[Chap. LIII THE VEGETATION OF NORTH AMERICA 773 



acacias, yuccas, and the various species of flat-jointed and cylindrical 

 cacti are most conspicuous. These plants either have a low rate of tran- 

 spiration, accumulate water during rainy periods, or are uninjured by 

 prolonged droughts. The yuccas are deep-rooted, the cacti shallow. The 

 annuals live as long as the soil water lasts. 



Fit;. 4UU. Colorado Desert, in soutiierii California. The "barrel cactus" is Fcnocactiis 

 ci/Iindraccus. Photo by W. S. Cooper. 



Forests of the western mountains. From southern Alaska to southern 

 Mexico is a series of high mountain ranges, and between the Sierras and 

 Rockies a series of high plateaus. The plateaus also have small north 

 and south ridses on them, manv of which are of sufficient height to 

 intercept rainfall in excess of that of the arid plateau. The vegetation of 

 all these mountains is dominated by coniferous forests as far south as 

 central Mexico. To be sure, there are fringes of deciduous and evergreen 

 oaks, poplars, willows, and, locally, other broad-leaved trees. Oaks are 

 proportionately more numerous and widespread on the mountains of 

 southern California and Mexico. In Central America they are gradually 

 replaced by tropical scrub and jungle. Considering the great extent of 

 the forests from the tundra to the tropics, and the frequency of local 

 differences in altitude of 3000 to 15,000 feet, the diversity of climatic 

 and soil conditions may be inferred. 



To describe the vegetation adequately would require consideration of 



