Relation of Plasmolysis to Shrinkage of Plant Tissue. 41 
pressure of the cell sap, but no attempt even was made in Thoday's 
note to bring forward such evidence. 
The question of the relation of loss in water by the cell to 
plasmolysis is one on which we had projected experiments, but 
circumstances have necessitated their abandonment for the present. 
In view of Thoday’s recent note we have therefore thought it worth 
while to publish a few preliminary results we have obtained in 
regard to this question in order to show the sort of assumption we 
made in supposing that with the potato tissue which underwent 
only slight change in weight when immersed in a salt solution, the 
latter could be regarded as approximately isotonic with the cell 
sap. 
In the experiments described below the gain or loss in weight 
of tissue immersed in solutionsof sodium chloride of differentconcen- 
trations was measured in the manner described in an earlier paper, 1 
while observations were made at the same time as regards the 
plasmolysis of the cells of the same tissue in various concentrations 
of the same salt. 
Thoday 2 implies that the solution which just brings about 
plasmolysis is to be regarded as isotonic with the cell sap. This 
neglects the observation of Pfeffer 3 that “at the commencement of 
plasmolysis the osmotic pressure of the external fluid is slightly 
higher than that of the turgid cell.” Generally that solution is 
regarded as isotonic which just fails to produce plasmolysis. Theor¬ 
etically these two concentrations should be the same within the limits 
imposed by the gradation of solutions used for the plasmolysis test. 
If Pfeffer’s statement cited above is to be accepted, it would there¬ 
fore appear that values for the osmotic concentration of the cell sap 
obtained by plasmolysis are more or less approximate. 
The matter is complicated in the case of a tissue, since, as is 
well-known, all the cells of a tissue do not require the same concen¬ 
tration of salt solution to bring about plasmolysis. Thus the value 
obtained for the concentration of a solution isotonic with the cell 
sap of a tissue is necessarily an approximation. 
While potato tissue is not a favourable material for plasmolytic 
observations, red beetroot on the other hand, is much more 
favourable for this purpose on account of the colour of the cell sap. 
In the experiments recorded below red beetroot, and not potato, 
was used. 
1 Ann. of Bot., 31, pp. 415-431, 1917. 
J New Phyt., 17, pp. 110, 111, 1918. 
* Physiology of plants, English Edit., Vol. I, p. 145, Oxford 1900. 
