294 THE INTERNAL WATER RELATIONS OF PLANTS 



pressure, and a gradual replenishment of the water content of the cells, the 

 latter in turn resulting in a progressive increase in the wall pressure of the 

 cells. 



Similar, although probably less marked diurnal variations undoubtedly 

 also occur in the osmotic quantities of the cells in the other organs of plants. 



Permanent Wilting. — This term refers only to wilting from which a 

 plant will not recover unless the water content of the soil is increased. It is 

 engendered by the development in the soil water of a diffusion pressure deficit 

 so great that the rate of movement of water into the plant is negligible. As 

 with transient wilting, visible symptoms of permanent wilting are apparent 

 only in thin-leaved species of plants, but physiologically equivalent conditions 

 may develop in practically all terrestrial species. 



In a soil which is slowly drying out temporary wilting will slowly grade 

 over into permanent wilting. Under such conditions each night recovery of 

 the plant from temporary wilting will occur more slowly and will be less 

 complete, until finally even the slightest nocturnal recovery fails to take place 

 and the plant passes into a continuous state of permanent wilting which grows 

 progressively more drastic the longer that it persists. 



Since plants enter the state of permanent wilting by a gradual transition 

 from a condition of temporary wilting, the early stages of this phenomenon are 

 not greatly different from those of transient wilting except that there is no 

 nocturnal recovery of turgor. As the available water in the soil becomes 

 depleted continuity of the soil water with the water in the plant is interrupted, 

 and the water mass in the plant becomes an isolated unit hydrostatic sjstem. 

 When this condition prevails the stress or tension in the hydrostatic system 

 gradually becomes intensified, since, even if the stomates are closed as they 

 usually are in permanently wilted plants, cuticular transpiration continues, 

 thus gradually reducing the total volume of water within the plant. It has 

 been shown experimentally that the water content of the leaves of permanently 

 wilted plants is less than that of plants in a state of transient wilting, and 

 that it gradually decreases during the continuance of permanent wilting. This 

 is also true of the other organs of plants. 



Continued gradual reduction in the volume of water in the plant ultimately 

 may throw some or all of the residual mass of water into a state of tension, 

 just as a reduction in the volume of water in a single cell — such as a cell of 

 the fern annulus — has the same effect. A tension generated in the water 

 columns, if of sufficient magnitude, will, in some species at least, be propa- 

 gated into the leaf cells and other tissues of the plant. Although it is cus- 

 tomary to think of the tensions developed in the internal hydrostatic systems 

 of plants largely in terms of the water columns, the existence of water under 



