INTERNAL REDISTRIBUTIONS OF WATER IN PLANTS 295 



tension in plants is not necessarily confined to the conductive system. Con- 

 tinued withdrawal of water from a cell after its turgor pressure has fallen to 

 a zero value can have one of two results; either the water in the cell ruptures 

 or else it is thrown into a state of tension. Tensions of a very considerable 

 magnitude undoubtedly develop in at least some of the cells of many species 

 when subjected to permanent wilting. According to Chu (1936), under 

 conditions of a severe internal water deficiency, the water in the leaf cells 

 of many species of trees, both coniferous and deciduous varieties, passes into 

 a state of tension. 



Shrinkage in the volume of water in a cell to the point at which it passes 

 into a state of tension results in the protoplasm and cell walls being subjected 

 to an inward pull because of the strong adhesive force between the water and 

 the cell walls. Under such a condition the wall pressure and the turgor pres- 

 sure of a cell have a negative value. The greater the tension to which the 

 water in the cell is subjected the greater the pull exerted by the contracted 

 mass of water upon the cell walls. The cell walls of plants are often dis- 

 torted by the centripetally directed pull which they sustain when the water 

 within them is under tension. It has been observed that the shrinkage in 

 the volume of the cells of a number of species during wilting results in an 

 inward folding or crinkling of the cell walls due to the centripetal pull to 

 which they are subjected (Thoday, 1921 ; Engmann, 1934). On the other 

 hand the walls of some plant cells are so rigid that they can sustain the de- 

 velopment of a considerable tension in the enclosed water without any apparent 

 distortion. 



It is generally believed that tensions of a very considerable magnitude 

 can develop in the water columns of permanently wilted plants, probably 

 ranging up to lOO atmos. and perhaps even higher. In some drought resistant 

 species the water columns apparently can be maintained in a state of high 

 tension for weeks or even months without breaking. In many species, how- 

 ever, gradual intensification of the tension in the water columns sooner or 

 later leads to the entrance of air and consequent breaking of the columns. 



Internal Redistributions of Water in Plants. — Whenever an internal 

 water deficit develops within a plant the resulting stress in the internal 

 hydrostatic system invariably results in increasing the diffusion pressure deficits 

 in the cells of some organs more than in the cells of other organs. Such a 

 development of unequal diffusion pressure deficits in different parts of a plant 

 frequently leads to the phenomenon often referred to as "internal competition 

 for water" among the various tissues and organs of the plant. This term, 

 although in common use, is not a very satisfactory one, as strictly speaking, 

 tissues within the body of the plant cannot "compete" with each other in any 



