardaia.

               28. Shoot and leaf habit of _Peganum harmala_, from
                     Ghardaia.

  Pl. 12, fig. 29. Habit of _Henophyton deserti_ at Ghardaia when
                     growing in a protected situation (cemetery).

               30. Leaf-habit of _Henophyton deserti_. Ghardaia.

  Pl. 13, fig. 31. View in a cemetery at Ghardaia, to show the
                     relatively abundant vegetation.

               32. View in an M’Zabite cemetery, Ghardaia.
                     _Haloxylon articulatum_ is the leading species
                     shown.

  Pl. 14, fig. 33. Habit of _Deverra scoparia_, from plain about 3
                     km. north of M’Zab Valley, Ghardaia.

               34. Root-habit of _Gymnocarpos fruticosum_, from
                     plain about 3 km. north of M’Zab Valley at
                     Ghardaia.

  Pl. 15, fig. 35. Root-habit of a mature specimen of _Peganum
                     harmala_, from the floor of the M’Zab Valley
                     near Ghardaia. The soil at the place is
                     comparatively deep.

  Pl. 15, fig. 36. Root-systems of _Helianthemum sessiliflorum_
                     (right), _Haloxylon articulatum_, and _Nolletia
                     chrysocomoides_ (left), from the flood-plain of
                     the Oued M’Zab, Ghardaia.

  Pl. 16, fig. 37. Shoot and root habit of _Citrullus colocynthis_.
                     Oued M’Zab, Ghardaia.

               38. _Euphorbia guyoniana_, in the valley of the Oued
                     M’Zab at Ghardaia.

  Pl. 17, fig. 39. Habit of _Euphorbia guyoniana_. Ghardaia.

               40. To the left, shoot of _Centaurea_ sp., showing
                     effect of grazing; to the right, shoots of
                     _Teucrium pseudo-chamæpitys_. From Ghardaia.

  Pl. 18, fig. 41. Habit of _Salsola_ sp. (below) and _Echinopsilon
                     muricatus_, from the M’Zab Valley, Ghardaia.

               42. Ghardaia to Ouargla. View overlooking the hamada
                     about 28 km. from Ghardaia. The relatively
                     abundant vegetation is associated with a
                     light cover of sand over the area shown. The
                     leading species are _Aristida_, _Deverra_, and
                     _Haloxylon_.

  Pl. 19, fig. 43. _Retama retam_, in dunes about 58 km. from
                     Ghardaia.

               44. Dates at the Bordj Zolfana, about 58 km. from
                     Ghardaia—one of two wells encountered between
                     Ghardaia and Ouargla.

               45. Ghardaia to Ouargla. View overlooking flood-plain
                     of the Oued M’Zab, or a tributary of this oued.
                     The adjacent upland is apparently without
                     plant life.

  Pl. 20, fig. 46. Vegetation on edge of the Oued M’Zab, about
                     63 km. east of Ghardaia, showing habitat of
                     _Rhantherium adpressum_ in foreground.

               47. Sandy flood-plain of the Oued M’Zab, about 63
                     km. from Ghardaia. _Retama_, _Genista_, and
                     _Ephedra_ are the leading plants in this place.

               48. Habit of _Ephedra alata_ in habitat shown in
                     figure 47. This specimen was 1.5 m. high.

  Pl. 21, fig. 49. View of habitat of _Ephedra alenda_, 138 km. from
                     Ghardaia.

               50. Detail of suckering habit of _Ephedra alenda_,
                     from habitat shown in figure 49.

  Pl. 22, fig. 51. Vegetation, mainly _Ephedra_ and _Retama_, of the
                     western edge of the Chott Mellala.

               52. Approach to western edge of the Chott Mellala,
                     showing characteristic rounded hills, or
                     mamelons.

  Pl. 23, fig. 53. View between the Ouargla plain and the Chott
                     Mellala, showing characteristic appearance of
                     eroded hills.

               54. Looking toward the Ouargla plain (reg).

               55. Shallow well about 25 km. north of Ouargla.

  Pl. 24, fig. 56. Ouargla to Touggourt. Leaf-habit of _Limoniastrum
                     guyonianum_. The surface of the leaves is
                     covered with an incrustation of salts.

               57. Habit of _Limoniastrum guyonianum_. About 25 km.
                     north of Ouargla.

  Pl. 25, fig. 58. Vegetation of the reg desert, about 25 km. north
                     of Ouargla. _Ephedra_ and _Retama_ are the
                     leading species of the area—a spreading dune.

               59. Shoot-habit of _Halocnemon strobilaceum_. About
                     28 km. north of Ouargla.

  Pl. 26, fig. 60. Habit of _Halocnemon strobilaceum_ on the edge of
                     a chott, about 28 km. north of Ouargla.

               61. Shoot and leaf habit of _Anabasis articulata_,
                     about 32 km. north of Ouargla.

  Pl. 27, fig. 62. Detail of the shoot-habit of _Salsola
                     tetragona?_, about 25 km. north of Ouargla.

               63. Habit of _Nolletia chrysocomoides_ near the edge
                     of a chott, about 80 km. north of Ouargla.

  Pl. 28, fig. 64. _Tamarix_ sp. as a sand-binder near Bled-el-Amar,
                     south of Touggourt.

               65. Biskra. Habitat of _Euphorbia guyoniana_, looking
                     toward the Djebel Maouya, with the Chaine de
                     Sfa in the background.

  Pl. 29, fig. 66. Characteristic vegetation on the north slope
                     of the Djebel Bou Rhezal, Biskra. _Haloxylon
                     scoparium_ is a prominent species.

               67. North base of Ed Delouatt hills, west of Biskra,
                     showing the low facing dunes. To the right is a
                     glimpse of an oued which pierces the hills in
                     the middle distance.

  Pl. 30, fig. 68. Flood-plain of the Oued Hamman es Salahin,
                     Biskra. Halophytes of various species occupy
                     the foreground.

               69. Habitat of _Phelyæa violacea_ shown in figure 70.

  Pl. 31, fig. 70. Young shoots of _Phelypæa violacea_, at north
                     base of Ed Delouatt hills, Biskra.

               71. Habit of young specimens of _Phelypæa_. Except
                     for about 15 cm., plants were buried by sand.
                     Biskra.

  Pl. 31, fig. 72. _Asphodelus_ sp. at north base of Ed Delouatt
                     hills, Biskra. Photographed in March.

  Pl. 32, fig. 73. Detail of square No. 1, on low hills north of
                     Biskra.

               74. Vegetation of north slope of the low hills which
                     are north of Biskra. _Ferula vesceritensis_ is
                     the perennial appearing in the figure.

  Pl. 33, fig. 75. Detail of square No. 2, near area shown in fig.
                     74. _Ferula_ and _Haloxylon_ are leading
                     species.

               76. Root-habit of _Haloxylon scoparium_, from a wash
                     near Biskra.

               77. Young shoots and mature roots of _Ferula
                     vesceritensis_. North of Biskra.

  Pl. 34, fig. 78. Root-habit of _Fagonia_ growing on edge of a
                     wash. Biskra.

               79. Large lateral root of _Haloxylon_, with numerous
                     deciduous rootlets, no longer functional.

               80. Shoot-habit, taken from above, of _Fagonia_ from
                     the flood-plain of a small oued near Biskra.

  Pl. 35, fig. 81. Spring annuals, March 17, on north slope of the
                     Djebel Bou Rhezal, Biskra.

               82. Root and shoot habit of _Peganum harmala_,
                     Biskra. The main root is especially well
                     developed, although the species has a
                     generalized root-system.

  Pl. 36, fig. 83. Annuals growing with _Peganum_, near Biskra.

               84. General view of the north face of the Djebel Bou
                     Rhezal, Biskra. Apparently barren, plants are
                     rather numerous in the rock crevices and small
                     washes of the mountains.




              BOTANICAL FEATURES OF THE ALGERIAN SAHARA.

                               * * * * *

                            =INTRODUCTION.=

The present paper is designed to give the results of some field-work
in southern Algeria in the autumn of 1910 and the spring of 1911. The
chief purpose of the tour was to examine the more obvious features
of the physiological conditions prevalent in the region in question
and, in connection with these observations, to make some detailed
studies of the root-habits of the most striking species of the
native flora. The route lay through the Atlas Mountains, over
the High Plateau, and for some distance into the Sahara itself,
returning by a somewhat similar way farther to the east. The country
traversed was extremely varied in topography and in plant life, and
probably the most southern regions seen may be taken as typical of
much of the western Sahara. The distance covered in the more arid
portions of the colony was about 1,000 miles.

An English-speaking tourist, or any tourist for that matter,
is something of a curiosity in southern Algeria. We were said to
be the second party of “Englishmen” who had passed over the
Ghardaia-Ouargla country in twenty-five years.

Leaving Algiers in October, diligence was taken to Ghardaia, the
most important town in southern Algeria and the end of the diligence
service. Beyond Ghardaia travel is by camel only. The diligence
journey, if made without stop, requires six days, running night and
day, except the first day’s run, which is by day only. Pauses in
the journey to Ghardaia were made at the leading towns or stage
stations (bordj), thus affording opportunity to observe several
localities in the High Plateau as well as the portion of the desert
just south of the Saharan Atlas. Stop-overs were made thus at Medea,
Boghari, Djelfa, Laghouat, and Tilrempt. A fortnight was passed
at Ghardaia, where the environs of the city were quite thoroughly
explored. As there are no roads between Ghardaia and Ouargla,
or between Ouargla and Touggourt, it was necessary to organize a
small camel-train, engage a cameleer (sokhrar) and servants, and
make special arrangements for the trip. Nine days were required for
the portion of the journey to Touggourt via Ouargla. At Touggourt
diligence was again taken for Biskra, two days’ journey. The
return journey to Algiers from Biskra was broken at Batna for the
purpose of visiting the fine forest of cedar (_Cedrus atlantica_)
in the mountains not far from the town. When Biskra was revisited,
in the spring of 1911, the northern portion of Algeria was crossed
on the way from Tunis, this affording an opportunity to observe
something of the picturesqueness of the mountainous regions as well
as the spring flora.

I wish to take this early opportunity to acknowledge my appreciation
of assistance received from different persons during the course
of the Algerian study, or as a means of preparing for it. I
am especially indebted to Prof. H. J. Hall, of Leland Stanford
University, who was my companion in Algeria and whose knowledge of
the American deserts made his advice doubly valuable. Dr. Keltie,
secretary of the Royal Geographical Society, provided letters
and information of much use. Dr. Trabut, government botanist
of Algeria, whose acquaintance with the country is extensive,
very kindly identified all plants sent him and gave valuable aid
in other ways. The map (plate A) was prepared expressly for this
study by Mr. Godfrey Sykes.

In a country where travel away from the beaten path is not without
discomfort, it is important that the way be made as smooth as
possible, not alone for comfort but for personal safety as well,
and it is therefore a pleasure to acknowledge the many kindnesses
shown by the civil and military authorities of the French colony,
as also by the American consul and vice-consul at Algiers.




              =GEOGRAPHICAL CHARACTERISTICS OF ALGERIA.=


The French colony of Algeria is of large extent and possesses a
highly varied topography and great range in climate. With an all-land
connection with the continent of Europe in earlier geologic times,
the flora and fauna of this portion of northern Africa are closely
allied to the fauna and flora of southern Spain, France, and Italy.

Algeria lies to the south of the Mediterranean Sea, between Tunis
to the east and Morocco to the west. The northern portion extends
somewhat beyond latitude 37°, or that of southern Spain, southern
Greece, and the southern part of Asia Minor. The limit of the colony
on the south is indefinite, reaching to about latitude 21°. Thus
east and west the extent is about 650 miles, and north and south
1,200 miles.

The most important topographical features of the northern part of
Algeria are the several mountain masses which together constitute the
Atlas range. Lowest in Tunis, where the Atlas Mountains do not exceed
5,000 feet altitude, they attain their greatest height in Morocco,
over 13,000 feet. In Algeria an altitude of about 7,500 feet is
reached in the Aurés, Dj. Chelia, and in the Great Kabylies. In
the eastern portion of Algeria the mountains extend to the coast,
but farther west a narrow strip of lowland separates them from the
sea. In the east (in the department of Constantine) they constitute
a single general uplift, although made up of several groups, but
as one proceeds westward the mountains separate into two ranges,
which at the Morocco border are about 125 miles apart. The two
ranges have been called by various names, among which are the
Great Atlas for the southern range and the Little or Maritime Atlas
for the northern one. But it seems best to use the names Atlas of
the Sahara or Saharan Atlas and Atlas of the Tell or the Tellian
Atlas, names which are self-explanatory. The Tellian Atlas and all
territory between this range and the Mediterranean Sea is known as
the Tell, or the land of hills. This is the most important part of
Algeria from an agricultural standpoint, and furnishes grain and
other products. Here also are the most important forests, oak,
pine, and cedar. Between the two ranges of the Atlas lies the
region of the steppes or plateaus, in this study referred to as
the High Plateau, inasmuch as the average altitude is over 3,000
feet. The steppe region is highest in the west and is wedge-shaped,
in conformity to the bounding mountains. In the eastern portion it
breaks up into mountain valleys. The topography of the High Plateau
is monotonous. The surface is gently rolling, and here and there
are undrained depressions, or _chotts_, where salts accumulate. In
rainy seasons the chotts contain water, but in the arid summers they
are dry. The region of the steppes is of no agricultural value,
although, as will appear below, the harvesting of the alfa grass,
which occupies vast areas, is of considerable importance.

South of the Saharan Atlas lies the desert, comprising about
2,000,000 square miles, the topography of which is extremely
varied. For present purposes it is sufficient to say that in the
extreme southern portion of the Algerian Sahara, and crossed by the
Tropic, there is an extensive highland, the plateau of Idghagh,
where an elevation exceeding 5,000 feet is reached. All of the
Sahara to the west of this plateau, or to the west of a line drawn
north from it, appears to be above sea-level (much of it having an
altitude of 1,000 feet), and of greater geological age than that
portion of the desert lying to the east. North from the plateau of
Idghagh the country gradually descends to the depression of which
the great Chott Melghir is a part, a region below the level of
the sea. Here extends also one of the longest oueds of the Sahara,
the Oued Igharghar, which takes its rise in the plateau of Idghagh
and empties in the Chott Melghir, an entire length exceeding 700
miles. In the western part of the Sahara the surface descends to the
Atlantic, but in the eastern part it falls away to the Mediterranean.

Turning now to consider the part of southern Algeria with which
this study especially deals—lying between Laghouat and Ghardaia,
between Ghardaia and Ouargla, and between the latter place and
Biskra, all to the south of the Saharan Atlas—we find topographical
details which are probably representative of much of the rest of
the Great Desert. Laghouat has an elevation of 2,400 feet. It lies
on the northern edge of the region of the dayas. This region is
characterized by the poor development of its drainage and has a
slightly undulating surface with frequent depressions, each the
center of an area of rather small extent, from which it receives
flood-waters. The dayas differ from the other undrained areas,
the chotts, in that they do not contain an excess of salt, owing
probably to efficient subdrainage. In the region of the dayas the
surface falls away to the south or the southeast until the region
of the Chebka is reached, which extends to the territory of the
Beni M’Zab. In the Chebka low and flat-topped mountains are so
irregularly disposed as to give rise to the name, meaning a net;
they are separated by valleys, narrow toward the northern portion
of the region, but expanding into small plains as one proceeds
towards Ghardaia. Ghardaia, the country of the Beni M’Zab,
marks the southern limit of the Chebka. At Ghardaia the altitude
is 1,600 feet. Between Ghardaia and Ouargla are undulating stony
plains, the Gantara (_hamada_), large salt spots, the chotts, and
a prominent range of sand mountains, _areg_ desert, possibly 1,000
feet high. At the eastern edge of the Gantara the general level
of the country drops suddenly about 200 feet to the Ouargla plain
(_reg_ desert), with an altitude less than 500 feet; this is an
eroded flood-plain of the Oued Igharghar or its tributaries.

There are no navigable rivers in Algeria. The most important river
is the Chelif, which takes its origin in the Saharan Atlas, crosses
the High Plateau, breaks through the Atlas of the Tell, and, turning
westward, traverses obliquely the Tell for a distance of about 108
miles before discharging into the sea. The Chelif is the only stream
which rises in the Saharan Atlas and pierces the northern range. To
the south of the Saharan Atlas are several important oueds. One,
the Oued Djedi, rises near Aflou and goes easterly, past Laghouat,
until it reaches the Chott Melrir, south of Biskra; two others, the
M’Zab and the Nessa, drain the region of the Chebka, and taking
an easterly or a southeasterly direction reach the Oued Rirh or its
upper extension. The Oued Rirh constitutes the northern portion of
the great Oued Igharghar, or a tributary of this oued, and extends
about 60 miles north from Touggourt to the Chott Merouan. The region
of the Oued Rirh is of great economic importance from the production
of dates, made possible through the development of artesian wells
by the French government. The oueds as a whole are very like the
arroyos of the southwestern portion of the United States, in that
they carry water for a small portion of the year only, when the
torrential rains fill them with a muddy, rushing flood.

In the plains adjacent to the oueds, at Ghardaia especially, the
natives usually dig their wells, from which water for domestic as
well as for irrigating purposes is obtained. The flood-water of
the oueds is also diverted into ditches, or impounded for later
use, although the latter has not met with uniform success. The
depths at which water has been found vary greatly. At Ouargla the
water lies within 3 feet of the surface, although there are also
very deep artesian wells, and at Ghardaia it varies from 10 to
50 feet or more. At the daya of Tilrempt the water in the deepest
wells stands as deep as 300 feet; it is drawn in a very primitive
fashion for purposes of watering flocks and for the bordj. The
heaviest vegetation is to be found along the oueds and the nearby
flood-plains. Here the water relations are the most favorable and
the oueds constitute highways along which plants venture into the
desert from the more humid regions.

Very little study appears to have been given the soils of Algeria. It
has been stated that there are vast areas of light, sandy soils, and
also extensive tracts of marls, clays, and alluvial soils.[1] Gypsum
is an important element in the soils, both those of the oases and
probably of the open desert also; it occurs in great quantity in
the large chotts of the desert, along with common salt and other
salts. In the soils of the oases it acts as a cementing material,
“uniting the finer soil-grains into aggregates which give the
soil a much more sandy appearance than would be suspected from the
results of mechanical analysis.” At Laghouat and at Ghardaia a
light-colored, hard substance, closely resembling the “caliche”
of the southwestern United States, was seen incrusting stones,
filling cracks in rocks and crevices between rocks, and in places
forming a stratum, horizontally placed (15 cm. more or less in
thickness) underneath the superficial soils. This is extremely
hard and can be broken or cut with difficulty. In the valley of the
M’Zab, where it constitutes a heavy substratum, it appears to be
impervious to water. This hardpan is used as threshing floors by
removing the superficial soils.

An unexpectedly small amount of sand was observed over the route
traversed. Near the southern edge of the High Plateau a sand belt
was encountered and a long stretch of low dunes was seen leading
to the eastward, which were said to reach nearly to Bou Saada; and
again at Laghouat there are dunes to the east of town as well as
to the west. Low dunes were seen in the valley of the Oued M’Zab,
and sand mountains, possibly 250 meters high, were passed on the way
from Ghardaia to Ouargla. Between Ouargla and Touggourt, also, sand
was encountered and the way lay across about 12 miles of low dunes;
to the north of Touggourt dunes are also to be seen; and finally,
some sand is to be found in the neighborhood of Biskra. Although,
thus, relatively little sand was met, much of the entire portion
of southern Algeria is covered by sand. Large areas of sand-covered
country lie to the east of the Oued Rirh, and especially southeast
of Touggourt, and also to the west of Ghardaia there is said to
be a large dune-covered territory. For the most part, however,
the surface of the plains crossed is covered with large or small
stones, mingled with which, or beneath which, there is a rather fine
clay-like soil. This constitutes the hamada, or stony desert, of
which the largest portion of the surface of the Sahara is probably
composed. Where stones are largely absent and the soil is fine,
usually of fluvian origin, the formation is known as “reg.” Reg
desert was encountered at and north of Ouargla, in the drainage of
the Igharghar or its tributaries, and south of Biskra. The latter
may not, strictly speaking, be reg, but a wide-stretching bajada,
and the soil is probably only in small part deposited by rivers.




                         =CLIMATE OF ALGERIA.=


The climate of Algeria is mild and temperate. This is due to several
factors, among which are its situation relative to the Mediterranean
on the north and to the Atlantic on the west, as well as to the great
desert which constitutes its southern portion, the great variation
in topography, and the fairly low latitude. Taking the colony as
a whole, there is a great range in temperature, precipitation,
relative humidity, and evaporation, to mention only such climatic
features as have been reduced and are recorded; and the range in the
intensity and in the quality of the light must also be great. The
climate of the northern portion of Algeria is coastal, while that
of the southern portion is continental.

The distribution in time and in space and the amount of precipitation
are of the greatest importance as climatic features of Algeria. The
rainfall is heaviest on the littoral, and especially heavy in the
eastern portion of the littoral. An average of 1,000 mm. is reported
from the immediate vicinity of the sea,[2] and as one goes southward
the amount of precipitation rapidly becomes less. In the Tell the
average rainfall is 570 mm., while on the High Plateau it is 310
mm. On the desert the rainfall is uncertain both in amount and
in time, except that when rains occur the time coincides with the
rainy season of northern Algeria. At Biskra the annual precipitation
is 199 mm., at Laghouat it is 198 mm., at Ghardaia it is 114 mm.,
and at El Golea it is 47 mm. In many places in the western Sahara,
five years or more go by without fall of rain.

The differences in the geographical distribution of precipitation
vary from year to year, as may be illustrated by referring to that
for the year 1908, which may be compared with the normal usual
distribution as given above. In the northern portion of the country
more rain than usual was reported; for example, there was over 1,000
mm. on the littoral east of Algiers, and over 500 mm. on the High
Plateau, but on the desert the amount was somewhat less. At Laghouat
it was 161 mm., at Ghardaia it was 89.2 mm., and at Ouargla it was
28 mm.

Besides the differences in amount of yearly rainfall, well-marked
seasonal amounts of precipitation are also to be noted. In the
northern portion of the colony rains are likely to occur in winter
and spring, the summer and early autumn being dry; and as one goes
south of the Saharan Atlas nearly the same conditions obtain; that
is, the rains usually fall during the rainy season of the coast. The
seasonal distribution of rain for the Tell, including the stations
of the littoral, the High Plateau, the Saharan Atlas, and the desert,
for a series of years including 1908, is given in table 1.


               TABLE 1.—_Seasonal distribution of rain._

  +-------+-----+------------+-------+------+-----------------------+
  |       |Tell |   High     |Saharan|Desert|                       |
  |Season.|(10).|Plateau (8).| Atlas |  (6).|      Remarks.         |
  |       |     |            | (10). |      |                       |
  +-------+-----+------------+-------+------+-----------------------+
  |       |_mm._|    _mm._   | _mm._ | _mm._|                       |
  |       |     |            |       |      |                       |
  |Winter |469.9|119.3 (50.8)|  113.5|  37.4|  The Tell is          |
  |       |     |            |       |      | represented by Fort   |
  |Spring |300.0| 80.5 (95.0)|   88.2|  39.0| National; the High    |
  |       |     |            |       |      | Plateau by Geryville  |
  |Summer | 38.1| 66.5 (35.8)|   50.4|   3.7| (with Ain Sefra,      |
  |       |     |            |       |      | for 6 years, in       |
  |Autumn |287.2|139.5 (58.5)|  123.1|  19.6| parentheses); the     |
  |       |     |            |       |      | Saharan Atlas by      |
  |       |     |            |       |      | Djelfa; the desert by |
  |       |     |            |       |      | Ouargla. The number   |
  |       |     |            |       |      | of yearly records on  |
  |       |     |            |       |      | which averages are    |
  |       |     |            |       |      | computed are given    |
  |       |     |            |       |      | at the head of each   |
  |       |     |            |       |      | column in parentheses.|
  +-------+-----+------------+-------+------+-----------------------+

The seasonal percentages of precipitation give a more graphic
conception of the rainfall conditions for the four physiographic
provinces. In the Tell this percentage in winter is 42, in spring 27,
in summer 4, and in autumn 27. On the High Plateau the percentages
are 30, 20, 16, and 34 for the four seasons respectively. In the
Saharan Atlas 30 per cent of the rain occurs in winter, 24 in spring,
13 in summer, and 33 in autumn. In the desert the percentages of
rainfall are 37, 39, 4, and 20[3] for the four seasons.

It is of interest also to note the number of days on which the rain
fell on an average each year over a period running from 7 to 12
years. Thus at two typical stations on the Tell rain was reported on
102 and 118 days; at two stations on the High Plateau it rained 65
and 83.8 days; at a station in the Saharan Atlas rain was reported
on 70 days; at desert stations, at Ouargla rain fell on an average
14.2, and at Laghouat 49 days each year. As a comparison, it may be
mentioned that for ten years at Wady Halfi, Egyptian Sudan, there
were only 22 days on which rain-drops were seen to fall. (Engler,
_loc. cit._)

The amount of precipitation varies greatly for the different desert
stations, usually becoming less as one goes south from the High
Plateau. As has already been mentioned, the average rainfall at
Laghouat, which lies at the southern base of the Saharan Atlas, is
198 mm., the average at Ghardaia is 114 mm., while that at El Golea
is 47 mm. The latter station is about 225 miles south of Laghouat, in
the midst of the Sahara. The amount of rainfall, however, is greatly
influenced by altitude, although lack of adequate precipitation data
for the desert makes impossible a detailed presentation of this phase
of the subject. As the amount of the yearly precipitation is less in
the extreme southern part of Algeria than it is nearer the Saharan
Atlas, where the altitude also is greater, it might be expected that
the number of rainy days would vary in a like manner. Such records as
are at hand, however, do not show this to be the case. For instance,
at Ouargla rain falls on an average 14.2 days, average of 7 years,
while the rainfall is 90.2 mm.; yet at El Golea, with a rainfall
of 47 mm., there are 23.4 rainy days each year.[4]

On the desert the rains are often of a torrential nature, as facts
presented above would indicate, and sometimes as much rain falls
within a few hours, or even a few minutes, as usually occurs in
an entire year. How much of the annual precipitation is of this
character and how much is of the non-torrential kind the usual
summaries leave entirely out of the account. It is well known that
the former type of storm is more destructive and less useful to
plants than the latter type. To illustrate the irregularity of
the rainfall in the northern Sahara the monthly precipitation at
Ouargla for several years is presented in table 2.


            TABLE 2.—_Rainfall at Ouargla, in millimeters._

  +----+-----+-------+----+----+----+-----+----+
  |    |     |No. of |    |    |    |     |    |
  |Year| Amt.| days  |Jan.|Feb.|Mar.| Apr.|May |
  |    |     | rain  |    |    |    |     |    |
  +----+-----+-------+----+----+----+-----+----+
  |1900|     |     1 |    |    |    |     |    |
  |    |     |       |    |    |    |     |    |
  |1901|     |    18 |    |    |    |     |    |
  |    |     |       |    |    |    |     |    |
  |1902| 70.5|     8 |    |    |11.3|     |    |
  |    |     |       |    |    |    |     |    |
  |1903|135.6|    14 |55.0|    |    |     |    |
  |    |     |       |    |    |    |     |    |
  |1904| 35.0|    26 |20.0|    |    |     |    |
  |    |     |       |    |    |    |     |    |
  |1905|     |       |    |    |    |     |    |
  |    |     |       |    |    |    |     |    |
  |1906|     |       |15.0|    | 2.0|  4.0|    |
  |    |     |       |    |    |    |     |    |
  |1907|210.0|    18 |    |68.2|  67|325.0|16.2|
  +----+-----+-------+----+----+----+-----+----+
  [**Continued]

  +----+----+----+----+-----+----+----+----+
  |    |    |    |    |     |    |    |    |
  |Year|June|July|Aug.|Sept.|Oct.|Nov.|Dec.|
  |    |    |    |    |     |    |    |    |
  +----+----+----+----+-----+----+----+----+
  |1900|    |    |    |     |    |    |    |
  |    |    |    |    |     |    |    |    |
  |1901|    |    |    |     |    |    |    |
  |    |    |    |    |     |    |    |    |
  |1902|    |    | 1.8|     |46.4|11.0|    |
  |    |    |    |    |     |    |    |    |
  |1903|    |    |    |     |    |30.6|50.0|
  |    |    |    |    |     |    |    |    |
  |1904|    |    |    |     | 9.0| 4.0| 2.0|
  |    |    |    |    |     |    |    |    |
  |1905|    |    |    |     |    |    |    |
  |    |    |    |    |     |    |    |    |
  |1906|    | 4.0|80.0| 41.0|80.0|    |    |
  |    |    |    |    |     |    |    |    |
  |1907|20.0|    |    |  5.0| 3.3| 3.8| 1.3|
  +----+----+----+----+-----+----+----+----+

The seasonal distribution of rainfall at four desert stations
shows also marked irregularities. The distribution (in percentages)
is given in table 3.


      TABLE 3.—_Seasonal distribution, in percentages, of rain._

  +----------+---------+---------+---------+---------+
  | Station. | Winter. | Spring. | Summer. | Autumn. |
  +----------+---------+---------+---------+---------+
  |Ouargla   |    37   |    39   |     4   |    20   |
  |Laghouat  |    11   |    27   |    31   |    31   |
  |Ghardaia  |    15   |    23   |    13   |    49   |
  |Touggourt |    13   |    30   |     3   |    53   |
  +----------+---------+---------+---------+---------+

The relative humidity at the desert stations is often very low,
sometimes running in midsummer between 7 and 9 per cent for 6 days,
and occasionally being too low to measure with the instruments
employed. The mean relative humidity (table 4) shows the general
very dry condition of the air of the desert as contrasted with a
station in the Tell, and also indicates something of the monthly
variations in this factor experienced among the desert stations
themselves. The averages given are from 4 to 8 years, except the
mean annual for In Salah, which is for 2 years only.


