24 PLANT PHYSIOLOGY AND ECOLOGY 



The same is true of cover, which is usually of far greater and 

 more universal importance. Dead and living cover retard run- 

 off and reduce the amount of water lost from the surface of the 

 soil. By their decay, plants add humus to the soil, thereby in- 

 creasing its capacity for absorbing and retaining water. Dead 

 nJ cover is of less importance, since it is found chiefly during the 

 resting period alone, but it plays a part of some value by catch- 

 ing and holding drifting snow. The cover of living vegetation 

 reacts upon the habitat in a much more vital fashion. While 

 it has a powerful effect in increasing water content, it reduces 

 it also by reason of the water lost through evaporation from the 

 plants. Cover can be expressed only in general terms of density 

 and height at present, and it suffices, as a rule, to indicate the 

 character of the plant covering. In this connection it should 

 be noted that other biological factors, viz., man and other ani- 

 mals, often exert an influence upon water content. Except in 

 special cases, such as irrigation and drainage, this effect is exerted 

 through other factors, and does not need further consideration 

 here. 



31. The influence of climatic factors. All the atmospheric 

 or climatic factors of a habitat have an effect upon water con- 

 tent, either immediate or otherwise. Such factors are humidity, 

 temperature, wind, pressure, and light. The influence of the 

 last is slight and very indirect. Temperature, wind, and pressure 

 can change the amount of water in the soil only through humidity, 

 and hence they will be further considered under the latter. 

 Humidity acts upon the plant and upon soil water in the same 

 way, i.e., by controlling evaporation. It affects water content 

 directly through water loss from the soil, and indirectly inas- 

 much as the water lost by the plant is first draw^n from the soil. 

 Since it is a direct factor, it will be more fully considered in the 

 following sections. It is mentioned here merely to complete 

 the list of factors that play a larger or smaller part in modifying 

 or controlling the water content of the habitat. 

 -<S532. Humidity. The moisture of the air, which is in the form 

 of^apor, is termed humidity. Owing to the nature of the medium 

 in which it occurs, humidity is much more uniformly distributed 

 than the water content of the soil. For the same reason it fluctu- 

 ates to a much greater degree. It differs from water content 

 also in that a part of the latter, i.e., the echard, is always non- 



