CONDUCTIVITY AND PERMEABILITY 201 



of the protoplasm increases in NaCl; and that in CaCl 2 

 there is a decrease followed by an increase. 



This is in complete agreement with results obtained 

 when permeability is measured by such methods as 

 plasmolysis, specific gravity, tissue tension, exosmosis, 

 diffusion through living tissue and direct determination 

 of penetrating substances. 



L 



w P 



FIG. 88. A vegetable cell showing plasmolysis. At left, normal; in the center, plasmolyzed; 



at the right, nearly recovered. 



In order to show the bearing of these measurements 

 on the problem, the methods employed will be 

 briefly described. 



The measurement of osmotic pressure by means of 

 plasmolysis depends upon the fact that when the osmotic 

 pressure within a cell is greater than that of the surround- 

 ing solution, water is absorbed. In the case of plant cells, 

 there is usually a central vacuole, the contents of which 

 exert pressure against the protoplasmic sac which sur- 

 rounds the vacuole : in consequence the protoplasmic sac 

 is pressed against the cell wall. If, however, the osmotic 

 pressure of the external solution exceeds that of the 

 vacuole, water is withdrawn from the cell and the proto- 

 plasmic sac contracts. This contraction is called 

 plasmolysis (Fig. 88). At the moment when contraction 

 begins, the osmotic pressure of the solution within the 

 cell is regarded as slightly less than that of the external 



