CHAPTER VIII 

 WATER RELATIONS OF PLANTS 



64. General Coacept of the Water Relations of Plants. The 

 Amount of Water Used by Plants. — As has already been seen in 

 Chap. I, the normal functioning of the plant cell requires its 

 saturation with water. In submerged plants, as for instance in 

 algae, which perhaps are the most ancient chlorophyll-bearing 

 plants on the earth, saturation with water is a natural result of 

 the ecological conditions of their surrounding medium. But 

 in plants inhabiting dry land, the necessity of maintaining the 

 protoplasm of the cells in a sufficiently saturated state represents 

 the most important limitations to their growth. It determines 

 their structure and all their life activity. To maintain satis- 

 factory photosynthetic activity, the chlorophyll-bearing cells of 

 land plants must be in continuous contact with the surrounding 

 atmosphere, which provides them with the indispensable carbon 

 dioxide. However, such exposure results in evaporation of 

 water from the cell into the surrounding air Sunlight, which 

 furnishes the plant with the energy necessary for photosynthesis, 

 on being absorbed by the chlorophyll is taken up also by other 

 cell structures and raises the temperature of the leaf, accelerating 

 the rate of evaporation. There must be a continuous stream of 

 water passing toward the chlorophyll-bearing cells to compensate 

 for this loss. The seemingly stationary condition of saturation 

 is maintained in the plant by the coordination of absorption and 

 evaporation of water, thus creating water relations that are 

 specifically different for each plant. This has been called by 

 L. A. Ivanov the ''water exchange" of the plant. 



Very few plants, and those mostly of a very simple structure, 

 such as mosses and lichens, can endure a prolonged interruption 

 of the water supply. Mosses and lichens can survive long 

 periods of desiccation in an air-dry condition. Among the higher 

 plants, only a few representatives of the desert flora, such as the 

 sand sedge, endure such desiccation. For most of the higher 



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