Plants of Different Regions. 331 



souriensis abounds along the crests of limestone hills. 

 Grasses, and other plants with creeping underground 

 stems, are well adapted to take possession of the shifting 

 soil of sand dunes where competition with other plants 

 less suited to such situations is not sharp, and in this indi- 

 rect way the unstable sandy soil gives a distinct character 

 to its vegetation. 



If all plants were able to accommodate themselves 

 equally well to different kinds of climate and soils, we 

 might expect to find them uniformly distributed, except- 

 ing where broad waters intercept their migration ; but 

 they show great diversity in their adaptability to different 

 external factors, most of them being unable to traverse 

 deserts or lofty mountains. Some are unable to maintain 

 themselves where the annual precipitation falls below a 

 certain amount, where the ranges of temperature pass cer- 

 tain maximum and minimum limits, or where the prevailing 

 winds exceed a certain velocity. It is because of its bear- 

 ing on this diverse capacity that the subject of plant dis- 

 tribution is one of high importance in the study of plant life. 



213. Vegetation in the Tropics. In the tropics the tem- 

 perature is uniformly high, excepting in the mountains, 

 averaging from 20 to 28 C. (68 to 82.4 F.) throughout 

 the year. In general the tropics are regions of high 

 annual rainfall (see rainfall map, Fig. 179), the precipita- 

 tion along the coast and in the mountains amounting in 

 some localities to 4 or 5 meters. The greater part of Cen- 

 tral and South America has an annual rainfall of 130 to 

 200 centimeters, and for the most part of the East Indian 

 Archipelago, and of the east coast of tropical Asia, the 

 annual precipitation exceeeds 200 centimeters. In tropical 

 Africa there is a zone 10 broad where the rainfall amounts 

 to 1 30 to 200 centimeters. 



