CHAPTER XI 

 OSMOSIS 



The shrinking of plant protoplasts away from their cellulose 

 walls — the phenomenon known as plasmolysis (page 194, Fig. 106) 

 — led botanists to study the causes underlying the pressure which 

 keeps plant cells turgid. The process responsible for the turgor 

 of plant cells is osmosis. Osmosis in the living world is most 

 often studied in association with plant cells, but it is now recog- 

 nized as a property of animal cells as well. In plants, it is the 

 cell vacuole which is regarded as the osmotic mechanism; but in 

 animal cells, it is the cell as a whole, thus including the cytoplasm, 

 which is the osmotic system. How far osmosis, as a force dis- 

 tinct from imbibition, is operative in animal cells is still a moot 

 question. 



An Osmotic System. — Osmosis was discovered in 1748 by the 

 Abbe Nollet and not again referred to until a century later, when 

 the Frenchman Dutrochet investigated it further. He found 

 that when a solution, e.g., of salt or sugar, is separated from 

 pure water by an animal membrane, water diffuses through 

 the membrane more rapidly from the pure-water side than from the 

 solution side, resulting in a rise in the level of the solution and the 

 consequent production of a (hydrostatic) pressure on the solution 

 side of the membrane. The Dutch and German botanists Hugo 

 deVries and Wilhelm Pfeffer next (1877) took up the work in their 

 studies of plasmolysis. Nowhere in science is there a more 

 dramatic series of events than those which led from a simple 

 botanical experiment in plasmolysis to the laws of solutions. 

 Nageli discovered the selective permeability of the plasma 

 membrane; deVries formulated the principles underlying it; 

 Pfeffer determined the pressures developed within plant cells; 

 van't Hoff enunciated the solution laws ; and Arrhenius set forth 

 the dissociation theory out of which arose the science of elec- 

 trochemistry. The last substantial work on osmosis was done 

 in America, where Morse and Frazer made exceedingly precise 

 measurements of osmotic pressures. 



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