33^ 



General Botany 



Fig. 2IO. 



W.S.Cooper 



Small-leafed desert shrub vegetation on dunes, Monterey Bay, California. 



the small flowering plants. Throughout the Rockies the streams 

 are bordered by alders, willows, and poplars. 



The southwestern desert formation. From the plateau of 

 Mexico, extending northward into California, Arizona, Nevada, 

 Utah, and Idaho, and eastward to New Mexico and western 

 Texas, is the desert. This great region is, for the most part, a 

 more or less broken plateau, with a rainfall of from 3 to 20 inches, 

 and with an evaporation rate five to thirty times as great. 

 Temperatures as high as 120° F. occur in the summer, and frosts 

 are not unknown even in southern Arizona. Farther north 

 the winters are severe, and, due to the great intensity of the sun- 

 light, the summers are very hot. Toward the south there are 

 two rainy periods, one in July and another in January. Follow- 

 ing these rains the desert is green with a covering of summer or 

 winter annuals that spring up quickly between the perennials 

 and, within a few weeks, flower, fruit, and die. 



At other seasons the vegetation is scattered, of a gray-green 

 color, and consists of thorny and spiny shrubs, large and small 

 cacti, fleshy-leafed agaves, yuccas, and other small succulent 



