270 PHYSIOLOGY OF ALIMENTATION. 



able to pass through the cell membrane plasmolysis must 

 in consequence be impossible, for under these circumstances 

 the diffusion of the dissolved substance into the cell equalizes 

 the pressure on both sides of the membrane, and the difference 

 between the osmotic pressure inside and outside of the cell 

 which is essential for plasmolysis does not come to pass. A 

 movement of water does not occur in the direction toward 

 the region of higher osmotic pressure as when a semipermeable 

 membrane is present, but a movement of dissolved particles 

 takes place from the region of higher osmotic pressure to that 

 of lower osmotic pressure. Only in case the cell membrane 

 possesses but a limited permeability do both water and dis- 

 solved substance move, for in this case a cell will give up 

 some of its water to the more concentrated solution surround- 

 ing it before the substance dissolved in this solution has had 

 time to diffuse into the cell. Under such circumstances a 

 temporary change in the volume of the cell concerned is pos- 

 sible. Herein lies the explanation of the behavior of the 

 cells of the alimentary mucosa in responding only temporarily 

 to great and sudden changes in the osmotic pressure of liquids 

 surrounding them. 



But the intensity with which a solution can bring about 

 the plasmolysis of a cell is a function not only of the degree 

 of difference between the osmotic pressure without and 

 within the cell, but also of the velocity with which the sub- 

 stance can penetrate the cell membrane. It is evident that 

 with the same degree of osmotic difference a substance capa- 

 ble of diffusing rapidly into a cell will be less likely to plas- 

 molyze the cell than one which diffuses in more slowly, for 

 under the latter circumstances a movement of water is more 

 likely to occur than under the former. 



It becomes possible to differentiate in consequence be- 

 tween substances which enter cells slowly and those which 

 enter rapidly. Glycerine belongs to the class of substances 

 which diffuse slowly into a cell and as slowly leave it. If 

 algae are in consequence placed in a dilute glycerine solution 



