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.