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 Chili, in the interior 
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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