          TABLE 4.—_Mean relative humidity, in percentages._

  +--------------+------+------+------+------+------+------+------+
  | Station.     | Jan. | Feb. | Mar. | Apr. | May  | June | July |
  +--------------+------+------+------+------+------+------+------+
  | Ouargla      | 61.0 | 58.6 | 60.3 | 51.2 | 52.0 | 49.0 | 44.1 |
  | El Golea     | 60.8 | 56.8 | 41.3 | 41.3 | 41.0 | 31.3 | 28.5 |
  | In Salah     | 51.5 | 48.0 | 45.5 | 40.4 | 34.6 | 32.0 | 23.4 |
  | Ft. National | 87.4 | 80.3 | 87.6 | 86.6 | 96.6 | 80.4 | 75.8 |
  +--------------+------+------+------+------+------+------+------+
  [**Continued]

  +--------------+------+-------+------+------+------+-------+
  | Station.     | Aug. | Sept. | Oct. | Nov. | Dec. | Year. |
  +--------------+------+-------+------+------+------+-------+
  | Ouargla      | 44.7 | 50.7  | 56.8 | 59.1 | 52.7 | 54.6  |
  | El Golea     | 31.8 | 34.8  | 37.1 | 58.1 | 65.1 | 45.5  |
  | In Salah     | 24.6 | 17.2  | 38.0 | 52.2 | 58.7 | 42.6  |
  | Ft. National | 62.8 | 80.2  | 87.7 | 92.8 | 88.4 | 85.1  |
  +--------------+------+-------+------+------+------+-------+

A consideration of the evaporation statistics of Algeria for the year
1908 shows some interesting relations. It has already been observed
that the rainfall along the coast is less in the west than in the
east, and it will appear below that as a rule the temperature of
the western portion is lower than that of the corresponding regions
lying to the east. In both rainfall and temperature, however, the
greatest variation is to be found as one goes inland, when decreasing
rainfall and higher temperatures are encountered. A similar relation
obtains in evaporation, which becomes continuously greater as the
distance from the coast increases; that is, the average evaporation
for stations on the High Plateau is greater than for stations in
the Tell, and the evaporation at desert stations is greater than
the evaporation on the High Plateau. The total average evaporation
for the year, in millimeters, for 5 stations on the littoral,
was 1,365.3; for 7 stations in the Tell it was 1,378.6; for 4
stations on the High Plateau 2,352.2, and for 3 stations in the
desert, 3,977.5. The least evaporation reported was at Bouzarea,
which was 989.9 mm., and the greatest was at Ghardaia, 5,309.7 mm.[5]

Table 5 gives in detail the monthly as well as the total evaporation
for the year at three desert stations and at Algiers for 1908.


             TABLE 5.—_Evaporation in millimeters_, 1908.

  +---------+-----+-----+-----+-----+-----+-----+-----+
  |Station. |Jan. |Feb. |Mar. |Apr. |May  |June |July |
  +---------+-----+-----+-----+-----+-----+-----+-----+
  |Laghouat |88.9 |102.3|143.4|203.1|289.2|373.9|421.2|
  |Ghardaia |172.4|233.4|340.7|528.7|611.7|699.1|749.2|
  |Touggourt|     |163.9|240.5|274.3|385.2|459.4|565.8|
  |Algiers  |84.2 |96.8 |90.5 |118.3|151.8|158.6|175.0|
  +---------+-----+-----+-----+-----+-----+-----+-----+
  [**Continued]

  +---------+-----+-----+-----+-----+-----+-----+
  |Station. |Aug. |Sept.|Oct. |Nov. |Dec. |Year.|
  +---------+-----+-----+-----+-----+-----+-----+
  |Laghouat |379.6|264.8|173.9|153.8|159.2|2753 |
  |Ghardaia |693.0|468.8|329.4|257.7|225.5|5309 |
  |Touggourt|487.3|329.4|222.6|166.1|142.3|     |
  |Algiers  |205.0|165.5|129.0|136.1|143.2|1654 |
  +---------+-----+-----+-----+-----+-----+-----+

For a better personal appreciation of the rate of evaporation, as
well as for the purpose of comparison, I arranged an apparatus to
tell the water-loss from a free water-surface. It was also desirable
to determine the relative evaporation of the day and night. As
employed, the apparatus consisted of a flat tin dish, with parallel
sides, 10 cm. in diameter. To the side was attached, by means of a
rubber stopper, a bent glass tube of small diameter. The water-loss
was read on this tube. Observations were made at Laghouat and at
Ghardaia. Following is a summary of the results obtained at Ghardaia:
For a period of 7 days, after November 10, the daily water-loss
between 7 a.m. and 7 p.m. was as follows: 5, 8, 8.5, 10.5, 13, 9,
and 7.5 mm. The evaporation between 7 p.m. and 7 a.m. was so slight,
1 mm. more or less, that it could not be well determined by the
apparatus used.

A good idea of the intensely arid character of the Algerian
climate, taken as a whole, is to be obtained by a study of the
relation between the total rainfall and the total evaporation,
based on the official climatic reports. In tables 6 and 7 the
report for the year 1908 is used. The figures represent the ratio
(_e_/_r_) between evaporation and rainfall, in which the amount of
evaporation is used as the numerator and the amount of precipitation
as the denominator. In the monthly evaporation-rainfall table
(table 6), in all cases where no rainfall was reported for the
month it was called 1 mm. In table 7, however, which gives the
seasonal evaporation-rainfall ratio, the actual figures of the
government report were in all cases employed, since there was no
season during the year 1908, even in southern Algeria, when no
precipitation occurred.[6]


         TABLE 6.—_Evaporation-rainfall ratio, monthly_, 1908.

  +----------------+------+-------+-------+-------+-------+-------+
  |    Station.    | Jan. | Feb.  | Mar.  | Apr.  | May.  | June. |
  +----------------+------+-------+-------+-------+-------+-------+
  | Nemours        |  1.07|   1.3 |  1.39 |  1.9  | 131   |   5.4 |
  | Cape Falcon    |  1.5 |   1.8 |  1.46 | 16.0  |   8.8 |  10.3 |
  | Oran           |  1.3 |   1.9 |  1.6  |  1.3  |  17.4 |   8.5 |
  | Algiers        |  0.63|   1.3 |  0.5  |  1.1  |  22.3 |  11.7 |
  | Bouzarea       |  0.57|   0.67|  0.22 |  0.56 |   8.3 |   4.8 |
  | Maison-Carée   |  0.72|   0.64|  0.55 |  1.1  |  13.5 |  11.8 |
  | Ft. National   |  0.72|   0.29|  0.34 |  0.58 |   3.7 | 192   |
  | Sidi-bel-Abbès |  0.63|   0.76|  0.6  |  1.1  |  18.8 |  18.5 |
  | Saida          |  0.67|   0.64|  0.36 |  0.83 |   2.2 |   6.9 |
  | Batna          |  1.6 |   2.3 |  1.5  |  2.5  |  46.5 |  33.4 |
  | Tebessa        |  4.6 |   6.05| 21.2  |  3.9  | 252.0 |   2.8 |
  | Bou Saada      |  4.05|   5.4 | 12.1  |  2.9  |   6.1 | 165   |
  | Barika         |  2.4 |   8.7 |  9.9  |  5.0  |   5.03| 270   |
  | Ain Sefra      |  9.2 |  21.5 |  1.5  | 23.8  |  12.4 |  63.1 |
  | Geryville      |  6.3 |   1.9 |  2.1  | 23.9  |   2.5 |  16.7 |
  | Laghouat       |  3.09|   4.7 |  9.8  |203.0  |   7.0 | 373   |
  | Ghardaia       |  8.9 | 233.0 | 81.4  |529.0  |  38.9 | 699   |
  | El Oued        | 53.9 |  27.7 | 67.0  |629.0  | 369.0 | 465   |
  +----------------+------+-------+-------+-------+-------+-------+
  [**Continued]

  +----------------+-------+-------+-------+------+-------+-------+
  | Station.       | July. | Aug.  | Sept. | Oct. | Nov.  | Dec.  |
  +----------------+-------+-------+-------+------+-------+-------+
  | Nemours        |  23.6 |  43.3 |   8.6 |  3.1 |  10.6 |   5.7 |
  | Cape Falcon    |  29.7 | 136.0 |  38.1 |  1.9 |   8.9 |   2.2 |
  | Oran           |  59.0 | 146.0 |  18.0 |  2.45|   3.4 |   4.4 |
  | Algiers        | 219.0 |  28.8 |   5.07|  1.06|   1.2 |   1.1 |
  | Bouzarea       |  53.7 |  27.1 |   3.4 |  0.2 |   0.76|   0.5 |
  | Maison-Carée   | 149.0 |  12.0 |   5.2 |  0.9 |   0.35|   0.51|
  | Ft. National   |  42.5 |  15.5 |   2.7 |  1.6 |   0.71|   0.16|
  | Sidi-bel-Abbès |  77.8 |  37.8 |  14.8 |  1.4 |   1.9 |   0.8 |
  | Saida          |  86.7 |  12.1 |   4.7 |  5.8 |   1.2 |   0.78|
  | Batna          |  24.4 |   7.6 |   3.5 |  4.9 |   2.6 |   1.6 |
  | Tebessa        |   9.8 |  14.8 |   1.1 |  3.9 |   3.5 |   3.7 |
  | Bou Saada      |  43.5 |  19.5 |   8.3 | 13.9 |   5.6 |   8.3 |
  | Barika         |   6.1 |   4.1 |  11.4 |  3.7 |  90.6 |   1.3 |
  | Ain Sefra      |  16.4 | 124.0 | 328.0 | 14.7 |  13.2 |   1.2 |
  | Geryville      |  22.5 |  23.4 |   3.4 |  2.1 |   4.1 |   3.9 |
  | Laghouat       | 421.0 |  21.0 |  25.9 | 14.2 | 154.0 | 104.0 |
  | Ghardaia       | 166.0 |  25.6 |  66.9 | 23.2 |  49.7 | 225.0 |
  | El Oued        | 509.0 | 482.0 |  68.6 | 46.1 | 215.0 | 123.0 |
  +----------------+-------+-------+-------+------+-------+-------+


        TABLE 7.—_Evaporation-rainfall ratio, seasonal_, 1908.

  +-----------------+---------+---------+---------+---------+--------+
  |    Station.     | Winter. | Spring. | Summer. | Autumn. | Annual.|
  +-----------------+---------+---------+---------+---------+--------+
  |Littoral:        |         |         |         |         |        |
  |   Nemours       |   2.69  |  44.09  |   24.1  |    7.4  |   3.0  |
  |   Cape Falcon   |   1.83  |   8.75  |   58.6  |   16.3  |   3.7  |
  |   Oran          |   2.54  |   3.7   |   71.0  |    7.95 |   4.2  |
  |   Algiers       |   1.0   |   7.96  |   86.4  |    2.44 |   1.8  |
  |   Bouzarea      |   0.58  |   3.0   |   28.5  |    1.45 |   0.93 |
  |   Maison-Carée  |   0.62  |   5.0   |  117.4  |    1.48 |   1.5  |
  |Tell:            |         |         |         |         |        |
  |   Ft. National  |   0.39  |   1.4   |   83.2  |    1.67 |   1.1  |
  |   Sidi-bel-Abbès|   0.73  |   8.2   |   38.3  |    6.0  |   2.2  |
  |   Saida         |   0.69  |   1.0   |    3.5  |    3.9  |   1.9  |
  |   Batna         |   1.8   |   2.1   |    0.35 |    3.6  |   4.4  |
  |   Tebessa       |   4.7   |   4.05  |   88.2  |    2.8  |   6.0  |
  |High Plateau:    |         |         |         |         |        |
  |   Bou Saada     |   5.9   |   7.0   |   76.0  |    9.2  |  11.0  |
  |   Barika        |   4.1   |   6.6   |   93.5  |   35.2  |  12.2  |
  |   Ain Sefra     |   1.4   |  12.5   |   67.9  |   18.5  |  11.1  |
  |   Geryville     |   7.0   |   9.8   |   20.8  |    3.2  |   3.5  |
  |Desert:          |         |         |         |         |        |
  |   Laghouat      |   6.0   |  73.2   |  271.6  |   64.6  |  17.0  |
  |   Ghardaia      | 154.9   | 416.3   |  293.7  |  195.9  |  59.7  |
  |   El Oued       |  68.3   | 354.0   |  485.2  |  109.5  |  63.0  |
  +-----------------+---------+---------+---------+---------+--------+

When we reduce the evaporation-rainfall ratios of the different
physiographic provinces to simple expressions we gain a comprehensive
view of this important climatic factor for the colony as a
whole. Thus the annual evaporation-rainfall ratios are as follows:
the littoral, 2.5; the Tell, 3.5; the High Plateau, 9.4; the desert,
46.5. The relation of these expressions may also be given thus:
1:1.4:3.7:18.6, for the several regions above given, by which we
see how rapidly the aridity of the country increases as the desert
is entered. The present custom of considering the southern base
of the Sahara Atlas as the northern edge of the desert, in place
of including the High Plateau, as was done earlier, is thus well
grounded.

The temperatures of the air vary greatly for the different regions,
and usually the variation is to be directly related to the positions
of the stations as regards the coast and the altitude. Along the
coast, however, the temperatures vary even if the stations are
at approximately the same elevation. The mean annual temperature
at Oran is 16.9° C. and at La Calle 17.7° C., while a study
of the January temperatures shows that a similar relation holds
good for the entire south coast of the Mediterranean, between
Oran and Alexandria. Algiers furnishes one exception to this
statement, in that the mean annual temperature is 18.1° C. On
the High Plateau the mean temperature falls to 12.7° C. (Batna)
and 13.5° (Setif). No records appear to have been made for the
high mountains of eastern Algeria, although the fact that snow may
remain in sheltered places as late as the latter part of July[7]
would indicate that the mean temperature at 2,000 meters elevation
is much lower than any above given. On the Algerian Sahara the mean
annual temperature is usually higher than at any point nearer the
coast, but even here there is considerable variation, depending,
among other factors, on the altitude of the stations and their
relation to the Atlas. The mean temperature is given by Engler as
20.5° C. for Biskra, 21.0° C. for Ghardaia, and 22.2° C. for
El Golea. As increased temperature means increased evaporation,
we have in this one factor a powerful agent making for aridity,
and when this is associated with decreasing rainfall, as one leaves
the coast region, its influence as a determinative factor in the
environment of plants is thus seen to be of great importance.

The annual variations in the temperature of the air are very
considerable throughout the colony and are especially great on the
desert and the High Plateau. At Algiers the variation is 40.7° C.,
at Setif it is 48.2° C., and at Ghardaia it is 47.9° C. An annual
absolute variation of 50° C. is not uncommon on the desert, and
Engler cites a range of 57.0° C. at Ghardaia.

The daily range of the temperature is also considerable for all
stations, but is especially marked in those of the High Plateau
and the desert. The daily range as given by Engler for the former
is 17.4° C., and for the latter 20.0° C., but the range on the
High Plateau may be greater than 17.4° C., especially during the
summer months. For example, at Batna, in 1904, a range of 19.4°
C. was reported in April, 20.2° C. in June, 21.8° C. in July,
and 20.2° C. in December. Except in December, the great ranges in
temperature here cited were on days when the sirocco was blowing
from the desert; hence, the usual daily range in temperature would
be much less. In the Tell, and especially along the coast, the
range in temperature recorded for any day is relatively little,
although at the time of the sirocco, as well as during the season
of drought, the daily range is not inconsiderable. Table 8 gives
the maximum daily range for the months of 1904, for two stations,
along the coast, including the Tell, and two stations on the High
Plateau. The daily range reported for Batna in June, July, and
August, 1902, was somewhat larger than the maxima given in table
8. In 1902 the maximum daily ranges in temperature for the three
months at Batna were 23.9° C., 25.3° C., and 24.4° C.


         TABLE 8.—_Maximum daily range in temperature_, 1904.

  +--------------+-------+-------+-------+-------+-------+-------+
  |   Station.   | Jan.  | Feb.  | Mar.  | Apr.  |  May  | June  |
  +--------------+-------+-------+-------+-------+-------+-------+
  |              | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | Algiers      |  9.0  | 13.0  | 13.0  | 13.6  | 12.7  | 13.0  |
  | Ft. National | 10.4  | 13.8  | 13.0  | 12.4  | 13.0  | 12.8  |
  | Batna        | 11.5  | 15.4  | 16.4  | 19.4  |       | 20.2  |
  | Saida        | 14.0  | 17.0  | 14.0  | 18.0  | 18.0  | 19.0  |
  +--------------+-------+-------+-------+-------+-------+-------+
  [**Continued]

  +--------------+-------+-------+-------+-------+-------+-------+
  |   Station.   | July  | Aug.  | Sept. | Oct.  | Nov.  | Dec.  |
  +--------------+-------+-------+-------+-------+-------+-------+
  |              | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | Algiers      | 15.0  | 15.4  | 11.2  | 14.0  | 10.8  | 10.6  |
  | Ft. National | 12.4  | 14.8  | 16.4  | 11.2  | 11.6  | 11.0  |
  | Batna        | 21.8  | 16.4  | 16.0  | 19.4  |       | 20.2  |
  | Saida        | 19.0  | 18.0  | 18.0  | 14.0  | 13.0  | 13.0  |
  +--------------+-------+-------+-------+-------+-------+-------+

The temperature conditions for the year 1908 will serve very well to
further illustrate this phase of the climate of Algeria. No freezing
temperatures were reported from the coast stations, although at the
northern base of the Atlas of the Tell several degrees of frost were
recorded. At Blida, for example, the thermometer registered -4.0°
C. as the minimum. Away from the littoral at every station whose
records were seen the lowest temperatures were zero centigrade,
or below. The coldest weather was felt at Aflou, at the north base
of the Dj. Amours, of the Saharan Atlas, where -11.0° C. was
reported. The summer was warm along the coast and hot in the
interior. The records of temperature of 23 stations of the Tell
were seen, and of this number, at only 8 was a temperature of 40°
C. reached or exceeded. On the other hand, at 44 out of 56 stations
on the High Plateau and the desert, the temperatures reported are as
great or greater than 40° C. Where temperatures below this figure
were recorded the stations were situated in the mountains.

The western portion of the Sahara is apparently not so hot as the
eastern portion. The mean annual temperature for Biskra is 20.3° C.;
for Ghardaia it is 21° C.; for El Golea it is 22.2° C.; and for
Touggourt it is 23.4° C. The mean temperature at Cairo is 21.3°
C.; for Suez it is 21.5° C.; and for Djedda, on the Red Sea, it
is 27.3° C. The absolute maximum temperatures in southern Algeria
for as many years and for two of the most southern army posts, El
Golea and In Salah, are as follows: El Golea, 47°, 46.5°, 48°,
and 49.2° C.; In Salah, 50.0°, 49.2°, 50.2°, and 48° C.; at
Ouargla the maxima are 50.2°, 51°, 52°, 49°, and 48.4° C. On
account of the fact that in the western Sahara at these stations in
winter there are usually freezing temperatures, the yearly range
of temperature is 50.0° C., or over. The greatest range reported
for the western Sahara is that for Ghardaia as given by Engler,
namely, 57°, from -7° to 50° C. At Timmimoun, 1904, the range was
from -3.0° to 53.1° C., or one of 56.1° C. The extreme absolute
daily range in temperature on the desert appears not to exceed that
of certain stations on the High Plateau, although, as shown below,
the temperature variations on the desert may be much larger than the
records indicate. At Laghouat and at Ghardaia the thermometers which
I exposed showed a daily range of from 10.5° to 12.5° C. only. The
instruments were placed on the outside of buildings, and usually on
the second story. As a contrast to this observed diurnal variation,
an opportunity was given to take temperatures on the open desert at
a time when the days were fairly warm and the nights were rather
cold. The place was between Touggourt and Ouargla. At 3 o’clock
on the afternoon of November 26, 1910, the shade temperature at
the place in question was 23° C. During the night the thermometer
registered -1.7° C. as the minimum, thus showing a drop of 24.7°
C. in something over 12 hours. Table 9 gives the extreme daily
range in temperature for three years observed at the stations named.


            TABLE 9.—_Absolute daily range in temperature._

  +----------+-------+-------+-------+-------+-------+-------+
  | Station. | Jan.  | Feb.  | Mar.  | Apr.  | May.  | June. |
  +----------+-------+-------+-------+-------+-------+-------+
  |          | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | In Salah | 21.6  | 23.0  | 24.4  | 24.6  | 22.0  | 23.2  |
  | Ouargla  | 20.8  | 25.4  | 24.0  | 25.0  | 27.0  | 26.0  |
  +----------+-------+-------+-------+-------+-------+-------+
  [**Continued]

  +----------+-------+-------+-------+-------+-------+-------+
  | Station. | July. | Aug.  | Sept. | Oct.  | Nov.  | Dec.  |
  +----------+-------+-------+-------+-------+-------+-------+
  |          | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | In Salah | 23.4  | 22.0  | 29.0  | 20.0  | 23.4  | 22.6  |
  | Ouargla  | 26.0  | 27.8  | 24.6  | 23.0  | 20.0  | 23.8  |
  +----------+-------+-------+-------+-------+-------+-------+

A further examination of the maximum temperatures shows certain
climatic features of interest and of great importance as factors in
the environment of the desert plants, especially the high average
maxima and the large amount of heat received in the desert, as
indicated by a summation of the maximal temperatures. That the
greatest daily temperatures must be high is indicated by table 10,
which gives the average maxima for three to five years at three
typical desert stations.


               TABLE 10.—_Average maximum temperatures._

  +----------+-------+-------+-------+-------+-------+-------+
  | Station. | Jan.  | Feb.  | Mar.  | Apr.  | May.  | June. |
  +----------+-------+-------+-------+-------+-------+-------+
  |          | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | Ouargla  | 21.9  | 18.3  | 30.9  | 37.8  | 41.1  | 47.1  |
  | El Golea | 23.0  | 25.8  | 35.6  | 27.5  | 40.9  | 44.4  |
  | In Salah | 27.4  | 30.1  | 37.2  | 41.0  | 43.4  | 47.6  |
  +----------+-------+-------+-------+-------+-------+-------+
  [**Continued]

  +----------+-------+-------+-------+-------+-------+-------+
  | Station. | July. | Aug.  | Sept. | Oct.  | Nov.  | Dec.  |
  +----------+-------+-------+-------+-------+-------+-------+
  |          | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
  | Ouargla  | 49.4  | 47.7  | 44.3  | 40.0  | 31.0  | 25.0  |
  | El Golea | 47.5  | 46.0  | 43.8  | 36.7  | 29.9  | 22.4  |
  | In Salah | 44.3  | 47.1  | 45.9  | 41.3  | 37.0  | 27.1  |
  +----------+-------+-------+-------+-------+-------+-------+

The maximum temperatures show that the total amount of heat received
on the desert, as compared to that received in the climatic provinces
nearer the sea, is not only great, but also that the amount is
variable on the desert itself, one station receiving much more
heat at certain times of the year than another station. The total
heat for each station is arrived at by adding up the daily maxima
for the midseasonal months, January, April, July, and October, and
dividing by the number of years whose records were consulted. The
amounts given in table 11 are the averages for three to four years,
for three desert stations and for one station in the Tell.


          TABLE 11.—_Sums of the daily maximum temperatures._

  +--------------+----------+--------+-------+----------+
  |   Station.   | January. | April. | July. | October. |
  +--------------+----------+--------+-------+----------+
  |              |   _°C._  | _°C._  | _°C._ |  _°C._   |
  | Laghouat     |   439    |   476  | 1,135 |   668    |
  | In Salah     |   641    | 1,027  | 1,455 | 1,121    |
  | Ouargla      |   570    |   909  | 1,444 |   923    |
  | Ft. National |   230    |   470  |   697 |   564    |
  +--------------+----------+--------+-------+----------+

The variation in total heat received at the three desert stations
may probably be explained partly at least by their differences in
latitude and in elevation above the sea, as well as the different
relations they hold to the highlands of the northern portion of
the colony. Laghouat lies immediately south of the Saharan Atlas,
latitude 33°48′, and at an elevation of 780 meters. Ouargla is
approximately 200 miles south of the mountains, latitude 31°55′,
and altitude 150 meters. In Salah is in the midst of the western
Sahara, latitude 27°17′, and about 300 meters above the sea. Fort
National lies in the Tell about 30 miles from the sea, and at an
altitude of 916.3 meters. The latitude of Fort National is 36°38′.

There are no published records of the soil temperatures of southern
Algeria. In the following table is given soil-temperature data
taken at Ghardaia by M. Buret. Maximum and minimum standard chemical
thermometers were used. They were placed in a horizontal position
in fixed tubes which were about 30 cm. in length. Precautions were
taken to properly insolate the instruments. The depth was 15 cm.


   TABLE 12.—_Soil and air temperatures, Ghardaia, July_ 2-11, 1911.

  +---------+---------------------+---------------------+
  |         |        Soil.        |         Air.        |
  |  Date.  +----------+----------+----------+----------+
  |         | Maximum. | Minimum. | Maximum. | Minimum. |
  +---------+----------+----------+----------+----------+
  |         |  _°C._   |  _°C._   |  _°C._   |  _°C._   |
  | July  2 |   36.0   |   31.0   |   41.0   |   21.0   |
  | July  3 |   36.0   |   31.0   |   41.0   |   22.0   |
  | July  4 |   36.0   |   31.0   |   41.0   |   23.0   |
  | July  5 |   36.5   |   31.5   |   41.0   |   23.5   |
  | July  6 |   33.7   |   32.0   |   41.0   |   25.0   |
  | July  7 |   36.0   |   32.5   |   40.0   |   26.0   |
  | July  8 |   36.0   |   32.5   |   38.0   |   26.5   |
  | July  9 |   37.0   |          |          |   25.0   |
  | July 10 |   37.0   |   32.5   |   40.0   |   24.0   |
  | July 11 |          |   33.0   |          |   23.0   |
  +---------+----------+----------+----------+----------+

An examination of the meteorological reports for Algeria shows
that the direction of the prevailing winds varies considerably,
although the variation is possibly greater near the coast than in
the interior. The winds undoubtedly play an important rôle in the
environment of the vegetation of the country. At Algiers, in 1907,
winds were reported from the north 288 times, from the east 203
times, from the west 224 times, and from the south 51 times. At
Barika, on the High Plateau, the number of times and the directions
of the winds for the same year are as follows: northeast, 203
times; east 242; south 21, west 89, and southwest 93. At El Golea,
on the desert, the winds were as follows: north 192, northeast 614,
southeast 107. The winds from the north, or northerly winds, are cool
and laden with moisture, but they are most effective in regions near
the coast or in the mountains. Winds from the south are dry winds,
and are probably of great importance in limiting the distribution
of plants through the increased aridity caused by them.

The most important of these desert winds is the sirocco, a wind that
is most likely to blow in spring and summer, although occurring in
autumn also, and to a very limited degree in the winter season. The
sirocco crosses the Mediterranean and is felt in the southern
portions of France, on the Italian Riviera, and in other parts of
southern Europe. It is especially common on the High Plateau; for
example, during five years it was reported on an average of 28.4
days each year. It does not generally last more than three days,
but at Batna, in July, 1902, it was recorded on eight consecutive
days. The sirocco operates to lower the relative humidity and
to raise the temperature. For instance, on the first day of the
eight-day sirocco above alluded to, the average relative humidity
was 16 per cent, while the average on the preceding day was 25.6
per cent. It has already been remarked that on the days of the
greatest temperature variation the sirocco was usually blowing. In
the desert the winds often bring with them much dust and sometimes
last during several days; for example, one is reported at In Salah,
in May, 1904, for six consecutive days. Such winds on the desert may
come from the north, or at least may be northerly, and when storms
of this character are in progress, although possibly destructive of
animal life or at least comfort, the effects as regards decreasing
the relative humidity and raising the air temperature are not so
marked as is the case when the sirocco is blowing.




         =SOME CHARACTERISTICS OF THE VEGETATION OF THE TELL.=


Because of the similarity in the flora of Algeria and southern
Europe, a very good introduction to Algeria is by the way of southern
Spain, France, or Italy. As one approaches southern France, for
instance, he begins to see evidences of increasing aridity. Upon
passing Lyons grassy fields and heavy forests are left behind, the
hills become bare or covered with a chaparral-like growth, and the
practice of irrigation is observed on the plains. The vegetation,
especially of the region between Avignon and Nimes, recalls that
of portions of California, and one sees the mulberry, the olive,
and the pomegranate in abundance, and occasionally the orange. Very
much the same conditions greet one when he arrives in Algiers, except
that along the littoral, at and in the vicinity of Algiers, there is
a wealth of native and especially of introduced plants, which give
little hint of the arid regions close at hand. The hills which make
a part of the beautiful city of Algiers contain fine plantations
of foreign trees, such as eucalyptus, conifers of various species,
acacias, figs, and a variety of fruit trees like the apricot, peach,
plum, apple, almond, orange, and others. In the fine public squares
one sees large palms also, and in the botanical garden are bamboos,
palms, bananas, India-rubber trees, and a large number of forms from
the subtropics. The great variety of introduced plants which appear
to thrive on the littoral again recalls portions of California,
where the kinds and numbers of introduced plants which grow well are
likewise large, and where the floral strangers are gathered from
the antipodes. But among the species there are also many natives
to the colony and which one will see when he begins to travel away
from the coast; for example, the cedar from the higher mountains
(_Cedrus atlantica_) and the fine pine (_Pinus halepensis_) from
lower altitudes; there are junipers and oaks, among the latter
the cork oak (_Quercus suber_), and fine specimens of _Pistacia
atlantica_ and its protector the jujube (_Zizyphus lotus_), the
relationship of which will be given later in this paper. Both of
the last-named species are native to the Sahara and are of rather
frequent occurrence in the regions visited.

Upon leaving Algiers one soon encounters a striking change in the
character of the vegetation, evidences of a rather small rainfall
and a low humidity, and one appreciates the fact that even near the
coast the climate of the colony is fairly arid. A fine general view
of the region about Algiers, giving at a glance its setting and
these features, is to be had from the heights behind the city. In
the words of Tristram (The Great Sahara):


Here we turned to observe the magnificent panorama of the city and
the harbour below, with the bay stretching far beyond, the slopes
of Mustapha on the right studded with villas, the Sahel range
terminating beyond the massive tower of the seminary of Kouba,
the conspicuous Maison Carée . . . planted where the plain of the
Metidjeh opens to the sea, the range of the lesser Atlas in the
distance beyond, and the peaks of the Djudjura, the last stronghold
of the Kabyles, behind them, capped with snow.


