Walter Stiles 
78 
plant cells. Yet it is obvious that a determination of the rate of 
intake of a substance can give no indication of the permeability of 
the cell unless the position of the equilibrium in the intake is known. 
Some few writers have, indeed, had a clear conception of this. 
Thus Pfeffer pointed out that a cell might be permeable to a 
dye and yet the intake of the dye might be unobservable because 
no accumulation of the dye took place in the interior of the cell. 
Yet in employing the favourite method used in recent work for 
determining the rate of intake of dissolved substances, that is, the 
plasmolytic method, investigators most usually do not consider the 
position of equilibrium at all, while the possibility of an equilibrium 
with different concentrations of the substance inside and outside the 
cell, is never considered. Quite usually the rate of intake is regarded 
as a measure of permeability. This would only be so if the difference 
in concentration between the external solution and the cell sap 
remained constant, which, according to the theory on which the 
method is based, is not the case. Lepeschkin, it is true, takes account 
of the change in concentration difference between external solution 
and cell sap as deplasmolysis proceeds, and so in treating his ex¬ 
perimental data, is at any rate consistent with the theory on which 
he works. 
But is it correct to assume that the dissolved substance diffuses 
into the cell until there is equality of concentration of this substance 
inside and outside the cell? Pfeifer’s work with dyes showed most 
undeniably that this is not the case with the majority of dyes 
examined by him, while it is equally certain that many substances 
occur normally in the plant in considerably higher concentration 
than they do in the external medium from which they were obtained. 
Wherever chemical combination or adsorption takes place in the 
interior of the cell it is obvious that the assumption made in the 
plasmolytic and some other methods, that the penetration of the 
dissolved substance into the cell can be regarded simply as the 
passage through a membrane into a medium which does not react 
with the substance, is unsound. It is therefore very necessary that 
the evidence available with regard to the position of this equilibrium 
should be examined. 
We may notice in the first place that a number of observations 
are on record which indicate that the concentration of a substance 
inside a cell can remain greater or less than its concentration in the 
external medium. With animal cells the case of blood corpuscles 
and serum may be cited, and with plant cells the observations of 
