THE PRAIRIES OF THE PACIFIC NORTHWEST. 



45 



integrated basalt under an annual precipitation of 21 inches, most of 

 which falls during the resting season, and with an average daily sum- 

 mer evaporation of 30 c.c. 



In both of these semiarid regions it has been shown that the water- 

 content of the soil is reduced to the non-available point to a depth of 

 3 or 4 feet, at least during certain years. It has also been shown that 

 in both of these prairie communities all of the dicotyledonous plants, 

 as well as many of the grasses, extend their root systems to depths of 

 from 3 to 7 feet or more. 



The absence of very shallow, widely spreading laterals is charac- 

 teristic of plants of both prairie communities, perhaps being more 

 pronounced in the Palouse region. This is in distinct contrast to the 

 root habit of many plains species, to which we shall now turn our 

 attention. 



