332 



General Botany 



iV. 6. Cooper 



Fig. 204. Sitka spruce forest at Glacier Bay, Alaska. This forest has grown up since 

 the retreat of the glacier about 100 years ago. 



xerophytic, flowering herbs, as well as cacti, yuccas, legumes, and 

 composites that reheve the gray-green monotony of the grasses 

 by their vari-colored flowers. 



We have seen that the treelessness of the prairies is due, for 

 the most part, to an excessive transpiration rate in proportion 

 to the soil-water supply ; locally toward the east, to unfavorable 

 soil drainage also. The treelessness of the plains is due to in- 

 adequate water supply; intense summer and winter droughts 

 make it very difficult for tree seedlings to become established. 



Western evergreen forest formation. The western Cordillera, 

 extending from Alaska through the Rockies, the Sierra Nevada, 

 and the Coast Ranges to Mexico, are clothed with conifer forests 

 of pines, firs, spruces, hemlocks, and cedars. This forest roughly 

 has three divisions : (i) the northern coastal forest, extending 

 from Washington to southern Alaska, (2) the Coast Range and 

 Sierra forest, extending southward to southern California, and 

 (3) the Rocky Mountain forest, stretching from northern British 

 Columbia to Arizona and Mexico. The region from Washington 



