126 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
of this preference, but do not make it clear, nor in any wise probable, 
that no other factors are involved. The behavior of the sahuaro and 
Encelva on southern exposures, their high temperature requirement, and 
their limitation in altitude and latitude, taken in connection with tem- 
perature records, all point to temperature as being of prime importance 
in determining the aspect preference of these plants. Similarly, the 
behavior of Lippia wright, evidently a plant of somewhat lower tem- 
perature requirements, finding its home on the north side of Tumamoc 
Hill at an altitude of 2,600 feet, but at 3,000 to 4,000 feet greater ele- 
vation requiring the warmth of southern exposures and occurring exclu- 
sively on them, leads to the same conclusion. It may well be, however, 
that many of the plants which on the Laboratory domain affect northern 
exposures find there congenial conditions not simply because they can 
sustain a lower temperature in winter or because the heat of summer 
is tempered, but because evaporation is less there than on southerly 
aspects. And even if at the outset, as seems plain from observations 
in the gulch near the Laboratory, temperature is proven to have been 
the controlling factor, changes taking place with the lapse of time, such 
as the accumulation of vegetable mold and the better retention of soil- 
moisture, accompanied by a relatively low rate of evaporation, may 
together produce a total of conditions which are semi-mesophytic in 
comparison with those of the bare rocks and rapidly drying soil of south- 
erly aspects as they lie exposed to the fierce glare of the sun. 
With this complicated set of relations it is apparently impossible at 
present to analyze aspect preferences causally further than to say that 
differences of temperature, corresponding with different temperature 
requirements on the part of the plants themselves, are without doubt of 
primary importance, while the degree to which other factors are involved 
is, aS yet, uncertain. Measurements of evaporation which are now being 
made may throw light on the problem. 
Thus, along all the several lines that have been followed, the evidence 
has accumulated rendering the conclusion a necessary one that percent- 
age of soil-water and temperature are two factors of the first importance 
in determining the local distribution of desert plants. There is also 
abundant evidence that aeration of the soil is still another highly impor- 
tant factor, but the data in regard to this are not sufficiently definite 
to admit of quantitative expression. The influence, in some cases con- 
spicuous, of high percentages of alkali and the complex of factors deter- 
mining evaporation must be admitted to the same category. 
That this conclusion is what might have been expected does not affect 
its value. It has all along been obvious that desert plants are governed 
by the same general laws in their relation to environmental conditions 
as are those of other regions, and the factors determining their distri- 
bution might a priori be assumed to be the same. ‘They exhibit, how- 
