The Problems of Plant Distribution 273 



of the hottest and coldest months in San Francisco 

 is only 10 F., while in Washington, with nearly 

 the same latitude, and with the same mean annual 

 temperature (55), it is 44. The rainfall is very 

 heavy at the north, but rapidly diminishes towards 

 the south, so that San Diego, in Southern Cali- 

 fornia, receives only about one-tenth as much rain 

 as falls at some points on the northern coast. The 

 rain falls mainly in the winter and spring, the sum- 

 mer being almost absolutely rainless. These very 

 great differences in climate, compared to Atlantic 

 North America with its cold winters and wet 

 hot summers, result in a remarkable difference in 

 the type of vegetation. Moreover, in addition to 

 the great differences in rainfall in different parts, 

 there must be considered the differences in altitude, 

 which in California approach 15,000 feet, involving 

 of course very great variation in temperature and 

 exposure. Consequently in proceeding down the 

 coast of California from Humboldt and Mendocino 

 Counties, to the Colorado desert of the southeast, 

 one passes from forests of enormous trees with an 

 almost tropical luxuriance of undergrowth, to 

 barren deserts where no plant can live. The mild 

 climate induces a growth of evergreen plants, even 

 deciduous trees and shrubs retaining their foliage 

 for the greater part of the year. 



All through the West there is a predominance 

 of coniferous trees, this being especially the case 

 in California, where most of the forests are com- 



