WATER RELATIONS OF PLANTS 311 



intercellular spaces. Such decrease can take place only when 

 the cell walls Hning the mtercellular spaces become drier than 

 usual. Such a dehydration of the walls in turn is subject to a 

 lower degree of saturation of the cells themselves and, conse- 

 quently, an increase in their suction tension. As a result of 

 this mechanism, based on the equihbrium between the suction 

 tension of the cell contents and the colloidal cell walls, a water 

 shortage in the leaf, due to excessive transpiration or an insuffi- 

 cient water supply from the soil, is in itself a cause of decreased 

 transpiration, independent of the degree of opening of the 

 stomatal pores. This phenomenon has been called ''incipient 

 drying" (Livingston). It must be regarded as a most important 

 nonstomatal regulator of transpiration. 



72. Water Balance of Plants. The Phenomenon of Wilting 

 and Its Significance. — Under usual conditions, the transpiration 

 of plants is fairly well coordinated with the absorption of water. 

 The relation between transpiration and absorption, or the water 

 balance, is favorable for growth showing neither excess of water 

 nor any considerable deficit. The water content of plants, 

 expressed in percentage of the dry weight, represents under 

 usual conditions a fairly constant magnitude. 



An increase in transpiration such as may be observed during 

 weather with bright sunshine may disturb this equilibrium and 

 cause a water deficit, up to 5 or 10 per cent, even when the soil 

 moisture is sufficient. With lack of soil moisture, this deficit 

 may increase up to 25 per cent and more. A water deficit occur- 

 ring mostly in the afternoon represents quite a normal phe- 

 nomenon and is not injurious to the plant. Its excessive 

 increase is inhibited by the ability of the plant to regulate its 

 transpiration losses within a fairly wide range and even under a 

 high rate of water loss. 



The plant can regulate its transpiration only within certain 

 limits and wdth a considerable increase of transpiration; or with 

 drjdng of -the soil, when absorption of water is inhibited, the 

 water balance is disturbed significantly. This becomes evident 

 in the phenomenon of wilting. Wilting is the result of a loss of 

 turgor, the tissues losing the turgor pressure proper to them in a 

 state of saturation and the leaves and young stem tips droop- 

 ing down. Wilting, however, does not indicate a loss of 

 vital activity. If the plant is provided in time with water, 



