Relation of Plasmolysis to Shrinkage of Plant Tissue. 45 
approximately isotonic, on the osmotic hypothesis, with the cell sap 
in the case of the three beet roots examined. 
In order to avoid misunderstanding we would emphasize that 
the values found for the concentration of sodium chloride isotonic 
with the cell sap of a tissue, whether the change in weight method 
or the plasmolytic method is used, is of necessity only a crude 
approximation. Nothing more than this was claimed in our 
earlier paper, where on the basis of the change in weight method 
we spoke of a solution of sodium chloride of concentration CH25 N 
as approximately isotonic with the cell sap of potato. Of course 
the degree of approximation depends on the gradation of the solutions 
used in the swelling and the shrinkage experiments, as well as on 
the hydrostatic pressure of the solution in the cell, a quantity 
itself depending on a variety of conditions. It is perfectly clear 
that the change in weight method could not be used for the 
accurate determination of isotonic coefficients ; on the other hand 
the plasmolytic method for the measurement of isotonic coefficients 
is not free from pitfalls and difficulty, and requires considerable 
caution in its use, so that it will only give accurate results under 
special circumstances. 1 
However, if we assume that the water relations in potato are 
similar to those of beetroot, an assumption which appeal’s to us in no 
way unreasonable, the facts recorded in this paper indicate that 
our statement to the effect that a solution of sodium chloride of 
concentration O’125 N is approximately isotonic with the cell sap 
of potato, was correct. We may also point out that the two 
sentences in which we spoke of an isotonic solution were very 
incidental remarks and had nothing to do either with the facts or 
the general theme of that paper. Nevertheless, we think the 
observations recorded above indicate that, as passing remarks, they 
were justified. 
There are a few other matters connected with the contents 
of this paper which we think deserve mention. 
In the first place we would call attention to the length of 
time that beet-root tissue remains in apparent equilibrium with 
salt solution as regards exchange of water. In Pig. 2 are shown 
graphically the results obtained from experiments in which beet discs 
remained immersed in solutions of sodium chloride for a period 
extending over 200 hours in the cases of distilled water, 0125 N 
sodium chloride and 0*25 N sodium chloride, and for a shorter 
period in the case of 0’40 N and 0-45 N (see Tables I and II). No 
1 See especially H. Fitting, Jalirb. f. wiss. Bot., f>7, 553-612, 1917, 
