242 BOTANICAL GAZETTE [OCTOBER 
by these organs, and aside from those cases where the chemical 
content of the atmosphere or the character or intensity of illumina- 
tion exert ari influence upon the structure of the transpiring organs 
or upon the general tone of the plant, the simple relation between 
soil moisture and the evaporating power of the air appears to be 
of prime importance in determining the character of the natural 
vegetation. Over an area such as that here considered, where the 
air and light conditions are approximately uniform, and where the 
chemical content of the soil appears not to vary sufficiently to 
produce variations in plant growth, the possible rate of water 
supply to plant roots may be supposed to become the limiting 
condition. This rate of supply depends primarily upon the abso- 
lute amount of soil moisture and upon the water-conducting 
power of the soil. 
Soil moisture, of course, depends upon the rate of influx and the 
rate of removal of water from the soil. The former is determined 
by the ability of the soil to raise water from the subterranean 
water table, where such exists, by the amount and time of precipita- 
tion or other water entering the soil from above or laterally, and 
by the water-retaining power of the soil. Water removal from the 
soil is a resultant of evaporation at the soil surface, downward 
movement into lower layers, and plant absorption. 
Throughout the area here studied precipitation is approxi- 
mately uniform. Surface drainage, nearly uniform within the 
extent of each soil type, is quite different (because of variations 
in slope and porosity) for the four types. The water-retaining 
powers of the four soils are also different. Underground drainage 
is very different for the different soils, as is also the depth of 
permanent subterranean water, which is practically quite absent 
from two of the soils. 
THE som TyPes.—The geology and topography of the Tumamoc 
area are thoroughly described by Torman in the monograph of 
SPALDING above cited. Tumamoc Hill, on which the Desert 
Laboratory stands, rises abruptly from the broad valley of the 
Santa Cruz River. The valley floor slopes very gently from the base 
of the hill to the river floodplain and to the various “washes,” 
tributaries of the main river channel. The four soils here considered 
