224 
Hind. — Studies in Permeability . III. 
Methods. 
1. Electrical Conductivity Methods. — By measuring the change in 
electrical conductivity of a solution surrounding plant tissue, some idea of 
the effect of the solution on plant cells can be obtained. If the substance 
in solution enters without harming the cell and causing an exosmosis of the 
cell-contents, a decrease in conductivity will be the result ; whereas if the 
plasma-membrane is affected in such a way that the substances within 
the cell diffuse out, the conductivity of the external solution will be 
increased on account of this. The change in conductivity in such cases will 
therefore be the resultant of the diffusion of the external solute into the 
cell and of exosmosis of electrolytes from the cell. 
These experiments were carried out in the same way as the electrical 
conductivity experiments described in the first of these studies. Twenty 
discs of potato tuber were put in a stoppered bottle containing icoc.c. acid, 
the conductivity of which had been measured. The conductivity of the 
acid solution was then taken at intervals. Dilute solutions of each acid in 
various concentrations were used. The experiments were all done in 
duplicate; they were carried out at constant temperature — i8°C. — as it 
has already been shown that temperature exerts a considerable influence on 
the rate of absorption ( 13 ). 
The curves were obtained by plotting the increase or decrease in 
electrical conductivity against the time. 
2. Volumetric Analytical Methods. — In order to measure the acidity of 
the external solution after it had been in contact with the potato discs for 
different times, the solutions were poured off and titrated with standard 
alkali. No reliable results, however, were obtained, owing to the great 
dilution of the solutions, and possibly to the diffusion out from the cells of 
substances which interfered with the indicators. 
3. Electrometric Method. — The method of experimentation involving 
the use of electrical conductivity can only give a rough approximation to 
the actual course of absorption of electrolytes, owing to the diffusion out of 
electrolytes from the cell, which takes place with most substances and even 
to some extent in distilled water. By measuring the concentration of the 
hydrogen ions in acid solutions before and after they have been in contact 
with potato discs, the actual rate of absorption of the hydrogen ion can be 
measured. Experiments were carried out in the way described in the 
second paper of this series ( 13 ). 
The hydrogen-ion concentration was calculated in terms of the original 
pure acid solution. The curves were obtained by plotting the relative 
hydrogen-ion concentration against the time. The bottles containing the 
discs and acid were kept at a temperature of 18 0 C. throughout the 
experiments. 
