Permeability 245 
the plasma-membrane is a gel of which the more solid phase consists 
principally of amphoteric emulsoid colloids and the more liquid 
phase of a buffer mixture. At the iso-electric point the continuous 
phase will be in a state of minimum hydration and so will occupy 
a minimum volume, and also will be without charge. It will in con¬ 
sequence be most permeable in this condition. Above the iso-electric 
point the membrane will possess a positive charge and so will repel 
kations, while below the iso-electric point the membrane will have 
a negative charge and will therefore repel anions. Only in the 
immediate neighbourhood of the iso-electric will such a membrane 
be permeable to ions. Modification of permeability, brought about 
by the addition of various substances, will, on this theory, be due 
to changes produced in the reaction of the buffer mixture of the 
more liquid phase of the protoplasmic membrane, and the phenomena 
of antagonism can be similarly explained. 
CHAPTER XV 
CONCLUDING REMARKS 
I N this review of our present knowledge of the permeability of 
plant cells and related phenomena, I have attempted to bring 
together and correlate the work done by many different workers on 
different lines and by the use of different methods. Anyone who takes 
the trouble to read the literature of the subject can scarcely fail to 
be impressed by the isolation in thought of the majority of workers 
in this field. That this neglect by the individual investigator of the 
work of others in the same subject is not a satisfactory condition of 
affairs a single example will suffice to make clear. It had become 
evident from the work of Nathansohn in 1903 that it is possible that 
the two ions of a salt are not absorbed in equivalent quantities by 
plant cells, and that the position of the equilibrium attained in the 
absorption of these ions is not one of equality of concentration outside 
and inside the cell. If these possibilities are facts, as they have now 
been shown to be, then plasmolytic methods of measuring salt intake, 
in which the rise of osmotic concentration of the cell sap is taken as 
a measure of the quantity of salt absorbed, cannot give results that 
are beyond question. Nevertheless, during the last five years, the 
results of quite a number of investigations with the use of plasmo¬ 
lytic methods have been recorded without the authors questioning 
