Stiles and J ergensen. — St tidies in Permeability. IV. 67 
the permeability of potato cells to electrolytes is shown diagrammatically 
in Figs. 13 and 14. It will be observed that the curves are similar to those 
obtained with other organic substances examined, but that, in spite of the 
equality of the surface tensions of the two substances, aniline is considerably 
more efficient in bringing about exosmosis than pyridine. 
No doubt here again Czapek would refer to the specific action of these 
basic substances, but that of course is only another way of stating that the 
surface tension is not the deciding factor. It may be mentioned that with 
both these substances the external liquid becomes coloured a bright yellow 
colour when exosmosis is complete, as contrasted with the dull pale brown 
colour produced by the monohydric alcohols. 
A General Criticism of Czapek’s Theory of the Plasma 
Membrane. 
The criticisms to which Czapek’s work has been subjected by earlier 
writers deal chiefly with his theoretical considerations. In this contribution 
to the subject we have dealt mainly with the trustworthiness of the experi- 
mental evidence on which those conclusions are based. The steps, both 
experimental and theoretical, which lead up to Czapek’s theory of the 
plasma membrane are these : 
1. The measurement of the concentration of a solution of a substance 
required to bring about exosmosis from plant tissue. 
2. The conclusion that there is a critical concentration below which 
exosmosis from the cell does not take place. 
3. The measurement of the surface tension of the solutions used. 
4. The conclusion that the surface tension of the solution of critical 
concentration against air is o-68, that of pure water being taken as unity. 
5. The conclusion that therefore the surface tension of the outer layer 
of the plasma against air is also o-68. 
6. The measurement of the surface tension of emulsions of neutral fats 
shows that their surface tension cannot be lowered below o-68. 
7. The conclusion from this that the substances in the outer layer 
of the plasma important in permeability phenomena are the neutral fats. 
We propose now to consider each one of these steps in detail. 
1. We have shown in the part of this paper dealing with methods that 
Czapek’s method is crude and cannot be used for following changes quanti- 
tatively. By its use only very approximate results are to be expected. 
Sections of leaves are left in the solutions until an equilibrium condition has 
been reached. There is no definite time allowed for equilibrium to be 
obtained, the only criterion for equilibrium being apparently that the desired 
result is reached. Thus, in the case of tertiary butyl alcohol Czapek 
required a solution of 3 per cent, by volume to produce exosmosis of tannin 
