ABSORPTION OF WATER 59 



diffuse through the cytoplasm between the two limiting 

 or surface membranes. 



56. Plasmolysis. — If the closed sac, formed by the 

 porous membrane, contains the less dense, instead of the 

 more dense liquid, then the reverse of turgor will take 

 place, and the sac will collapse. This may be easily 

 demonstrated under the microscope by irrigating root- 

 hairs, or other plant cells, with solutions more dense than 

 the cell-sap. For example, the root-hairs shown in 

 Fig. 41, mounted in water on a glass slide, under a cover- 

 glass, were found, by microscopic examination, to be 

 turgid. Then they were irrigated with a 5 per cent, 

 solution of common table salt. This solution is denser 

 than the cell-sap of the root-hairs, so that exosmosis was 

 more rapid than endosmosis, and cell-sap was withdrawn 

 from the vacuoles faster than liquid entered from without. 

 The salt-solution having soaked through the cell-wall, 

 passed with difi&culty through the limiting membrane of 

 the cytoplasm, and thus began to exert an osmotic pressure 

 from without, which loosened the protoplasm {plasmoly- 

 sis^), and caused it to collapse. When this results, the 

 cell is then said to be plasmolyzed. 



57. Importance of Osmosis. — No physical phenomenon 

 is more important than osmosis. Upon it depends the 

 life and death of every living thing. By it, not only do 

 plants take in necessary substances from the soil, but all 

 the food assimilated by man and the lower animals passes 

 into their cells. It has been demonstrated that the mainte- 

 nance of turgidity is necessary in order that cells may con- 

 tinue to perform their normal functions. In a state of 

 plasmolysis they cannot do so. This is illustrated in a 



1 From the Greek, plasma + liicin, to loose, or set free. 



