332 J. DAVIDSON 



lytes, as calcium nitrate, potassium sulphate, etc., their mineral 

 content increases with their growth. The electrolytes of the 

 nutrient solution must, consequently, enter the plant cell. 

 We know further that sugars are translocated in plants and 

 that heterotrophic plants can absorb sugar directly from a 

 sugar solution; consequently, sugar must be capable of entering 

 and leaving the plant cell. But when the permeability of plant 

 or animal cells is measured by the plasmolytic method it is 

 found that such cells are permeable neither to electrolytes, nor 

 to sugar, for the plasmolysed cells never regain their original 

 form so long as they remain in hypertonic solutions of these 

 substances. 



Using the same plasmolytic method it is found that there are 

 in addition to the substances which produce irreversible plas- 

 molysis, many substances (monohydric alcohols, aldehydes, pep- 

 tones, etc.) which do not produce plasmolysis at all, and, con- 

 sequently, enter the cell very easily; and that there are numerous 

 substances (dihydric alcohols, glycerine, the amids of the mono- 

 basic acids, etc.) which produce reversible plasmolysis, the 

 plasmolysed cells recovering while in the hypertonic solution 

 with varying rapidity and, consequently, enter the cell slowly.^ 



The explanation of the plasmolytic phenomena is based on 

 the difference in osmotic pressure within and outside of the 

 cell and on the property of selective permeability. When a 

 living cell is placed in a hypertonic solution the osmotic pressure 

 in the surrounding medium is greater than within the cell, and, 

 as a result, water is drawn from the cell and plasmolysis takes 

 place. When the cell is impermeable to the substance in solu- 

 tion the plasmolysis is irreversible as long as the cell remains 

 in the hypertonic solution. When, however, the cell is perme- 

 able to the solute equal concentration and, consequently, equal 

 osmotic pressure are eventually obtained within and without 

 the cell through diffusion and the cell assumes its previous form, 

 i.e., the plasmolysis is reversed. 



Unless the interpretation of the plasmolytic phenomena is 



^ Overton, E., tjber die Allgemeine Osmotischen Eigenschaften der Zelle. 

 Vierteljahrschrift. Naturf. Ges. in Zurich 44: 88-135. 1899. 



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