THE COHESION OF WATER THEORY 235 



which favor a rate of water loss from the leaves considerably in excess of the 

 rate at which water enters the roots from the soil. 



The movement of water across the cells of the root from the soil into 

 the lower ends of the water-conducting elements must be considered as an 

 integral part of the translocational process. At their lower terminations the 

 xylem vessels or tracheids are in contact with the pericycle, or more rarely, 

 directly with the endodermal cells of the root (Fig. 70). Water moves from 

 the adjacent root cells into the conducting elements because the tension (dif- 

 fusion pressure deficit) developed in these elements exceeds the diffusion 

 pressure deficit of the cells of the root. When the tension in the water 

 columns is relatively low a gradient of diffusion pressure deficits will be 

 established across the root cells similar to those which are established in the 

 leaf mesophyll cells when water is moving through them. The diffusion 

 pressure deficits will increase from cell to cell along this gradient in the 

 direction in which water is moving, i.e. from the periphery of the root towards 

 the xylem. 



Since, however, the osmotic pressures of root cells are usually lower than 

 those of the mesophyll cells (Hannig, 19 12), it is probable that the tension 

 developed in the water columns often exceeds the highest osmotic pressures 

 of any of the root cells through which water passes on its way from the soil 

 to the xylem. Under such conditions the volume of water in the root cells 

 may continue to diminish even after their turgor has been reduced to zero. 

 As a result the walls of the cell are pulled inwards due to the adhesion 

 between them and the contracting mass of water. The counter pull exerted 

 by the elastic w-alls of the cell will stretch the encompassed mass of water 

 and throw it into a state of tension (Chap. XI). Whenever a tension of 

 sufficient magnitude has been generated in the conducting elements, therefore, 

 the water in the root cells may also pass into a state of tension (Livingston, 

 1927). Even under such conditions a gradient of diffusion pressure deficits 

 would be established across the root, but the mechanism responsible for the 

 development of this gradient would be more complicated than were no tension 

 generated in the cells. When the water in a cell is under tension its diffusion 

 pressure deficit is equal to its osmotic pressure plus the tension to which the 

 water is subjected (Chap. XI). 



Regardless of the exact mechanism involved, the essential fact is that 

 the development of tensions in the water columns induces the establishment 

 of a gradient of diffusion pressure deficits, increasing consistently from cell 

 to cell across the root from its peripheral cell layer to the conducting elements, 

 and water will move along this gradient from the epidermal layer to the 

 xylem tissue. Water enters the peripheral ceils of a root whenever the dif- 



