A. C . Halket. 
164 
ON VARIOUS METHODS FOR DETERMINING 
OSMOTIC PRESSURES. 
With a Description of the Application of Barger’s Method 
of Determining Molecular Weights to the Estimation of the 
Osmotic Pressure of the Cell Sap of Plants. 
By A. C. Halket, B.Sc., 
Demonstrator in Botany , Bedford College, London. 
[With Two Figures in the Text.] 
I.—Some Methods of Determining Osmotic Pressure. 
ARIOUS methods of determining osmotic pressure have been 
used at different times by workers investigating the properties 
of solutions of different kinds. A brief review of these may be useful. 
1. Plasmolytic method. This method, which we owe to 
de Vries, has been the one generally employed to determine the 
value of the osmotic pressure of the cell sap of plants. Cells of the 
tissue under examination are placed in salt solutions of various 
known strengths and, as the solution which has exactly the same 
osmotic pressure as the cell sap causes no change to take place, 
that solution which causes just a slight shrinking of the protoplasm 
away from the cell wall is taken as isotonic (isosmotic) with the 
cell sap. This method is, however, not without its difficulties. 
Ewart 1 comments on this point in his paper “ On the Ascent of 
Water in Trees,” where he speaks of its want of exactitude for deter¬ 
mining the osmotic pressure of the sap of small cells. 
Some of the recent work on the toxic action of many of the 
common inorganic salts on the protoplasm of plant cells shows that 
plasmolysis of cells is induced by salt solutions of concentrations 
far weaker than those isotonic with the cell sap. Osterhout 2 reports 
that in young plants of Vaucheria, produced from zoospores, the 
protoplasm contracted away from the cell-walls when the filaments 
were immersed in a ’0937 gram-molecular solution of sodium 
chloride. Contraction of the protoplasm was also caused by a 
solution as dilute as "0001 molecular if it was allowed to act for 
some days. This contraction was found not to take place if the 
1 “ On the Ascent of Water in Trees.” Phil. Trans. Roy. Soe., Vol. 199, 
1908, p. 355. 
2 “ On Plasmolysis.” Bot. Gaz., Vol. 46, 1908, p. 54. 
