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PERMEABILITY 
By WALTER STILES 
CHAPTER XIII 
REVERSIBLE AND IRREVERSIBLE CHANGES 
IN CELL PERMEABILITY 
I T is commonly supposed that the permeability of the plant cell 
is capable of undergoing changes, and statements to that effect, 
and involving that point of view, are common in the literature of 
the subject. But before discussing these changes it is necessary to 
enquire what exactly is meant by the term “change in permeability/’ 
In making this enquiry we are brought up at once against the difficulty 
referred to in the first chapter, namely, our present very inadequate 
analysis of the system involved. In the preceding chapters it has 
already been necessary to call attention to the fact that the rate of 
intake of a substance is not necessarily a measure of the permeability 
of any or all of the cell membranes, as the rate of intake will depend 
on the difference of concentration within and without the cell, and 
so also on the position of the equilibrium attained in the intake. Nor 
on the other hand can exosmosis be used as a measure of permeability. 
To make the position clear let it be supposed that when a plant cell 
is placed in a certain solution no diffusion out from the cell takes 
place at all. Suppose the cell is now transferred to a second solution 
into which a considerable exosmosis of substances takes place forth¬ 
with from the cell. It would be generally said that the solute in the 
second solution had brought about an increase in the permeability 
of the cell, or of the protoplasm, or of the plasma-membrane. This 
view is really based on the assumption that the protoplast or vacuole 
is surrounded by a membrane which is changed physically or chemi¬ 
cally by the solute in the external solution, so that the substances 
within are now able to diffuse through the membrane. This might 
indeed be the true explanation, but clearly others are possible. Thus 
the solute in question might affect non-diffusible substances of com¬ 
plex molecular constitution so that they broke down into a number 
