WATER IN PLANT CELLS. 31 



liquid rises in the tube by this pressure above the level of the 

 water in the jar outside of the thistle tube. The diffusion of 

 liquids through a membrane is osmosis. 



62. Importance of these physical processes in plants. Now 

 if we recur to our experiment with spirogyra we find that exactly 

 the same processes take place. The proptoplasmic membrane 

 is the diffusion membrane, through which the diffusion takes 

 place. The salt solution which is first used to bathe the 

 threads of the plant is a stronger solution than that of the cell- 

 sap within the cell. Water, therefore, is drawn out of the cell- 

 sap, but the substances in solution in the cell-sap do not readily 

 move out.. As the bulk of the cell-sap diminishes the pressure 

 from the outside pushes the protoplasmic membrane away from 

 the wall. Now when we remove the salt solution and bathe the 

 thread with water again, the cell-sap, being a solution of certain 

 substances, diffuses w r ith more difficulty than the water, and the 

 diffusion current is inward, while the protoplasmic membrane 

 moves out against the cell wall, and turgidity again, results. 

 Also in the experiments with salt on the tissues and cells of the 

 beet (see exercise 14), the same processes take place. 



These experiments not only teach us that in the protoplasmic 

 membrane, the cell wall, and the cell-sap of plants do we have 

 structures which are capable of performing these physical 

 processes, but they also show that these processes are of the 

 utmost importance to the plant, in giving the plant the power 

 to take up solutions of nutriment from the soil. 



Exercise 14. 



63. To test the effect of a 5$ salt solution on a portion of the tissues of a 

 beet. Select a red beet. Cut several slices about \cm in diameter and 

 about $mm thick. Grasp the slices between the thumb and forefinger and 

 attempt to bend them by light pressure. They are quite rigid and bend but 

 little. Immerse a few of the slices in fresh water and a few in a 5$ salt solu- 

 tion. In the course of an hour or less, examine the slices again. Those in the 

 water remain as at first quite rigid, while those in the salt solution are more 

 or less flaccid or limp. They readily bend by pressure between the fingers. 



The salt solution, we judge after our experiment with spirogyra, with- 



