PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 23 



of the osmotic pressure of the cell sap, as may be seen from the 

 formula S = P — T, where S varies inversely to T and reaches 

 its highest value when 7^ = 0. 



The relationship, between turgidity, osmotic pressure, and 

 suction tension may be illustrated graphically by the following 

 diagram (Fig. 7). On the abscissas are marked the various 

 stages of tension of the cell wall at different degrees of saturation 

 of the cell with water or at different volumes of the cell. On 

 the left is seen the condition of a cell when completely plas- 

 molyzed, or wilted, and, therefore, at its smallest volume, which 

 is denoted by 1. The concentration of the cell sap, and conse- 



Gi - — Qx — ^ 6t 



Fig. 7. — Diagram showing changes of the osmotic value in a cell passing from 

 saturation with water to wilting (from right to left), and vice versa (from left to 

 right) : T, turgor pressure; 0, osmotic pressure of the cell sap; *S, suction tension; 

 G, degree of distention of the membrane {after Hofler). 



quently its osmotic pressure attains its maximum. The 

 cell wall is in a completely flaccid state, its pressure on the cell 

 contents equals zero, and the suction tension >S attains the 

 magnitude of the total osmotic pressure. Now let the cell 

 absorb water. Its volume, and consequently the tension of the 

 cell wall, increases. This is denoted by the figures below the 

 diagram. A gradually increasing turgidity T becomes notice- 

 able. When the cell sap becomes diluted with water, its osmotic 

 pressure falls, and the suction tension rapidly decreases. On 

 the right side of the diagram is illustrated the condition of com- 

 plete saturation with water; the suction tension having fallen 

 to zero, the turgor pressure now balances the osmotic pressure 

 of the cell. Under the average conditions of a cell, denoted in 

 this diagram by the letter G, a part of the osmotic pressure is 



