Stiles and Jergensen. — S Hedies in Permeability. IV. 75 
a region during which the rate remains more or less constant, and a final 
region in which the rate gradually gets less and less. The first part of the 
curve corresponds to the period during which the impermeable substance in 
the plasma is rapidly breaking down. The rate at which this takes place 
gradually gets less until the changed substance remains practically constant 
in amount; the difference in concentration of electrolytes inside and outside 
is then the chief factor in determining the rate of exosmosis. This period 
corresponds to the last part of the curve, in front of which we have the 
approximately straight transitional part. 
With very high concentrations the first part of the curve is extremely 
short, and in practice readings have to be taken very frequently to show it, 
and it may be too brief to show graphically on the curves at all. 
In all these considerations we have assumed that the concentration 
of the external solution is so high that it remains practically unchanged 
throughout the experiments. But if this should become appreciably less as 
a result of the action, the rate at which the membrane is changed will 
be much lessened as time goes on. Consequently the rate of exosmosis will 
be considerably less than it would be if the concentration of the external 
solution remained constant. The curve we should expect under such 
conditions is precisely that obtained by us for the lower concentrations of 
the external solutions we have employed. 
Summary. 
1. The action of various organic substances on the permeability of 
plant cells has been investigated by estimating the exosmosis of electrolytes 
from plant tissue immersed in aqueous solutions of the substances. The 
exosmosis is estimated by measuring the rise in electrical conductivity 
of the solution in which the tissue is contained, due regard being had to the 
depression of conductivity resulting from the presence of the non-electrolytic 
experimental solutions. 
3. The method is compared with various methods employed by former 
workers, and evidence is brought forward to show that it is more exact and 
of more general application than those earlier methods. 
3. With each organic substance the rate of exosmosis depends on the 
concentration of the substance employed ; the higher the concentration 
the more rapid the exosmosis. 
4. Equimolecular solutions of different substances do not bring about 
the same exosmosis. Thus, in the homologous series of the monohydric 
alcohols, the more complex the molecule the greater the exosmosis produced 
by solutions of equimolecular strength. 
5. The rate of exosmosis produced by a solution is not a function of its 
surface tension alone. 
