50 Stiles and Jergensen . — Studies in Permeability. IV. 
problem, and to repeat some of his experiments with more exact methods, 
which will at the same time enable one to obtain an insight into the kinetics of 
the changes taking place. This we have done in the experiments described 
in this paper, and we shall show as a result of them, that not only is the 
theory founded by Czapek upon his experimental results untenable, but that 
the facts on which he bases his theory are merely illusions of experimenta- 
tion due partly to the crudity of the method employed, and partly to 
a selection and arbitrary arrangement of experiments. 
We do not intend in this paper to enter into a general discussion of the 
permeability of the cell, nor to differentiate between the permeability 
properties of different tissues. Nevertheless we would point out that the 
chemical composition and physical properties of membranes in different 
tissues are not likely to be identical, and their permeability properties are 
consequently not likely to be expressed by one general law. 
In order to avoid discussion which available information does not 
justify, we use the generally accepted nomenclature, including the term 
‘ plasma membrane \ By this latter expression we mean that part of the 
cell which is concerned in the phenomena of permeability, without reference 
to its actual location in the cell. 
Methods used in the Present Investigation. 
In this investigation we have employed a method for studying per- 
meability changes which is more sensitive and of more general application 
than the methods of earlier workers, and which at the same time allows the 
changes taking place to be followed easily from time to time. This method 
consists essentially of measuring the rate of exosmosis of electrolytes from 
plant tissue by means of measurements of the electrical conductivity of the 
solution in which the tissue is placed. In former investigations in which 
we have used this method the solution external to the tissue has been an 
electrolyte, and so changes in the conductivity of the external solution have 
been the resultant of diffusion of electrolytes into the tissue from the 
solution and diffusion of electrolytes out of the tissue into the solution. 
In the present investigation the substances examined are all practically 
non-electrolytes, and the electrical conductivities of solutions of them 
are almost identical with that of the conductivity of the water used in 
preparing them. Hence the increase in conductivity will give a much surer 
measure of the extent of exosmosis than when the effect of acids or salts is 
under investigation. It is, however, necessary to make allowance for the 
lowering of conductivity due to the presence of non-electrolytes, and this has 
been done in the cases cited in the following pages. 
A brief account of the method has been given in a previous paper (10), but 
we may here take the opportunity of describing the method in more detail. 
A suitable quantity of the solution under investigation is placed in 