Crossing the plain of the Mitidja (Metidjeh), the route runs
through a country devoted largely to the raising of wine grapes,
with bare hillsides, or hills covered with low shrubs and small
trees, and, following a custom derived from France, with either side
of the highway lined with trees, mulberry and ash. At the base of
the Atlas we see large orange-groves and numerous fig trees. The
lower slopes of the mountains are covered with a chaparral-like
growth, and as one penetrates the mountains, ascending gradually
alongside the Oued Chiffa, he sees, among other forms, extensive
areas of dwarf palm, doum (_Chamærops humilis_), which resembles
remotely the familiar saw-palmetto of the southern portion of the
United States. The leaves of the doum are gathered by the Arabs for
making into baskets, rope and other useful articles, and several
donkeys laden with doum bales were observed being vigorously driven
marketward by their small bournoused masters. On the sides of the
gorge one sees species of delicate ferns, bunch-grass in tumbling
masses (very luxuriant where there is moisture), chestnut, arbutus,
and masses of evergreen ivy overhanging the way in places. At one
place, fairly high in the mountains, a colony of native monkeys has
found a safe retreat and may be frequently seen gamboling among the
rocks and the shrubs near the stream. They were seen and described
by Tristram about 50 years ago, who states that they “are of the
same species as those of the rock of Gibraltar.” Here are a few
oaks, myrtles, some “dherou” (_Pistacia_ sp.), and a few and
scattering specimens of _Pinus halepensis_. The upper portion of the
mountain, at least of the part seen, is nearly treeless, and is under
some cultivation. After leaving the pass a drive of a few minutes
brought us to the old Roman town of Medea, which appears from the
character of the vegetation, both native and introduced, to have
a cooler as well as drier climate than Algiers. Conspicuous among
the ornamentals are the Judas tree and the plane, both planted much
about the town; among the fruits the apple is cultivated extensively.

From Medea to Berrouaghia the mountain range is broken up into large
rounded hills, in part cultivated, but almost wholly open, scantily
covered with shrubs or trees and mainly grazed over by large flocks
of sheep and goats. About midway between the two places we pass
through an open forest of oaks, from whose boles the bark has been
removed. A chance acquaintance told us that the cork was removed
about once in four years, but this is probably not the true cork
oak (_Quercus suber_), which grows under more moist conditions,
as between Tunis and Constantine, or between the latter place and
Algiers, along the littoral. The altitude of this region is somewhat
under 4,000 feet.

From Berrouaghia to Boghari, which is on the northern edge of the
High Plateau, the route runs through an open grazing country,
with scattering oaks (_Quercus ballota_), juniper (_Juniperus
oxycedrus_), and “dherou” (_Pistacia lentiscus_), and, among
other herbaceous plants, not a little bunch-grass, whose species I
did not learn. On the northern slopes of Mount Gorno, 3,500 feet, is
an open forest of oak of the species named, the formation recalling
the chaparral of California. Upon reaching the crest of the mountain
one is suddenly brought to a forest of pine (_Pinus halepensis_)
which covers the entire southern slope to the exclusion of other
species of trees. Spreading over the slope on the upper levels, it
avoids the gulches near the base and reaches out on the crests of
the ridges for a considerable distance. The width of forest where
crossed is about 50 kilometers. In its habitat, which comprises the
lower Atlas Mountains, _Pinus halepensis_ forms a rather small tree,
shapely, with rounded summit. When growing somewhat apart from its
fellows it is of a more squat appearance than when in the forest,
although the forest of the species is by no means a dense one.




                       =THE FORESTS OF ALGERIA.=


There are estimated to be between 5,000,000 and 8,000,000 acres
of forested land in Algeria. Although the forests lie mainly
in the Tell, certain species, notably _Pinus halepensis_, occur
in the Saharan Atlas as well. Trees also grow along the oueds,
especially on the High Plateau, but not in sufficient abundance
to constitute forests proper. The leading species are, among the
conifers, _Cedrus atlantica_ and _Pinus halepensis_. The junipers
are of importance mainly as a fuel, but do not form forests. Three
oaks, namely, _Quercus ballota_, _Q. suber_, and _Q. lusitanica_,
are forest-making species, but several other common forms, such
as the olive, the plane, the ash, and the betoum (_Pistacia_),
may share in the making of a mixed forest, but do not occur in
sufficiently large numbers each to constitute a forest. In 1908
in the civil domain the acreage of the leading kinds of forest
trees was as follows: oak, 1,853,520; pine (_Pinus halepensis_),
1,398,470; juniper (about one-half being _Juniperus communis_),
444,780; and _Cedrus atlantica_, 85,000.

The species of trees are distributed in well-defined zones,
because of which they may to a large degree be segregated, or
at least the specific composition of the forest, if a mixed one,
may be determined. In altitudinal range, the oaks are found from
sea-level to 6,000 feet, within which each species may have its
characteristic range. For example, _Quercus suber_ reaches from
sea-level to 2,500 feet; _Q. ballota_ from 2,500 to 4,000 feet;
and _Q. lusitanica_ from 3,500 to 6,000 feet. _Pinus halepensis_
grows from the sea-level to 3,600 feet, and probably much higher
in the Saharan Atlas, while _Cedrus atlantica_ is to be found from
4,000 to 6,000 feet. _Abies barborensis_ is said to attain a higher
altitude than the cedar.

Among the definitive physical factors by which the composition of a
forest is determined—the rainfall, the temperature, the soil, and
the altitude—probably of the first rank should be considered the
rainfall and the temperature, which are affected by the altitude. In
the case of the distribution of the cork oak, however, the character
of the soil plays an important rôle. This species grows only on
sandy soil, and where the rainfall exceeds 600 mm. Because of the
latter requirement the larger part of the cork-oak forested area
is east of Algiers, the annual rainfall to the west of that place
falling for the most part under 600 mm. Similarly the minor features
of distribution, inside of the specific range, may be explained. For
example, in the upper limits of its range, _Pinus halepensis_
appears chiefly on southern slopes, as on Mount Gorno, while at the
low altitudes it is to be found on the northern face; temperature
reactions apparently—familiar phenomena in mountainous districts.

Because of the unison of response to common environmental factors,
much of the forested area, especially at the higher altitudes, is
composed of one species or one species largely predominates. This is
true of the pine and the cedar forests to a marked degree; _e.g._,
in the pine forests on Mount Gorno and the cedar near Batna.

Commercially speaking, the cork oak is at present the most important
species in Algeria. It occupies about 600,000 acres, and the yearly
yield is valued at nearly $1,000,000. It may be seen along the
railroad between Tunis and Constantine and east of Algiers. When
old the species has a peculiarly gnarled appearance, with a short,
stout bole, usually hollow, which may become 10 feet in diameter,
with irregular, straggling branches. It is less symmetrical in nature
than under cultivation, as in the Santa Clara Valley and Santa
Barbara, California, where a few specimens may be found. Another
species of oak, _Quercus ballota_, of no great commercial importance,
provides the source of the acorns in general use among the natives
for food. The acorns are found in all markets, even (as in Ouargla
and Ghardaia) where transportation for considerable distance
is necessary; they are less astringent and hence more pleasant
than those of most species. Although _Q. ballota_ appears not to
be planted for its fruit, it has been stated (Kearney and Means,
_loc. cit._) that the Kabyles preserve such selected trees as have
superior fruit, which would perpetuate the best-liked varieties. It
may be remarked in passing that the seeds of the pine are also
in common use among the Arabs as a food, although not employed so
generally nor in so many ways as the acorn.

The cedar (_Cedrus atlantica_), the most beautiful tree in Algeria,
is found in high altitudes only and on mountains separated from
one another, but always in the more northern Atlas ranges. In the
following mountain groups are to be found the main cedar forests,
namely: Ouarensis, Teniet, Blida, Babor, Maadid, and the Aurés. The
forest seen was that near Batna, near the western base of the most
important range in Algeria, the Aurés. Somewhat below the lower
limit of the Batna cedar forest is an open forest of oaks (_Quercus
mirbeckii_), _Juniperus oxycedrus_, and _J. phœnicea_, mingled with
which are shrubs suggesting those of the desert, such as _Acanthyllis
numidica_ and _Retama sphærocarpa_, as each genus is represented
south of the Saharan Atlas on the open desert. These species are
mainly confined to the southern facing, and hence on the side of
the mountains opposite the beginning of the cedar. By the roadside
are to be seen also several specimens of _Juniperus oxycedrus_
badly infested with the mistletoe (_Arceuthobium oxycedri_). In
one instance the unusual condition was observed of a mistletoe
group remaining alive on a host branch which appeared to be dead
for several inches below the point of attachment.

The first representatives of the cedar forest were encountered as
stragglers in the dry wash at the north base of the mountain on
which the forest is situated. In part these trees were shapely, with
a taper summit, and in part they were short, with a summit broad
and flat, in effect like an inverted cone. When the main forest
was entered the trees were mainly of the type first characterized,
with widely reaching lower branches and slender summits. In exposed
places or in older parts of the forest the trees of the second type
were often seen; and on the crest of the mountain the most bizarre
shapes (induced by wind action), the trunks nearly parallel to the
ground and the branches hugging the ground. In the upper portion of
the forest the trees were more widely separated than in the lower
portion, and here and there we met with really large specimens, which
must have been very ancient. One of the large trees had a bole which
1 meter above the ground was about 5 meters in circumference. The
trees were fruiting freely, but we did not see many seedlings. Why,
was not apparent. There were no indications that fires had swept
over the mountain recently.




                          =THE HIGH PLATEAU.=


The vegetation of the High Plateau, taken as a whole, is sparse, due
in part to rather light annual precipitation, but largely to the lack
of efficient drainage, for which reason large areas are so heavily
charged with salts as to be inimical to most plants. Halophytes
form an important element in the flora of this region. In the most
intense salt areas no plants are to be found at any season. Along
the oueds such shrubs as species of _Tamarix_ and _Zizyphus_ occur,
and juniper may be seen in the more elevated places, such as near
Guelt-es-Stel or further toward Djelfa.

The route followed across the High Plateau ran from Boghari to Ain
Ossera, Guelt-es-Stel, and Djelfa, which is in the midst of the
Saharan Atlas, and from thence to Laghouat.




                        =BOGHARI TO LAGHOUAT.=


Boghari, situated at the place where the Oued Chelif, having come
across the High Plateau, enters the Tellian Atlas on its way through
them to the Mediterranean, lies on the northern edge of the High
Plateau and in what appears to be a fairly arid region. The oued
at this place is rather narrow and has low banks. In its bed,
in October, were a few pools of water. On either side is the
flood-plain of the oued, several meters in width, sometimes partly
under cultivation. Tilled fields are to be seen to the west and
not far from the town. From the low mountains immediately to the
west of the town the steppes stretch unbroken (save by low hills)
to mountains bounding the southern horizon, possibly 100 miles
straight away. The mountains are the Dj. Sahari, the Saharan Atlas,
beyond which lies the desert.

The vegetation in the neighborhood of Boghari is at present meager
in amount and of small size. Along the banks of the Chelif are
a few tamarisks, and on the plain not far from the oued are a
few specimens of _Zizyphus vulgaris_ and _Pistacia lentiscus_
and other low-growing shrubs. Owing to the large number of sheep,
goats, and camels which are driven through the pass of the Chelif,
or which are kept in the neighborhood by their Boghari owners, few
plants thereabouts fail to exhibit indications of being eaten. In
fact, at Boghari the effects of grazing were first noticed, though
afterwards repeatedly seen. Only such plants as are poisonous,
distasteful, or heavily armed survive the predatory attacks of the
countless numbers of domestic animals.

The habitat of _Pinus halepensis_, which thus extends to the very
edge of the steppes, is to the east and west of the town. To the
west it forms an open forest and is associated with _Quercus
ballota_, growing on the crest and on the northern slopes of
the low mountains. Its abundant fruit, together with acorns, is
gathered assiduously by the Arabs for food. _Juniperus oxycedrus_
and _J. phœnicea_ also occur.

After leaving Boghari and the plain by the Oued Chelif, the route
goes among low, rounded hills for a distance of about 24 kilometers,
when it strikes boldly across the wide-extending plain. The general
appearance of the vegetation, away from the intensely salty chotts,
is that of low-growing shrubs on the plain, and of somewhat higher
shrubs or low trees along oueds. Of the former, perhaps the most
abundant are _Noæa spinosissima_ and _Haloxylon_ sp., and by the
oueds _Tamarix_ sp. and _Zizyphus_ sp. Near Ain Oussera is a wide
belt of _Stipa tenacissima_, the alfa grass, which occurs nearly to
the exclusion of other species, and a second belt of alfa, several
kilometers in width where crossed, was seen very soon after leaving
Guelt-es-Stel. At each of these places were seen large piles of
the grass baled ready for hauling to Algiers.

The alfa, or bunch-grass, covers large areas in Algeria as also in
Spain. In November the long leaves of the grass are dry, tightly
rolled, and rush-formed, in place of being flat as during the
rainy season or period of growth. The species reproduces largely
by means of much-branched rhizomes, from which spring the young,
fleshy leaves, enlarged at the base. In Algeria, “situées en
territoire civil,” there are 543,620 acres of alfa, mostly on
the High Plateau, but a part along the littoral in the province of
Oran, west of Algiers. The leading environmental influence upon the
peculiar distribution of the species is apparently that of rainfall,
reacting in this respect very like plants with storage organs,
avoiding alike regions where the rainfall is excessively heavy or
where it is so little as to cause marked desertic conditions. It
is apparently confined to sandy soils and is replaced by others
wherever the soil of a region (otherwise appropriate for its growth)
is of clay or is charged with any considerable amounts of salts. It
is an important article of export from Algeria. Its total tonnage
is said to amount to 80,000 each year, bringing approximately
$1,500,000. It is sent to England, Belgium, and France, and used
in the manufacture of fine grades of paper, light, strong, and of
a silky texture; also in making baskets, hats, and mats, for which
a superior grade of the grass, commanding especially high prices,
is employed (Kearney and Means, _loc. cit._).

Among other species commonly seen in crossing the steppes are
various salt-bushes, such as _Haloxylon articulatum_ and _Anabasis
articulata_ and especially _Artemisia herba-alba_, with _Tamarix_
and a few specimens of _Pistacia_ along the oueds or where
water conditions are most favorable. Between the belt of alfa
last mentioned and El Masserane, a bordj, there is a broad plain
surrounded by low mountains, which are really the northern extension
of the broad Saharan Atlas, where salt-bushes occur in a formation
several kilometers, possibly 24, across. Here in the summer the
nomads, coming up out of the desert, find grazing for their flocks,
and even in October we saw countless numbers of sheep and goats,
and hundreds of camels, browsing the shrubs.

At El Masserane are specimens of large _Tamarix_, really the size
of small trees, growing near the bordj; and to the south of the
bordj we passed the first dunes encountered on the plateau. These
are part of a series of dunes which were seen to extend to the
horizon to the northeast, as we approached El Masserane, and
which, we were informed, reached as far as Bou Saada, nearly 90
kilometers distant. The dune flora was quite different from that of
the surrounding plain, owing to the total absence of salt plants,
and to the presence, among other species, of a _Tamarix_ and a large
grass, the “drinn” (_Aristida pungens_), which was subsequently
frequently seen.

Soon after passing the dunes the way lay through a country with low
mountains, almost bare of vegetation, where scattering oaks and
junipers constitute the only species of plants, until we reached
the walled town of Djelfa.




                         =DJELFA TO LAGHOUAT.=


The bleakness and the bareness of the environs of Djelfa come with
a surprise when one considers that the rainfall of the place is not
inconsiderable, about 375 mm., and that the altitude is about 1,110
meters, which insures a fairly low temperature and hence a relatively
low evaporation rate. The sparseness of the vegetation is probably
partly due to the fact that the rainfall does not occur at one or at
two seasons, as nearer the coast, but is distributed fairly evenly
between the four seasons, and also to the long occupancy by the
Arabs and the French, by which possibly most of the useful native
plants, large and small, have long since been destroyed. Somewhat
removed from the town, particularly on the mountains to the west,
is a forest of pines. Along the streets are many shade trees,
as Lombardy poplar, ash, locust, and others, and within the town
limits is a small but fine public park and experimental garden with
a large variety of shrubs and trees.

From Djelfa to Laghouat the road runs through barren mountain passes,
and is dreary and of little interest. Tristram’s description of
the approach to Laghouat, written about 1860, gives very well the
present condition of things:


The next day’s journey was through a rocky desert country. . . . We
afterwards passed a low-lying strip of sand-hills on the west, with
the marks of an ancient ocean beach; on the east a high range of
mountains, with the stratification regular and horizontal. . . . Our
next day’s ride was by a base of a continuous chain of steep
ridges, again with an even water-line very near the crest, and
presenting a singular serrated appearance (the Djebel Lazareg). I
counted no less than 127 little peaks rising above this straight
horizontal line, almost all of them of equal height, like the
crests of a long sea-reef; and lower down the sides were many tidal
strings, if I may so term them. Turning around to our left and
crossing the dry channel of an evaporated and aged “Wed,” we
had some low headlands close behind us—Ras Ainyah of the Arabs,
“Prise d’eau” of the French—the scene of a bloody combat
under General Yusuf. Through an opening between the mountains we
debouched on a wide plain, and suddenly before us stood an isolated
rock. Two cliffs facing each other bore each a bastioned tower,
and in a depression between these lay a town.


The town, whose situation is thus so graphically presented, is
Laghouat, of which the leading present interest lies in the fact that
it is on the very edge of the Sahara proper. From the rocky hills
by the town one can see the serrated Saharan Atlas to the north,
extending northeast and southwest, and, turning, to the south,
an expansive and gently undulating bare plain, stretching without
a break to the horizon.




                              =LAGHOUAT.=


The ancient Arab town of Laghouat, which is also an important
military post, is a very favorable place from which to begin a study
of the plants and the environment of the plants of the northern
Sahara. Its altitude (780 meters) is greater than that of Ghardaia
(520 meters), as also that of the latter place is greater than the
altitude of Ouargla (150 meters). The annual rainfall of Laghouat
is more than that at either of the places mentioned and more
dependable. The surface of the desert at Laghouat is, for example,
not of one type only, but characteristic of much of the Sahara; that
is, it is mainly stony, a hamada, but there are also sand areas, a
oued and its flood-plain. Finally, the plants growing in the vicinity
of Laghouat are largely typical of those found farther to the south,
as at Ghardaia and Ouargla, or even deeper in the desert. It is of
great interest to observe the change in the habits of the plants,
in their number, distribution, and other features, when one leaves
a less arid region and goes toward a region of gradually increasing
aridity, as when passing southward from Laghouat.

The leading plant habitats are the oasis, the arid plain, and
the dunes. It is not likely that any of these habitats have been
greatly changed because of the settlement by Arabs. The arid plain
and the dunes surely have _per se_ not suffered marked alteration,
and the oasis itself is probably not so different from what it was
formerly, as the great difference in plants growing in it might at
first lead one to suspect. More water is brought to the surface at
present than in primitive times, but if it were possible to remove
all introduced plants, and restore all native plants peculiar to the
oasis, there is no apparent reason why they should not live there
quite as successfully as in earlier times. It does not follow that
there has been no modification of the flora itself, a result of the
founding of a town at the oasis, and it will be pointed out later
that such has surely been the case, but to what extent or in what
way does not appear.


                        THE OASIS OF LAGHOUAT.

The oasis of Laghouat is situated on the Oued Mzi, the upper portion
of the Oued Djedi, one of the most important oueds of Algeria. The
Oued Djedi runs in an easterly direction from Laghouat, receiving
many tributary oueds _en route_, by a rather long course to the
great Chott Melrirh, which is southeast of Biskra. Like other desert
rivers, the Oued Djedi is dry most of the year, but is occasionally
filled to overflowing with a rushing flood, which is of great erosive
power and may be very dangerous to the traveler. Above the town of
Laghouat, where the Oued breaks through the last pass of the Atlas,
the flood-plain is narrow, but upon leaving the pass the plain
widens until in the immediate proximity of the town it is about 1.5
kilometers in width. On either side of the flood-plain stretches the
arid plain (hamada), usually stony, but near the mountains covered
with low, slowly moving sand ridges. To the south of the oasis,
the arid plain merges into the topography characteristic of the
region of the dayas.

The portion of the oasis devoted to the cultivation of date and
other trees, and to gardens, is about 3 square kilometers in size,
but arable land extends above and below the town, so that outside of
the oasis, as delimited above, there are about 6 square kilometers,
all of which have at times been under cultivation. The last referred
to is the flood-plain of the oued and is used mainly for growing
barley. By the edge of the oued, or along the irrigating ditches,
are several characteristic species of plants, which may point to the
character of the primitive flora; for example, willows, oleander,
and _Tamarix_, with a few palms. The betoum (_Pistacia atlantica_),
which must surely have been an inhabitant of the oasis formerly,
is now apparently wholly absent. The species just mentioned are to
be found between the town and the pass above; but below the oasis,
owing to an apparently poorer water-supply, there are fewer large
species. Among those found are a few specimens of _Rhus oxyacantha_
and _Zizyphus vulgaris_, and it was probably below the town that
the betoum was to be found in earlier times.

The oasis is under intensive cultivation (see fig. 1). There are
about 300 gardens, each bounded by mud walls, and often separated
by picturesque, meandering lanes. The plant life, almost wholly
introduced, is luxuriant. In some gardens the effect is tropical,
where vines reach from tree to tree, making a canopy nearly sun-proof
and separating the spreading tops of the palms from the wealth of
shrubbery and herbaceous plants beneath. First among the trees of
the gardens, in numbers as well as in economic importance, is the
date palm, of which there are said to be about 30,000. Although
this is small in comparison to the number of date palms at Ouargla,
Touggourt, or the Oued Rirh, the dates are of great importance to
the dwellers at Laghouat, where the products of the gardens are
almost all consumed. The living tree provides shelter against the
intense heat and light of the desert, and the dead leaves constitute
an important source of fuel in a land where fuel is extremely
hard to obtain. The flesh of the date fruit is eaten by the Arab
and the cracked seeds are given to the camels. Without the date
a continuous occupancy of a remote oasis by the Arab is clearly
impossible. Besides the date palm fruit trees of other kinds are
abundant, among them the apricot, fig, mulberry, peach, pear, and
orange. The pomegranate and the table grape are also very generally
grown. The lowest story of the vegetation of the gardens is composed
of garden vegetables, such as artichoke, bean, carrot, melon,
pea, potato, squash, and radish. Among the ornamentals one sees
roses, asters, and chrysanthemums, and occasionally very luxuriant
cannas. One or two parks contain interesting introduced trees and
shrubs. We recognized among the trees _Ailanthus_, _Eucalyptus_,
umbrella, plane, poplar, pine, cypress, ash, locust, willow, and
St. John’s tree. The Barbary fig (_Opuntia ficus indica_) is also
common, but does not stray away from the best-watered situations.


                              THE PLAIN.

The part of the plain (hamada) studied lies to the west of the
oasis, between it and the adjacent hills, Mountains of the Nomads,
which are to the west of the pass of the Oued Mzi. Emerging from
the oasis, one finds himself on the arid plain, the transition
from the one to the other being abrupt. The plain, at first view,
with a covering of small stones and pebbles, gives the impression
of total barrenness. Not a tree, shrub, or herb appears to hide
the bare ground. The mountains are naked rock, while the harsh
outline of desert ranges and the distant low sand ridges give no
evidence of plant life. But a closer examination of plain, dune,
and mountains reveals the presence either of living forms or of
the dried remains of plants of a preceding moist season, in numbers
and in kinds not at first suspected.

Close to the oasis the plain forms the highway for caravans as well
as the drilling ground for army recruits, so that the herbage is
either trodden under foot or eaten to the roots. Somewhat farther
away, where the plain rises to meet the mountains, we first encounter
perennials large enough and abundant enough for consideration, the
most prominent being quedad (_Acanthyllis tragacanthoides_), adhidh
(_Zollikoferia spinosa_), rempt (_Haloxylon articulatum_), and drinn
(_Aristida pungens_); _Acanthyllis_ is perhaps the most numerous.

Quedad is the most striking plant native to Laghouat. It is a shrub,
related to _Astragalus_, usually not over 40 cm. high. A single
specimen consists of a group of unbranched or little-branched stems,
rather stout, of a grayish-green color, and provided with long and
stout spines. As a whole the plant has a very close resemblance
to small specimens of ocotillo (_Fouquieria splendens_) of the
southwestern United States. During dry seasons the stems are bare,
but when the rains return leaves are put out in the axils of the
spines, which are the rachides of the leaves. The habit of quedad is
shown in figs. 4 and 6. Although the species is so well protected
against attack by animals that it rarely, if ever, suffers on that
account, it is made a supplemental food through the burning off of
the spines. When thus prepared the half-woody stems are eaten with
avidity by camels.

The census of _Acanthyllis_ was taken on the upper portion of the
arid plain at a place where the plant seemed to be most abundant. On
an area 16 by 16 meters, 92 specimens were found living. This was
the dominant species. Other species, present in less number, were
so badly eaten by animals as to be quite unrecognizable.

The root-system of _Acanthyllis_ offered some points of interest
and a short study of it was made in the field. All of the specimens
whose roots were examined were growing in the habitat above referred
to and within a meter of one another. The leading results of the
observation are as follows: The largest specimen studied possessed a
tap-root 3 cm. in diameter at the crown. Growing rapidly smaller as
it ran downward, the root gave off four laterals, of which a portion
dipped at an acute angle to a depth of 20 to 30 cm., sending off
branches by the way. One of the largest of the laterals was traced
75 cm. and where left was 2 cm. beneath the surface. The branches,
at least of the main laterals, that is, the roots of the tertiary
order, for the most of their course ran thus near the surface of
the ground. One of the leading laterals was followed to the base of
a neighboring specimen of quedad, where it lay close to the crown
of the main root. The depth of the penetration of the tap-root
of this specimen was not learned. The tap-root of a neighboring
specimen ran directly downward 20 cm.; then, turning sharply,
it extended in a horizontal direction for a distance of 70 cm. As
the large laterals were wholly lacking on this plant, the tap-root
was the entire system—surely an anomalous condition. On a third
plant the tap-root penetrated the ground about 4 cm. only, after
which it turned and ran the rest of its course within 4 cm. of the
surface. The more superficial of the roots of a fourth specimen were
found to extend to the base of the last plant mentioned. Thus it was
found that the root-system of _Acanthyllis_, as growing naturally,
extends both widely and deeply for a considerable distance, and that
it is flexible to a degree; in short, is generalized[8] and closely
resembles that of certain species of the Tucson region, particularly
_Covillea tridentata_, which grows under similar conditions.

The root-systems of three or four other species were also
examined. Of these _Zollikoferia spinosa_ and _Artemisia campestris_
were growing in a little hollow in the plain close by the habitat
of _Acanthyllis_. _Zollikoferia_ has a very close habit of growth
with dichotomous branching (compare figs. 5 and 7). When dry the
branches are slender and of a woody hardness. The root-system of
this species is characterized by a pronounced tap-root and by the
absence of large laterals, at least near the surface. The leading
feature of the root-system is, in short, its tap-root. A root-system
of a similar type was found in _Artemisia herba-alba_.

Rempt (_Haloxylon articulatum_) also occurs on the plain. This is
a shrubby perennial (half shrub?) which is possibly the most often
met of any species, or at least genus, in southern Algeria. It is
capable of enduring very arid conditions, is unarmed, and is much
eaten by all herbivorous animals. The plant will be figured and
especially referred to later in this study, so that at this time
only a brief definition will be given of its root-system, which is
a typically generalized one, penetrating the ground fairly deeply if
the character of the soil permits, but also lying near the surface;
there is also a relatively large number of secondary and tertiary
roots, characteristic of the generalized type.[9]

Inspection of the soil showed it to be a sandy clay with a large
percentage of pebbles and stones and with greater depth of earth in
the hollows on the plain than on low ridges. It probably contains
some gypsum, since an outcropping of it occurs on the southern
face of the low mountains to the north. Taken as a whole, the soil
appeared very like that of the plain by Tilrempt and Ghardaia,
as well as between Ghardaia and Ouargla (Gantara). The soil will
be described later in this paper.


                       THE DUNES NEAR LAGHOUAT.

Dunes are not present in large enough numbers, or of large enough
size, to figure very prominently in the topography of the environs
of Laghouat; but they occur both to the east and to the west of the
town, those to the east being the larger. Sand is found facing the
south side of the Rocher des Chiens, a rocky hill on the western
edge of the oasis, and the south side of a portion of the Nomad
Mountains, to the north. There is also a succession of low dunes
between the Nomad Mountains and the oasis. The Rocher des Chiens
dune is moving from west to east, but the dune along the face of
the Nomad Mountains is probably stationary. The series of dunes on
the plain are moving toward the northeast.

An inspection of the dunes shows that the number of individuals,
as well as the number of species, is very limited. In fact there
are apparently fewer plants here than on the dunes of large size
subsequently encountered between Ouargla and Touggourt. The most
characteristic plant is “drinn” (_Aristida pungens_), but it
is by no means common; there are also a few specimens of _Tamarix_
growing near the Nomad Mountains. As frequently happens where
there are moving dunes, the passage of the dune greatly changes the
character of the flora. Although this feature was not especially
studied, it was noted that the number of grasses where the dune
had swept was greater than it was before this. _Acanthyllis
tragacanthoides_, a plant typical of the plain, was found to
survive the passage of the sand, although changed in appearance in
a characteristic manner. It will be recalled that the species has
a top consisting of several shoots, springing from the root-crown
at the level of the ground, and a root-system in which the laterals
and the main root are alike well developed. In order that a plant,
already established, can maintain this general relation during the
heaping up of the sand, the main root must grow at the crown as
fast as the sand encroaches. This is exactly what happens, so that,
when the dune has passed on, the shoot of the species is elevated
for a space equaling the depth of the sand, which is half a meter
or more. From the probability that _Acanthyllis_ is of very slow
growth and that the dunes are low, it follows that the rate of dune
movement must be slow.


                     THE MOUNTAINS ABOUT LAGHOUAT.

The southern spurs of the Saharan Atlas lie to the north of Laghouat,
within 3 miles of the oasis. This range (the Nomad Mountains)
is not over 266 meters in height. Higher mountains extend both to
the west and to the east, but only the Nomads are visited. There
are also two high, rocky hills on opposite edges of the oasis,
the town lying between, and it is perhaps because of their presence
that the water is forced to the surface, or near the surface, and
the oasis is formed. The hill to the west of the town is called
the Rocher des Chiens. On the south side of the Nomad Mountains
is a long outcropping of gypsum; the south face of these mountains
is also abrupt, carrying little soil; but on the northern face the
slope is gradual and there is much soil, although rocks are abundant.

Two localities only in the Nomad Mountains were seen. One was
in a pass through which caravans travel _en route_ to Laghouat,
from the High Plateau or the more distant Tell; the second to the
east of the pass is much less often visited by flocks. The physical
plant conditions appeared to be the same in both locations. As one
approaches the mountains from the south the number of plants grows
less until on the south face they quite disappear, but on attaining
the crest they quickly increase and form a noticeable element in
the landscape. In fact, the flora of the northern gentle slope,
where the soil conditions are relatively favorable, is much richer
than on the plain or the dunes. Had the shrubs been of good size
they would have been very conspicuous.[10]

A certain area on the north slope of the Nomad Mountains, not
far from the pass through which the Oued Mzi goes, was selected
for making a census of the plant population. Here the number
of individuals, as well as their size, showed that the moisture
relations were good as compared with those of the plain. On an
area 16 meters square 422 living perennials were found, mainly of
the following species: _Acanthyllis tragacanthoides_, _Asparagus
spinosa_, _Deverra scoparia_, and _Zollikoferia spinosa_. The most
numerous, _Zollikoferia_, was represented by 96 individuals. The
general character of the habitat and the plants is sufficiently
well shown in fig. 8 and does not need further mention in this place.


