48 Stiles and Jorgensen . — St zz dies in Permeability. IV. 
to chemical combination between the solute in the external medium and 
some substance of the plasma membrane, the rate at which the permeability 
changes will depend upon the active masses of the reacting substances. We 
know also that according to the principle of Willard Gibbs the lower the 
surface tension at the interface, the greater will be the concentration of 
solutes there. The actual rate of the reaction will nevertheless be controlled 
by the laws of mass action, although the lowering of the surface tension may 
increase the active mass of the dissolved substance at the interface and so 
increase the rate of reaction and, in consequence, the rate of change of per- 
meability. On the other hand, if the surface tension of the solution is raised, 
chemical action will still take place, though it may be at a reduced rate. 
But other physical factors may be of importance in determining the rate of 
reaction besides surface tension. Thus, if the reacting substance is removed 
by chemical action faster than it can be replaced by diffusion, this latter 
factor will assume greater prominence in determining the rate of reaction. 
However, on account of its undoubted complexity the plasma membrane 
cannot be assumed to be merely a system in which permeability alters 
simply as a result of chemical action. So, for instance, Schryver (8) thinks 
of the substances as cemented together in a gel, on the alteration of which 
the phenomena of permeability changes mainly depend, and in a later paper 
(9) he points out how many physical factors influence the state of aggrega- 
tion of the gel. 
Czapek investigated the exosmosis of tannin substances from the * sub- 
epidermal mesophyll 5 cells of Echeveria leaves and from other objects. 
When treated with various reagents, notably with an aqueous solution of 
caffeine, the tannin in cells is precipitated in a characteristic and easily 
recognizable manner. Sections of Echeveria leaves, for example, are placed 
in 50 c.c. of the solution under investigation, and removed when equilibrium 
has been attained and tested for the presence of tannin. If tannin is absent 
it is assumed that exosmosis has taken place. There is no definite time in 
which equilibrium is attained ; this has to be found in each case. ‘ Bei 
fortgesetzter Beobachtung lasst sich die Zeit leicht ermitteln, in welcher ein 
langer dauernder Gleichgewichtszustand hinsichtlich der Exosmosegrenze 
besteht, und diese Grenze stimmt mit der theoretischen Tetisionsgrenze in 
der Regel befriedigend uberein.’ 
Plant organs containing coloured cell-sap were also tried, for the 
exosmosis of anthocyan can be observed without the aid of a microscope. 
But most of these objects had the disadvantage that they were damaged 
after less than 24 hours’ immersion in solutions of higher surface tension 
than the critical value. So ‘resistant objects’ such as Echeveria leaves, 
in which ‘ secondary damage * does not occur, were chiefly used by Czapek 
throughout the investigation. In some cases these were left for 24 hours, 
and even for 48 hours, for the equilibrium condition to be reached. 
