HANS BURSTROM 



49 



tion between turgor and wall pressure is illustrated in the osmotic 

 diagram in Figure 3. 



In order for absorption of water to occur it is necessary that T exceed 

 W. We may therefore say that the pressure from the inside causes an 

 expansion of the cell; but from the equations it follows that in such a case 

 S must also exceed E to the same extent, so we may quite correctly 

 also say that the suction of the cell is greater than that of the external 

 medium. Hence water is absorbed and the cell expands. Expansion is 

 caused by a difference, T — W, or in other words, S — E; it may be 

 called a turgor-over-pressure or water deficit of the cell. It is thus im- 

 material if we call the resulting increase in volume an absorption of water 

 owing to a water deficit or an expansion owing to a turgor-over-pressure. 



Volumel ,4 



Figure 3. Diagram showing the osmotic conditions of one cell at equilibrium 



or nonequilibrium with one external medium. O O osmodc value 



of the cell sap, T T turgor pressure, W W wall pressure, 



E osmotic value of the external medium, S suction of the entire cell. I = 

 incipient plasmolysis, II = the cell absorbing water. III = equilibrium with 

 the medium E. 



