22 



pl.a:\t physiology and ecology 



compensation is already made, may be used. In elementary work 

 it is impracticable to measure the rainfall in different habitats. It 

 is fairly satisfactory to use the reports of rainfall obtained from a 

 neighboring weather station when the latter is not more than a 

 few miles distant. The effect of rainfall upon water content is best 

 ascertained by taking soil samples in different habitats immediately 

 after a rain, and then determining the increase in water content. 

 In open, hilly regions there sometimes occur in spring differences 

 in water content which can only be explained by a knowledge of 

 the distribution of the late snows of winter. 



28. Physiography. Physiographic factors are altitude, expos- 

 ure, slope, antl surface. There are in addition certain physio- 



Fig. 5. Mountain sides showing differences of slope, exposure, and cover. 



graphic processes, such as weathering, erosion, and sedimen- 

 tation, which play a fundamental part in changing habitats, but 

 these do not act directh^ upon water content. The latter is di- 

 recth^ influenced by slope and surface, while altitude and exposure 

 operate only through humidity. Cover, either dead or liAing, while 

 not exactly a physiographic feature, affects water content in much 

 the manner of surface, under which it ma}?^ well be considered. 



