30 DISTRIBUTION AND MOVEMEN’S OF DESERT PLANTS. 
as the aspect becomes more southerly. But south aspect involves greater 
heat, and here, too, a steep slope also results in greater heat, at least 
during parts of the year, because of the more nearly perpendicular inci- 
dence of the sun’s rays. The accompanying map (plate 13) shows the 
almost entire absence of Encelia from north slopes and from the gentle 
slopes below the 2,500-foot contour. 
It appears, then, that FEncelia farinosa is limited to areas having a 
relatively high degree of heat and moisture. It grows more densely than 
any other large plant of the Laboratory domain, and is well limited 
to its own particular habitat. Within this habitat it is limited (a) by 
sheer cliffs; (6) by such an increase of rocks as to amount to bare talus; 
(c) by certain areas of creosote-bush, which may possibly prove signifi- 
cant as denoting an unusually great distance between surface and water. 
LARREA TRIDENTATA. 
The creosote-bush is the most abundant, and with the exception of 
Parkinsoma microphylla, and possibly Acacia constricta, the most widely 
and continuously distributed ligneous plant of the Laboratory domain. 
At the same time it is no less limited than Encelia, as regards its best 
habitat, by certain physiographic features. It occupies, either exclu- 
sively or as the major element, the gentle lower slope between the moun- 
tains and the flood-plain. 
Its upper limit, as a major element, is often very definitely marked 
by the same few degrees of difference in gradient as the lower one of 
Encelia, hence the two come together on the contour in such places (com- 
pare plates 13 and 14). Its lower limit, along the valley, is marked with 
absolute definiteness by the edge of the flood-plain, and accordingly 
its principal habitat is seen to coincide with the areas of coarse, gravelly, 
whitish, transported soil, devoid of large fragments of rock, which char- 
acterize the lower slope as already defined. 
Outside of its main belt the Larrea comes in strong in certain places of 
gentle slope and deep soil, places that have evidently been filled up in the 
past, but at present are wearing down. Some of these places are situated 
in the heart of the Encelia belt, but Encelia is absent. This confirms 
what the main belts show, namely, that Encelia and Larrea are of dis- 
tinctly opposite tendencies in their relation to certain determining factors 
of distribution. Encelia, as we have seen, follows areas having a relatively 
high degree of heat and moisture. Larrea, on the other hand, is preemi- 
nently a plant of well-drained erosion areas. ‘The main Larrea belt 
between the hills and the flood-plain is constantly wearing down, as is 
seen, among other things, from the numerous small gullies over this 
area, and the exposed sahuaro roots on the plains beyond the wash, where 
there is a strong growth of Larrea, tell the same story; even the bottom 
of the wash, where Larrea is present in large numbers, is at present wearing 
