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.=