CHAPTER V 

 ABSORPTION OF WATER 



41. General Idea of the Water Balance. Amount of Water 

 Absorbed by Plants. — In the preceding chapter it has been shown 

 that to function normally the plant cell must be saturated with 

 water. Naturally, no difficulty is experienced in this respect by 

 submerged plants. For land plants, a constant supply of water 

 from the soil is necessary. Most plant cells, especially those per- 

 forming the work of assimilation, must be in close contact with air 

 in order to obtain CO2. As the air is very seldom saturated with 

 water vapor, the water within the cell is continually given off. To 

 make up for this loss, a constant current of water must ascend 

 towards the cells. 



Only a few of the lower plants, such as mosses and lichens, are 

 able to stand a prolonged interruption of the water supply and 

 to remain alive in the state of complete desiccation. For 

 higher plants, such desiccation would result in death; hence, 

 they possess a peculiar water balance. The absorption of water 

 usually equals its loss. Plants adapt themselves to maintain this 

 balance without persistent or serious deficit. Usually, they pos- 

 sess a well-developed root system which absorbs water from the 

 soil with sufficient rapidity. A well-developed conductive system 

 carries this water to the principal evaporating organs, the leaves. 

 Finally, a series of protective adaptations checks the escape of 

 water, if necessary. 



The amount of water which passes through the plant is im- 

 mense. Experiments have shown that during 1 hr. of a hot sum- 

 mer day more water passes through a leaf of our common plants 

 than the leaf itself contains at any particular time. During the 

 whole vegetative period, a sunflower or corn plant evaporates as 

 much as 200 kg. (440 lb.) of water, i.e., the contents of a barrel of 

 considerable size (55 gal.) (Fig. 49). Of the total amount of this 

 water, the plant assimilates but an insignificant fraction. This 



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