THE OSMOTIC QUANTITIES OF PLANT CELLS 147 



of simplicity the effects of such volume changes upon the osmotic pressure, etc. 

 of the cell have been disregarded in the discussion up to this point. It is as- 

 sumed that the wall pressure of the cell increases proportionately with an 

 increase in the volume of the cell. When the cell is completely flaccid (rela- 

 tive volume =1.0) its diffusion pressure deficit is equal to its osmotic pres- 

 sure (20 atmos.) while its turgor pressure and hence its wall pressure are both 

 zero. As the volume of the cell increases, due to an influx of water, the 

 osmotic pressure decreases due to dilution of the cell sap. The turgor pres- 

 sure and wall pressure increase equally and progressively with increase in the 

 volume of the cell, the diffusion pressure deficit of the cell showing a corre- 



2SR 



20-- 



I5-- 



I 

 a 



5 



e.-- 



1:2 1.3 



RELATIVE CELL VOLUME 



Fig. 32. Inter-relationships among the osmotic pressure, turgor pressure, wall pres- 

 sure, diffusion pressure deficit, and cell volume of a plant cell. Modified from Hofler 

 (1920). 



sponding but more rapid, progressive decrease. When the cell attains a con- 

 dition of maximum turgidity (relative volume =1.5) its turgor pressure and 

 wall pressure are equal to its osmotic pressure while its diffusion pressure 

 deficit has fallen to zero. 



The dotted extension of the line for wall pressure to the left is intended 

 to suggest that it sometimes has a negative value. Conditions under which 

 this may occur are discussed shortly. The diffusion pressure deficit of a cell 

 in which the wall pressure is negative is greater than its osmotic pressure. 



In many types of cells volume changes are much less marked than indi- 

 cated in this figure, and hence they can often he disregarded in generalized 

 considerations of the water relations of plant cells. In some species m.am^ of 

 the cells have walls which are totally lacking in elasticity. This is especially 



