150 THE OSMOTIC QUANTITIES OF PLANT CELLS 



the cell sap of X has the higher osmotic pressure. Although such an ex- 

 planation is commonly offered, even in some textbooks, a little consideration 

 shows that this cannot possibly be true. 



The water in the cell sap of X would have a diffusion pressure deficit of 

 12 atmos. were it under no pressure. The wall pressure of 6 atmos., how- 

 ever, reduces the diffusion pressure deficit of the cell to 6 atmos. Similarly 

 the water in the vacuole of cell Z would have a diffusion pressure deficit of 

 lO atmos. were it not also influenced by a wall pressure of 2 atmos. Hence 

 the diffusion pressure deficit of cell Z is 8 atmos. The diffusion pressure 

 deficit of Cell X is therefore less than that of cell Z. Water will therefore 

 move from cell X to cell Z. Water will continue to show a net movement 

 from X to Z until the diffusion pressure deficits of the two cells are equal. In 

 the movement of water from cell to cell in plants it is the diffusion pressure 

 deficits and not the osmotic pressures which tend to equilibrate. This is only 

 a special aspect of the fundamental tendency of the diffusion pressure of water 

 to attain a uniform value throughout any system. It is by no means impos- 

 sible, therefore, for water to move from a cell of higher to one of lower 

 osmotic pressure, although it is not to be inferred that this is generally or even 

 usually true. Any cell will "absorb" water from a cell or solution of lower 

 diffusion pressure deficit and, correspondingly, will lose water to a cell or 

 solution of higher diffusion pressure deficit. 



It should not be supposed that after a dynamic equilibrium has been es- 

 tablished between two cells that their diffusion pressure deficits will be an 

 exact average of their initial diffusion pressure deficits, as this is seldom true. 

 Differences in the original volume of the cells and the elasticity of the cell 

 walls usually make this impossible. The only general statement which can be 

 made is that, at equilibrium, the diffusion pressure deficits of the two cells 

 will be equal, and this value will lie somewhere between the two origmal 

 values of the cells. 



Whenever the diffusion pressure deficits of two adjacent cells are dis- 

 similar we may speak of a diffusion pressure deficit gradient as existmg be- 

 tween them. Movement of water from one cell to another can occur only 

 when such a gradient exists. Other conditions being equal the "steeper" this 

 gradient, i.e. the greater the difference in diffusion pressure deficits, the more 

 rapidly one cell will gain water from the other. The term diffusion pressure 

 deficit gradient can also be applied to a chain of cells in which the diffusion 

 pressure deficit increases serially from cell to cell. Several examples of such 

 gradients will be discussed in subsequent chapters. 



Water Relations within Individual Plant Cells. — Imbibition of water 

 is due fundamentally to a lesser diffusion pressure of water in the imbibing 



