574 RELATION TO ENVIRONMENT. 



on the west coast of South America from 5 to 30 south latitude. 

 The annual rainfall in these deserts varies, does not exceed ^ocm 

 (12 in.), while it is usually much lower, from $-8cm (2-3 in.), 

 and in one place in Chili it is as low as icm. In the great desert 

 of north Africa, and its continuation in southwestern Asia, the 

 annual rainfall is about 2ocm (8 in.). 



1061. Time of maximum rainfall. The maximum rainfall 

 occurs at different seasons in different deserts; for example, 

 during the spring in Sahara, autumn in north Chili, summer in 

 Australia. The rainfall is so slight that the vegetation periods 

 (except the rainy-season flora) are independent of the maximum, 

 but they are dependent upon temperature, and the austere period 

 occurs during the hottest period of the year. (In Sahara the 

 July temperature rises to 36C. = 97F.) According to Schim- 

 per the vegetation is more dependent on the ground- water than 

 on the moisture from rainfall, though there is a general ephemeral 

 rain flora in the spring. When the rainy season is over the green 

 covering of the earth disappears, and the ground is as nearly 

 devoid of plant life as before. Many perennials are also bene- 

 fited by the spring rains, chiefly because transpiration is 

 checked. 



1062. Character of Sahara Desert. The great desert of 

 Sahara consists of elevated, furrowed, and stony plains, or rolling 

 and stony lands, sandy lowlands, long sand-dunes, with here 

 and there an oasis. Strange as it may seem, the places are rare 

 where vegetation is not in sight from any point, but this consists 

 of low, compact, shrubby plants in small clusters or streaks. 

 The oases are populated by a rank growth of trees and herbs and 

 parts of them are cultivated. The line between oasis and desert 

 is closely drawn. 



1063. Two ecological vegetation types in the desert. The 

 plants of the desert may be grouped into two ecological cate- 

 gories: the existence of the one is dependent directly on the rain-, 

 the other on ground-water, ist. To the first category belong 

 the rainy-season plants. They germinate their seeds, grow, 

 flower, and seed again during the period. There are also per- 



