CHAPTER XVIII 

 THE INTERNAL WATER RELATIONS OF PLANTS 



In preceding chapters the loss of water from plants, the movement of water 

 through plants, and the absorption of water by the roots of plants have been 

 considered more or less as independent processes, although it should have 

 become increasingly evident as the discussion progressed that the internal 

 hydrostatic system of plants is essentially a unit in its behavior. This point 

 cannot be too strongly stressed as no adequate picture of the water relations 

 of plants can be drawn in terms of these processes considered separately. 

 Many of the more succulent species of plants may, in fact, be regarded as 

 physically little more than a mass of water held in position and shape by an 

 amount of cellular structure which is remarkably small in relation to the 

 volume of water thus retained. Even in woody species of plants, in which 

 the proportion of cellular material to water is greater, the water in the plant 

 is maintained as a continuous unit system. Within this unit system diffusion 

 and mass flow of water are continually in progress. A shift in the diffusion 

 pressure of the water in any part of this unit system will show its influence, 

 sooner or later, but usually within a relatively short time, in other parts of the 

 system. Such effects of changes in the diffusion pressure of the water in one 

 part of this system upon the diffusion pressure of water in other parts of the 

 same system are more pronounced when the rate of absorption of water is 

 slow, or has ceased entirely, than when water is moving into the roots of a 

 plant at a relatively rapid rate. 



V/ilting. — One of the commonest of observations among agriculturists 

 and gardeners is that the leaves of many species of plants wilt on many 

 hot summer afternoons, only to regain their turgidity during the night even 

 if the plants are not provided with additional water by rainfall or irrigation. 

 In dry, hot regions, or during hot weather in more temperate regions, such a 

 phenomenon may be a regular daily occurrence, even during periods when the 

 soil is well supplied with water. This familiar response of plants is usually 

 called temporary or transient wilting and is clearly due to a temporary excess 

 of the rate of transpiration over that of absorption. As a result the total 

 volume of water in the plant shrinks, although not equally in all the tissues. 



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