PRAIRIE PLANTS OF SOUTHEASTERN WASHINGTON 



241 



out this condition in general, with higher average daily evapo- 

 ration on both slopes (23:9 cc. on northeast and 33.0 cc. on south- 

 west) and a ratio on the two slopes of 72 : 100. 



*^ 



1 



Fig. 5. One end of a trench used in excavating root systems. Many roots 

 of prairie plants reach depths of over 10 feet. 



When we consider that the daily evaporation in July and 

 August often reaches a maximum of 40-55 cc, and at a time when 

 the available soil water may be depleted to a depth of two feet, 

 we can see the necessity for extensive root-systems in the sub- 



