194 



PROTOPLASM 



by a membrane (Fig. 2). Both membranes are of protoplasmic 

 material and function osmotically; i.e., they are semipermeable. 

 Within the vacuole is a solution of salts, carbohydrates, pro- 

 teins, etc., certain ingredients of which cannot readily pass 

 through the membrane. The vacuole is, therefore, an osmotic 

 system, a solution of relatively high concentration within a 

 selectively permeable membrane surrounded by a solution of 

 low concentration. The outer solution, in the case of an aquatic 

 plant, is the water of the pond or, in the case of the root hairs of 

 terrestrial plants, the soil water; if the cell is part of inner tissue, 



A B I C 



Fig. 104. — Stages in the plasmolysis of a plant cell. 



then the plant juices or intercellular sap constitute the sur- 

 rounding solution. 



Water in an osmotic system moves in excess toward the solu- 

 tion of higher concentration. If the concentrations are reversed, 

 the movement will be reversed, and any pressure already existing 

 on the one side will be released. This holds true for the plant 

 cell. If the osmotically active substances dissolved in the water 

 of the vacuole are the equivalent of, say, 3 per cent of common 

 salt, then, if the cell is bathed in 5 per cent salt, osmosis will be 

 reversed, and the turgor in the cell relieved, so much so that the 

 cell (protoplast) may shrink away from the (relatively) rigid 

 cellulose wall (Fig. 104). This shrinking is known as plasmolysis. 

 Naturally, if the inner and outer solutions are equivalent (in 

 osmotic activity), there will be no movement of water in either 

 direction and no change in pressure. If plant tissue, such as the 

 leaf of Elodea, is put in salt or sugar solutions of different con- 

 centrations, varying from 1 to 10 per cent of salt (potassium 



