FACTORS RENDERING THE PLASMOLYTIC METHOD INAPPLICABLE IN THE ESTIMATION OF OSMOTIC VALUES OF PLANT CELLS

In 1884 de Vries (10) introduced a new method, that of determining the osmotic value of plant cells by immersion in solutions of known concentrations of cane sugar or potassium nitrate. He determined the concentration of each solute which would bring about plasmolysis or the withdrawal of the protoplasm from the walls of the cells. He reasoned that all solutions inducing the same minimum degree of visible plasmolysis of cells must be of equal strength. Various aspects of the phenomenon of plasmolysis have been elaborately studied since then by de Vries (11), Fitting (4), Hannig (5), Iljin (8), Hofler (6, 7), Ursprung and Blum (9), Beck (1, 2), Weber (12, 13), and many others. These workers have interpreted the results obtained by this method as furnishing adequate data for the estimation of the osmotic value of the cell sap. Objection may be raised to this interpretation on two grounds. In the first place, the material used by de Vries, and by most of his followers, cannot be considered to have been in normal, uninjured condition. Thin sections were prepared for microscopic examination by sectioning, thus subjecting the cells to the deleterious osmotic and other actions of sap exuded from adjacent ruptured cells. That this influence may be important has been pointed out elsewhere (3). Moreover, the phenomenon commonly designated as plasmolysis, although usually regarded as purely osmotic in nature, may be induced by several other causes. Mechanical injury induced by pressure brings about a contraction of the protoplasm which in its early stages resembles plasmolysis and which is often reversible if the injury is slight. It passes through a plasmolysis-like stage to an irreversible granulation if the pressure is considerable or of long duration. Strong light (13) and high temperature likewise cause retraction of the protoplasm from the cell walls, and light and temperature conditions which are not markedly injurious may influence the time required for plasmolysis. In the second place, the assumption made by Ursprung and Blum (9), and with few exceptions accepted without experimental proof by subsequent workers, that the cell contents and applied solution come into equilibrium within 20-30 minutes, can be shown to be untenable both on theoretical i This work was done at the Imperial College of Science and Technology, London. The writer wishes to express her thanks to Professor V. H. Blackman for his. interest and advice.