254 BOTANICAL GAZETTE [OCTOBER 
of the hill, this soil is undoubtedly deficient in moisture for by far 
the greater part of the year. It usually supports spring and sum- 
mer annuals for a few weeks during each of its moist periods. 
The soil of the wash shows an even more inadequate moisture 
supply for the month of June than does that of the slope. This 
soil contains practically no gravel in its superficial layers and its 
moisture content at this time (2.1 per cent by dry weight) was only 
8.4 per cent of its water-holding power. From the general aspect 
of the vegetation it would be concluded that this soil was better 
supplied with moisture than that of the slope, but the graph fails 
to explain this view. The discrepancy is apparently due mainly 
to the great depth of the soil, which acts as a much more extensive 
reservoir for water than the shallow soils of hill and slope. The 
presence of underground water is doubtless effective in case of 
those trees and shrubs the roots of which penetrate deeply. The 
continuously active forms which occur here must be considered 
as much more deeply rooted than is possible on the hill and on the 
slope. ; : 
The soil of the river plain exhibits a much more favorable mois- 
ture condition, not only in June, but throughout the year, than any 
other of our four soil types excepting that of the hill. Its average 
water content for the drought period was 6.3 per cent by dry 
weight. In terms of its moisture-holding power, this becomes 
16.1 per cent. This is a deep soil, with unknown, but surely con- 
siderable, amounts of moisture below the limits of sampling. It 
also has constant subterranean water at depths which can probably 
be reached by tree roots. This latter fact seems to explain the 
occurrence here of mesquite, acacia, and other deep-rooting shrubs. 
In the dry seasons (early summer and late autumn) this soil shows 
very few active plants besides those just mentioned, but in the 
rainy seasons it supports a dense growth of annuals, being apparently 
as well suited to such forms as is the soil of the hill itself. That 
many annuals of the hill fail generally to appear on the plain, and 
conversely, is perhaps to be explained by the considerably higher 
moisture content occurring during the rainy seasons on the former 
soil. It may also be related to conditions which hinder surface 
