Stiles and Jergensen . — Studies in Permeability . IV. 49 
The surface tension of the various solutions used was measured by 
means of a capillary manometer, the essential feature of which is the 
measurement of the pressure just required to force a bubble of air through 
a capillary tube containing the liquid under examination. From this may 
be calculated the surface tension of the solution in contact with air. Of 
course what we are actually concerned with in exosmosis is the surface 
tension between the solution and the protoplast, which may be a very 
different thing. But because ‘ all the experience we possess * indicates that 
increase in concentration of the solution external to the protoplast causes 
a lowering of surface tension, and since increase of concentration of these 
same solutions also brings about a lowering of their surface tension against 
air, Czapek neglects the essential difference between the conditions in his 
experiments with the plant and the conditions under which the surface 
tension is measured. 
A large variety of organic substances was used by Czapek in these 
experiments, including a series of monohydric alcohols, glycol, glycerine, 
esters, & c. The general conclusion is reached that exosmosis only takes 
place from the cell when the surface tension of the external liquid when in 
contact with air is reduced to o-68 times the surface tension of pure water 
against air. From this rather remarkable conclusion the surprising deduc- 
tion is made that the surface tension of the outer layer of the protoplast 
in contact with air must also have this value. Since Czapek found by 
means of his capillary manometer that emulsions of various fatty 
substances occurring in plants, unsaturated triglycerides such as castor 
oil, linseed oil, and triolein, have the same surface tension, and that the 
surface tension can never be reduced below this value of about o-68, he 
further concludes that the surface-tension properties of the plasma membrane 
are chiefly due to the presence in the outer layer of the protoplast of such 
unsaturated triglycerides. These neutral fats are supposed to be present in 
the form of an emulsion with water, the globules being coated with a layer 
of soap which acts as a protective colloid. Czapek admits that he has not 
disproved Overton’s view that lecithin and cholesterine may be the essential 
substances in the plasma membrane, and he is also prepared to agree that 
there may be some protein present as well. The important substances, 
however, in his opinion are the neutral fats. 
Czapek’s theoretical considerations have already been subjected to 
severe criticism, to which, as we shall draw attention later, he has been 
unable to make any adequate reply. But, apart from his theory, it becomes 
of interest to examine whether it is permissible for him to draw the 
conclusion he does from his experiments. The method he used was laborious 
and crude, and he regards the measurements he made as referring to 
equilibrium conditions which he supposed to have become established in 
the systems he investigated. It is necessary, therefore, to reinvestigate the 
E 
