236 Hind. — Studies in Permeability. III. 
of the lecithin showed that it had little effect on the hydrogen-ion 
concentration of the acid. 
Discussion. 
The results of the experiments described in this paper lead one to 
conclude that acids in dilute solution, or at any rate their hydrogen ions, 
readily enter plant cells. 
The curves obtained in the electrical conductivity experiments show 
that plant cells are affected in different ways by different acids, and also 
give an idea of the rate and the manner in which the acids affect the cells ; 
but considering the complexity of the system, and the number of actions 
possible, it cannot be assumed that they give a definite measure of either 
the rate of absorption of the acid, or of the exosmosis of the substances in 
solution within the cell. They do suggest, however, that in dilute solution 
some acids can penetrate into the cells without doing much injury to them 
for some time, while others produce such an effect that some of the 
substances within the cell pass out almost as soon as the potato discs are 
immersed in the acid. 
The hydrogen-ion concentration measurements show the actual rate 
of absorption of the hydrogen ions by the cells. From the curves, it can be 
seen that the hydrogen ions of various acids are absorbed in the same way 
for some time, but that, after some hours, acids such as formic acid produce 
results different from those produced by the mineral acids. The anions 
have obviously an effect, and determine whether the acid is toxic or com- 
paratively non-toxic, toxicity being roughly indicated by the rate of 
exosmosis of electrolytes from the cell, as indicated by a comparison of the 
conductivity curves and those of hydrogen-ion concentration. 
The increase in the hydrogen- ion concentration of the external solution 
which takes place after some hours in the case of formic and acetic acid is 
a curious fact which requires explanation. 
It may be supposed that in penetrating into the cell, these acids, which 
are both chemically active, may react with some of the cell contents and 
produce other acids, probably organic acids which cannot normally pass 
through the limiting layer of the protoplast. But in entering the cell, the 
acid will have probably reacted with the outer layers of the protoplasm, 
and will have so altered their chemical constitution that the acids produced 
in the cell are able to diffuse out, and so increase the hydrogen-ion con- 
centration of the external solution. 
Considerable interest attaches to the mechanism by which acids enter 
the cell so rapidly. So far, opinion has been divided as to whether it is the 
protein or lipoid substances which are active in absorption. The experi- 
ments described in the third section of the experimental part of this paper 
all appear to suggest that, in regard to absorption of acids, it is the protein 
