Deserts and Desert Flora of the West 

 Sonoran region comprises southern Arizona, the 

 Colorado Desert of southern California, and the 

 Mexican states, Sonora, Sinaloa and Baja (Lower) 

 California. 



Scanty rainfall, low humidity, high tempera- 

 tures, and excessive evaporation are the principal 

 causes of desert conditions throughout all these 

 regions. 



Character of the Desert Vegetation. — The 

 severe arid conditions of the deserts are fatal to 

 plants not furnished with special drought-resisting 

 devices. Following favorable rains when the tem- 

 peratures are moderate, annuals lacking these 

 special devices spring up; but like arctic plants 

 they vegetate, flower and fruit in a remarkably 

 short time. The desert is transformed at such times 

 into a riot of color, more varied and brilliant than 

 an alpine meadow, — an ephemeral carpet that van- 

 ishes with the return to normal arid conditions. 



The dearth of forest trees and the prevalence of 

 low stragglv sparselv scattered shrubs are predom- 

 inant features of the desert. These shrubs are 

 usually of a light gray tone due to thickened cuticle 

 or other protective covering. Their short stubby 

 or spiny branches are thinly clad with small leaves 

 or are entirely destitute of foliage. As a rule they 

 present a monotonous landscape, such as the sage- 

 brush plains of Utah and Nevada. But in the deserts 

 of southern Arizona and southern California the 

 dull monotony of the northern deserts gives way to 

 a weird and" fantastic vegetation. The great col- 

 umnar trunks of the giant cactus, and the tree 

 yuccas bristling at every point with tufts of bayonet- 

 like leaves, offer an unfamiliar flora, more foreign 

 than Europe or Asia. 



Origin of the Desert Flora. — Arid conditions 

 have prevailed in the Mexican plateau region since 

 Cretaceous time, even during the time when the 

 United States was covered with rich deciduous for- 

 ests from the Atlantic to the Pacific. While the 

 common genera familiar to us all were evolving in 

 the moist temperate climates to the north, and 

 spreading over northern North America, Europe and 

 Asia by means of land connections that have dis- 

 appeared, drought-resisting plants were taking form 

 on this great arid plateau. Here originated the cacti, 

 yuccas, dasylirions and most of the other genera pe- 

 culiar to the American deserts. At the end of the 

 Glacial Period this Mexican flora pushed northward 

 into the western United States following increased 

 aridity. It is this ancient Mexican element that 

 gives "the unique character to our desert vegetation. 



169 



