SALT AND WATER IN LIFE AND GROWTH 



115 



The expanding action of osmotic pressure can be observed 

 also in the cells of a muscle. If a freshly excised muscle of 

 a frog is placed in distilled water, it swells; in concentrated 

 salt solution, it shrivels. The muscle therefore seems to 

 consist of cells surrounded, like the red blood cell, by a semi- 

 permeable film. A closer observation shows that the muscle 

 does not consist entirely of such osmotic cells, but also of 

 fibers which support them. A similar support of the tender 

 osmotic cells is found in growing plants. In a Tradescantia 



Fig. 33. Diagram Illustrating the Effect of Osmosis on Spiderwort 



Cells (Tradescantia) 



In spiderwort cells only a part of the cell will shrink if placed in a con- 

 centrated solution. The tender semi-permeable film is imbedded in a 

 supporting framework of cellulose fibers; the latter remain unchanged in 

 any solution. 



(spiderwort) cell, for instance, the osmotic cell is held in a 

 rigid framework (Fig. 33). Place this cell in a concentrated 

 salt or sugar solution and the innermost portion will shrink 

 as indicated on the diagram. The surrounding cellulose 

 fibers remain unchanged. The conclusion is, therefore, 

 that only the inner part of this plant cell is surrounded by a 

 film which lets water pass. 



It seems likely that the expanding forces of osmotic pres- 

 sure must also be of importance for the natural growth of 



