The Water Balance in Plants 95 



the physiological processes of plants, and how plants are modified 

 in size and structure by growing under different conditions of 

 water supply. 



Most plants cannot be grown to maturity in all kinds of wet 

 and dry conditions. Each kind of plant has a rather definite 

 water requirement for its best development. Hence in nature 

 plants live only in those situations in which they are able to main- 

 tain a suitable water balance. Three great classes of plants are 

 distinguished on this basis : 



(i) The plants that naturally live where the evaporative 

 power of the air is intense and the available water is hmited are 

 called xerophytes (Greek : xeros, dry, and phyton, plant). These 

 are the plants that are adjusted to a nearly continuous dearth of 

 water ; the cacti, agaves, yuccas, and sagebrush of our Western 

 plains and deserts are striking representatives. In the eastern 

 United States there are less marked examples of xerophytes in 

 the plants that live on dry cliffs and sand beaches, and in the 

 mosses and Hchens that grow on trees and rocks. 



(2) The plants that live partly or wholly submerged in the 

 water are known as hydrophytes (Greek : hudor, water, and phy- 

 ton, plant). These plants have an excessive water supply, and 

 transpiration is reduced or entirely wanting. In this class are 

 included the water lihes, pondweeds, cat-tails, bulrushes, and 

 many sedges. They are the common plants of fresh-water ponds, 

 swamps, and marshes throughout the world. 



(3) Between these extremes are the mesophytes (Greek : meso, 

 middle, and phyton, plant), by far the largest class of seed plants. 

 They have a medium rate of transpiration and grow best with a 

 moderate water supply. In this group are included the plants 

 that yield most of our garden, -field, and meadow crops; also 

 most of the forms that are found in the maple, beech, and elm 

 forests of the Eastern states and in the fir and spruce forests of 

 the canyons and bottom lands of the Western states. 



Xerophytes, hydrophytes, and mesophytes are readily dis- 



