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.