CHAPTER VI. 



HOW WATER MOVES IN AND OUT OF PLANT 

 CELLS. 



Absorption, Diffusion, Osmose. 

 Demonstration 1 1 . 



58. Osmose in spirogyra. — Mount a few threads of the alga spirogyra in 

 a drop of the 5$ salt solution on a glass slip, and place on a cover glass for 

 microscopic examination. Let each pupil examine the preparation to ob- 

 serve the protoplasm contracted away from the cell wall. The protoplasmic 

 layer contracts slowly from the cell wall, and the movement of the mem- 

 brane can be watched by looking through the microscope. The membrane 

 contracts in such a way that all the contents of the cell are finally collected 

 into a rounded or oval mass which occupies the centre of the cell. 



Now add fresh water and draw off the salt solution. The protoplasmic 

 membrane expands again, or moves out in all directions, and occupies its 

 former position against the inner surface of the cell wall. This indicates 

 that there is some pressure from within, while this process of absorption is 

 going on, which causes the membrane to move out against the cell wall. 



The salt solution draws water from the cell-sap. There is thus a ten- 

 dency to form a vacuum in the cell, and the pressure on the outside of the 

 protoplasmic membrane causes it to move toward the centre of the cell. 

 When the salt solution is removed and the thread of spirogyra is again 

 bathed with water, the movement of the water is inward in the cell. This 

 would suggest that there is some substance dissolved in the cell-sap which 

 does not readily filter out through the membrane, but draws on the water 

 outside. It is this which produces the pressure from within and crowds the 

 membrane out against the cell wall again. 



59. Turgescence. — Were it not for the resistance which the 

 cell wall offers to the pressure from within, the delicate proto- 

 plasmic membrane would stretch to such an extent that it would 



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