VEGETATION AND ENVIRONMEN'T IN CALIFORNIA 227 



ularly of such as are south of San Francisco bay, is the air 

 movements. For example, in the morning the wind blows north, 

 or down the \'alleys, which should j^robably be considered as 

 cold air drainage from higher altitudes. But in the afternoons 

 the prevailing westerlies, being deflected from their course, blow 

 up the valleys with much force and with a decidedly dessicating 

 effect. In certain instances, as, for example, near the head of the 

 SaUnas valley on the route to Los Angeles, the wind action is so 

 effective that the general features of the topography are deter- 

 mined by it. 



Another feature of the valleys may also be mentioned, although 

 however important, it is at present but imperfectly known. I 

 refer to the conditions of soil moisture. So far as superficial 

 observations give an idea of such conditions, it would appear that 

 the moisture utilized by shrubs and trees in the valleys lies either 

 3 to 6 feet from the surface or at a considerably greater depth, as 

 for example 12 feet or more. In the first instance it is clearly 

 derived directly from the rains, or from superficial runoff from 

 higher ground, and in the second it owes its presence to a more 

 deeply placed water table. The importance of a water table in 

 semi-arid, or sub-arid regions (as many of the valleys may be 

 considered to be) , is to be found in connection with the presence 

 of a forest, or of trees, of a more or less mesophytic nature. This 

 relation with the en^'ironment will be referred to in more detail 

 below. 



THE DESERTS 



To the south of the Sierra Nevadas and separated from the 

 oeean by high mountains, lie two of the most arid areas of the 

 United States, the Mojave and the Colorado deserts. The 

 surface of these regions comprises about 14 % of the surface of 

 the state. As is well known the leading climatic feature of the 

 deserts are the low precipitation, which is also irregular in amount, 

 the high summer temperatures, and the high rate of evaporation. 

 The annual rainfall for several stations lies between 2.46 and 3.61 

 inches, all of which occurs in winter. Under so extremely arid 

 conditions the vegetation partakes of a nature of great specializa- 

 tion, as is verv well known. 



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