12 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
mesquite, which attains the size of a tree on the plain, is a mere shrub on 
the hill, and Bigelowra hartwegu, which grows with the luxuriant habits 
of a weed in the former situation, is scattering and small in the latter. 
It appears, then, that the flood-plain is the natural habitat of a number 
of species, many of which are incapable of successful growth elsewhere, 
while a few grow fairly well, but not at their best, under different condi- 
tions beyond these limits. Taken as a whole, the plants of the flood 
plain find there their real home, and they exhibit—such of them, at least, 
as have been carefully observed with reference to this—a striking con- 
formity of the root-system to the peculiarities of the soil in which they 
are growing. This, as already stated, is of fine texture, retentive of 
moisture, and of great depth, with the water-table varying in level, but 
apparently never beyond the reach of the long tap-roots of the mesquite 
and Acacia. 
The root-system of these plants consists of a tap-root which grows 
rapidly downward, and when developed is always within reach of a per- 
manent, deep water-supply, and a system of widely spreading lateral 
roots which are in relation to more superficial layers of the soil. Thus 
the plant is admirably fitted to absorb water largely from the upper 
layers when these are moist, and at the same time, and also in times of 
drought, without any interval of precarious supply, to draw on the deeper 
sources. below. The contrast between this and the shallow root-systems 
of many of the great trees of eastern mesophytic forests, familiar to 
everyone who has seen them uprooted by heavy winds, is highly instruc- 
tive. There is little wonder that the mesquite and Acacia constricta 
have tenaciously held their places through all vicissitudes and promise 
to be dominant in their habitat until actually rooted out. 
There are evident movements of vegetation now taking place within 
this association. Bzigelowia hartwegu, a native weed, has spread with 
wonderful rapidity under the mesquite within the last three years of 
favorable seasons; and other species, both weeds and useful plants, have 
been brought in at various times and by different agencies to such an 
extent as to give to the vegetation, in places, a distinct change of char- 
acter. It should be added that Bigelowia (and presumably other low- 
growing plants of the flood-plain) has a far less extended root-system 
than the mesquite, obtaining water from relatively near the surface. 
If the root-system does not reach to the water-table, and this can hardly 
occur, it would seem that this plant, and others of this association of the 
same physiological class, must be more xerophytic in habit than the mes- 
quite. It seems that the flood-plain may be thought of as the home of 
two quite different sets of plants, one with a tap-root and its branches 
reaching the water-table, and the other depending upon the water con- 
tained in upper layers of the soil; the former approaching mesophytic, 
the latter more definitely xerophytic habits. 
