[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 205 



bottle filled with a solution and then immerse the bottle in water; or 

 one may convert the membrane into a small bag. 



Demonstrations that are even more instructive may be made directly 

 with living plant cells. In a series of small glass dishes or hollow ground- 

 glass slides we may place distilled water containing different amounts 

 of sugar varying all the way from none at all on one end of the series to 

 as much as 20 per cent on the opposite end. We may now place living 

 plant cells in these different solutions and with a microscope watch what 

 happens. At one end of the series where there is little or no sugar in the 

 external water, the plant cells may become larger; a little farther along 

 in the series the cells appear to remain unchanged in size and shape; still 

 farther along they will decrease slightly in size. Toward the end of the 

 series the protoplasm is partly separated from the cell wall, and in the last 

 of the series the protoplasm has become a small mass in the cell, since 

 the vacuole has entirely disappeared. That is, the water that was formerly 

 in the vacuole has diffused out of it. 



All these observations may be interpreted on the basis of the laws of 

 diffusion and the differential penueability of membranes. Perhaps it is 

 sufficient to add here that where the protoplasm just began to separate 

 from the wall the concentration of the water in the vacuole was prac- 

 tically the same as that of the water in the dish if the temperature was 

 the same; where the cells became larger the concentration of the water in 

 the vacuoles was originally lower than that in the dish; and where the 

 protoplasm was separated from the wall the concentration of the water 

 in the vacuole was originallv higher than that of the water in the dish. 

 Since the protoplasm separated from the cell wall, it is the differentially 

 permeable membrane of the cell. The bounding surfaces of plastids and 

 nuclei within the protoplasm are also differentially permeable. Instead 

 of single cells, pieces of plant tissue may be placed in the series of solu- 

 tions and the relative changes in turgidity and size of the tissue may be 

 observed. 



When fresh-water plants are placed in salt lakes, water diffuses out of 

 them into the lake because the concentration of the water in the lake is 

 less than it is inside the plant cells. Such conditions are sometimes re- 

 ferred to as physiological drought. There is an abundance of water out- 

 side the plant but its concentration is too low. A whole lake full of salt 

 water may be as drv as a desert to a plant. 



Plasmolysis and turgor. When the protoplasm of a cell separates from 

 the wall because water is diffusing out of the vacuole, the cell is said to 