            EFFECTS OF GRAZING ON VEGETATION NEAR LAGHOUAT.

It is generally recognized that a potent influence is exerted by
man, and the lower animals, in shaping the flora of an arid region,
not only as regards the kind and number of plants, but also as
concerns certain of the leading characteristics of the plants
themselves. The action is largely such as brings about a survival
of the “useless” forms, so that we do not know, from the plants
we meet in the desert to-day, how many or what kinds of plants it
might support. It also may be true that no desert shows the modifying
effects of the causes suggested more than the northern Sahara.

The gazelle (_Gazella dorcas_) is the leading wild animal preying
upon the desert plants in the northern Sahara. It is frequently
seen by the traveler to-day and was present in large numbers no
longer ago than 50 years (Tristram). It is said by the Arabs to
feed on the fresh shoots and leaves of many species of shrubs and
trees, especially the betoum (_Pistacia atlantica_), as well as on
the annual vegetation for the relatively short period when it is
to be had. Other species of gazelle, according to Tristram, range
farther to the south. Other herbivorous animals of the region are
hares, antelope, moufflon, and bubale. The moufflon, at the time of
Tristram’s visit, was “far from uncommon throughout the whole of
the mountain districts, whether wooded or bare,” but the bubale,
“the wildest of the wild game of North Africa,” appears not to
go north of the Oued Rirh, while its home is farther south. It is
impossible to know the number of wild animals subsisting on native
desert vegetation, but the list given, which might be extended,
suggests that it is by no means small. Although the population
of wild herbivorous animals is at present considerable, it was
probably much greater a few years ago, the decrease being due,
as Tristram says, not so much to the greatly increased population
as to the more efficient weapons used by the Arabs in hunting.[11]

Laghouat has been inhabited by the Arabs continuously for about
1,000 years, during which time the oasis and its environs have
been the source of supply of all of the fuel used and for much,
if not the most, of the food consumed both by the Arab and his
flocks. Naturally, the food for the inhabitants of the oasis is
won from the oasis itself, but that for the beasts is derived from
the desert. While it is impossible to learn the number of sheep,
camels, and goats which from century to century have ranged over
the desert pastures, there is no reason for supposing it was not
large, as at present is the case. For example, in the department
of Algiers, in 1907-08, the number of sheep is reported to have
been 2,109,071; of goats 1,156,500, and of camels 23,912.[12] Of
these a large percentage is to be found in and to the south of the
Saharan Atlas Mountains. In portions of the colony farther south,
or in regions even more desertic than at Laghouat, the number of
camels, sheep, and goats is surprisingly large. The statistics
for 1907-08 give the population of the three classes of animals in
the southern territories, that is, the territories of Ain-Sefra,
Ghardaia, Touggourt, and the oases of the Sahara, as follows: sheep,
1,932,392; goats, 588,121; camels, 126,088.

The flocks of sheep and goats range at various distances from the
source of their water-supply. It is quite usual for the goatherds
to gather their flocks in early morning, returning to the oasis
in the evening to distribute the goats to their various owners;
but probably in most cases the flocks of sheep, with some goats,
return to water once in two or three days, thus being able to stray
from 20 to 40 kilometers into the desert. Being less dependent on
water, the camels range a much greater distance. It thus appears
that the area grazed over by the flocks of the Arabs is fairly
circular in form, with a radius of 40 or more kilometers from the
wells, and that the range of the camels may be much greater than
this. It should also be noted that the caravan routes are broad
tracts where all vegetation fit for fuel or food has been utilized
Between the two sources of destruction referred to, the stationary
and continuous and the frequent but not continuous, little territory
passes untouched as a source of food-supply.

A visit to the wood markets of the town indicates to what ends the
natives resort to obtain fuel. (See figs. 2 and 3.) Among many kinds
of wood, some are from the oasis itself, such as the willow, plane,
and palm; some from the mountains, like juniper, pine, and oak;
and some from the oued, like the jujube and _Tamarix_. Much of the
fuel is at present brought three days’ journey by camel. Usually
the subaerial portions only of the plants are used, but in the
case of the jujube both root and branches are gathered. The fuel
requirements have apparently brought about the extinction of some
species from certain of their earlier habitats; for example, the
betoum (_Pistacia atlantica_) probably formerly either grew in the
oasis of Laghouat or near by, since it was formerly in common use
as a fuel. Tristram mentions the betoum, and no other species,
as a source of fuel, but inquiry failed to show that at present
this species is in general use for this purpose; but the betoum
is a familiar sight in the region of the dayas south of Laghouat,
where it is the only arboreal species of the region.

From what has been said regarding the large numbers of domestic
animals that gain their entire living on the desert, it follows
that of plants growing within the range of the flocks only such
as are poisonous, distasteful, armed, or otherwise protected,
escape partial or complete consumption. Only such species as are
too small for fuel or can not be eaten by animals attain to the
usual development year by year. In the vicinity of Laghouat the
most prominent of the immune plants is the quedad (_Acanthyllis
tragacanthoides_), which is well protected by its stout spines;
and even this species is not wholly undisturbed. The jujube, also,
although not common here, is provided with short spines. It is
stated by Massart[13] that the camels used by him, not having eaten
for five days, consumed the branches of the jujube in spite of the
spines, and that _Anvillea radiata_, a composite with acrid juice,
was passed by. Perhaps the plants most frequently eaten are of
the genus _Haloxylon_, generally distributed from the High Plateau
southward into the region of the M’Zab. That this genus is not
exterminated is interesting, since it is rarely permitted to come
into flower and fruit, and it appears not to reproduce to any extent,
if at all, in a vegetative way.




                     =FROM LAGHOUAT TO GHARDAIA.=



                         REGION OF THE DAYAS.

From Laghouat the way lay through a gently rolling country,
ever dipping to the south and southeast. No mountains relieved
the monotonous horizon. “A hard stony desert alternated with
rolling sandhills,” followed by a “vast level plain dotted with
dayas,” to quote again from Tristram’s narrative. Somewhat to
the south of Laghouat depressions are met here and there, separated
from each other by the low ridges of the plain, which receive the
drainage each of a limited territory. (See figs. 10 and 11.) These
are dayas, and are in fact oases with an uncertain water-supply but
with favorable soil conditions, so that such rain as falls on them,
or is conducted to them from higher ground, sinks deeply and creates
relatively favorable conditions for plants. In that the daya is
the center of a drainage system, and has no visible outlet, it is
comparable to the chott or salt-spot, but it is to be distinguished
from the chott by the absence of salts in excessive amounts. That
salts are not present in the dayas in quantity is probably due
to subterranean drainage, the daya being in fact similar to the
bolsons of western America. On the plain in the northern portion
of the daya region the following may be observed:[14]

  Echinops spinosus.
  Acanthyllis tragacanthoides.
  Thymelæa microphylla.
  Peganum harmala.
  Euphorbia guyoniana.
  Citrullus colocynthis.
  Artemisia herba-alba.
  Artemisia campestris.
  Haloxylon articulatum.
  Anabasis articulata.
  Teucrium polium.
  Aristida obtusa.
  Stipa gigantea.
  Noæa spinosissima.
  Asteriscus pygmæus.

In the dayas one sees _Zilla macroptera_, _Peganum harmala_, but
chiefly _Zizyphus lotus_ and the betoum (_Pistacia atlantica_),
which is perhaps the only species of tree outside of the oasis in
this portion of the Sahara. Massart mentions not seeing any tree
away from oases from the time he left Biskra until he reached the
region north of Berrian, the northern portion of the Chebka. As
the betoum is so conspicuous among the Saharan plants, and also
from the very remarkable relationship existing between it and the
jujube, the species is of very great interest. The relationship
will be described under an account of one of the largest dayas of
the region, that of Tilrempt.


                           DAYA OF TILREMPT.

The daya of Tilrempt is one of the largest (about 103 hectares) and
is the most southerly of all. It lies near the southern margin of
the daya region, and is surrounded by a gently undulating plain
(hamada), whose surface is strewn with stones and pebbles, with
apparently an underlying stratum of impervious material, since
such is to be seen wherever erosions have laid it bare. The floor
of the daya is free from stones, being composed of soils of a
fine texture which have been washed or blown by the wind from the
surrounding higher country, and is apparently not underlaid by a
hardpan. Judging from the depth of the two wells at the daya (which
were dug, not bored), the deeper of which is said to be 95 meters,
there is an abundance of earth for the roots of the plants. Besides
the wells there is a cistern, sunk below the general level of the
daya floor, which receives and stores up flood-water; it is rarely
filled, but occasionally contains considerable water, possibly
up to one-fourth of its capacity; its filling is very uncertain,
depending on the rare and scant rains. According to Massart, no
rain had fallen during the two years previous to his visit, and
the cistern was empty when we were there. These observations are
given to show under what intensely arid conditions such a tree as
the betoum (mature specimens of which carry an immense evaporating
area) can become of large size, giving but slight evidence of a
severe struggle against such adverse conditions. The altitude of
the daya is about 600 meters.

As one crosses the plains in the vicinity of Tilrempt he notes
the scantiness of the vegetation. (See fig. 9.) Here and there in
the depressions are a few betoums, often only one specimen, but
usually more, and in the erosion channels leading to the dayas is a
sparse population of low, gray shrubs. Over the higher portions of
the plain, one is aware that small perennials, 20 to 30 cm. high,
are widely scattered, but it is the bare ground which gives the
character to the landscape. Among other forms are species of
_Aristida_ and _Stipa_, _Anabasis_ and _Haloxylon_, with dwarfed
specimens of _Zizyphus lotus_ in the washes near the dayas. The
leading species, _Haloxylon articulatum_, is much eaten by the
flocks of sheep and goats (over 7,000 sheep are said to obtain
water at the daya), but occasionally it is present in surprising
numbers. For example, on a slope to the northeast of the daya,
and but a few meters distant, 227 living specimens of _Haloxylon_
were counted on an area of 16 meters square; in another place,
near the crest of a low hill to the west of Tilrempt, where the
conditions were probably as unfavorable as any in the region, 118
specimens were found on an area of the same size. In both squares
there appeared to be no other species present. Tilrempt is said
to contain 2,400 betoum trees, although the visitor would not be
likely to estimate the number at nearly so high a figure.[15]
Numerous specimens of jujube are also scattered through the daya,
with some _Peganum harmala_ and _Francœuria crispa_.

As a person visits the floor of the daya he is struck by the
great beauty of the betoums. They are of a compact habit of
growth, shapely, and cast a dense shade, an unusual feature in
a desert tree. Attaining a height of 15 meters or more, the tree
may have a spread equaling or exceeding this amount. The bole of
the largest specimens is of large size; one was found 4.56 meters
in circumference, another 4 meters, while a third measured 3.36
meters. All measurements were made 1 meter from the surface of
the ground. No betoums, however, were seen to have developed in
a perfectly normal way, and this observation applies not only to
Tilrempt but to all other dayas seen, but they were disfigured
in a peculiar manner, the lower branches giving the appearance of
having been cut and removed at a height of 2.85 meters. In fact,
this had been done, and the lowest existing branches marked the
highest point to which a browsing camel can reach.

The leaves of the betoum are compound, consisting of 7 to 9 large
leaflets. The branches are unarmed and the twigs, younger branches,
and leaves are eagerly eaten by whatever herbivorous animals can
reach them. (Fig. 12.) So much is the betoum sought after as a food
that it would be exterminated if it were not protected by another
plant, namely, the jujube.[16] The character of the leaves and young
shoots of the betoum may be seen in fig. 15.

In considering the main characteristics of the jujube we find that
it is a spreading shrub, frequently attaining a height of 3 to 4
meters, with branches well armed with stout spines. The leaves are
small, simple, and leathery. (Compare fig. 14.) The spines are so
efficient as a means of protection that the shrub shows no signs of
being eaten by animals, although, as quoted above, Massart remarks
that after his camels had gone five days without food they ate the
jujube branches in spite of the spines.

The unarmed betoum and the armed jujube have a very interesting
relationship, which is as follows: When the seeds of the betoum
germinate the seedling is eagerly eaten by animals if it chances,
as is usually the case, that the germination occurs on the open daya
floor; but if the seeds are carried to a _Zizyphus_ and germinate
in its midst, the young plants may attain to a considerable height
before being seen by animals, and, being protected by the encircling
jujube, will continue growing until they are too large to be easily
killed through grazing. It usually happens that once the betoum
plant appears above the top of the protecting shrub the camels
attempt to reach the attractive shoots and the jujube is trodden
under foot. The jujube is thus ultimately destroyed and a mound
around the base of the young betoum is all of it that remains. If the
jujube is relatively small and the developing betoum is discovered
while still small, it will be much eaten, and probably killed; but
if it reaches a considerable height before the discovery is made,
only the lower branches will be devoured and the specimen will
survive. Massart was unable to find any young betoums, but when
my visit to Tilrempt was made, November, 1910, there were several,
although so well hidden as to cause much trouble in finding them. A
view of one of these is shown in fig. 13.

The betoum is eaten by the gazelle as well as by domestic animals,
which are abundant enough in this region, and this fact is probably
of great influence in restricting its distribution. Fairly numerous
on the desert at present, according to Tristram the gazelle was
very abundant in earlier times; he speaks of their tracks marking
the plain like sheep-walks.

From what we have already seen regarding the ill effect of grazing,
it will appear that the relation between the betoum and the jujube
is a very vital one to the former; and it probably is not too much
to say that the distribution of the betoum in the daya region
is entirely dependent on that of the jujube, since there is no
other armed shrub in the region to afford the protection essential
to its survival; with relatively favorable moisture conditions,
considerable depth of soil, and a protecting jujube, the betoum
will flourish and reproduce now quite as well as in former years.


                              THE CHEBKA.

No dayas were seen after leaving Tilrempt, and the aspect of
the country changed markedly and suddenly as the drainage became
better defined. The hills were more abrupt and in systems, and
the valleys became broad and continuous. At first the valleys were
wide and shallow, the hills being low and with flat summits, but
as the distance from the daya region increased, the valleys became
deeper, until at Ghardaia the effect was that of low, flat-topped
mountains with broad valleys between, thus remotely suggesting
the topography of southern Arizona. However, in southern Algeria
the mountains are not so high nor is the “mesa” (hamada) so
extensive as in Arizona. The general level of the daya region is
prolonged as the summits of the mountains of this the chebka region,
while the valleys are eroded to a new level, that of the plain of
the M’Zab. A similar condition is seen as one goes from Ghardaia
to Ouargla, so that in fact there are several immense terraces,
reminders of that remote period when there was more rainfall in
this portion of the Sahara than at present.

Although the drainage to the south of the daya region is well
developed, the valleys and mountains run in a rather confused way,
so as to give to the fancifully inclined Arab the idea of a net,
from which the name “chebka” is said to be derived.

The country from Tilrempt to Ghardaia is characterized by a
continuously decreasing amount of vegetation. In place of the
country as a whole having a covering, however sparse, as in the
daya region, one is apt to consider the chebka a barren desert,
absolutely devoid of plant life; but closer inspection dispels this
illusion and reveals the presence, in the more favorable situations,
of not a little vegetation.

In the northern portion of the chebka region one sees here and
there, on the bottoms, specimens of the jujube and the betoum,
as well as _Zilla macroptera_, _Retama sphærocarpa_, and _Coronilla
juncea_. On the rocks at Settafa, Massart reports finding lichens,
the first he had seen after leaving Biskra. However, crustaceous
lichens occur on the flat tops of the low mountains by Ghardaia.
Massart suggests that the absence of lichens in the Sahara
(possibly they are not to be found south of Ghardaia) is because
of the intense dryness and the great heat, the temperature of
the rocks becoming from 60 to 70° C.[17]

At Berriane, one of the M’Zab cities, there are over 30,000
palms of a superior sort, watered from over 400 wells. The
surroundings of this oasis are extremely desertic and a casual
survey of the route between this place and Ghardaia, 44 kilometers
distant, reveals almost no vegetation. Here the calcareous plain
of Cretaceous origin, the Chebka, is even more eroded than in the
portion farther to the north, and the valleys are wider. The soil
is a fine clay without an admixture of sand. It is only in the most
favorable places, along the washes, that plants are to be found, and
here are _Deverra chlorantha_, _Anabasis articulata_, _Gymnocarpon
fruticosum_, _Artemisia herba-alba_, _Ononis angustissima_, _Linaria
fruticosa_, _Antirrhinum ramosissima_, and _Haloxylon articulatum_
(Massart, _loc. cit._). _Peganum harmala_ occurs very sparingly
by the roadside. The habits and the habitats of certain of the
above-mentioned species will be described in greater detail later
in this paper.




                              =GHARDAIA.=


The Ghardaia region can be characterized as a vast plain, broken to
the north by low, irregularly disposed mountains, and stretching for
a great distance to the south and southeast with a fairly monotonous
surface, diversified only by oueds, chotts, or occasional dunes,
which may be the size of small mountains. Thus on the one hand one
finds the fairly diversified and stony Chebka and on the other the
hamada, which has been aptly described by Brunhes[18] as “le désert
par excellence, la vrai désert . . . les grandes plaques pierreuses
indéfinies des hamadas!”

The leading oued of this region is the M’Zab, which extends for
about 270 kilometers in a direction south of east across the southern
part of the Chebka. It takes its origin about 80 kilometers west of
Ghardaia and extends to the vicinity of Ouargla, where it debouches
on the Ouargla plain. At Ghardaia the oued lies in a valley, with
abrupt sides, which is sunk about 60 meters below the surrounding
plain and which at this place is about 3 kilometers in width. (See
fig. 16.) There are four main tributaries of this oued, all of which
join it from the north or the Chebka side. The valley of the M’Zab
becomes more and more shallow as one proceeds eastward and at last
lies but little below the general level of the country. Like the
other deeper valleys of the Chebka, the M’Zab Valley represents
the work of erosion by water at an earlier geological epoch, when
the great terraces were formed. The filling of the eroded valleys
has perhaps taken place during the long arid period since that time
and has probably proceeded very slowly.

It appears to be uncertain how long the M’Zab has been inhabited
by man, or, more accurately, by the race now dwelling there;[19]
but it has probably been not less than nine centuries.[20]

At Ghardaia are several well-defined plant habitats, which may or
may not be distinct topographical areas, and which differ from one
another in exposure, soil conditions, and water relations. These are
the plain of the Chebka (hamada), the low and flat-topped mountains
resting on the plain of the Chebka, the walls of the M’Zab Valley,
and the valley floor with its gardens, cemeteries, and waste lands.

The soil conditions of the areas mentioned are very diverse. On the
valley walls and the mountains there are bold rock outcrops with soil
in the interstices only, and here the most intensely arid conditions
prevail. The soil on the hamada also is exceedingly meager. Rocks of
various sizes strew the surface. It is only between them, as well
as in the washes of gentle gradient, that the best soil conditions
of the plain are to be found. Here a cursory examination shows a
large admixture of small stones to the fine clay, the prevailing
soil type, and that at a depth less than 50 cm. A white hardpan,
similar in appearance to the caliche of the southwestern United
States, may usually be encountered. In the drainage depressions the
soil is relatively more coarse than on the more level portions of
the hamada. There is also great variation in the character of the
soils of the valley. Above the upper palm gardens, which are about
2 kilometers above the town of Ghardaia, will be found much sand
and fairly large stationary dunes, while smaller dunes, shifted by
the winds, are to be found at various places in the valley. About
10 kilometers down the valley, toward the east, the sand is blown
against the valley sides, and in certain places where the walls are
low it has been sifted in a thin layer over the plain itself. At
the sister city of El Ateuf the drifting sand is a continuous menace
to the gardens.

Between Ghardaia and the upper palm gardens, and also between this
city and Beni Isguen and Melika, are bare areas, free from sand
or clay, where the soil is so hard as to be used for threshing
floors and where the small amount of grain grown in the valley is
threshed and winnowed. The hardpan is similar in appearance to the
caliche of the southwestern United States and may be essentially the
same. It is of wide extent in the valley and probably underlies the
largest portion of it. Near the threshing floors the upper portion
of the hardpan stratum is from 2 to 3 meters above the bed of the
oued M’Zab; the stratum is about 30 centimeters in thickness
and is of fairly uniform structure throughout. Beneath this is
another stratum, less well defined perhaps, of approximately the
same thickness, and with nearly the same character, but carrying
a noticeably large percentage of sand. The lower stratum is less
hard than the upper one. Underneath the second stratum is soil,
largely sand, containing rocks of various sizes. Where erosion of
the oued banks has occurred the soft lower hardpan stratum and the
yet more soft underlying soil are both removed, leaving the upper
stratum projecting as a shelf, sometimes as large as 2 by 4 meters
in extent. When the shelving banks break they remain practically
intact, partly buried by the sandy floor of the oued. (See fig. 17.)

Along the sides of the valley, and at a distance more remote
from the oued, there are occasional washed-out areas, really box
cañons, where the banks show a slightly different condition of the
hardpan from that just described. Here there may be three strata of
hardpan. The uppermost is of the same stratum as the top stratum
by the oued, and the second stratum also resembles the lower one
just described. There is also a third hardpan stratum of a much
different character, in that it has a very large admixture of sand
and gravel, and large as well as small stones, making it more easily
eroded than either of the upper strata. The soil in which the roots
studied were found varied from a fine sand, with waterworn pebbles,
near the oued, to a clay mixed with sand nearer the sides of the
valley. In places the sand is cemented so as to resemble one of the
hardpan strata above described, but it is less hard and apparently
is penetrated by water without great difficulty.

The Oued M’Zab, whose channel is 15 meters more or less in width,
is dry most of the year, containing water for only a few hours
following the rare storms. Wells are very numerous in the valley and
furnish a good supply of water. At the time of my visit to Ghardaia
the water lay from 10 to 25 meters from the surface, depending on
the position of the wells. The depth to water in a single well is
said to vary from 1 or 2 meters to 15 meters; in other words, the
water-table of the valley varies 13 to 15 meters between the dry and
the moist seasons. No analysis of the water is available, but it is
reputed to be noticeably saline near and below the town of Ghardaia,
while above the town this quality is not apparent to the taste. The
water relations of the plain are much less favorable for plants
than those of the M’Zab Valley. In addition to the fact that the
soil of this area receives only such water as falls directly on it,
there is so little depth that the water escapes shortly after it
falls, leaving only the most favorably situated soils, for example,
those beneath shallowly placed rocks, or between rocks, or in deeply
penetrating cracks or the depressions, with sufficient moisture for
long use by plants. The depth to water on the hamada is so great
that successful wells have never been dug.


                        THE OASIS OF GHARDAIA.

Each of the cities of the M’Zab has its palm gardens as well as
gardens in which grains of various sorts and vegetables are grown.
Intensive gardening is practised and the fruits of the soil, although
won with great labor, are nevertheless not inconsiderable.[21]

Perhaps the most palms are to be found about 2 kilometers above
Ghardaia, where they are so abundant as to form a small forest. Here,
in the most thickly planted portions, one finds a veritable jungle
in which the desert glare is softened by the spreading tops of
the palms and by the close canopy of grape-vines which reach from
one palm-stem to another. There is a second story of apricots,
peaches, almonds, and figs, and on the floor one finds a variety
of vegetables. Outside of the palm gardens, and adjacent to them,
are the plots in which cereals are grown. These gardens are divided
into diminutive fields, frequently not larger than 1 by 1.5 meters,
which are separated by small irrigating ditches and smaller laterals,
from which they are given water (fig. 19). Here barley, oats, and
wheat are raised, and often with them are planted carrots, turnips,
or other vegetables. The main ditches are rendered impermeable by
heavy coats of plaster, making it possible to use with the least
waste the difficultly acquired water.


                    THE PLAIN (HAMADA) OF GHARDAIA.

A superficial view of the plain (hamada) which lies both to the
north and to the south of the M’Zab Valley does not suggest
any vegetation whatever, at least during the dry season. The
desert is quite as barren in appearance, as, for example,
portions of the Libyan Desert are in reality. In every direction
one sees grayish-brown stones and bowlders, with little earth,
and in some places blackened stones, blackened by “fires from
heaven” the Arabs believe, but nothing to indicate the presence
of plants. Tristram has described the plain as “one mass of
naked rock, rough stone, and coarse débris, from the neighboring
mountain, but without a scrap of earth or a vestige of the minutest
vegetation.” But close study of the plain makes out a better case
than this; in fact, where the soil has accumulated in pockets, where
there is a slight drainage depression, or where spaces between the
rocks are filled with soil, careful examination shows the remains
of annuals and not a few living perennials; but like other intense
deserts, plants, even when relatively numerous, are not present in
sufficient numbers and not of sufficient size to give character to
the landscape or to hide the surface of the ground.

The plain on both sides of the valley was studied and a few areas
carefully examined with results which are summarized in the following
paragraphs.

It has already been mentioned that the walls of the M’Zab Valley
at Ghardaia are precipitous, rising between 60 and 100 meters from
the valley floor, their summits being the general level of the
plain. Both to the north and to the south of the valley there are
short but steep tributary gulches. In these gulches, and especially
at the heads of the gulches, are pockets filled with earth, and here
may be found some perennial vegetation. For example, at the head
of such a ravine, 3 kilometers north of the valley, 10 undetermined
living species were found, of which 6 were perennials and the balance
were long-lived annuals or biennials. (See fig. 20.) In an analogous
situation, but on the plain to the south of the valley, the aspect
being similar, a larger number of plants were found, including,
among other species, _Aristida_ sp., _Centaurea pubescens_,
_Deverra scoparia_, _Fagonia bruguieri_, _Peganum harmala_, and
_Teucrium pseudo-chamæpitys_. A census of plants was taken, where
the individuals were most numerous, with the following result:
On an area 16 by 16 meters there were 330 living specimens. The
three dominant species were _Aristida_ sp., _Deverra scoparia_,
and _Helianthemum sessiliflorum_.

On the level portions of the plain one sees almost no perennials and
only the dried remains of annuals, although here and there may be
found an isolated specimen of _Peganum harmala_ or even of _Citrullus
colocynthis_, the latter strangely out of its proper surroundings. In
one place, also, a small date palm was found surviving the extremely
arid conditions. But on the hamada it is only in relatively favorable
situations that plants are to be found. One such was given above
and another was found on the open plain, but near the base of a
low mountain, where there was a slight depression and where some
water was received from the mountain run-off. The area alluded to
is 10 kilometers north of the north valley wall; the south base of
the nearest mountain is 100 meters to the north of the area. The
ground inclines gently to the south, and rises slightly both to the
east and to the west. The surface is thickly strewn with stones and
the soil is clay mixed with sand, the latter predominating in the
center of the depression, where there is also a relatively large
proportion of small pebbles. The area studied, 16 meters square,
was so selected that the depression crossed the middle portion,
leaving the two sides as representing the larger part of the
plain. (Fig. 21.) In a country where the conditions of plant life
are so severe it is of interest to observe how slight advantages
of whatever kind, such as in the square under consideration, work
for the betterment of the vegetation. The dominating species was
a bunch-grass, probably _Aristida_ sp., but there was also present
_Haloxylon_ sp. (eaten to the surface of the ground by the passing
flocks) with other undetermined forms. On the area given 414 living
perennials were found with numerous dead annuals. All of the plants
were growing in the depression, there being, in fact, none on the
adjacent but somewhat higher parts of the hamada. The character of
the soil of this square and a discussion of the root characters of
plants growing in it are given in another place.

How far the paucity of plants on the plain is owing to the arid
conditions obtaining there, apparently a sufficient explanation in
itself, and how far to the fact that herbivorous animals, wild as
well as domestic, for several centuries have been gaining their food
from the plain, can not at present be well told. Observations given
below, however, indicate that if areas are protected against the
depredations of animals, the plants are noticeably more numerous and
of a larger size than when there is no protection. This conclusion
applies to portions of the plain as well as to the other habitats
under discussion.


                     THE MOUNTAINS ABOUT GHARDAIA.

The vegetation of the low mountains and of the rocky valley walls
is extremely meager, mainly on account of the small amount or total
absence of soil. In certain places (for example, near Melika) the
plants of the hamada descend the rocky gulch nearly to the floor of
the valley, and a similar condition has already been noted at the
heads of two of the larger gulches. In such places we find species
of grass and _Haloxylon articulatum_ mainly, but these species are
not typical of this habitat. Only two forms appear to occur on the
walls of the valley or on the mountains, and nowhere else. These are
the “kabar,” _Capparis spinosa_, and one or more crustaceous
lichens. (See figs. 22 and 23.) The kabar is a large shrub, 1 to
2 meters high, which bears persistent and fairly large leaves. The
shrub is uneaten by animals, owing to some disagreeable flavor,[22]
but is provided with small spines. The species is poorly represented,
there being but few individuals, and it does not exhibit exposure
preference, but grows in crevices between rocks, sometimes at the
base of the walls, or wherever it can attain a foothold. I have
seen lichens on the flat and horizontal upper surfaces only of a
ridge of low mountains about 4 kilometers north of the valley of
the M’Zab; search failed to reveal any on the north surface of
the mountains or on any rocks vertically placed; their position
would thus subject them to the greatest temperature ranges and to
the most intense aridity (fig. 26).


                       THE VALLEY OF THE M’ZAB.

The bottom lands, as already has been shown, are relatively favorable
for plant life; here the soil is the deepest and the water relations
are the best. Accordingly we should expect to find in the valley of
the M’Zab more plants than we have seen on the plain, and we will
not be disappointed in these expectations; but it is almost certain
that in primitive times the vegetation of the valley was even richer
than at present. In fact we now find in the unprotected places only
such plants as are too small for fuel or are not good for food, and
the useful sorts are largely wanting. In other and similar valleys,
which have not been so much disturbed by man as the M’Zab and where
primitive conditions still largely obtain, there is a surprising
wealth of vegetation. Such conditions were seen between Ghardaia
and Ouargla and will be specially noted on another page.

