THE RELATION OF PLANTS TO WATER 



147 



From the above discussion it will be evident to the student 

 that the amount of water available for the use of the plant must 

 exercise a profound effect upon its form and structure. This 

 available water is dependent also, in the case of land plants, first, 

 upon the amount of water in the soil available for the roots and, 

 secondly, upon the atmospheric conditions, such as temperature 



FIG. 70. Mesophytic vegetation 



Meadow vegetation occuring in zones, herbaceous plants of mint (Monarda) in 

 the foreground, scrub and woodland in the background. After Clements 



and relative humidity, which determine the amount of evapora- 

 tion of water vapor from the leaves. If the amount of available 

 water in the soil is abundant and the atmospheric conditions are 

 such as to restrict excessive evaporation, a medium type of plant 

 life is developed, which is usually designated as mesophytic. 



The mesophytes include the broad-leaved forest trees, shrubs, 

 and herbs with which we are most familiar, and constitute the 

 typical vegetation of the great forests of the temperate and 

 tropical zones as well as that of the more productive lowland and 



