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.