Chapter VII 

 THE OSMOTIC QUANTITIES OF PLANT CELLS 



A Description of the Quantities and of their Interrelations : — The 



osmotic quantities of plant cells are osmotic pressure (OP), diffusion pres- 

 sure deficit (DPD), and turgor pressure (TP). That the dynamics of 

 water movement and retention can be explained by these three quantities 

 has been shown by the work of Thoday (1918) and Hofler (1920), and 

 later by Beck (1928), Ursprung (1935), Meyer (1938), and others. A 

 diagram that has proved extremely valuable in presenting these concepts 

 and their interrelations is shown in Figure 19. One fundamental equa- 

 tion necessary to the interpretation of this figure is 



OP = DPD + TP (i) 



This diagram resembles Figure 15 of Chapter V with the exception 

 that the diffusion pressure deficits are shown as positive quantities and are 



/./ /.z /.a 



Relative cell volume 



/■s 



Fig. 19. — A diagrammatic presentation of the relations among osmotic 

 pressure, diffusion pressure deficit, and turgor pressure as applied to plant 

 cells. Redrawn with modification from Meyer and Anderson (1939). 



placed above the base line. The curved upper boundary labeled osmotic 

 pressure designates the concept used by Meyer (1945), that is, the index 

 of the value which the sum DPD + TP may attain in the cell. It is sloping 

 to account for the increase in volume and decrease in concentration that 

 occurs as water passes into the cell between the states of limiting plasmol- 



