THE PROTOPLASMATIC MEMBRANE 



well serve the purpose, but they are not so easily 

 cut with the razor. When such sections are put 

 into salt solution of sufficient concentration, e.g. 

 potassium nitrate 2 per cent, after some minutes 

 all cells show their protoplasm shrunk away 

 from the cell wall. The cell protoplasm forms a 

 red ball lying free in the cell. When the sections 

 are put back into water, the plasmolysis disappears 

 and the cells regain their normal condition. 

 Plasmolysis is therefore a normal, merely physical 

 phenomenon, not at all a pathological one. 



How can plasmolysis be explained ? Micro- 

 scopical inspection immediately convinces us of 

 the fact that the volume of protoplasm is reduced 

 in plasmolysis. It was only possible for this to be 

 brought about by the expulsion of water fronr'the 

 sap vacuole of the protoplast. By loss of water the 

 concentration of the sap is increased, until the 

 osmotic value of the outer solution is greater than 

 the osmotic value of the cell sap. This state being 

 arrived at, equilibrium is regained. We learn 

 from this process that the protoplasmatic mem- 

 brane cannot be permeable for the salt in solution. 

 If it had been permeable, the equilibrium would 

 have been reached simply by endosmosis into the 

 cell, as long as the concentration inside and outside 

 had not become equivalent. Or osmotic substances 

 would have penetrated the protoplasmatic mem- 

 brane from the inside of the cell when plasmolysis 

 disappeared in water. Consequently, wo may say 

 37 



