242 JOHN ERNST WEAVER 



stratum, as well as aerial structures for enduring drought. 

 Another fact of importance must not be overlooked. Owing to 

 the cool nights the greater bulk of these high water losses (82.5 

 per cent as determined for three days in July) occurs during the 

 day, and thus very xerophytic atmospheric conditions are brought 

 to bear upon the plant. 



Summarizing briefly the factors of the habitat in which these 

 studies were carried on, we find a region of moderate winter and 

 low summer precipitation. The soils are composed of a fine 

 silt-loom of high water-holding capacity, they are usually very 

 deep, and the soil water extends down many feet beyond the 

 greatest root depth. 7 In early summer the superficial layers 

 of soil soon lose all of their w T ater available for plant growth, and 

 as the season advances this condition occurs in the deeper soils, 

 while the entire soil mass to a depth of 5 feet and beyond, gradu- 

 ally yields up most of its available water. Soil temperatures 

 at 3 inches show a daily range of from 3°F. to 24°F., while at 1 

 foot the daily range is seldom over 1°F. The seasonal range 

 (April— August) of the soil temperatures varies from 22°F. at 1 

 foot to 16°F. at 5 feet. Air temperatures show a mean daily 

 range varying from about 25°F. in April and May, to 38°F. in 

 July and August. The cool nights on the high plateau tend to 

 counteract the low humidity of the day and to reduce the high 

 daily rates of evaporation. 



North and northeast slopes are less xerophytic than the south 

 and southwest slopes. This is due in part to actually greater 

 precipitation caused by blowing snow and in part to soil texture 

 which is more open, has more humus, and a greater water- 

 holding capacity. These factors are reflected in the greater 

 amount of soil. water and in lower soil temperatures. Likewise, 

 these slopes are sheltered from the drying southwest winds, and 

 from the perpendicular rays of the sun. This is reflected in 

 slightly lower air temperatures and greater humidity, and 

 especially in the lower evaporating power of the air. 



7 Wells dug only into the soil often afford a good supply of water, although 

 usually they are drilled into the porous layers of basalt. See also Landes, Henry, 

 Underground Waters of Washington. Water Supply and Irrigation Paper, No. 

 Ill, 1905. 



