Permeability 131 
Kahho (1921 b) found that upper epidermal cells of the leaf of 
the red cabbage placed in a 0-48 N solution of sodium iodide at 
a temperature of 23-24 0 C. were killed more rapidly in light than in 
the dark. Similar results were obtained with sodium bromide and 
sodium thiocyanate. These results were attributed by Kahho to a 
more rapid entrance of the salts in question in the light than in the 
dark. 
Influence of the thickness of the tissue on the rate of absorption. 
Hofler and Stiegler (1921), using the plasmometric method, observed 
that urea entered the epidermal cells of the stem of Gentiana Stur- 
miana much more slowly in very thin sections which contained only 
the epidermal layer, but this uninjured, than in thick sections. The 
explanation of this is not forthcoming; it might, perhaps, be corre¬ 
lated with the observations of Trondle (1921) on the effect of wounding 
on permeability to which reference is made in the next chapter. 
On the course of absorption in general. The curves obtained by 
Stiles and Kidd for the course of absorption of salts show that there 
is a rapid intake of salt for the first 15 or 30 minutes after which 
absorption follows an approximately logarithmic course towards the 
position of equilibrium. How exactly the latter part of the curve, 
or the whole of it, is represented by a logarithmic equation, cannot 
be said; in the case of the absorption of hydrogen ions by potato tuber 
the approximation was near enough for the assumption of this 
relation in the calculation of the influence of temperature. 
As the position of equilibrium differs in the case of every substance 
and of every concentration of the same substance, at any rate as far 
as existing data indicate, it is perfectly clear that the rate of absorption 
cannot be regarded as a measure of the permeability of cell mem¬ 
branes to the substance. For if the permeability of the cell membranes 
to potassium chloride and calcium chloride were the same, the rate 
at which the calcium salt entered the cell would soon slow down as 
compared with the rate at which the potassium salt entered, on 
account of the different position of the equilibrium. 
The explanation given by Fitting (1915) of the slowing down of 
the rate of deplasmolysis is that the salt itself lowers the permeability 
of the cell to the salt. That this explanation is inadequate becomes 
clear from the fact that the position of equilibrium bears a definite 
relation to the concentration of the external solution, and that in 
low concentrations the absorption ratio may be many times unity, 
a result which cannot be explained on the ground of changes in 
permeability of the membrane alone. As the concentrations used by 
9—2 