At present no trees occur naturally in the valley in the vicinity of
Ghardaia or any of the sister cities. The French portion of Ghardaia
contains ornamental or shade trees, such as the ash, sycamore, and
betoum. The largest native shrubs are a species of _Tamarix_, growing
by the oued, and a few specimens of _Zizyphus lotus_, the latter
confined to the side gulches and numbering half a dozen specimens.
Among the most generally distributed plants in the valley are
_Peganum harmala_ (figs. 27 and 28) and _Haloxylon articulatum_; the
former is a half-shrub of wide distribution in southern Algeria,
occurring from Biskra on the north, and although not strictly
confined to the flood-plains of the oued is most abundant where the
soil is relatively deep. The leaves are rather large and do not appear
to have unusual protection against drought.[23] Like its relative
in the southwestern portion of the United States, the creosote bush
(_Covillea tridentata_), it is not eaten by any animals, although not
armed and not poisonous. This species, therefore, is one of the few
which to-day probably retains essentially the same distribution and
appearance it had before the country was inhabited. It is interesting
to note that _Peganum_ is generally distributed through the M’Zab
Valley, being especially abundant between Ben Isguen and Melika. Here
in November _Peganum_, except where trodden under foot by the flocks
and caravans, retained much of its foliage, although rain was said
not to have fallen for twelve months. In the protected areas also,
as will be mentioned below, this species was found to be fresh green,
showing little or no indication of the long drought. Of other species
found in unprotected places in the valley, _Haloxylon articulatum_
and _Henophyton deserti_, although eaten by animals so as to be
recognized only with difficulty, were also fairly abundant. There
were found also _Euphorbia guyoniana_, called “le bain” by our
French-speaking Arab helper, because it is used by the natives as
a soap, and _Nolletia chrysocomoides_, _Æluropus_ sp., and others.


                    PROTECTED AREAS NEAR GHARDAIA.

Of the plant habitats whose leading characteristics have been briefly
given above, only the oasis and its gardens are secure against the
inroads of animals. However, owing to the long settlement of the
region, rather large tracts of land are at present in what must
nearly approach their primitive condition. The areas referred
to are the cemeteries, which, for the reason suggested, possess
special interest to the botanist, showing briefly what plants might
be expected to occur in the region naturally. The typical M’Zab
cemetery is of varying size and surrounded by a stone wall. No plants
are introduced to decorate it and no irrigation is practised within
its walls. The only disturbance of the natural condition of the
land is in the use for which it is set aside. The interments are so
conducted that the ground appears to be always used progressively,
that is, there is always an older portion and always a newer portion,
and the part once used is never afterwards made use of again. From
this fact, as well as others which need not be entered into, the
more ancient portion of the cemeteries, after a lapse of several
centuries, or even several decades, without disturbance, must be
in essentially the same condition as regards the soil and water
relations, which would be most affected by the fact of interment,
that they were in pre-M’Zabite times.

Several cemeteries near Ghardaia and the other M’Zab towns vary
greatly in their position as well as exposure; some are on the valley
floor below the town of Ghardaia, and others are in side cañons;
one is on the south wall of the valley with a northern exposure,
and another is on the opposite wall and hence with a southern facing;
one cemetery is on the edge of the plain itself. So far, therefore,
as the flora of the older portions of these areas represent the
ancient vegetation of the same areas, we have in them at present a
means of learning something of the kinds as well as the abundance
and the habits of the plants which formerly occurred here, and (by
inference) of the plants which were in the region in primitive times.

Below and not far from Ghardaia, in the valley floor, is a very
ancient cemetery, or rather a cemetery with a very ancient part. In
the old portion the drifting sand has obliterated all traces of
graves, which have long since been forgotten by the citizens of the
town. In the newer portion, farther from the edge of the oued and on
higher ground, the sand gives place to clay. In the older portion
of the cemetery may be found a fairly rich flora and rather large
plants—a striking contrast to the vegetation of the unprotected
area immediately without the wall. Here one finds _Haloxylon
articulatum_ and _Henophyton deserti_, both species eagerly eaten
by animals, as well as _Deverra scoparia_, _Lithospermum callosum_,
_Zilla macroptera_, and _Helianthemum sessiliflorum_; also grasses
and other plants which I did not know. Something of the abundance and
the large size of the plants is indicated in figs. 29, 31, and 32.

In one of the cemeteries situated against the south wall of the
valley, but not including the wall, the conditions are somewhat
different from those just sketched; the soil is a sandy loam, with
rocks of various sizes in abundance, and here may be found a fairly
rich flora. In the ancient portion of this cemetery the most numerous
species is perhaps _Haloxylon articulatum_, also _Fagonia glutinosa_,
_Fagonia bruguieri_, _Cleome arabica_, _Echinopsilon muricatus_,
_Helianthemum sessiliflorum_, _Zollikoferia resediflora_, _Salsola_
sp., and others. The plants are relatively abundant and of fairly
large size.

In a cemetery on the north wall of the valley, reaching from the
floor to the plain above, the wall is less precipitous than at other
places and there is a small amount of earth. The number of species
here is very limited, being confined almost wholly to _Haloxylon
articulatum_, which is fairly abundant; but in the upper portion of
the cemetery are also found _Peganum harmala_ and _Capparis spinosa_.

The cemetery situated wholly on the edge of the plain has an
unexpectedly large number of plants, almost all of them _Haloxylon
articulatum_, which is of good size. Outside this protected area the
species is neither large nor abundant, since it is eagerly sought
after by camels, sheep, and goats, and a shoot no sooner appears
than it is eaten to the base.

The sentiments of the residents of Ghardaia, which led to the
establishment and protection of the cemeteries, made a close
botanical study of them injudicious. Enough was seen, however,
to establish several points, the most important being, at least
for the areas considered, that there is growing in them, without
irrigation, a somewhat rich flora composed of relatively large
perennials. And from this fact it seems probable, if also protected
against the predatory attacks of animals, that other areas under
the present rainfall and other physical environmental conditions
now obtaining would support a much heavier vegetation than is
generally the case. How far the presence of man and of his flocks has
otherwise modified the flora, especially as regards its composition,
is another question, but it certainly has not been without its
effect. In the portions of the M’Zab region, except certain
areas rather remote from the towns, where there is no protection,
the only forms which are at all abundant, or at least conspicuous,
are such as are armed, poisonous or distasteful, or too small for
use as fuel. Among these are _Peganum harmala_, _Zilla macroptera_,
_Tamarix_, and others seen later. However, _Haloxylon articulatum_,
although eaten by all animals so as never to develop in a normal
manner, is surprisingly abundant, although by no means conspicuous.


                  ROOT-HABITS IN THE GHARDAIA REGION.

Owing to the small amount of soil, close observation of the
root-systems of the plants growing on the mountains and the plain
was difficult; examination of the roots in the field was therefore
carried on mostly in the valley, although enough was seen of the
roots of plants in the other habitats to permit a characterization
of them. The roots of several species growing on the hamada close to
the valley were examined with the following as the leading results:
_Deverra scoparia_ was found to have a main root running directly
downward 20 cm. without giving off large laterals; at that depth it
forked, the resultant branches running thereafter in a horizontal
direction. (See fig. 33.) _Teucrium pseudo-chamæpitys_, _Centaurea
pubescens_, and _Salvia ægyptica_, all from the hamada to the north
of the M’Zab Valley, have pronounced main roots. A similar type of
root was seen in _Zollikoferia resedifolia_ and _Fagonia bruguieri_
from the plain to the south of the valley. Grasses growing on the
plain had roots which, as usual with grasses, showed more diversity,
but on the whole penetrated rather deeply. The root-system of
_Haloxylon articulatum_ is of the modified generalized type,
penetrating deeply also, and will be best described as an inhabitant
of the valley.

From these observations it would appear that the roots of most of
the plants growing on the plain have a well-developed main root,
and that few, if any, perennial roots lie near the surface of
the ground. This condition is rather different from that seen
at Laghouat, where the root-system of typical inhabitants of a
similar habitat is of the strictly generalized type and may point
to a difference in some character of the habitat—for instance,
the precipitation at Ghardaia.[24]

In certain portions of the floor of the M’Zab Valley the soil
conditions favor full and normal development of all forms of
root-systems, but in other portions, because of the presence of an
impervious hardpan, such development is not possible. Where there
is hardpan the presence of species with obligate tap-roots is
precluded, while such as have a more flexible root-system (e.g.,
_Peganum harmala_) can to a degree accommodate themselves to
the unfavorable soil conditions. However, one instance was seen,
which will be reported below, where a plant with a tap-root was
found growing on a hardpan stratum so hard as to be excavated with
the greatest difficulty. The character of the root is very greatly
modified by the presence of the hardpan. Following is a sketch of
the root-systems of a few plants growing naturally in the valley.

_Peganum harmala_ is one of the most characteristic plants of
the floor of the M’Zab Valley; so far as my observations go, it
usually occurs where there is much soil and where the water relations
are the most favorable. Several studies of it were carried out on
plants growing in different parts of the valley. In order to make
the leading study of its roots, a typical habitat was selected east
of Ghardaia and nearly in the midst of the valley. Here the upper
soil, to a depth of about 20 cm., is a fine sand carrying water-worn
pebbles and coarse stones. Below the sand is a denser stratum a few
centimeters in thickness, but not the hardpan described in another
place. Below the harder stratum, to an undetermined depth, are
mingled sand and gravel coarser in texture than the superficial sand.

The first specimen of _Peganum_ studied in this habitat was about
30 cm. high and in full leaf. Its roots were fibrous, that is, they
were not fleshy. The root-system consisted of a main root, which ran
directly downward 17 cm., and several laterals. Probably owing to
the influence of the hard stratum, the main root at length turned
abruptly and maintained a horizontal course for 1.5 meters. It was
1.35 cm. in diameter at the crown and 2 mm. in diameter where left,
at a depth approximating 27 cm. The main root gave off a lateral
5.5 cm. from the surface of the ground, which also branched; the
ultimate branches followed a fairly level course for 45 cm. At the
sharp bend of the main root another branch arose and this branched
in turn, the daughter branches going somewhat downward for over
43 cm. Branches from the latter roots descended to within 8 cm. of
the surface. (See fig. 35.)

Differing in details, the roots of other individuals of the same
species in the same habitat have on the whole a root-system
essentially like the one just sketched. Even in relatively or
actually deep soil the roots of this species do not penetrate deeply,
but reach rather widely. They are of the generalized type,[25]
similar in many ways to the root-system of _Covillea tridentata_
of the Tucson region. When growing in a habitat where hardpan comes
close to the surface, the main root is not so well developed, but
there is a better development of laterals than in the specimen more
favorably situated as regards soil, in the habitat above alluded to.

Growing in or near the habitat of _Peganum harmala_, whose
root-system has been sketched, were other species whose roots
were also examined. Among these were _Haloxylon articulatum_ and
_Euphorbia guyoniana_; the former is one of the native plants most
eagerly sought by animals for food, for which reason it was not
found possible to secure for study specimens whose shoots were
entirely normal; but the plants finally chosen were the least
damaged of any found outside of the protected areas. The shoot
of the specimen of _Haloxylon_ examined was about 50 cm. high;
its gnarled base showed that it had been subject to intermittent
attacks by animals. It was found to have a main root which ran
directly downward more than 30 cm.; at a depth of 10 to 15 cm. a few
small laterals took their origin. (Fig. 36.) The other specimens
studied showed the same type of root-system, so that _Haloxylon_
has here a well-developed main root.[26]

Growing not far from the two species whose root-systems have
just been sketched, and under similar soil conditions, were
several specimens of _Euphorbia guyoniana_, whose roots were also
examined. This plant has a habit of growing in groups whose members
are more or less widely separated. The first specimen studied was
one of a colony of a half-dozen whose habit of growth is shown in
fig. 38. Its shoot was about 15 cm. high and bore several narrow,
smooth leaves, and was 2.5 mm. in diameter at the base. The shoot
was found to go directly downward until it joined a horizontally
placed fleshy root, from which the other individuals of the colony
were seen to take their origin. This, apparently a root-stock,
was 8 mm. in diameter and gave off two branches, 3 and 5 mm. in
cross-section, which were also horizontally placed.

On the southern side of the oued, where the soil is cemented into
a sort of hardpan, _E. guyoniana_ grows in greater abundance,
frequently in groups but also singly. Other observations confirm
those above reported, namely, that the species reproduces largely
vegetatively, new plants springing from old roots, which at
Ghardaia are essentially water- and food-storage organs. Only two
other species were seen to have water-storage capacity, namely,
_Citrullus colocynthis_, to be described directly, and _Phelypæa
violacea_, which was seen at Biskra and will be described later.

In the habitat of _Peganum_, and a few meters distant, was found
a specimen of _Henophyton deserti_, which had been little injured
by flocks. The shoot of the plant was about 30 cm. high, and,
in spite of the long dry season, was in full leaf. It possessed
a long tap-root, of which 75 cm. was recovered. No laterals were
given off along the portion of the root seen. Other specimens of
the same species were also examined, and in every instance a similar
type of root was found.

Somewhat nearer the side of the valley, but growing where there was
considerable depth of sand, were several specimens of _Tamarix_,
the roots of one of which were partly exposed. The plant studied
was over 3 meters high and had not been injured by animals. Its
root was of the tap-root type, since the main root went unbranched
directly downward.

There occur in the valley a few specimens of _Citrullus colocynthis_
described by Schimper, in “Plant Geography,” as follows:


A cucurbitaceous plant resembling our cultivated pumpkin, and its
long, juicy, relatively thick-foliaged and large-leaved shoots remain
green throughout the summer, producing fruits as large as a child’s
head. It presents, therefore, the appearance of being protected in
an unusual manner against the loss of water. As a matter of fact,
however, severed shoots dry up in a few minutes. The extraordinary
length of the roots of colocynth alone renders its existence possible
in the desert.


While the description of Schimper is a good one, it conveys the
idea of much greater luxuriance of growth than was observed for
the species at Ghardaia. As a matter of fact, the leaves of this
plant are small and much dissected, and the fruit is the size of an
orange. How deeply the roots penetrate the ground, or their length,
was not learned, but a very striking characteristic of the root is
its fleshiness, which is shown by fig. 37, and the ability of the
species to successfully withstand drought may lie in the fact that it
is thus provided with a very well-developed and very well-protected
water-storage organ, as much as in having a great length of
root. _Citrullus_ occurs typically where there is considerable
depth of earth, especially close to oueds, although it is rarely
to be found on the plain and only where the soil is deepest. In
the latter habitat the possibility of a permanent connection with
a perennial water-supply is without question excluded. The species
is rather to be considered one of the forms, rare to the Sahara,
which possess a water-balance, and which has the power of storing
sufficient water during the widely separated rainy seasons to last
it during the period of drought. It can be pointed out here that
plants having hypogeous water-storage organs have a very different
relation to the climate of the desert than such as have such organs
above ground. The inclosing soil is a protection, nearly perfect,
against drying. Where the soil is removed the protected parts
quickly become dry.[27] Given two species, both having water-balance,
but one with the storage epigeous and the other hypogeous, other
conditions being equal, the latter should survive under more arid
conditions than the former. It is rather surprising, therefore,
that there are not more of the latter type in the Sahara.

A few measurements of the root of a typical _Citrullus_ from the
M’Zab Valley will give a concrete idea of the water-storage
capacity of the species. A specimen growing in the oued near
Ben Isguen was selected for examination (see fig. 37); it had a
luxurious shoot and several fruits. The shoot was found to arise
from a large and aged root 26 cm. in diameter at the crown, but
tapering rapidly, it was only 12 cm. in circumference at a point
14 cm. beneath the surface, where it forked, giving off one branch
which was 1.7 cm. and another 1.8 cm. in diameter; several smaller
ones took their origin close to the surface of the ground. In its
general appearance the main root of _Citrullus_ is very like that
of _Cucurbita digitata_, which grows on the domain of the Desert
Laboratory and in a similar habitat.

Up to this point the plants whose roots have been described were
found growing where the soil was favorable to a fairly normal
development, but the soil of the valley is not all of this character,
and where hardpan is present the substratum is often extremely
hard and, one would think from inspection, impermeable to water
as well as impenetrable to roots. In such habitats the number of
individuals and also of species is very naturally limited.

About 2 kilometers west of Ghardaia is an area where the hardpan
reaches nearly or quite to the surface. Here _Peganum harmala_
is to be found, since its generalized type of root-system is
capable of not a little variation, adjusting the species to a
variety of soil conditions not otherwise possible. Associated with
_Peganum_ was a single specimen, of dwarfed and badly eaten form,
growing out of the hardpan itself. The position of the plant was
so unusual that its root-system was in part excavated. The form,
an undetermined chenepod, was found to have the exceptional form of
root, thus proving the rule just suggested, for it had a well-marked
tap-root. A gnarled main root was found to take a zigzag course
through 27 cm. of hardpan to the softer stratum beneath. The root
did not follow a crack, but struck boldly downward through soil so
hard that it was removed by the use of a sharp iron instrument and
only with great difficulty.


                  LEAF-HABITS IN THE GHARDAIA REGION.

Observations of the leaf characters of the desert perennials, as
shown in November, offer some points of interest, especially since
there had been no rain for a year previous to the visit. As would be
expected, the leaf-habit is various, ranging from scale-like or none
to fair size. Several species appear to be evergreen, including
_Artemisia herba-alba_, _Capparis spinosa_, _Echium humilis_,
_Fagonia bruguieri_, _Gymnocarpon_ sp., _Haloxylon articulatum_,
_Helianthemum sessiliflorum_, _Henophyton deserti_, _Herniaria
fruticosa_, and _Salsola_. Certain plants probably, as _Henophyton_,
are facultative evergreens, retaining the leaves if the season is
moist and dropping them if it is excessively dry; several of this
genus were seen without leaves, but with green stems. (See, also,
figs. 39, 40, and 41.)

Some idea of the variation in size of the leaves of a single species,
as well as the average size of the leaves, will be had from the
results of a few measurements. The leaves of _Henophyton deserti_
range in length from 2 to 3.1 cm., and in breadth from 0.2 to 0.5
cm. The average length of 12 leaves from a single branch 7 inches
in length was found to be 2.82 cm.; the average breadth was 0.35
cm. On another branch, 23 cm. long, were 35 mature leaves which
averaged almost exactly the size of those just given. Since these
leaves are rather numerous and of good size, it will appear at once
that the species has a relatively large leaf-surface, even if the
area can not be stated more definitely. (Fig. 30.)

The evergreen shrub _Capparis spinosa_ has the largest leaves of
any plant native to the Ghardaia region, and probably of southern
Algeria. Its leaves are bilateral and nearly round and are placed
upright on the branches; a series of 29 leaves from one branch 33
cm. long varied from 2 to 3.2 cm. in length and slightly less in
width, the average being, length 2.9 cm., breadth 2.2 cm.

_Euphorbia guyoniana_ has a deciduous leaf-habit. Its leaves are
rather small but numerous. A shoot 22 cm. long bore 40 leaves
varying in length from 1.5 to 2.1 cm. and in breadth from 1.2 to
2.0 mm., with the average length and average breadth 1.7 cm. and
1.6 mm. respectively.

It is worth noting that the surface of the leaves, as shown by
inspection, is, possibly, most often smooth, as, for example, in
_Capparis_, _Henophyton_, and _Haloxylon_. Dense hairiness is an
exception among the species seen, _Lithospermum callosum_ being
the only plant striking in this regard.


          GROWTH AND FLOWERING HABITS IN THE GHARDAIA REGION.

Upon arriving at Ghardaia in November, it was surprising to find
several species of perennials, under strictly desert conditions,
putting on new growth, forming fresh leaves, or coming into
flower. Especially was this unexpected in view of the fact that
no rain had fallen in the region for twelve months. A study of the
roots in relation to the depth to water showed also that most of the
native plants, during the dry season, could not have penetrated to
a depth anywhere near that of the water-table of the valley, and the
water relations of the plants growing on the plain above the M’Zab
Valley were even more severe. Further, most of the species do not
possess water-storage faculty. It should be noted that the only
apparent difference in the water relation between November and in
early autumn, or summer, was that of raising the relative humidity
of the air through the lower temperature. In a preceding section it
has been stated that little or no evaporation took place at night
in November at Ghardaia, and possibly less in the daytime than
would have been supposed. The leading environmental changes were,
of course, the lower daily temperature and the really cool nights.

It is not uncommon in the Arizona desert for a species to form
flowers or leaves, following a change in temperature, the moisture
conditions being not otherwise changed, but, so far as I know,
the temperature changes bringing about this result are always from
a cooler to a warmer condition and not the reverse. It would not
be expected, consequently, that in the present instance a renewal
of vegetative activities would follow as a _direct_ result of a
lower temperature, although analogous changes are necessary before
certain species, after rest, will start development. Whatever may
be the immediate cause of the renewal of activity on the part of the
plants at Ghardaia as noted, the following species were seen to have
formed new leaves: _Henophyton deserti_ and _Zilla macroptera_;
also the following fresh flowers with or without shoot growth:
_Fagonia bruguieri_, _Haloxylon articulatum_, _Henophyton deserti_,
_Ononis polyclada_, and _Zollikoferia resedifolia_, and three other
species not determined.




                       =GHARDAIA TO TOUGGOURT.=


From Ghardaia the route, consisting of camel trails only, pursued
a course south of east to Ouargla, and from Ouargla a direction
east of north to Touggourt, over 400 kilometers, Ouargla being about
half-way. There are no villages between Ghardaia and Ouargla and none
between this place and a point 20 kilometers south of Touggourt,
so that in accounts of the vegetation or the topography, lacking
convenient points around which to center descriptions, the device
will be adopted of using distance estimations to or from the three
chief towns.

The trail followed the valley of the Oued M’Zab, or kept close to
it, for 63 to 73 kilometers before finally leaving it. It passed
the sister towns of Ghardaia and crossed short intervals of plain,
descending occasionally to the oued. The walls of the M’Zab
Valley, 60 meters more or less at Ghardaia, become lower and
less precipitous as one goes down the drainage, until at length
they become little more than rounded banks. The low, flat-topped
mountains, which are a feature of the topography about Ghardaia,
were soon left behind, and nothing similar was encountered until
the vicinity of Ouargla was reached. Between Ghardaia and El Ateuf
are small dunes in the valley and sand is drifted along the base of
the walls at various places. Near and immediately east of this town
the sand is especially abundant and, being shifted by the winds,
constitutes an ever-present menace to the small gardens belonging
to the inhabitants of the place; in order to control its drifting,
fences of palm leaves are made or the sand is removed when it becomes
too abundant. Often the gardens are abandoned, leaving the palm
fences to mark their sites after the sand has gone beyond. Where
the air-currents are most powerful or most consistent, or the walls
are broken down, the sand may be carried in small quantities onto
the plain, where it constitutes a mulch, influencing in a striking
manner the character of the vegetation (fig. 42).

Finally leaving the valley of the M’Zab about 73 kilometers from
Ghardaia, the trails wound upwards through low rounded hills to the
hamada. This is the northern edge of the region of the Gantara, 100
by 150 kilometers or more in extent, reaching from the valley of the
M’Zab on the north to the region of the dunes to the south. It
slopes towards the Oued Igharghar, or the drainage depression
connected with this great oued. The Gantara has a few chotts and
is crossed by three oueds in the southern portion. It is probably
the most arid part of southern Algeria. About 60 kilometers of the
plain were crossed and here it was gently rolling and stretched
without a break to the horizon. The surface resembles that of the
hamada at Ghardaia, that is, stones of various sizes, usually small,
lie on its surface, but never forming a continuous cover, as in some
portions of the Arizona desert. The soil is brown, of fine grain,
and with little or no addition of sand. In the innumerable little
hollows the soil is deeper than on the slight rounded ridges. Wind
is apparently the most potent erosive agent.

Two chotts were encountered between Ghardaia and Ouargla, one
unimportant, the other large and with many features of interest. The
latter, the Chott Mellala, is about 10 by 15 kilometers in size. The
trail descends from the plain, winding through a zone of rounded,
cone-shaped hills or mamelons, to the floor of the chott, which lies
about 60 meters lower than the general level of the plain. The chott
was quite dry in November when we visited it, but at rare intervals
water is said to flood the central portion. Toward the outer edges
the floor is thrown into waves, where the heavy incrustation of
salts is broken. In the center the salt crust forms an unbroken
and level surface. Gypsum (calcium sulphate) is the predominant
salt. On the eastern side a long and high ridge of sand rears its
uneven summits. The height of this ridge was estimated to be 250
meters, and was said by Massart to be the largest seen by him in
the Sahara. This dune we had seen lying on the eastern horizon for
one or two days before reaching the chott.

The relation of the Chott Mellala to the country to the north or
the south was not seen, but between it and Ouargla there lies a
succession of smaller and more irregular chotts, which together
form a fairly well-connected chain. These chotts are separated
by low passes and flat-topped hills whose summits are on a level
with the neighboring plain. Many of the hills are cone-shaped
and in other topographic features the region shows the eroding
action of wind. About 7 kilometers from Ouargla an opening in the
mamelons gives a view of a plain extending on a lower level to the
horizon. This is the reg, or fluvial desert. The Ouargla plain, or
reg, is connected with the drainage of the great Oued Ighaghar and
has a character which in many ways is different from the Gantara,
over which we had just passed.

With the descent to the reg desert a more monotonous region is
encountered. In the vicinity of Ouargla and for some kilometers to
the north the topography is quite flat and gives the impression of
a flood-plain. To the east it stretches unbroken to the horizon,
but to the west it is bounded by a fairly abrupt wall, the Gantara
escarpment. At intervals of several kilometers low sand ridges cross
the route over the plain, and on the second day somewhat higher
ground was traversed and a sand ridge about 4 kilometers across
was encountered. The country then becomes somewhat more broken and
presents the appearance of being the remains of an ancient and more
elevated plain. About 56 kilometers from Ouargla are the largest
dunes crossed; where traversed, these were 10 kilometers from north
to south and extended beyond our vision both to the east and the
west. This is apparently the edge of extensive dune regions which
lie mainly east of Touggourt and of the Oued Rirh. For possibly the
last 30 kilometers of the journey to Touggourt there are dunes and
chotts in alternation.

A word should be said regarding the hydrography of the region whose
surface features have been sketched above. Between the cities of Beni
M’Zab and Ouargla two wells were passed, although a route could
have been taken which could have included three wells. The wells are
125 meters or less in depth and are maintained by the government for
the benefit of the caravans, as well as to provide water for the
large number of goats and sheep pastured in the neighborhood. The
situations of the wells are always in depressions, either along the
Oued M’Zab or in similar although smaller drainage areas, and none
are on the Gantara. At Ouargla and on the reg desert to the north of
the town the water lies very close to the surface of the ground. It
can be dipped with buckets and the roots of the palms reach to the
water-table. The water from the shallow wells is strongly impregnated
with salts. Before reaching Touggourt standing water was seen where
the trail crossed certain chotts. Numerous artesian wells have been
made by the government which penetrate the ground several hundred
meters and give a large and continuous supply of sweet water.


                    GHARDAIA TO OUARGLA—VEGETATION.

The plants seen during the first day’s march from Ghardaia
were such as have already been observed to be characteristic of
the valley of the Oued M’Zab or of the neighboring hamada. The
vegetation of the hamada, usually very sparse, was noticeably more
abundant wherever the sand had been drifted over it from the dunes
of the valley, even if the thickness of the sand was so slight
as to be little more than a mulch. Here low grasses, much eaten,
were the prevailing forms. On the dunes in the valley of the
M’Zab, 20 kilometers from Ghardaia, the number of species and
individuals is relatively large, the most abundant species being
drinn (_Aristida pungens_), although _Deverra scoparia_ is also
fairly numerous. Somewhat farther on the route, and in a sandy flat,
besides these two species, there is much _Ephedra alata_. On the
slopes leading from this flat and on the plain above, there is an
almost pure stand of _Rhantherium adpressum_.

Crossing relatively small portions of the plain, in place of
always following the bends of the Oued M’Zab, about noon of
the second day we reached the bordj Zolfana, in the valley of
the Oued amidst low and narrow dunes, which are moving slowly
across the flats. (Fig. 44.) No vegetation appears on the dunes,
but on the fixed sand between them, or on the stationary dunes
at the border of the flats, some plants are to be found. Among
these the most abundant, but really not numerous, are _Euphorbia
guyoniana_ and retam (_Retama retam_), which was often seen later
along the line of march as well as in the vicinity of Biskra. Retam
superficially resembles _Ephedra alata_ in having rudimentary leaves
and green, reed-like branches; it is carefully avoided by animals,
although _Genista saharæ_, a very similar plant, which grows in
like situations between Ghardaia and Ouargla, is said by Massart
(_loc. cit._) to be eaten greedily by them. (Fig. 43.)

A short distance beyond the bordj the bottoms suddenly widened,
the dunes disappeared, and for the remainder of the day’s march
we passed through the richest vegetation we had seen since reaching
the M’Zab region. (Figs. 45, 46, and 47.) Here the shrubs were
of fair size and of sufficient abundance to give character to the
landscape. About the 63 kilometers camp the leading species are
_Retama retam_, _Ephedra alata_, and _Haloxylon schmidtianum_. On the
hamada adjoining the flat are several species, including _Aristida
ciliata_, _Artemisia herba-alba_, _Farsetia ægyptiaca_, _Farsetia
linearis_, _Gymnocarpon fruticosum_, _Helianthemum eremophilum_,
_Henophyton deserti_, _Marrubium deserti_, _Salsola vermiculata_,
_Teucrium polium_, _Thymelæa microphylla_, and _Zollikoferia
mucronata_. On the hamada just adjoining the place of our camp,
however, there appeared to be _Haloxylon schmidtianum_, to the
exclusion of other species.

After crossing the bottoms of the oued the trail climbed up to
the hamada and we did not see the M’Zab Valley afterward. The
vegetation of the hamada soon becomes very sparse and as far as the
eye can reach the appearance is that of entire bareness. But, as
was found to be the case near Ghardaia, close examination revealed
the presence of many living perennials as well as the remains of
the previous annual flora. This plain, the Gantara, of wide extent,
is the most arid region seen in southern Algeria. Massart estimates
that there is an average precipitation of 15 cm. on the desert,
but, from data previously cited, it would appear that this amount
is rarely attained; indeed, several months, or even two or more
years, may pass without any rainfall whatever. When one searches
the hollows he finds a few small perennials; plants are almost
wholly absent on the low ridges. But in some areas on the plain,
where superficial examination does not show any plants, a surprising
number were found. For example, on the level hamada, 96 kilometers
from Ghardaia, on an area 16 meters square, 389 living plants were
found, but only 24 were so large as to be seen from a distance;
all were either eaten badly or trampled to the ground, so that it
could not be learned what the flora of the area might have been
had no animals interfered with its full development.

