216 BURTON EDWARD LIVINGSTON 



(2) Thickness of soil water films (soil moisture 

 content). This depends upon: 



a. Permeability of upper and lower soil 



surfaces to water (percolation 

 drainage) . 



b. Position of subterranean water table 



with reference to soil surface. 



c. Slope of the two soil surfaces (slope, 



drainage) . 



d. Climatic conditions of the habitat. 



These involve: 

 d 1 Evaporating power of the air, 

 which depends upon: 

 d 1 (1) Temperature. 

 d 1 (2) Humidity. 

 d 1 (3) Air movement. 

 d 2 Seasonal distribution of rainfall. 

 d 3 Amount of rainfall and rapidity 

 of fall in the separate show- 

 ers. 

 d 4 Rate of influx of radiant heat to 

 the soil surface. The latter 

 depends upon: 

 d 4 (1) Intensity of sunshine. 

 d 4 (2) Quality of sunshine. 

 d 4 (3) Radio-absorptive power 

 ( >f the soil surface (color, 

 etc.) 

 2.* Power of roots to absorb water, which depends 

 upon : 

 (1)* The water-imbibed condition of the periph- 

 eral cell walls (whether thoroughly 

 imbibed or not) , which depends upon : 

 a. The general rate of water entrance 

 from soil (B, in general). 

 . b.* Rate of water conduction away from 

 absorbing surfaces (toward the 

 transpiring surfaces) . 



