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).