16 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
age. In view of their habits, it may fairly be questioned whether such 
xerophytic structures as some of the salt-bushes exhibit stand primarily 
in relation to “physiologically xerophytic”’ soil conditions or not; in 
fact, it seems not at all improbable that they may be found to fall into 
the same category with the mesquite and catclaw, exhibiting certain 
xerophytic structures which are useful in the dry air of the desert, and 
which also in cases of extraordinary drought successfully regulate tran- 
spiration and make less draft on the resources of the plant. 
(6) THE MESA-LIKE SLOPES. 
(a) CREOSOTE-BUSH ASSOCIATION. 
The long, gentle slop2 of the “mesa,” as it is commonly but incorrectly 
called, rises from the flood-plain, at first with an almost imperceptible 
grade, becoming steeper as the ascent continues to the hill above. Its 
soil, as already stated, is mainly gravelly or sandy, with but little loam 
and with a considerable amount of caliche. ‘The slope and the nature 
of the soil insure perfect drainage and aeration. But with the poor soil, 
with relatively little capacity for the retention of water, it is plain that 
we have passed, almost at a step, from soil conditions that may be classed 
as mesophytic, or semi-mesophytic, to those that are suited only to dis- 
tinctively xerophytic plants, such as grow where peculiarly trying con- 
ditions prevail. Under these conditions, as might have been anticipated, 
nearly all of the perennial species of the flood-plain abruptly cease; in 
fact, the creosote-bush is the single one that. is capable of successfully 
holding the ground on the worst places, and even this is dwarfed in the 
struggle to maintain existence where the water-supply is never abundant 
for any length of time, and through a large part of the year is necessarily 
meager in the extreme. Yet, in spite of these untoward conditions, the 
Larrea claims this zone as its own, and has produced upon it an almost 
pure growth of a single species (plate 8). 
It is impossible, as yet, to enumerate completely the peculiarities of 
this plant, structural and physiological, which have enabled it, more 
successfully than any other, to cope with the worst of desert conditions 
as they exist in the Southwest. Some of these, however, are obvious. 
It is provided with a root-system (plate 7) which both penetrates deeply 
and also branches widely near the surface, thus being in a position to 
avail itself of water that accumulates at either level; it is capable, as few 
other species are, of absorbing appreciable quantities of water through 
its leaves, and these are admirably protected by varnish and in other 
ways against excessive transpiration. With the coming on of drought 
a large percentage of its leaves are thrown off and a still further reduction 
of surface is accomplished by the leaves becoming checked in their devel- 
opment long before they have attained their full size, as seen in individ- 
