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DESERT BOTANICAL LABORATORY 



The seasonal distribution of the rainfall in a region in which the 

 normal precipitation bears a low ratio to evaporation may not only 

 exert a marked influence on the character of the vegetation, but may 

 also operate to produce desert conditions and make possible the sup- 



FiG. I. Location of and annual precipitation at certain stations in the arid 

 region of Western America. Chiefly from records of the U. S. Weather Bureau. 



port of xerophytic vegetation only, in districts with a large amount of 

 rainfall. Thus if the greater part of the precipitation should occur 

 during a season of low temperature, or in the quiescent period of the 