After leaving the 96 kilometer station the perennials were seen
to diminish rapidly in numbers and to decrease in size, until the
ridges of the low undulations were absolutely without plants, and
there were but few in the hollows. There was no apparent change in
the character of the hamada or in that of the soil. This sterile
condition persisted for 26 kilometers, when vegetation similar in
character to that previously seen was again encountered. The zone of
better vegetation lasted for 3 kilometers, when the country became
barren once more, which condition lasted for 10 kilometers. The
presence of barren belts on the Gantara, where the plants at the
best are insignificant in size as well as number and without change
in topography or soil, points to an especially arid belt. This
conclusion is further strengthened by the observation that the
plants found were relatively of very small size.[28]

The flora was also sparse upon the route followed by Massart across
the Gantara, which was apparently somewhat farther to the north than
the one now being described. He mentions having found _Argyrolobium
uniflorum_, _Asteriscus graveolens_, _Fagonia microphylla_,
_Deverra chlorantha_, _Fagonia glutinosa_, _Halogeton alopecuroides_,
_Helianthemum_ sp., and _Herniaria fruticosa_. He says that _Deverra_
is one of the rare glabrous forms on the hamada. It is said to have
the odor of parsley, and the Arabs have a belief that camels which
eat it become blind, but Massart’s camels were not injured by
eating the plant.

As we drew near the Chott Mellala, on the eastern edge of the barren
zone mentioned in the preceding paragraph, we suddenly encountered
a belt of _Ephedra alenda_, stretching to the north and to the south
as far as could be seen (fig. 49). On one side was the barren zone,
on the other the immense plantation. Upon examination it was found
that the _Ephedra_ was the only species of perennial. It had several
points of interest, but the short time at our disposal precluded
more than a superficial examination. Usually keen about the desert
species, it is curious to note that the Arabs appear not to recognize
this, called by them “alenda,” as being related to the larger
species of _Ephedra_ with which they are well acquainted. It is a
small species, growing from 30 to 40 cm. high. It does not occur
singly, but has the habit of growing in groups of a half dozen or
more. Between the _Ephedra_ colonies were only the dried remains
of the annual flora of the last rainy season. An examination of
the root-system of the species showed that, like other species of
the genus, it has a well-developed tap-root (fig. 50).

_Alenda_, however, has a root-habit which, although not peculiar
to it, is at least very striking and of great importance to its
survival. From the root-crown a stolon arises which extends away
from the parent for a distance of about a meter. From this stolon
there arise shoots which develop into daughter plants. Through this
method of reproduction the small colonies of the species are formed
and possibly the species mainly multiplied. From the stolons small
roots arise, giving aid to the mother root in providing the offspring
with moisture. This habit is very like that of _Kœberlinia spinosa_
of the Arizona desert, which has a similar type of root-system
and which reproduces vegetatively in a similar manner. _Alenda_
continued to be the dominant species until near the edge of the big
chott (Mellala), a few kilometers east of where it was first seen.

Practically no plants were seen when descending through the eroded
portions of the hamada to the floor of the chott (fig. 52), but on
reaching the bottom of the great chott a surprisingly large number
was observed, including _Anabasis articulata_, _Aristida pungens_,
_Ephedra alata_, _Limoniastrum guyonianum_, _Retama retam_, and
_Traganum nudatum_ (fig. 51). In the wide central part of the chott,
where the salts are perhaps most dense, there are no plants; but on
the eastern side are _Euphorbia guyoniana_, _Anabasis articulata_,
and _Zygophyllum_ sp. These small species were growing far apart and
were badly injured, either by being trodden under foot or by being
eaten by the passing animals. To the south of where we crossed
the chott and also to the north the sand mountains arise. After
leaving Chott Mellala we ascended gradually to go over a low pass
separating it from a small chott to the east. Here we obtained
a backward view of the sand mountains, low as seen from the
pass, with sharp, wind-made ridges, and bearing a few specimens
of _Aristida pungens_. Descending somewhat, another but smaller
chott was crossed. Between the latter chott and the Ouargla plain
the plants were very few and confined to the slopes and the higher
ground, avoiding almost wholly the depressions. Of those recognized,
_Traganum nudatum_ was the most abundant. This is the “vamran”
of the Arabs and is an inconspicuous shrub frequently seen later
on the way to Touggourt.




                  =OUARGLA TO TOUGGOURT—VEGETATION.=


Ouargla is an ancient, rambling town, somewhat in decay, set in
the midst of extensive palm gardens. There are said to be 500,000
date palms at the oasis. It was founded in the tenth century by the
M’Zabites and later taken possession of by the Arabs. The town
is peculiar in its situation and its gardens, lying, as before
described, in the flood-plain of an ancient river. The plants
cultivated in the town as ornamentals are fewer than at Ghardaia,
but of the same kinds, and (besides dates) the gardens contain
fewer fruits and apparently fewer kinds of vegetables. Between the
gardens one finds _Tamarix_ in some abundance.

Leaving Ouargla by the western gate and turning north, we soon
passed through the crooked streets and reached the reg desert to
the north. There is little vegetation near Ouargla, but on some
low dunes extending over the reg we found _Aristida pungens_ and
_Phragmites_ sp., growing in a hollow. On the flood-plain there
were a large number of individuals and probably a large number of
species. Among the most conspicuous of the shrubs were _Retama retam_
and _Limoniastrum guyonianum_, the “zaita” of the Arabs. (See
figs. 56 and 57.) Zaita is a handsome shrub with cylindrical leaves
often covered with a fairly heavy salt incrustation. So abundant
is the exudation that in localities where the species is especially
abundant, as at our camp 32 kilometers south of Touggourt, the plants
have the appearance of being covered with snow. Although, like many
other species between Touggourt and Ouargla, zaita can live where
there are salts in excess, it appears not to be an extreme type of
halophyte and does not occur where the salts are most dense. It
was seen both on the dunes and on the low lands. Other common
halophytes are “souid” (_Salsola tetragona_), “belbel”
(_Anabasis_ sp.), and _Halocnemon strobilaceum_, which appears
especially resistant. (See figs. 59, 60, 61, and 62.)

Other forms appear on the higher ground and on the plain about
50 kilometers from Ouargla, where _Ephedra alata_ especially
is common. There are no dunes on the plain, but some sand swept
across it from the fluvial desert to the west passes eastward and
augments the dunes farther east. The larger specimens of _Ephedra_
are rather effective sand-binders and bring about the formation
of diminutive dunes. The effect on the growth of the species by
the piling sand is peculiar. As the sand accumulates it covers the
lower branches of the shoot; these are stimulated to unusual growth
and new branches may spring from them. Thus the effect is similar
to that habitually occurring in _E. alenda_, but in _E. alata_
the habit is not a fixed one. As an instance of the length which
such submerged branches may attain, it may be mentioned that one,
2 to 3 cm. in diameter, was over 4 meters long.

About 75 kilometers from Ouargla, and on a plain similar to that
just referred to, “dhamran” (_Traganum nudatum_) was the most
common species, but was by no means abundant. The census of an area
16 by 16 meters, taken in a locality where dhamran was dominant,
resulted in finding 31 living and a few dead plants, probably all
_Traganum_. It is to be understood that the vegetation of the reg
desert not far distant is much richer, not only in species but
especially in individuals, and also that the plants are much larger.

From 56 kilometers to about 71 kilometers from Ouargla the route
lay over more or less continuous dunes, where the leading species
seen were _Aristida_ sp., _Ephedra_ sp., _Euphorbia guyoniana_,
_Limoniastrum guyonianum_, and _Traganum nudatum_.

In or about the edges of the chotts, which are the leading
topographical features of the region immediately south of Touggourt,
the most common species are _Haloxylon_ sp., _Salsola tetragona_,
_Limoniastrum guyonianum_, and _Arabis aphyla_. Besides these forms,
about 28 kilometers south of Touggourt fine specimens of _Halocnemon
strobilaceum_ were seen growing in an extremely salty situation,
to the total exclusion of other species. _Tamarix_ also is to be
found in and about salt spots near Touggourt. (See fig. 64.)

Both Massart and Doumet-Adanson (Bull. Soc. Bot. France, 39, 1892)
have discussed the flora of the Touggourt-Ouargla region. The
following exhaustive list is given by the latter author as having
been collected between the two desert towns:

  Henophyton deserti.
  Matthiola livida.
  Malcolmia ægyptica var. linearis.
  Moricandia cinerea.
  Sisymbrium pendulum.
  Savignya longistyla.
  Reseda stricta.
  R. arabica.
  Helianthemum sessiliflorum.
  H. ellipticum.
  Randonia africana.
  Frankenia pulverulenta.
  Silene nicoeensis.
  Monsonia nivea.
  Erodium glaucophylum.
  Fagonia glutinosa.
  F. frutescens.
  Polycarpæa fragilis.
  Gymnocarpus decandrus.
  Retama retam.
  Genista saharæ.
  Astragalus gumbo.
  Anthyllis henoniana.
  Neurada procumbens.
  Ammodaucus leucotrichus.
  Mesembryanthemum sp.
  Deverra chlorantha.
  Nolletia chrysocomoides.
  Senecio coronopifolius.
  Anthemis monilicostata.
  Tanacetum cinereum.
  Ifloga fontanesii.
  Centaurea purpuracea.
  Amberboa omphalodes.
  Rhantherium adpressum.
  Atractylis flava.
  A. prolifera.
  A. microcephala.
  Tourneuxia variifolia.
  Catananche arenaria.
  Spitzelia saharæ.
  Zollikoferia chondrilloides.
  Z. squamosa.
  Scorzonera undulata.
  Echium humile.
  Echiochilon fruticosum.
  Arnebia decumbens.
  Statice pruinosa.
  Limoniastrum guyonianum.
  Plantago ciliata.
  P. psyllium.
  Scrophularia saharæ.
  Linaria fruticosa.
  Euphorbia guyoniana.
  Atriplex dimorphostegius.
  Caroxylon tetragonum.
  Echinopsilon muricatus.
  Calligonum comosum.
  Haloxylon articulatum.
  Anabasis articulata.
  Cornulacea monocantha.
  Thymelea hirsuta.
  Th. microphylla.
  Aristida flaccosa.
  A. pungens.
  Cyperus conglomeratus.
  Ephedra fragilis.
  Erythrostictus punctatus.
  Scilla sp.
  Dipcadi serotinum.
  Asphodelus pendulinus.
  Ruppia maritimus.
  Chara fœtida.




        =TOUGGOURT TO BISKRA—PHYSICAL FEATURES AND VEGETATION.=


From Ouargla to Touggourt we have found that the country gradually
descends, the former place being 124 meters and the latter 77 meters
above the level of the sea. From Touggourt, also, for a distance of
about 120 kilometers, the descent along the route continues until
at the Chott Merouan a level of 6 meters is attained. The lowest
places in every case are of course the chotts and the connecting
oueds. The most important chott of this series is Melrirh,
11 meters or more below sea-level. This drainage system is the
northern culmination of the vast one of which the Oued Igharghar
is the most important part. In an earlier age water came north in
the oued from the highlands of central Sahara and poured into the
Chott Melrirh, having passed successively through the lesser chotts
farther south. At that time, also, the Chott Melrirh probably was
connected with the Gulf of Gabes.

From Touggourt to Chott Merouan, the lowest portion of the route
to Biskra, the topography is that of a region of chotts; that is,
there are salt spots surrounded on every side by higher ground,
which in many cases is of sand. From Chott Merouan the route
passes over a higher desert of a different character, which in
part bears a remote resemblance to the Gantara and in part to
the Ouargla plain. It is a vast plain, with little topographical
diversity, which rises to meet the Atlas Mountains to the west
and north. In the eastern portion it is somewhat rolling, stones
are strewn plentifully on its surface (hamada), and there has been
considerable erosion, so that gullies are formed. In the portion
nearer Biskra the surface is more level, the soil is fine (reg),
and there has been comparatively little erosion.

The soil of the chott region is largely of sand; on the hamada there
is much clay, while on the reg it is fine and easily blown by the
wind, and this in spite of the fact that the most vegetation seen
in the Sahara was in this region. A slight breeze picks up the
dust and carries it long distances in dark clouds. It fills the
throat, nose, and eyes of the traveler and makes crossing the reg
exceedingly disagreeable.

The plant life as seen along the portion of the route through
the chott region consists almost wholly of halophytes, as would
be expected, in addition to which, where for short distances the
hamada or dunes were crossed, there were forms characteristic of
such areas. The _Tamarix_ is especially common in the region and
_Limoniastrum guyonianum_ is also often met.

The date gardens of the Oued Rirh are justly famous. One passes
numberless plantations where the date is cultivated, and in
the neighborhood of each group of gardens one sees squalid Arab
villages. Over 19,000 tons of dates are said to be carried each
year by camel from the Oued Rirh to Biskra, whence they go to the
markets of the world. One day we passed 700 camels laden with dates
going to Biskra.

As soon as the chott region is left and the higher ground is
reached a different as well as a richer flora is encountered. Here
diversity of topography favors diversity of plant life. On the reg
near Biskra vegetation is especially abundant. Here, in fact, one
passes through large thickets of _Tamarix_ and other species, and
sees that the desert is much less intense than that in the south,
especially in the region of the M’Zab, where in many respects the
topography is similar. The kinds of plants also, as the list below
will indicate, are different in the main from those farther to the
south. Where the surface is most rolling we find _Tamarix_ sp. on the
heights, _Zizyphus lotus_ in the hollows, and the following grasses:
_Stipa tortilis_, _Hordeum maritimum_, and _Phalaris minor_. A
salsolaceous shrub (_Arthrocnemon macrostachyum_) may be found in
washes, _Nitraria_ and _Limoniastrum guyonianum_ on sandy places,
and _Odontospermum pygmæum_ and _Anastatica hierochuntica_ occur
between rocks. On clay flats one finds _Halocnemon strobilaceum_
and _Suæda vermiculata_, indicating the presence of salts.

The preceding notes are in part from Massart. The following species
are given by Doumet-Adanson as having been collected by him between
Biskra and Touggourt:

  Savignya longistyla.
  Eremobium lineare.
  Lonchophora capiomontana.
  Monsonia nivea.
  Erodium pulverulentum.
  Fagonia sinaica.
  Haplophyllum tuberculatum.
  Lœfflingia hispanica.
  Paronychia nivea.
  P. cossoniana.
  Herniaria fruticosa.
  Zygophyllum album.
  Z. cornutum.
  Peganum harmala.
  Nitraria tridentata.
  Astragalus gyzensis.
  Ononis serrata.
  Ammodaucus leucotrichus.
  Cyrtolepis alexandrina.
  Anacyclus clavatus.
  Pyrethrum fuscatum.
  Nolletia chrysocomoides.
  Tanacetum cinereum.
  Ifloga fontanesi.
  Artemisia herba-alba.
  Anvillæa radiata.
  Atractylis flava.
  A. prolifera.
  Nonnæa micrantha.
  Lithospermum callosum.
  Heliotropium undulatum.
  Plantago ciliata.
  P. albicans.
  Limoniastrum guyonianum.
  Statice pruinosa.
  Echinopsilon muricatus.
  Traganum nudatum.
  Ephedra alata.
  Cutandia memphitica.
  Erythrostictus punctatus.
  Asparagus albus.
  Aristida pungens.




                         =THE BISKRA REGION.=



                           TOPOGRAPHY.[29]

Biskra lies immediately south of the Atlas Mountains, in the
Department of Constantine, 220 kilometers from the Mediterranean
and about 400 kilometers from Ouargla. To the northeast of the oasis
lie the Aurés and to the west the beginning of the Saharan Atlas,
which run south of west across Algeria into Morocco. Just north
of the place are small hills and low, jagged mountains—detached
spurs from the main ranges. These are the Djebel Bou Rhezal, running
nearly northeast and southwest. The highest of them, directly west
of Biskra and about 8 kilometers distant, has an altitude of 463
meters. The Bou Rhezal Mountains have a precipitous southern face,
but fall away more gradually to the north, where the slope joins
a wide and undulating plain. The latter extends to the base of the
main Atlas ranges. Southwest of the oasis, and about 2 kilometers
distant, a range of rocky hills extends for a distance of about 6
kilometers, or until they join the mountain range of the Saharan
Atlas. These hills are called Ed Delouatt. To the south of Biskra,
as has already been stated, there extends a vast plain, the reg,
which dips gently to the south and drains into the Chott Melrirh,
50 kilometers or more distant. The situation of Biskra relative to
the mountains on the one side and the desert on the other, together
with its altitude, governs the climate of the place. Except Laghouat
no vicinity in the desert proper was seen with so great an amount of
precipitation (about 200 mm.) as Biskra, which is indicated by the
relatively rich flora. One who has seen only Biskra can not draw
conclusions regarding the vegetation or the conditions of plant
life of those portions of the Sahara that lie farther to the south,
where much more intense conditions of aridity obtain.

The soils of the vicinity of Biskra are various. That of the low
hills between the town and the Djebel Bou Rhezal is only a few
centimeters in thickness, but in the washes from the hills it is
a meter or more. Here the soil is a sandy loam with an admixture
of small stones and pebbles. On the flat ground to the north and
to the south of these hills it is of a finer texture, approaching
the adobe of the southwestern United States. On the reg to the
south of the oasis the soil is also fine, and in some places, if
not underlying the plain as a whole, there are strata of gravel
at varying depths beneath the surface. This soil in places carries
considerable salts. It dries to a powder during the long dry seasons
and is easily blown by the winds. Owing to outcropping rock, the
south face of the Bou Rhezal Mountains has but scant soil, but that
of the northern side resembles the soil of the low hills to the
south, which has already been characterized. Near the town are dunes
of good size. Especially to the southwest the sand banks against
Ed Delouatt hills is in large amount. In the opposite direction,
but farther from the oasis, the dunes are fairly extensive.

The Oued Biskra is of great importance to the oasis, since it
carries water for several weeks of the year and furnishes water for
irrigation. Its channel lies about 3 meters, or possibly more, below
the general level of the oasis, and possibly in earlier times may not
have been so well defined as at present, spreading its waters over
its flood-plain during high water. The oued is made up of several
tributary oueds which cross the plain north of Djebel Bou Rhezal,
unite where there is a pass in these mountains, and finally debouch
on the reg to the south of the town, where the channel becomes
continually less well defined. Another oued takes its origin in the
Bou Rhezal Mountains in several independent branches which unite at a
pass in Ed Delouatt hills and extend for a distance of 15 kilometers
or more into the reg. One of the feeders of this oued is from hot
springs, Hamman es Salahine, about 8 kilometers northwest of Biskra.


                 PLANT HABITATS OF THE BISKRA REGION.

From the preceding sketch of the leading topographical conditions
of the vicinity of Biskra it will be seen that the plant habitats
are more diverse than at any other place visited. For the present
purpose the habitats may be distinguished as follows: The alluvial
desert (reg), which lies on every side of the oasis save the north;
low hills adjoining the oasis on the north; Ed Delouatt hills,
southwest; the oueds and their flood-plains; the Dj. Bou Rhezal; the
hamada (?) lying both to the north and to the south of the latter.


                   VEGETATION OF THE BISKRA REGION.

A glance over the list of plants which grow naturally in the vicinity
of Biskra[30] shows that many are the same as occur farther south,
with many unlike these, having affinities outside of the desert
proper; also, the number of plants as well as species is greater at
Biskra than farther south. This would be expected from the greater
rainfall and more diverse topography.

The flora of the Biskra oasis, according to the authors referred to
above, consists of 175 or more species. Of cultivated plants there
are 25 or more species, the most conspicuous being the date. The
other species are mostly the same as have already been noted at
other Algerian oases, except that both the peach and the apricot
are wanting at Biskra, although cultivated at Laghouat, Ghardaia,
etc. On the outskirts of the town the fairly extensive flood-plain is
given over mainly to the cultivation of grain, barley predominating.

The hills and mountains, and the bajada at their base, as well as
certain oueds with water relations, exposure, and soils different
from those of the oasis, have also a very different flora, which, for
the most part, is desertic in character. For purposes of comparison,
some of the leading characteristics of the plants growing in a
half-dozen localities will be given.

As stated above, to the southwest of Biskra there runs a range
of hills, Ed Delouatt, to the south and the north of which may be
found interesting plants and plant conditions. On the south side
the slope (bajada) descends gradually to the great reg, and near
the base of the range, in the vicinity of the place where tradition
says a Roman town formerly existed, there is a wide, sandy plain,
reaching from the Oued Melah, which pierces Ed Delouatt hills,
nearly or quite to the western extension of the oasis. There are
no large dunes here, but sand billows about a meter in height and
sand hillocks diminutive in size. Between these the plain is fairly
level. Over this whole tract there seemed to be only one species
(_Euphorbia guyoniana_), but this was fairly abundant. (Fig. 65.) As
at Ghardaia and elsewhere, this species grows in small colonies
because of its suckering habit, and acts to a small degree as a
sand-binder, each group being situated on a sandy hillock. It will
be remembered that this species at Ghardaia, as well as at a certain
bordj east of that place, had roots which were somewhat fleshy as
well as roots which were fibrous, on one and the same plant. I was
interested to learn whether similar conditions should obtain at
Biskra, since it had been learned, in the case of two species of
_Opuntia_ in the Tucson region, that a different habit followed
certain differences in habitat. Several specimens of _Euphorbia_
were removed with care from the soil and in no case was it found
that the roots were fleshy, but in every instance they were entirely
fibrous. It is supposed that the reason for the variation in behavior
may possibly be traced to differences in the water relation, as
was found to be the case of the variation in cactus roots cited,
but no experiments have been made on _Euphorbia_ to prove this.[31]

There is a variety of habitats on the opposite (north) side of the
hills, ranging from the large dune (which reaches the summit to the
west of the pass through which the oued passes and which must be 100
to 150 meters above the oued) to the flood-plain, with water very
near the surface, during rainy season at least. There are rocky
slopes, also, and sandy slopes apart from the dunes referred to,
as well as a fine clay with sand admixture on the flood-plain. In
all of these habitats, except the large dune, the vegetation is
actually or relatively abundant. (See fig. 67.) Among the rocks
are small shrubs or half-shrubs, and also on the plain below. Here
one may find, among other species, _Echiochilon fruticosum_,
_Helianthemum_ sp., _Atractylis serratuloides_, _Gymnocarpos
fruticosum_, _Thymelæa microphylla_, _Nitraria tridentata_, and
_Acanthyllis tragacanthoides_. On the sandy slope was growing a very
numerous population of liliaceous forms, mainly, perhaps wholly,
_Asphodelus fistulosus_ (fig. 72).

It has been noted that the oued which pierces Ed Delouatt is made
up of the confluence of all of the small oueds lying between the
Bou Rhezal Mountains to the north of Biskra and the hills lying
directly north as well as those (Ed Delouatt) to the southwest. The
united oueds reach the base of the hills nearly 1 kilometer east
of the pass, and turning abruptly follow the base the remainder
of the distance. At the place where the oued touches the range
the soil is moist nearly to the surface of the plain (reg) and for
several meters back from the oued. Here, then, the water relations
of the plants are such as to favor, probably most of the year,
the growth of mesophytes (?) or even of hydrophytes, but for some
reason there is not much vegetation there; whether large species
have been removed as for fuel, have been destroyed by animals, or
never existed, was not learned. The most interesting plant found
was the well-known parasite _Phelypæa violacea_, which grows,
according to Mobius, on a salsolaceous host.[32] Only a few specimens
were seen at the time of the second visit to Biskra, in March,
and they were just appearing above the ground. (See figs. 69, 70,
and 71.) When removed from the soil the longest specimen was found
to have penetrated over 59 cm.; it was 6.5 cm. in diameter at a
point 50 cm. from the tip. The plants were exceedingly heavy, being
gorged with sap, and appeared to be able to absorb moisture from
the wet soil through the white and delicate epidermis of the entire
portion submerged. If this is the case the parasitical relation
is of especial interest, as the species is wholly dependent on its
host for organized foods, but not for water—a condition opposite
that found in such semi-parasites as the mistletoes, which obtain
water and unorganized foods in solution from the host, but which,
save for the fact of attachment, are otherwise independent of it.

To the northeast of Ed Delouatt hills and north of Biskra, but
immediately adjoining the town, is an irregular group of low,
rounded hills, mostly very arid, which support a scant vegetation
of typical desert plants. These hills, without a name, are eroded
to a degree and have shallow washes leading from them in every
direction. The soil appears to be thin, except in the washes, where
it has accumulated to the depth of a meter or more. On the rounded
summits it is rather fine, but in the washes there is much gravel
and larger stones. Here one finds the greatest range in exposure,
and probably also an accompanying difference in the temperature of
the air and soil.

In considering the vegetation of the hills it must be remembered
that no plants suitable for forage or large enough for fuel would be
left untouched, as the biotic factor is quite as much in evidence in
modifying the Biskra flora as that of the other regions visited. As
the plants are at present, however, and for whatever definitive
causes, there is a considerable variation in numbers and apparently
also in kinds. An examination of different parts of the hills
by which various exposures as well as other conditions are seen
bears out the hypothesis. On the southern slopes, particularly
the upper portions, the plant covering is especially poor. The
population of an area 16 by 16 meters in the upper south slope
included 134 perennials and numerous annuals. Of the species,
_Haloxylon scoparium_ was the most numerous; there were also
_Dæmia cordata_, _Thymelæa microphylla_, and _Fagonia sinaica_
(figs. 78 and 80). All of these species were small, so that from
superficial examination the area appeared fairly barren (fig. 73).

When one crosses the hills to the north, so that northern and
northwestern exposures are seen, a striking difference is immediately
apparent. The northern slopes are much richer in perennials than
the opposite facing. The population of an area of the size before
taken shows this observation to be correct. On a square 16 by 16
meters, with north exposure, 536 living perennials and numerous
annuals were found, with _Haloxylon scoparium_ again the dominant
species. The species second in number was _Ferula vesceritensis_,
which is to be found chiefly on the north exposure (figs. 74, 75,
and 77). There were also a few specimens of _Acanthyllis numidica_,
and many annuals, which, unlike those on the south facing, were
not in flower.

On the lower slopes, in the small and open gulches and the lower
portions of the washes and their flood-plains, the plants were most
numerous and include a variety of forms, with _Peganum harmala_
the most conspicuous and most numerous.

The roots of several species growing on the hills were studied
with the following as perhaps the leading results: The root-system
of _Haloxylon scoparium_, as has already been seen to be the case
with an allied species in the country of the M’Zab, may be said
to be a modification of the generalized type. One specimen, partly
exposed by erosion in a wash, had a tap-root over 113 cm. in length
(fig. 76). As this root was 4 mm. in diameter where left, and was
8 mm. in diameter at the crown, it may have penetrated much beyond
the point where it was left, provided the soil conditions continued
favorable. The main root gave off two large laterals, of which one
left the parent root 8 cm. and the other 18 cm. beneath the surface
of the ground, but there were also numerous filamentous roots about
2 cm. in length, which were borne in tufts. (See fig. 79.) These
resembled the deciduous rootlets found on many perennials in the
Tucson region and doubtless function quite as they, namely, they
are organized at the beginning of the rainy season, operate to
increase the water-absorbing area of the plant quickly and greatly,
and die as soon as unbearably arid conditions set in.

In the same wash, where the soil was deep with an admixture of
small stones and pebbles, the roots of other species were also
examined. One of these was _Peganum harmala_, whose roots were
examined at Ghardaia. The root-systems of the plant in the two
regions were similar in being generalized. A main root was found
extending downward over 61 cm. and it gave off three good-sized
laterals, arising from 15 to 27 cm. beneath the surface. The
uppermost lateral took a horizontal course. Figure 82 gives a fairly
good idea of the general character of the root-system of the species.

There were many specimens of _Plantago albicans_ growing in the
same habitat as _Haloxylon_ and _Peganum_. Its root-system was
also studied with the following results: The tap-root is strongly
developed. In one instance the slender main root was found to go
straight down over 71 cm. Numerous laterals were borne between 8
and 20 cm. beneath the surface of the ground. The species has an
interesting habit of propagating vegetatively by means of fleshy
stolons. An examination of the stolons showed them to be of very
unequal age, some having been lately formed, while others had been
organized in previous years and were no longer living. Many specimens
were examined to learn the probable service of the fleshiness of its
stolons, and the conclusion was that this factor probably enables
the species to pass over periods of excessive drought.

Running across country are the Bou Rhezal Mountains, of which
the north and south faces were examined in one or two places, on
each side somewhat in detail. The south face is precipitous and
has little vegetation, but there are many plants on the opposite
face, especially near the base, where the soil conditions are
relatively favorable (fig. 84). Among the perennials were found a
few specimens of _Rhus oxyacantha_ and _Zizyphus lotus_, which,
although dwarfed, were the largest plants seen away from the
oasis, being about 1.5 meters high. In another gulch were found the
following species: _Acanthyllis tragacanthoides_, _Dæmia cordata_,
_Ferula vesceritensis_, _Haloxylon scoparium_, and other plants
unknown. Toward the upper portions of the gulches _Haloxylon_ was
seen to be especially abundant, but in the bed of the washes and on
their flood-plains _Peganum_ grows in large numbers (fig. 83). In
the most favorable places on the northern slope in March the annuals
were the most abundant observed in the vicinity of Biskra, although
even there they failed to completely conceal the ground. Where the
situation was less favorable as regards soil and water conditions
(for example, on the side of the gulches, on the summit of ridges,
and the like) there were almost no annuals and those present were
relatively small.

In this brief account of some of the most striking features of
the Biskra flora it should be noted that several habitats are
omitted as not being pertinent to the points in view. Especially,
nothing has been said regarding the flora of the salt flats along
the Oued el Hamman or that of the dunes, since the influences here
are largely edaphic, while the present interest lies mainly in the
relation between plants and climate; and the flora of the oasis
has been largely neglected for similar reasons.

Not only is there in certain regards a larger number of plants
in the Biskra region than had been previously seen in southern
Algeria, but there are certain types, mostly new, which point to
more favorable conditions of plant life. These are such forms as
have a water-storage habit, like _Asphodelus_, _Ferula_, _Plantago_,
and _Phelypæa_, which, although not wholly absent farther south,
appear to be much more numerous near Biskra. The presence of bulbous
plants is well known as being one of the floral characteristics
of the High Plateau, and it is also known that similar forms are
not to be found where the arid conditions are the most severe,
which probably accounts for the facts noted. It may be pointed
out also that plants at Biskra exhibit exposure preference where
soil conditions appear to be parallel. This condition is not so
marked farther in the desert as at Ghardaia, for example, where,
provided there is sufficient depth of soil, apparently any species
may be found on any exposure. In other words, exposure preference
implies a certain amount of water as well as sufficient soil.

So far as shown by observations of the root-systems carried out on
similar species growing in the Biskra region and at Ghardaia, the
essential root characters of plants growing in the two regions are
the same. The single exception to this so far noted is that of the
roots of _Euphorbia guyoniana_, which, contrary to the expectation,
are apparently wholly fibrous at Biskra, although they are in
part fleshy farther south, where the soil conditions are surely
more arid. At Biskra, also, the only plant observed with typical
generalized root-systems (_Peganum_) does not grow where the soil is
shallowest, but where it is relatively deep. So far as seen, also,
plants growing where the soil is shallow have either a generalized
root-system or a root-system approaching this type, even if the
branching is only relatively deep.




