THE OSMOTIC QUANTITIES OF PLANT CELLS 149 



The water in the vessels and cells of plants frequently passes into a state 

 of tension. Imposition of pressure from an external source raises the diffusion 

 pressure of water. Imposition of a tension (which is equivalent to a negative 

 pressure) has precisely the opposite effect. In cells and vessels this can only 

 happen if the enclosed water contracts in volume to such a point that the 

 encompassing walls are pulled inwards due to adhesion between the water 

 and the walls. The counter pull exerted by the walls on the water results in 

 throwing it into a state of tension. Under such conditions the wall pressure 

 and hence the turgor pressure of the cell are negative in value (Fig. 32), 

 and the tension ("negative pressure") developed within the water will be 

 equal in magnitude to the negative wall pressure. The equation for the 

 diffusion pressure deficit of a cell under such conditions becomes: 



Diffusion pressure deficit = Osmotic pressure - (- Wall pressure) 



In other words the diffusion pressure deficit of the water in a cell or vessel 

 when subjected to a tension is equal to the osmotic pressure plus the tension 

 imposed on the water. 



The diffusion pressure deficit of any plant cell, if we disregard for the time 

 being the possible effect of tensions, may range in value from zero to the os- 

 motic pressure of that cell when flaccid. The range of diffusion pressure 

 deficits in plant tissues therefore corresponds very closely with the range of 

 osmotic pressures, as discussed earlier in this chapter. 



Dynamics of the Intercellular Movement of Water in Plants.— In 



order to simplify this part of the discussion changes in the osmotic pressure of 



cells due solely to volume changes 



(as shown in Fig. 32) will again 



be disregarded, as usually these are 



not great enough to modify seriously 



any generalized picture of the water 



relations of plant cells. 



Let us imagine a certain cell j- » „ n 



^ Fig. 33. Diagram of two adjacent cells 



(X) to have an osmotic pressure of ^^^^ -^^ explanation of the mechanism of 



12 atmos., and a wall pressure of cell-to-cell movement of water. 



6 atmos.; and a second cell (Z) to 



have an osmotic pressure of 10 atmos., and a wall pressure of 2 atmos. Let 



us further suppose that these two cells are brought into such intimate contact 



that a normal osmotic movement of water can occur between them, under 



conditions that no evaporation can occur from them (Fig. 33). 



Which cell will gain water under these conditions? An uncritical inter- 

 pretation would answer that water will move from cell Z to cell Xj since 



