THE PRAIRIES OF THE PACIFIC NORTHWEST. 



43 



summer precipitation. The soils are composed of a fine silt loam of 

 high water-holding capacity and they are usually very deep. In early 

 summer the superficial layers of soil soon lose all of their water avail- 

 able 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) gradually yields 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 to 

 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 precipita- 

 tion 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. On the part 

 of the plants the most obvious response to these severe environmental 

 conditions is the development of extensive root systems. For just as 

 the evaporating power of the air and the nature of the transpiring 

 organs determine the water-loss of plants, likewise the soil-water 

 and the nature of the root systems determine the supply. 



Table 13. — Average root depths {in inches) of plants on different slopes. 



It is instructive to note that according to the types of root systems 

 as set forth by Cannon, all the roots here described, with the possible 

 exception of Leptotoenia muliifida, fall under the generalized class. This 

 was true also for all but a few of the eastern prairie species. By a 