                  =GENERAL SUMMARY AND CONCLUSIONS.=



        THE ENVIRONMENTAL CONDITIONS OF PLANTS IN ARID REGIONS.

The environmental conditions encountered by plants in
the arid regions are widely different from those of moister
regions. Precipitation is not only slight, but it shows an enormous
range in variation from year to year; the rate of evaporation is
high; the temperature of the air and soil varies widely both during
the day and with the seasons; the light is of great intensity,
and the soil is low in humus content and may contain an excess
of salts. These, the most striking physical factors of deserts,
are present in different combinations with resultant differences
among deserts, and an arid region may be so large as to include
such variation within its borders. Also, a desert may be so far
from the ocean, or it may include such diversity of topography,
as to show great variation in biological features as well as in
its surface phenomena.

In the flora of any arid region, the mutual relations of the
constituents of the flora and its general and detailed relation to
the physical environment are quite different in the main from these
features in the flora of the more humid regions. Thus, the leading
relation is the relation to water, and on the response of the plants
to this relation much of the phenomena associated with plant life in
the desert directly depends. For example, in the extreme deserts it
is probable that the elements of competition between the perennials,
which is an important factor in the survival of a species in the
moister regions, is wholly lacking. It should be noted, however,
that in the less extreme deserts, as in the vicinity of the Desert
Laboratory, competition exists between plants, although this is not
at first apparent. In this case the competition is not for room,
but for water, and is not manifest by palpable crowding, but by
invisible competition of the roots. Thus the reactions are with
the physical environment and are exhibited in a variety of ways,
some of which concern the plants themselves in an intimate manner,
being morphological and physiological, some being concerned with
the flora as a whole. The environmental responses are often obscure
and complex, but in other instances they are less obscure and
apparently direct.

As is well known, a desert flora is in part perennial, lasting
with but little outward change from season to season, and in part
ephemeral, consisting of short-lived species which appear with the
rains and which disappear with the return of the dry season. The
ephemeral flora differs in no essential regard from annuals of the
moister regions; also, the environment to which they are exposed
closely resembles the environment of the annuals of such regions. But
the perennial desert flora, on the other hand, offers very striking
departures from the corresponding flora of the moister regions,
just as the environment to which they are exposed for the most of
the year is also different. It will be sufficient, for the purpose
of bringing out the point of view, to notice a few of the leading
characteristics of desert plants and of their physical environment
and, in a few instances, to observe possible relations between
the two.

The most obvious features of desert plants are associated,
in whatever way, with the subaerial portions. Leaves are usually
greatly reduced or wanting, during the dry seasons at least. Spines
are frequently present and the exposed parts are often well covered
with hairs. The stomata are sometimes deeply sunken, the cuticle
often very heavy, and a waxy substance may cover leaves or stems. The
chlorophyll-bearing cells are arranged with the long axis at right
angles to the leaf or stem surface. All or most of these characters
are associated with the low humidity of the air. In certain deserts
plants are also to be found with greatly enlarged stems and branches
which serve as water-storage organs. It should be noted, however,
that plants which do not have a constant surface undergo many marked
changes with a betterment of the water relations, particularly if
this comes when the temperatures are favorable. For example, many
cacti organize leaves which are unsuited in structure for periods
of extreme drought, and which consequently fall away soon after
the close of the rainy season. These leaves enormously increase
the rate of transpiration at a time when this is not injurious.

It is not in the subaerial parts alone, however, that the plants
of one desert are different from those of another, that plants
are unlike in the same desert, or that plants of a desert are
different from those of the more humid regions. The root-habits
also exhibit not a little diversity and show marked reactions
to the pressure of their environment. For instance, the desert
shrubs of the region surrounding the Desert Laboratory at Tucson
have well-marked root-systems, apparently constant under natural
conditions, which may roughly be designated as the tap-root type,
the superficial type, and the generalized type. Other conditions
being equal, species with characteristic root-types have also
characteristic distribution, or exhibit in other regards consistent
reactions. Thus, the widely extending and superficial type of roots
is confined, among independent plants, to such as have water-storage
capacity. The relation of this root-type to the distribution will be
mentioned later. Plants with a dominating tap-root are confined to
areas where the soil is relatively or actually deep, while species
having generalized roots have a local distribution which may be
considered the maximum.

The relation of the superficial type of root-system to the
distribution of the species is not so apparent in this relation
in plants as with the other types of roots, but is undoubtedly
close and possibly definitive. The absorption roots of plants with
water-balance mostly lie less than 10 cm. beneath the surface and
are thus subject to extreme desiccation for the maximum time, or
in other words they are exposed to favorable moisture conditions
the minimum time. How long the optimum water-absorption time
may be for such species is not known, but that it is longer than
might be supposed (from the period certain species can survive
without water) is highly probable from the facts concerning their
distribution. In brief, the best development of the fleshy species
in question occurs where the rainfall is periodic, occurring twice
each year, and they are wanting or sparse where the rainfall is
uncertain or occurs but once annually. Had these plants a deeply
penetrating type of root-system the local as well as the general
distribution would be very different from what it now is. Owing
to the unfavorable character of the rainfall in southern Algeria,
plants with a water-balance are wanting there, just as they are
wanting in portions of southwestern United States where the amount
or the character of the precipitation is likewise unfavorable.

Besides the characteristics of the root-systems of the desert
perennials whose significance has been sketched, there are other
features of importance. For example, the roots lying near the
surface bear tufts of delicate roots, which are formed during
periods of active growth and perish when such seasons cease. By
the organization of deciduous rootlets the absorption area of the
species is enormously increased, and quickly, without at the same
time increasing the distance of water transport.

There need be mentioned here only one additional feature of the
desert plants. It is now known, in brief, that the non-fleshy
perennials of the desert, not halophytes, may possess a very dense
cell-sap. This fact has been demonstrated in the subaerial parts
of several species, and is assumed to hold for the roots also of
the same species. As suggested, not all of the desert plants,
however, are capable of developing dense juices. Thus, certain
fleshy species, and such mesophytes of the desert as have been
studied in this connection, do not have more highly concentrated
cell-sap than the ordinary plants of the humid regions. Further,
it appears that desert species which, under natural conditions
and during the dry seasons, form extremely dense juices, lose this
capacity when grown under humid conditions. So far as is now known,
species capable of developing a cell-sap with high osmotic power have
generalized roots, although this may be of no especial significance;
but the relation of this capacity to survival in an arid substratum
is apparent and vital.


            ENVIRONMENTAL FEATURES OF THE FLORA OF ALGERIA.

The physical environment of the plants of southern Algeria is, in
a few broad features, similar to that of the southwestern portion
of the United States. These regions have about the same latitude,
both are separated from a large sea by mountains, and the range in
altitude is similar. There are other features, however, particularly
as regards the amount and the distribution of the precipitation,
in which the two widely separated regions are very unlike, and a
correlated difference in the habit and composition of the floras
of the two regions is apparent.

The Algerian climate as a whole is a mild, temperate one, but very
diverse. The latitude and topography taken in connection with the
presence of large seas to the north and west, and a large continent
leading away to the south are its chief determinants. The climate,
therefore, of the northern portion is coastal; that of the southern
portion continental.

Probably the most important of the secondary factors which modify
the climate of Algeria is its highly varied topography. An important
mountain system, the Atlas, made up of many more or less detached
groups and secondary systems, a plateau or steppe lying 3,000 feet
more or less above the sea, and finally the northern edge of the
Sahara, which has a very diverse topography of its own—such is
the surface of Algeria.

Algeria is divided into three climatic provinces corresponding to the
leading topographical differences: the Tell, including the littoral,
or portion between the maritime Atlas and the Mediterranean; the
High Plateau, or steppe, which lies between the Tellian Atlas and the
Saharan Atlas; and the desert. These provinces have marked individual
differences in rainfall, temperature, and other climatic features.

In the Tell and the High Plateau the winds from the sea deposit most
of their moisture. Along the coast as much as 700 mm. of rain is
recorded, while in other parts of the Tell it is about 570 mm. On
the High Plateau the yearly precipitation sinks to 310 mm. In the
desert south of the Saharan Atlas, however, where the altitude
is lower and the temperature greater than in either of the other
provinces, the yearly rainfall is 200 mm. and less. In some years,
in fact, no precipitation whatever is reported in the desert.

The seasonal distribution of the rains in any arid or semi-arid
region is of great importance as a factor in shaping the character
of the vegetation. For example, in the semi-arid region of the
southwestern part of the United States, in the Tucson region, there
are two distinct rainy seasons—the rains of winter and those of
summer—and here the plants with a water-balance are an important
feature; but farther to the west, where there are no summer rains,
there are no succulents. In Algeria, also, there is but one rainy
season and it has already been noted that the absence of plants with
water-storage facilities is one of the leading characteristics of
its vegetation. The seasonal distribution of rains is as follows:
In the Tell, in winter it is 42 per cent, in spring 27 per cent,
in autumn 27 per cent, and in summer only 4 per cent. On the High
Plateau the percentages are 30, 20, and 34 for winter, spring,
and autumn, and 16 for summer. In the desert the percentages for
winter, spring, and autumn are 37, 39, and 20 respectively, while
in summer 4 per cent of the entire rainfall occurs. On the High
Plateau, however, and in the Saharan Atlas the distribution of rain
is much more equable, since 16 per cent falls on the High Plateau in
summer and 13 per cent in the Saharan Atlas the same season. We
have, therefore, the interesting result that both in the Tell and
on the desert there is a long, dry summer season, but in the
intervening country more or less rain falls at this time of year.
It seems very probable that a careful study of the plants of these
regions would show reflected in the vegetation this peculiar
character of climate.[33]

The mean relative humidity changes in a marked manner as one passes
from the Tell, across the High Plateau, and enters the desert. For
example, at Fort National the mean relative humidity is 85 per
cent. On the desert it varies from 54.6 per cent at Ouargla to 42.6
per cent at In Salah. At times in midsummer the humidity in the
desert is too low to measure with instruments; it is often 7 or 9
per cent. On the other hand, the humidity in autumn is surprisingly
high, owing in part to the lower temperature and in part to the
northerly winds. However, no dew is reported and probably its
occurrence is rare.

With so great difference between the Tell and the desert in
relative humidity is associated marked variation in the rate of
evaporation. For example, at Algiers the total annual evaporation
is 1,654 mm., while at Ghardaia it is 5,309 mm., which is possibly
the greatest amount of evaporation thus far reported. Thus, the
difference in evaporation between the Tell and the desert is nearly
as the ratio 4 to 1.

The evaporation-rainfall ratios for the Tell, High Plateau, and
the desert are of great interest. The seasonal evaporation-rainfall
ratio for the littoral is 2.5 to 1; that of the Tell is 3.5 to 1;
that of the High Plateau is 9.4 to 1; and that of the desert is
46.5 to 1. If we represent the evaporation-rainfall ratio as unity,
the ratio for the Tell becomes 1.4 and the ratio of the High Plateau
becomes 9 to 3.7, while the desert ratio is 18.6.

The annual and daily variations of temperatures in the desert are
naturally relatively great. At Algiers the annual variation is
approximately 40.7° C., while at Ghardaia it is 47.9° C. As great
an annual variation as 57° C. has been observed at Ghardaia. The
daily variation of temperature is especially marked on the High
Plateau and the desert, ranging 17° C. more or less on the High
Plateau, and 20° C. more or less on the desert. These figures are
occasionally overstepped: for example, at Batna in July, 1904, when
the maximum daily range was 21.8° C.; and an observation made by the
writer in the open desert between Ouargla and Touggourt, in November,
1910, showed a variation of temperature of 24.7° C. between 3
o’clock in the afternoon and 6 o’clock the following morning.

The absolute maximum temperatures in southern Algeria are fairly
high. At El Golea, for example, they are 47°, 46.5°, 48°,
and 49.2° C., while at In Salah, about 700 miles from Algiers,
the maximum temperatures for four years have been found to be
50°, 49.2°, 50°, and 48° C. It is interesting to note that at
Ouargla, which is much nearer the coast than In Salah, even higher
temperatures have been recorded. The maxima for as many years are
as follows: 50.2°, 51.0°, 52°, 49°, and 48.4° C. Usually in
winter freezing temperatures are experienced at all stations in
southern Algeria.

Very little has been done on the soil temperatures in the desert
region, but at Ghardaia, in July, 1911, the temperature of the
soil 15 cm. beneath the surface of the soil ranged, maximum from
36° to 37° C., minimum from 31° to 33° C., giving an absolute
range of 6° C.

In addition to the rainfall, the evaporation, and the temperature,
there is another important climatic factor which should be taken
into account, but which can not be stated in accurate terms, _i.e._,
air-currents. It is a matter of common experience that one rarely
observes a calm day on the desert, but that usually the wind,
which is often of considerable force, is found to be blowing. This
is unquestionably an important factor in raising the total of
evaporation and therefore in increasing the arid conditions of
this region. The winds which are most effective in the direction
mentioned are those which come from the desert and are known as the
“sirocco;” these are most likely to blow in spring and summer,
although they occur in autumn also, and to a very limited degree in
winter. When the winds blow from the north cooler conditions occur,
the relative humidity is lower, and therefore the evaporation rate
is less. The sirocco, or desert wind, crosses the Mediterranean and
is sometimes felt in southern Europe. It does not generally last
more than three days at one time, but at Batna, in July, 1902, it
was reported for eight consecutive days. When the sirocco blows the
humidity is likely to be markedly affected; for example, at Batna
during the eight-day sirocco alluded to, the relative humidity fell
from 25.6 to 16 per cent on the first day.


     SOME EFFECTS OF TEMPERATURE AND RAINFALL IN SOUTHERN ALGERIA.

While it is recognized that, generally speaking, climate shapes the
character of the vegetation, its immediate effects can not well be
measured, or, at least, have not been accurately measured, so that
it seems necessary to confine one’s observations to supposed
or probable effects, however unsatisfactory this may be. So far
as suits the present purpose, climatic effects may conveniently be
separated into those which are direct and those which are not direct,
remembering at the same time that the division is purely arbitrary,
since the climate is a complex of various factors and its effects
on vegetation are also complex. Among the climatic factors whose
effects are most striking are the air temperature and the rainfall,
and only certain effects resulting from a variation of these will
be commented on here.

In parts of the Sahara visited where the most rain is reported,
especially Laghouat and Biskra, plants were observed to exhibit
exposure preference. Here the south or southerly facing slopes may
have a floral composition different from the opposite exposure. In
each instance the soil conditions, and apparently the moisture
conditions also, were alike. Exposure preference was not noticed in
the southern portions of the colony. Another temperature relation
was observed, namely, the renewal of growth in the autumn. This
is probably direct effect, although the point is not certain. In
Ghardaia it was seen that many of the perennials were taking on
new growth and coming into flower, although no rain had fallen for
12 months. Analogous conditions, with a significant difference,
occur each year in the Tucson region. Here with the change from
a cooler to a warmer temperature, as from winter to spring, or
from spring to summer, fairly independent of the rainfall, many
perennials organize flowers or shoots. But, so far as is known,
no species renews its vegetative activities with the coming of
winter, or with a decreased temperature and also _independent_
of the rains, although there are characteristic winter and summer
plants. Judging from analogy, therefore, it would appear that
the stimulus to development on the part of the M’Zabite plants
may be from the relatively better water relations made possible
by a lower temperature without rain. In November at Ghardaia the
evaporation rate is much below that of summer, that during the
night being very small. Further, it was told me by good authority
that the same species seen growing in autumn renew growth whenever
rain chances to come, whatever might be the season. But it should be
remembered that rain most commonly occurs in this region in winter,
so that the plants may have a rhythm to which they usually conform,
but from which they may depart, and that both stimuli (better water
relations and lower temperature) are the annually recurring factors
by which it may have been induced. Reference, of course, is made to
perennials only, as no annuals were seen until the rains of spring
made conditions favorable for their appearance.

The effects of a varying amount of precipitation are naturally the
most marked of any climatic factor. It is especially striking as
one goes south from the Mediterranean, crossing the Tell and the
High Plateau and entering the desert proper. Whether the effects
would be increasingly striking with deeper penetration of the
desert is doubtful. As is well known, a leading characteristic of
the vegetation of the littoral and of the Atlas Mountains is the
presence of forests of whatever species. As the littoral is left
behind the forests disappear until on the High Plateau there are only
straggling trees along the dried water-courses. This steppe bears
mainly shrubs, many of which are halophytes, with the perennial
grass, _Stipa tenacissima_, and _Artemisia herba-alba_ away from
salt spots. Such low forms are present in sufficient numbers as to
give character to the landscape and to conceal the surface of the
ground fairly well.

South of the Saharan Atlas a marked change occurs. Here, with a
rainfall of 200 mm. and less, the trees are confined to the dayas,
a narrow belt, the vicinity of oueds, and the oases, exclusively. The
shrubs of the hamada also decrease in numbers as one goes south,
and where the annual precipitation is least, as on the Gantara
between Ghardaia and Ouargla, large barren areas extend. At no place
on the hamada of the M’Zabite region are the shrubs present in
sufficient numbers or size to conceal the surface of the ground or
to give character to the landscape.

Aside from the effects following a lessened annual precipitation
there is also to be taken into account the increasing uncertainty
of rain, or its irregularity, which is also a marked characteristic
of the Saharan climate. In the desert, also, storms are likely to be
of the torrential type. I did not observe vegetation characteristics
which appeared directly traceable to the irregularity in rainfall,
but Hayward reports an interesting condition observed by him in the
southern Sahara, near Kidal, where large tracts of _Mimosa_ had
died from an unusually long period (five years) without rain.[34] It
is not at all improbable that to the cause named much is directly
traceable which is generally attributed to insufficient rainfall
taken in the usual sense. It is probable that the vegetation of
the desert—the amount as well as kind—is due to the capacity
of desert forms to meet successfully the occasional, even rare,
conditions, of whatever sort.


                THE SOIL RELATION IN SOUTHERN ALGERIA.

A very important environmental factor, although one which can not
at this time be adequately presented, is the soil relation. Nowhere
is the edaphic factor more important than in the desert, where
quantity and quality are always important and occasionally even
determining factors. In this connection I do not refer particularly
to dunes or to chotts, but to country soils, that is, the sort
most commonly to be found, which in southern Algeria is a clay with
sand present in greater or less amount. So far as the relation of
plant to soil refers to the presence or the absence of the plant,
the problem can be briefly stated thus: Given similar kinds of soil
and an equal precipitation, areas where, within limits, there is
greatest depth of soil will have the largest number of plants, and
areas with light soil covering will have few or no plants. Also,
having given sufficient soil, the kind of plants present, together
with certain root-types, will depend on the soil depth. It should
be understood that these generalizations are supposed to apply to
southern Algeria and not the deserts in general, or at least not to
semi-deserts where in certain regards a very different condition
obtains. The plant distribution thus dependent on soil depth, and
the root character also having relation thereto, will be presented
below; the soil conditions, so far as they are known to me, can be
given briefly in this place.

From a few excavations on the hamada near Laghouat, on an analogous
area by Tilrempt, on the hamada at Ghardaia, between Ghardaia and
Ouargla (Gantara), and by Biskra, in each instance with analogous
topographical conditions, it was learned, in short, that on
formations of this sort the soil is usually less than 50 cm. in
depth, although where there are rocks or in drainage depressions the
depth may be greater. In the oueds, where a different type of soil
occurs, greater soil depth is naturally found; also on the reg,
or alluvial plain, frequently at least the flood-plain of oueds,
the soil conditions are peculiar and the soil is deeper than on
the higher hamada. The special significance lies not so much in
the differences in the soil _per se_, but in the differences in
the water relation occasioned by variation in depth. Owing to
want of data in regard to penetration of water and its retention
on the Algerian soils, the soil-moisture relation can only be
gathered from inference. As regards rains, it is probable that
light showers, those so slight as not to penetrate over 1.0 cm.,
have little or no direct influence on the perennial vegetation,
but greater penetration directly benefits such plants. The first
conclusion is drawn from the observation that filamentous rootlets
(seemingly like the deciduous rootlets of the perennials of the
Tucson region) on _Haloxylon_ at Biskra were not found nearer
the surface than 8 cm. Should the more superficial soil layer
be moist for any considerable time, there would apparently be no
reason why such temporary rootlets should not be formed nearer the
surface. However, should there be sufficient moisture in the soil
to permit absorption by roots, provided a slow rate was adequate
to replace the transpiration loss, which rate was made lower by a
more moist air, a slight rain would be of great significance, even
if it did not penetrate to any appreciable depth. On the intensely
arid desert such slight modifications of the water relations as the
lowering of the temperature as winter approaches, causing decreased
evaporation or rains, although actually small in amount, may be of
large moment to plants. Such a condition was noted at Ghardaia,[35]
where there had been a drought for over twelve months, but on the
return of the cool season, with a lower evaporation rate, growth
was resumed and several plants came into flower. This appeared to be
not wholly the stimulus of lower temperature, since I was informed
by good authority that the plants renewed their various activities
whenever rains chanced to come, whatever might be the season.

From the observations last given it appears that sufficient moisture
persists in the soil to tide perennials over the long periods of
drought, although not in sufficient amount to permit active growth
during the dry seasons. This is not an uncommon occurrence with
desert plants. For example, whenever in the Tucson region the arid
seasons are uncommonly long, or there has been a relatively small
rain, as 25 per cent less than usual, much of the vegetation may
remain dormant. Under such conditions an evergreen like _Covillea
tridentata_ drops all save the youngest and smallest leaves and
maintains this nearly defoliate condition for long periods, with
little other change. The plant behavior noted is always connected
with insufficient moisture.

The soil in the oueds and probably also in the reg, as well as
that of the dayas, is of considerable depth. The soil depth, or
rather the depth before solid rock is encountered in the valley
of the Oued M’Zab at Ghardaia, is from 20 to 30 meters, or even
more; at the Daya Tilrempt the depth to water in one of the wells
is about 95 meters. Although there is much gravel and sand in the
fill of these depressions, it is likely that they afford the most
favorable soil conditions in the desert for the development of
a large root-system. Be that as it may, it is certain that large
plants, such as _Tamarix_ and _Pistacia_, occur only in such places.

Where there is most soil on the hamada, as in certain pockets near
the old hill town of Ghardaia, abandoned several centuries since,
one finds also the most and largest plants of this, the hamada,
formation. Moreover, at the time in November when growth was noticed
in the oued plants, it was also taking place in these favoring
situations, about 50 meters above the level of the valley of the
M’Zab and much above any possible permanent water-supply such as
is afforded by a water-table.

The various habitats, therefore, not including the oases, are
naturally closely associated with soil differences. These are few
and, in nearly each case, to name is sufficient definition. There
are dunes (areg), hamada or stony desert, reg or alluvial desert,
the daya, and the flood-plain of the oueds (reg?). Modification
of the hamada, reg, and oued flood-plain occurs whenever white
hard-pan (caliche) is present. The soil of the reg, often that
of the oued flood-plain and that of the daya, is fine alluvial
and is relatively or actually deep. The hamada has the poorest
soil condition, being underlaid by rock, and often or always by
hard-pan as well. Large stones and boulders are embedded in the
soil or lie on the surface. A modification of the hamada occurs
whenever sand is strewn over its surface, even if the sand is only
a few centimeters in thickness. This acts as an effectual mulch,
increasing the retentive capacity of the soil, and very strikingly
changes the character of the vegetation. The final habitat to be
mentioned is the salt spot, or chott, where gypsum constitutes an
important salt.

As regards their relative importance the habitats in southern Algeria
can probably be grouped in the following sequence, a relation which
very possibly holds good for the Sahara taken as a whole: Hamada,
dune, oued, flood-plain, reg, daya.[36]

The habitats are unlike as regards the relation to the rainfall
and its effects. This is in part due to the differences in soils,
or their depth, and in part to topographical differences. By the
latter the low-lying areas receive a relatively large amount of
water; and since their soil is relatively deep the water is retained
longer than on the hamada, for instance, where the soil is shallow.

The habitat preferences of the plants of southern Algeria are
marked, as would be expected from the striking differences in
the habitats. On the dunes, for example, we find drinn (_Aristida
pungens_) as possibly the most commonly occurring and the most widely
distributed sand-plant. One finds on sandy areas also _Tamarix_ sp.,
_Euphorbia guyoniana_, _Ephedra_ sp., _Retama retam_, _Limoniastrum
guyonianum_ (zaita), and other forms in smaller numbers. On the
oued banks there are _Tamarix_, _Nerium oleander_ (Laghouat),
and, near the oases, date palm and other introduced plants. On the
flood-plains will be found a large number of species, among the most
typical of which are _Peganum harmala_, _Retama retam_, _Ephedra_,
_Genista_ sp., and _Haloxylon_ sp. The typical plants of the dayas
are _Pistacia atlantica_ and _Zizyphus lotus_, the latter occurring
on flood-plains and the reg as well. On the chotts we find mainly
such halophytes as _Anabasis articulata_, _Halocnemon strobilaceum_,
_Salsola_ sp., _Limoniastrum guyonianum_, and others. The flora of
the reg, so far as my observations show the conditions obtaining, is
essentially like that of the flood-plains, which would be expected
from the relation of the two habitats. South of Biskra, however,
where the reg is probably not of fluvial origin, one finds a forest
of _Zizyphus lotus_, and much _Ephedra_ sp. among the most striking
forms. Finally, the flora of the hamada, which has a peculiar stamp,
can be briefly characterized.

On the hamada are to be found the fewest species and the smallest
individuals. Probably most perennials of the hamada are under 30
cm. in height. Among the species characteristic of the hamada are
_Artemisia_, _Teucrium_, _Deverra_, _Centaurea_, _Acanthyllis_,
_Thymelæa_, _Echinops_, _Henophyton_, and _Haloxylon_. The last
named is possibly the most widely distributed, occurring in other
habitats as well.


     ROOT-CHARACTERS AND SPECIES DISTRIBUTION IN SOUTHERN ALGERIA.

The general principles bearing upon the relation between the type
of root-system and the distribution of the species, as observed in
southern Algeria, can be briefly stated. Often the relation is close
and apparent, but not always. For example, large perennials, such as
_Tamarix_ and _Zizyphus_, have an obligate specialized root-system,
with a long tap-root. These plants naturally occur where there is
considerable depth of soil, and hence are not to be found on the
hamada, for instance, where it is shallow. On the other hand, such
species as have a generalized root-system, like _Acanthyllis_ and
_Haloxylon_, are to be found on the hamada, but they occur also in
other habitats where the soil is deep. The last type of root-system
is flexible, accommodating the species to a wide range of soil
conditions. In doing this the change in form is almost a change
in type; for example, the roots of _Haloxylon_ on the hamada at
Ghardaia develop both laterals and a main root, but in deeper soil,
as at Biskra and Ghardaia also, the laterals are nearly suppressed
and the tap-root is the striking feature. A marked exception to
the rule that plants with a generalized type of root-system have
also the widest local distribution lies in _Peganum harmala_,
which, having roots of this character, is nevertheless restricted
to habitats where the soil is deep. The species is a half-shrub,
having a perennial subterranean portion and a short-lived subaerial
portion, the life of which appears to depend on the character of
the water-supply. As learned by Fitting, the species can develop
in its leaves a very dense sap, enabling it to extract water from
a very dry soil.[37] In spite of this fact it appears to act like
an annual in certain regards, requiring at all times, particularly
during the most arid season, a relatively good water-supply.

Thus, in brief, a study of the relation of the root-type of the
Algerian plants to the plant’s distribution leads to the same
general conclusion already obtained by similar but more extended
study in the Arizona desert, namely, that the connection is often
a very close one and often of definitive importance. Where the
root-type is an obligate type the distribution of the species is
much restricted, but where it undergoes modification with changed
environment the distribution of the species is much less confined. It
is of interest to note especially that as a rule it is the latter
kind of root-system that is developed by such plants as occur
where the soil conditions are most arid, that is, on the hamada
or its equivalent, and not the former, from which it follows that
the generalized type of root-system is really the xerophytic type
_par excellence_, and not the type with the most deeply penetrating
tap-root, as might be supposed.


                          THE BIOTIC FACTOR.

It will be well to summarize some of the main facts regarding another
and important environmental feature of the Saharan plants, namely,
the relation to herbivorous animals. In the western Sahara, wherever
there is any forage, animals which subsist on it are to be found. Of
the wild animals the gazelle is probably the most numerous and the
most destructive. All travelers across the desert have noted the
presence of this animal. In the northern Sahara Tristram remarked its
abundance fifty or more years ago, and it may be frequently seen by
the traveler at the present time. In addition to the native animals,
the domestic animals, especially the sheep, goats, and camels, are
very numerous, very destructive of plants, and range great distances
for food. As a result, an area around every well or oasis, extending
as far as 40 kilometers or even much more than this,[38] is repeatedly
grazed over and has been utilized in this manner for centuries. As a
result only the poisonous or the distasteful species, or the plants
especially well armed, are left undisturbed to grow and reproduce,
while the balance are more or less consumed, frequently so much so as
to be quite unrecognizable. From the large number of camels, sheep,
and goats which range the desert pastures it might be concluded
that the leading types of plants to be found would be such as are
not eaten by them, but this is not the case. On the other hand,
possibly the most generally consumed form is _Haloxylon_, which
grows on the oued flood-plain, the reg, and the hamada. Thus,
so far as this type is concerned, there is probably little or no
diminution in numbers because of the attacks of animals.

A similar conclusion would doubtless be drawn after study of other
forms, but there lies at least one notable exception, namely,
the influence of animal grazing on the distribution of the betoum
(_Pistacia atlantica_). The betoum, which is the largest arboreal
species in the Sahara, is confined to the region of the Dayas;
that is, to the country immediately south of Laghouat. The tree
is unarmed and is eagerly sought after by all herbivorous animals
for its foliage and tender twigs. Owing to the presence of such
animals, wild and domesticated, the young tree would have no chance
to survive were it not that, growing in association with it, is
the jujube (_Zizyphus lotus_), which is armed and is not eaten by
any animals. The jujube affords safe protection for the seedling
betoum, and in its capacity as nurse prevents predatory attacks by
animals during the critical period. The survival (and probably the
distribution as well) of the betoum is mainly conditioned on the
presence of its protector.

When I first visited southern Algeria it seemed improbable that any
portion of it, or at least any portion that I should be likely to
see, would exhibit the possibilities of plant growth as unaffected
by herbivorous animals. But finally there were found two classes of
plant formations in which animals either had not intruded at all,
or not to a harmful degree. One of these is the wide flood-plain
of the Oued M’Zab, or one of its tributaries, lying about 50
kilometers east of Ghardaia. Despite the fact that the flocks are
very numerous in the vicinity, and that the flood-plain is on the
regular caravan route between Ghardaia and Ouargla, there are few
signs of grazing. The entire plain is so well covered by shrubs
that the vegetation gives the tone to the landscape—a rare thing
in the desert. Here one finds _Retama retam_, _Genista saharæ_,
and _Ephedra_ sp., as well as other species in fair abundance. Thus
the plants are not only numerous, but are of a good size. It should
be observed that on either side of the flood-plain, on the hamada,
scarcely any vegetation may be found.

