REVIEW AND DISCUSSION. 125 
groups is confined to the flood-plain and wash, but their habits and 
physiological requirements are such as to make these habitats available 
for their occupation. 
The account that is given of the march of the soil-moisture content 
is no less important in its bearing on the relations of desert plants to 
soil-moisture. The lagging of the effect of precipitation behind the 
march of the precipitation itself, and a lagging in the opposite direction, 
graphically shown by curves, go far to explain the fresh condition which 
plants of this region, particularly those of the hill, exhibit after a com- 
paratively long period in which no rain has fallen. 
An important observation to the effect that the moisture conditions 
of the surface layers of the soil must be regarded as the prime factor in 
determining germination, thereby imposing inevitable limitations to what 
can be accomplished by the plants of the Laboratory domain in even 
making a start toward its continued occupation by a natural renewal, 
corresponds perfectly with what is observed, for example, in the deport- 
ment of the winter and summer annuals, which has already been described. 
Without entering further into details, which, however, can be given 
in any number by one familiar with the ground, it may be said that so 
far as relates to determinations of soil-moisture the facts established by 
this investigation show a remarkable degree of correspondence with the 
facts of distribution as already observed. Such correspondence can not be 
accidental, and the only possible explanation points to a definite causal 
relation. 
It is to be regretted that it was not found practicable to extend the 
investigation to include the ‘“‘alkali’’ soils of the salt-spots, one of which 
lies close to the Laboratory domain. As regards these, we are in posses- 
sion of the facts of distribution, but the causal relations involved are 
not wholly clear. How far the peculiar vegetation of the salt-spots and 
its arrangement, and the absence of most other plants from these areas 
are due, to use Dr. Livingston’s words, to the concentration of the soil 
solution, which is often too great to allow osmotic absorption by the 
roots of most plants, or how far the cause may lie in poor drainage and 
consequent defective aeration is still uncertain. It is suggested by the 
author, though no exerimental work in this direction is reported, that 
the distribution of plants is perhaps more often determined by avail- 
ability of oxygen than by that of water. This suggestion gains in force 
by what, as a conspicuous example, are shown to be the excellent drain- 
age conditions of the Larrea slope, and by what we have seen of the 
avoidance on the part of the creosote-bush of areas of defective drainage. 
Observations of aspect preference, which have been made both on 
the Laboratory domain and in other places at different elevations, have 
resulted in a large accumulation of facts which point to the conclusion 
that temperature is first of all a controlling factor in the determination 
