ECOLOGY 145 



primarily to scanty rainfall rather than to soil conditions, 

 as in the case of rocks and sand-dunes. They are so 

 situated that the prevailing winds have been previously 

 deprived of their moisture by passing over mountains. 

 The chief desert region of the United States is found in 

 the Great Basin from the plains of the Columbia in eastern 

 Washington southward through Arizona to the Mexican 

 plateau. The aridity increases southward and reaches 

 its maximum in the Colorado Desert of southeastern 

 California. The annual rainfall is less than 20 inches, 

 often less than 10 inches. On account of the higher tem- 

 perature and longer summers the aridity increases 

 southward even though the rainfall may remain the 

 same. Other desert regions are found along the 

 Pacific slope in Peru and northern Chih, in the interioi' 

 of Australia and Asia, and the Sahara Desert of north 

 Africa. 



The perennial grasses of deserts are for the most part 

 bunch-grasses and on account of the scarcity of moisture 

 the bunches are widely scattered. In contradistinction 

 to the other xerophytic regions, deserts are inhabited by 

 several species of annual grasses. Such grasses are adapted 

 to the distribution of the rainfall. This usually comes in 

 occasional heavy showers. Immediately after such a 

 shower the seeds of annuals germinate, develop rapidly 

 and mature seed before the effects of the shower have 

 passed away. This adaptation to seasonal moisture is 

 especially marked if the showers are concentrated within 

 a certain period of the year forming a rainy season. In 

 southern Arizona there are usually two such rainy sea- 

 sons, one in winter and one in summer, with a correspond- 

 ing growth of annuals, many of them grasses, after each 

 period of rainfall. In all desert regions the grasses tend 

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