The second formation referred to is that of protected areas,
especially at Ghardaia, which have been little disturbed at any
time, and portions of them not at all disturbed for centuries. These
are the cemeteries. Such areas are situated not only in the floor
of the M’Zab Valley, where the plant conditions are relatively
favorable, but also on the hamada, where they are relatively very
unfavorable. Both in the valley and on the hamada, as well as on
the valley wall between the two wherever there chances to be soil,
the plants are relatively numerous and of fair size. This fact has
been detailed under the section on Ghardaia and need not be more
than mentioned here.

From these two general observations (exceptions to the usual
conditions), that on the flood-plain of the Oued M’Zab and the
cemeteries at Ghardaia, it is concluded that the grazing of animals
has had a very marked influence in modifying the flora of southern
Algeria. So far as could be told from the limited opportunity to
observe, the modification has gone along on at least two lines,
which are, of course, closely related. The size of the plants eaten
is much under normal for the particular locality, and at the same
time the capacity of the plant for reproduction has been greatly
lessened. That such species as are not touched by animals have not
spread more rapidly, or even have not become the dominant forms,
is an interesting problem, and one that would have to be worked out
for each species. It can be suggested, however, that the restricted
distribution probably lies in the fact that the struggle of desert
plants is mainly with an adverse physical environment rather than
with one another, and that such conditions would not be affected
by grazing animals. For example, large shrubs do not occur on the
hamada, hence _Tamarix_, _Zizyphus_, and _Rhus_, as well as _Peganum
harmala_, are limited to situations where the soil is fairly deep
and the water relatively favorable. _Euphorbia guyoniana_ also
occurs only in sandy soil, and the number of plants limited in
their distribution to soil characters is necessarily a large one.


   COMPARISON OF SOME GENERAL FEATURES OF THE VEGETATION OF SOUTHERN
                   ALGERIA AND OF SOUTHERN ARIZONA.

When we compare the most striking characteristics of the vegetation
of the Algerian Sahara with that of the Tucson region where
the Desert Laboratory is situated, we find some interesting
differences, which may be summed up in the terms “desert”
and “semi-desert,” as applied to the two widely separated
regions. What is meant by these terms will be apparent from the
following short characterization:

Passing into the Sahara from the Saharan Atlas, over the route which
I followed, one encounters a great variety of topography, of which
the most extensive may for the moment be classed as plains. The
plains are divided into three well-marked regions, that of the
dayas, the Chebka, and the Gantara (hamada). The topography is
further diversified by oueds and their flood-plains and by low,
flat-topped mountains. On the northern portion of the plains one
encounters a sparse population of low perennials, and as Ghardaia
is approached the plains vegetation becomes continuously poorer
until at Ghardaia there appears to be none. On the hamada between
Ghardaia and Ouargla areas are to be crossed, several kilometers
in width, where perennials are wholly lacking. The decreasing plant
population of the plains, until it entirely disappears, is entirely
due to the increase in aridity as one goes from the mountains to
and across the Ghardaia-Ouargla country. The low mountains are
almost entirely barren. The flood-plains of the oueds, however,
support a surprisingly luxuriant population of perennials.

Should we contrast the topography and vegetation of the Algerian
Sahara with that of southern Arizona we would find little that
is similar and much that is different. The wide-stretching plains
(bajada) of southern Arizona are well covered with perennials of
good size. The water-courses are fringed with trees, and often an
open forest is to be found on the flood-plains. The low mountains
have a fairly dense plant population, partly of trees, and the
lower mountain slopes are often covered with a mixed flora of
shrubs and trees. It may be said that there is probably no large
area in southern Arizona, where the soil conditions are favorable
for plants, where the water conditions are too meager to support a
perennial flora of some sort. The greater aridity of the northern
portion of the Sahara is evident, therefore, from the great contrast
in its flora.

In crossing the plains of southern Algeria one is likely to call a
region barren when close inspection will show that this is not the
condition. In fact, it was found that areas on the plain, 16 by
16 meters in extent, carried as many as 330 or more perennials,
although a casual glance did not reveal the presence of any
conspicuous vegetation.[39] Both of these conditions are the immediate
result of the small rainfall. The reason for the large number of
plants in certain areas, as above noted, lies in their small size,
since it would probably be difficult to find an equal number on
this area were the plants as large, for example, as in southern
Arizona. The fact that the perennials are inconspicuous is in part
because they are small and in part because the leaves are either
absent or greatly reduced.

When viewed somewhat more closely, one finds other features
in which the flora of southern Arizona and of southern Algeria
are unlike. Travelers, botanical as well as non-botanical, have
described the armed condition of the Saharan plants until the
impression is general that such plants as persist from season to
season are usually well provided with spines. What may be the
proportion of armed to unarmed plants in the northern Sahara I
do not know, but to a person familiar with the plants of southern
Arizona, where spinose forms are very numerous, the Algerian plants
do not appear especially well protected. As this appears to be a
general condition, it is scarcely an accident that the spines of
the American species of the genus _Zizyphus_, for example, are much
better developed than are those of the Algerian representative of
the same genus. From the circumstance that grazing by wild as well
as by domestic animals is very destructive in Algeria, apparently
more so than in Arizona, where the results of grazing are scarcely
to be noted, the general facts regarding spininess in plants, as
given above, suggest the really small influence such animals play
in shaping such a character in desert plants.

Finally, it need only be remarked that plants with a water-balance
are wanting in southern Algeria, and that they constitute one of
the striking features of the flora of the southwestern United States.




[Plate 2:

Illustration: Fig. 4. Shoot-habit of Acanthyllis tragacanthoides.
 Laghouat.

Illustration: Fig. 5. Shoot-habit of Zollikoferia spinosa. Laghouat.

Illustration: Fig. 6. Acanthyllis tragacanthoides on sandy plain.
 Laghouat.

Illustration: Fig. 7. Zollikoferia spinosa in habitat, plain
 (hamada). Laghouat.]

[Plate 3:

Illustration: Fig. 8. Detail of north slope of Nomad Mountains where
 Zollikoferia spinosa is the dominant species. Laghouat.

Illustration: Fig. 9. Vegetation of plain (hamada) at Tilrempt. The
 conspicuous shrub is Haloxylon articulatum.

Illustration: Fig. 10. The Daya of Tilrempt from the plain, showing
 the character of the depression.]

[Plate 4:

Illustration: Fig. 11. Near view of the Daya of Tilrempt. The
 fortified stage station, bordj, and a nomad camp are to be seen. The
 flattened, level, lower surface of the trees is the effect of
 grazing, mainly by camels.

Illustration: Fig. 12. Jujube (Zizyphus) shrubs and betoum (Pistacia)
 at the Daya of Tilrempt.]

[Plate 5:

Illustration: Fig. 13. A young specimen of betoum in the midst of a
 protecting jujube, at the Daya of Tilrempt.

Illustration: Fig. 14. Leaf and shoot habit of the jujube (Zizyphus
 lotus). Daya of Tilrempt.

Illustration: Fig. 15. Leaves of the betoum (Pistacia atlantica),
 from the Daya of Tilrempt.]

[Plate 6:

Illustration: Fig. 16. South wall of the valley of the Oued M’Zab at
 Ghardaia.

Illustration: Fig. 17. Detail of an eroded bank of the Oued M’Zab at
 Ghardaia. The overhanging stratum is hardpan similar to the “caliche”
 of southwestern United States.]

[Plate 7:

Illustration: Fig. 18. Shoot-habit of Haloxylon articulatum, from the
 plain near the Daya of Tilrempt.

Illustration: Fig. 19. Agriculture at Ghardaia. The fields are
 divided into plots about 3 by 4 feet in size, for the more economical
 use of water. Young barley is shown growing (November).]

[Plate 8:

Illustration: Fig. 20. Vegetation in upper part of a “draw” on plain
 north of the M’Zab Valley Ghardaia.

Illustration: Fig. 21. Situation of square No. 2. on plain north of
 valley at Ghardaia.]

[Plate 9:

Illustration: Fig. 22. Capparis spinosa at base of valley wall at
 Ghardaia.

Illustration: Fig. 23. Leaves of Capparis spinosa, from Ghardaia.]

[Plate 10:

Illustration: Fig. 24. Leaf-habit of Dæmia cordata. Ghardaia.

Illustration: Fig. 25. Shoot-habit of Salvia ægyptica. Ghardaia.

Illustration: Fig. 26. Upper surface of rocks, showing small
 incrusting lichens, on a low mountain about 5km. north of the M’Zab
Valley, Ghardaia.]

[Plate 11:

Illustration: Fig. 27. Habitat of Peganum harmala, at Melika,
 Ghardaia.

Illustration: Fig. 28. Shoot and leaf habit of Peganum harmala, from
 Ghardaia.]

[Plate 12:

Illustration: Fig. 29. Habit of Henophyton deserti at Ghardaia when
 growing in a protected situation (cemetery).

Illustration: Fig. 30. Leaf-habit of Henophyton deserti. Ghardaia.]

[Plate 13:

Illustration: Fig. 31. View in a cemetery at Ghardaia, to show the
 relatively abundant vegetation.

Illustration: Fig. 32. View in an M’Zabite cemetery, Ghardaia.
 Haloxylon articulatum is the leading species shown.]

[Plate 14:

Illustration: Fig. 33. Habit of Deverra scoparia, from plain about
 3km. north of M’Zab Valley, Ghardaia.

Illustration: Fig. 34. Root-habit of Gymnocarpos fruticosum, from
 plain about 3km. north of M’Zab Valley, Ghardaia.]

[Plate 15:

Illustration: Fig. 35. Root-habit of a mature specimen of Peganum
 harmala, from the floor of the M’Zab Valley near Ghardaia. The soil
 at the place is comparatively deep. (See text for further
 explanation.)

Illustration: Fig. 36. Root-systems of Helianthemum sessiliflorum
 (right), Haloxylon articulatum, and Nolletia chrysocomoides (left),
 from the flood-plain of the Oued M’Zab, Ghardaia.]

[Plate 16:

Illustration: Fig. 37. Shoot and root habit of Citrullus colocynthis.
 Oued M’Zab, Ghardaia.

Illustration: Fig. 38. Euphorbia guyoniana, in the valley of the Oued
 M’Zab at Ghardaia.]

[Plate 17:

Illustration: Fig. 39. Habit of Euphorbia guyoniana. Ghardaia.

Illustration: Fig. 40. To the left, shoot of Centaurea sp., showing
 effects of grazing; to the right, shoots of Teucrium
 pseudo-chamæpitys. From Ghardaia.]

[Plate 18:

Illustration: Fig. 41. Habit of Salsola sp. (below), and Echinopsilon
 muricatus, from the M’Zab Valley, Ghardaia.

Illustration: Fig. 42. Ghardaia to Ouargla. View overlooking the
 hamada about 28km. from Ghardaia. The relatively abundant vegetation
 is associated with a light cover of sand over the area shown. The
 leading species are Aristida, Deverra, and Haloxylon.]

[Plate 19:

Illustration: Fig. 43. Retama retam, in dunes about 58km. from
 Ghardaia.

Illustration: Fig. 44. Dates at the bordj Zolfana, about 58km. from
 Ghardaia. One of two wells encountered between Ghardaia and Ouargla.

Illustration: Fig. 45. Ghardaia to Ouargla. View overlooking
 flood-plain of the Oued M’Zab, or a tributary of this oued. The
 adjacent upland is apparently without plant life.]

[Plate 20:

Illustration: Fig. 46. Vegetation on edge of the Oued M’Zab, about
 63km. east of Ghardaia, showing habitat of Rhantherium adpressum in
 foreground.

Illustration: Fig. 47. Sandy flood-plain of the Oued M’Zab, about 63
 km. from Ghardaia. Retama, Genista and Ephedra are the leading plants
 in this place.

Illustration: Fig. 48. Habit of Ephedra alata in habitat shown in
 figure 47. This specimen was 1.5m. high.]

[Plate 21:

Illustration: Fig. 49. View of habitat of Ephedra alenda, 138km. from
 Ghardaia.

Illustration: Fig. 50. Detail of suckering habit of Ephedra alenda,
 from habitat shown in figure 49.]

[Plate 22:

Illustration: Fig. 51. Vegetation, mainly Ephedra and Retama, of the
 western edge of the Chott Mellala.

Illustration: Fig. 52. Approach to western edge of the Chott Mellala,
 showing characteristic rounded hills, or mamelons.]

[Plate 23:

Illustration: Fig. 53. View between the Ouargla plain and the Chott
 Mellala, showing characteristic appearance of eroded hills.

Illustration: Fig. 54. Looking toward the Ouargla plain (reg).

Illustration: Fig. 55. Shallow well about 25km. north of Ouargla.]

[Plate 24:

Illustration: Fig. 56. Ouargla to Touggourt. Leaf-habit of
 Limoniastrum guyonianum. The surface of the leaves is covered with an
 incrustation of salts.

Illustration: Fig. 57. Habit of Limoniastrum guyonianum. About 25km.
 north of Ouargla.]

[Plate 25:

Illustration: Fig. 58. Vegetation of the reg. desert, about 25km.
 north of Ouargla. Ephedra and Retama are the leading species of the
 area a spreading dune.

Illustration: Fig. 59. Shoot-habit of Halocnemon strobilaceum, about
 28km. north of Ouargla.]

[Plate 26:

Illustration: Fig. 60. Habit of Halocnemon strobilaceum, on the edge
 of a chott, about 28km. north of Ouargla.

Illustration: Fig. 61. Shoot and leaf habit of Anabasis articulata,
 about 32km. north of Ouargla.]

[Plate 27:

Illustration: Fig. 62. Detail of the shoot-habit of Salsola
 tetragona?, about 25km. north of Ouargla.

Illustration: Fig. 63. Habit of Nolletia chrysocomoides near the edge
 of a chott, about 80km. north of Ouargla.]

[Plate 28:

Illustration: Fig. 64. Tamarix sp. as a sand-binder near
 Bled-el-Amar, south of Touggourt.

Illustration: Fig. 65. Biskra. Habitat of Euphorbia guyoniana,
 looking toward the Djebel Maouya, with the Chaine de Sfa in the
 background.]

[Plate 29:

Illustration: Fig. 66. Characteristic vegetation on the north slope
 of the Djebel Bou Rhezal, Biskra. Haloxylon scoparium is a prominent
 species.

Illustration: Fig. 67. North base of Ed Delouatt hills, west of
 Biskra, showing the low-facing dunes. To the right is a glimpse of an
 oued which pierces the hills in the middle distance.]

[Plate 30:

Illustration: Fig. 68. Flood-plain of the Oued Hamman es Salahin,
 Biskra. Halophytes of various species occupy the foreground.

Illustration: Fig. 69. Habitat of Phelypæa violacea shown in figure
 70.]

[Plate 31:

Illustration: Fig. 70. Young shoots of Phelypæa violacea, at north
 base of Ed Delouatt hills, Biskra.

Illustration: Fig. 71. Habit of young specimens of Phelypæa. Except
 for about 15cm., plants were buried by sand. Biskra.

Illustration: Fig. 72. Asphodelus sp. at north base of Ed Delouatt
 hills, Biskra. Photographed in March.]

[Plate 32:

Illustration: Fig. 73. Detail of square No. 1, on low hills north of
 Biskra.

Illustration: Fig. 74. Vegetation of north slope of the low hills
 which are north of Biskra. Ferula vesceritensis is the perennial
 appearing in the figure.]

[Plate 33:

Illustration: Fig. 75. Detail of square No. 2, neat area shown in
 fig. 74. Ferula and Haloxylon are the leading species.

Illustration: Fig. 76. Root-habit of Haloxylon scoparium, from a wash
 near Biskra.

Illustration: Fig. 77. Young shoots and mature roots of Ferula
 vesceritensis. North of Biskra.]

[Plate 34:

Illustration: Fig. 78. Root-habit of Fagonia growing on edge of a
 wash. Biskra.

Illustration: Fig. 79. Large lateral root of Haloxylon, with numerous
 deciduous rootlets, no longer functional.

Illustration: Fig. 80. Shoot-habit, taken from above, of Fagonia from
 the flood-plain of a small oued near Biskra.]

[Plate 35:

Illustration: Fig. 81. Spring annuals, March 17th, on north slope of
 the Djebel Bou Rhezal, Biskra.

Illustration: Fig. 82. Root and shoot habit of Peganum harmala,
 Biskra. The main root is especially well developed although the
 species has a generalized root-system. (See text.)]

[Plate 36:

Illustration: Fig. 83. Annuals growing with Peganum, near Biskra.

Illustration: Fig. 84. General view of the north face of the Djebel
 Bou Rhezal, Biskra. Apparently barren, plants are rather numerous in
 the rock crevices and small washes of the mountains.]




FOOTNOTES:


[Footnote 1: Kearney and Means, Agricultural explorations in Algeria,
Bul. No. 80, Bur. Plant Ind., U. S. Dept. Agric., 1905.]

[Footnote 2: A. Engler, Die Vegetation der Erde IX. Die Pflanzenwelt
Afrikas. 1 Bd., 1910, page 902.]

[Footnote 3: The seasonal distribution of rain (by percentages)
is somewhat different from that given by Engler, which is given in
the accompanying table.

  +---------+-----------+-------+---------------+
  | Season. | Littoral. | Tell. | High plateau. |
  +---------+-----------+-------+---------------+
  | Winter  |    41     |   36  |      45       |
  | Spring  |    27     |   32  |      46       |
  | Summer  |     4     |    7  |      11       |
  +---------+-----------+-------+---------------+

]

[Footnote 4: The most recent records available, 1897-1908, do not
give a satisfactory account of the precipitation on the desert. For
instance, meteorological records covering eight years for El Golea
do not report on the rainfall. Records of five years at In Salah
take the rainfall into account on one year only, and on that year
no precipitation occurred.]

[Footnote 5: Unless otherwise stated, the climatological statistics
given in this paper were taken or compiled from Observations
Météorologiques du Riseau Africain, 1907-1908. The evaporation
data are based on readings of the Piche evaporimeter. The amount of
evaporation given in the text can be reduced to the evaporation from
a free-water surface by multiplying by 0.737 (Meteorological Notes,
J. I. Craig. Cairo Scientific Journal, vol. vi, May 1912).]

[Footnote 6: At Touggourt, however, no rain was reported during
the summer season of 1908.]

[Footnote 7: Kearney and Means, _loc. cit._]

[Footnote 8: The Root Habits of Desert Plants. W. A. Cannon. Carnegie
Institution of Washington Publication No. 131. 1911. This paper gives
a descriptive classification of the main root-types in the deserts
of the southwest, in which such a root as found in _Acanthyllis_
is called _generalized_, in distinction to roots like those of
most of the cacti, or _Zizyphus_, the former having a system
wholly superficial and the latter a system wholly deeply placed,
as _specialized_. The specialized root-systems appear to be so fixed
in character as to be not easily changed, while the generalized type
is flexible. It will be self-evident that the type of root-systems
may be of great importance in determining the local distribution
of a species.]

[Footnote 9: Compare the root-system of _H. scoparium_ at Biskra,
p. 64.]

[Footnote 10: The most striking change in the general character of
the vegetation which the traveler notices as he goes from the less
arid to the more arid portion of southern Algeria is its decrease
in amount. This occurs through dwarfing effects of whatever cause
and through a decrease in the number of individuals. Within certain
limits the results observed are to be attributed mainly to the
first of these, since there is often a surprisingly large number of
perennials on any given area. But in other and more intensely arid
regions (as portions of the Arabian-Egyptian desert, and indeed
a limited area on the hamada between Ghardaia and Ouargla) plants
are wholly wanting. Whether such is generally the case on the reg
or the hamada farther south in southern Algeria is not known.]

[Footnote 11: Tristram remarks that it “seems that the larger wild
animals have been rapidly decreasing in numbers and are in process
of speedy extinction. Dr. Shaw, 150 years since, enumerates in his
travels . . . five species of ruminants, which from his descriptions
must be the bubale, the aoudad or wild sheep, the addax, and the
gazelle, as well as the stag. . . . As the population has not
increased, but rather retrograded, we can only surmise that the
substitution of the flint and steel gun for the matchlock of the
Bedouin . . . has been fatal in its results to all larger game.”
It may be remarked that the French impose such restrictions on
the Arab as regards the character of the guns he may use (only the
army and certain officials of the government employing modern arms)
that for the region visited Tristram’s description holds fairly
well for to-day.]

[Footnote 12: Statistique Générale de l’Algérie, 1908.]

[Footnote 13: Un voyage botanique au Sahara, Bull. Soc. Bot. Belg.,
1898.]

[Footnote 14: Massart, _loc. cit._]

[Footnote 15: Joan’s Guide de l’Algérie et de la Tunisie.]

[Footnote 16: Massart, _loc. cit._, p. 314, suggests that the Sahara
may be gradually becoming more arid and says that as a result the
betoum is becoming more and more rare and may become extinct. He says
“L’extinction du Pistacia atlantica présente le caractère, tout
à fait exceptionnel, d’ètre uniquement l’effet du climat.” It
will appear from what is said in this study regarding the effects of
grazing in general, as well as the especial effects on the betoum,
that if the betoum is becoming extinct the sole cause, or perhaps
the main cause, is not the adverse climate.]

[Footnote 17: Dr. Charles Amat, Le M’Zab et les M’Zabites,
p. 70, gives a somewhat higher temperature for the rocks of the
southern Chebka, placing it at 90° to 100° C., or even higher.]

[Footnote 18: Les Oasis du Souf et du M’Zab, La Géographie, 1902.]

[Footnote 19: Foureau, d’Alger au Congo par le Tchad, 1902,
mentions having met with indications of early settlement of the
Sahara by people now forgotten, and whose tombs, inscriptions, and
other remains, were well known by his Touareg servants, although
not at all understood by them. So far as I have learned, however,
it is not supposed that the region of the M’Zab was inhabited
before the coming of the Beni M’Zabs.]

[Footnote 20: There are seven cities of the Beni M’Zab, of which
five lie in the M’Zab Valley, close to one another. These are
El Ateuf, Ben Noura, Melika, Beni Isguen, and Ghardaia. In the
pre-French times these cities were bound together in a confederacy
with Ghardaia as the capital. The M’Zabites are at present,
and probably always have been, a peaceful trading folk. They are
heterodox Moslems. In an early time they aroused the antagonism of
their more warlike as well as more orthodox Arab neighbors of the
Tell, who drove them away from the coast region, and again from
Ouargla and other places settled by them, until safety was at last
secured in the eleventh century in the “inhospitable Chebka.”
Palm gardens were established which for centuries have been irrigated
laboriously by very primitive methods, and the inhabitants have
accumulated wealth in flocks and by barter. The relatively large
population (there were 92,761 inhabitants in 1908), the really great
number of domestic animals, and the great length of time which the
region has been occupied, are all factors of importance in bringing
about a modification in whatever way of the primitive flora.]

[Footnote 21: In 1908, according to the Statistique générale de
l’Algérie, there were cultivated in the territory of Ghardaia
572,114 fruit trees, among which were: almond, 5,850; fig, 101,722;
date palm, 209,898; other sorts of fruits, 211,761. There were also
17,268 hectares of grain under cultivation.]

[Footnote 22: We were informed by our Arab attendant that the kabar,
particularly the fruit, made such animals as ate it insane. The
spicy flavor of the plant might otherwise be distasteful to animals.]

[Footnote 23: Fitting (Die Wasserversorgung und die osmotischen
Druckverhältnisse der Wüstenpflanzen, Zeitschr. f. Bot., 4,
209-275, 1911) states that water-storage tissue is wanting, that
in addition to being large, the leaves are much divided, without
trichomes, and provided with thin cuticle. The stomata are fairly
large, rather numerous, and not sunk. The leaves transpire rapidly
and wilt soon after being removed from the stem. The osmotic
pressure of the cell-sap of the leaves was found to equal 35.3
to 64 atmospheres, from which it is assumed that this plant, like
others growing under desertic conditions, has a root cell-sap of
great osmotic pressure, which permits it to extract water from a
fairly dry soil or at a rather rapid rate. But neither at Biskra
nor elsewhere, so far as I know, does _Peganum_ grow where the
conditions are extreme, as might be concluded from the habit of
the plant as given above.]

[Footnote 24: It has already been shown that the number of days on
which rain may be expected to fall each year is greater at Laghouat
than at Ouargla, and probably at Ghardaia also. According to reports,
the rainy days at Laghouat vary from 20 to 84 (seven years’
observation), with an average of 49 each year. The average number of
days on which rain falls at Ouargla is 14.2. The amount of rain at
Laghouat is 200 mm., at Ouargla 90.2 mm. It would appear, therefore,
that the average rain at Laghouat is less in amount than the average
rain at Ouargla; or, in other words, it points to the torrential as
being the type of the desert storm. Since, other things being equal,
the greater storms would penetrate the ground the most deeply, we may
here have an explanation of the emphasis at Ghardaia on the tap-root
as against the generalized root as the type of the root-system.]

[Footnote 25: The Root Habits of Desert Plants, _l. c._]

[Footnote 26: Compare the root-system of the species at Biskra,
p. 64.]

[Footnote 27: In the vicinity of Tucson (see The Root Habits
of Desert Plants) is to be found a slender-stemmed _Opuntia_
whose roots are fleshy and are placed within 2 to 4 cm. of the
surface of the ground. It has been observed that if the roots are
examined in the midst of a dry season, as in June, they are gorged
with water, but if the soil is removed for a few hours they become
shriveled. A similar habit was seen in another species of the same
genus. Two other genera of the cacti from the Tucson region have
the water-storage organs wholly or partly protected by the soil. In
_Cereus greggii_ the subterranean portion forms an organ 15 to 30
cm. in diameter, and in the other form the fleshy subaerial stem
is partly drawn under the surface of the soil, so that only the
flat upper surface is visible.]

[Footnote 28: In the case of annuals the differences in development
of the shoot between plants well watered and those with only a meager
supply are very striking. In one instance in the Tucson region
specimens of _Parietaria debilis_ growing in extreme conditions,
one moist and the other arid, varied in length between 39 cm. and
8 mm., or a difference with a ratio of 49 to 1. (Root Habits of
Desert Plants, _loc. cit._)]

[Footnote 29: The vegetation in the vicinity of Biskra is so well
known that a sketch will suffice as a basis of comparison with the
flora and conditions of plant life farther south.]

[Footnote 30: Liste des plantes observées aux environs de Biskra
et dans l’Aurés, Trabut _et al._, Alger, 1892.]

[Footnote 31: Briefly the case is as follows (see The root systems of
desert plants, _loc. cit._): _Opuntia arbuscula_ growing near Tucson
develops fleshy roots, but what is probably the same species growing
about 100 miles distant has fibrous roots. Also, seedling opuntias
have fleshy roots. _Opuntia vivipara_, which occurs naturally in
the bottom of an arroyo (oued), may or may not have fleshy roots. By
preliminary series of experiments it was learned that all opuntias
tested which had an abundant water-supply developed fleshy roots,
and it is assumed from this that the differences in this character
as observed in nature had also such a physiological basis.]

[Footnote 32: Eine botanische Exkursion nach Algier and Tunis,
Bericht der Senckenbergischen Naturforschenden Gesellschaft in
Frankfort a. M., p. 76, 1910.]

[Footnote 33: Plants with subterranean water-storage organs—bulbous
plants—are said to be a feature of the High Plateau.]

[Footnote 34: Through Timbuctu and across the Great Sahara, 1912,
p. 266.]

[Footnote 35: Soil samples were taken from the plain about 3
kilometers north of Ghardaia from an area where the vegetation is
relatively good (see p. 40). The surface of the soil at the place
is slightly depressed. Soil samples from the depressed area and
samples from portions of the plain adjacent to it were settled
under water, with the result that the proportion of fine soil was
found to be less in the depressed area. Samples of the soil from
the lower area were placed in air-tight cans and the moisture
content determined subsequently. The soil was found to contain
0.8 per cent water. Through the kindness of the Bureau of Soils,
U. S. Department of Agriculture, the critical moisture-content of
the same soil was determined, which was 5 per cent. The critical
moisture-content of mesa soil, taken from the creosote-bush slope
at the foot of Tumamoc Hill, Desert Laboratory, as determined by
the Bureau of Soils, is 10.5 per cent, which forms an interesting
comparison of nearly similar situations.]

[Footnote 36: The mountains have been disregarded, since in southern
Algeria they are nearly barren. The only exception to this that
I saw was that of the crustaceous lichens in small numbers at
Ghardaia. In the central Sahara, however, where the mountains are
of great elevation, the mountain climate brings about favorable
conditions for plant life.]

[Footnote 37: Die Wasserversorgung und die osmotischen
Druckverhältnisse der Wüstenpflanzen. Zeitschr. f. Botanik,
4, 1911.]

[Footnote 38: Hayward, _loc. cit._, p. 320, says that at In
Salah camels are driven 200 kilometers before finding suitable
grazing-grounds.]

[Footnote 39: It should be understood that such densely populated
areas are separated by wide stretches where are few or no plants.]




Transcriber's note:


pg 15 Added period after: vegetation of the country

pg 17 Changed: _Quercus bollota_ to: _ballota_

pg 24 Changed: _Rhus oxycantha_ to: _oxyacantha_

pg 25 Changed: _Artistida pungens_ to: _Aristida_

pg 32 Changed: Astericus pygmæus to: Asteriscus

pg 35 Changed: _Petama spherocarpa_ to: _Retama sphærocarpa_

footnote 21 (pg 38) Changed: Statique to: Statistique

pg 43 Changed: were in in pre-M’Zabite to: were in pre-M’Zabite

pg 43 Changed: iteslf to: itself

pg 50 Changed: _Zillikoferia_ to: _Zollikoferia_

pg 53 Changed: Fig. 441. to: 44.

pg 54 Changed: _Astericus graveolens_ to: _Asteriscus_

pg 55 Changed: _Tragonum nudatum_ to: _Traganum_

pg 56 Changed: _Phargmites_ to: _Phragmites_

pg 57 Changed: Maricandia cinerea to: Moricandia

pg 57 Changed: Haloxylon articulata to: articulatum

pg 57 Changed: Sorzonera nudulata to: Scorzonera undulata

pg 59 Changed: P. albans to: albicans

pg 63 Changed: _Ferula visciritensis_ to: _vesceritensis_

pg 65 Changed: _Ferula vesciritensis_ to: _vesceritensis_

pg 73 Changed: edaphacic to: edaphic

Other spelling and formatting inconsistencies have been left
unchanged