CHAPTER VI 



THE OSMOTIC EQUILIBRIUM BETWEEN THE CELL AND 

 ITS SURROUNDINGS 



NAKED AND WALLED CELLS. 



WHEN a naked cell is placed in a medium the osmotic 

 pressure of which is lower than that of its vacuoles, water 

 is taken up, causing distension of the protoplasm, and the 

 process may ultimately cause the cell to burst. This 

 occurs commonly in the lower marine animals when trans- 

 ferred suddenly to water of a lesser degree of salinity than 

 that of their normal surroundings. 



It has been suggested by Dixon that one function of the 

 contractile vacuole is to remove crystalloids from the 

 interior of the cell, for by their accumulation these would 

 cause the vacuole to increase greatly, and so stretch the 

 protoplasm unduly. Contractile vacuoles are never met 

 with in cells which have a continuous wall of cellulose or 

 other similar substance, for in these the wall sets a limit 

 to the distension of the protoplasm. The latter lines the 

 wall, and owing to its semi-permeable nature may be 

 pressed against it and subject it to a considerable distend- 

 ing force. In some cases, as, for instance, when certain 

 kinds of pollen grains fall into water, this force may be 

 sufficient to rupture the cellulose wall also. As a general 

 rule, however, the wall is able to withstand the osmotic 



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