1917] BROOKS—PERMEABILITY 309 
4. The Laminaria thallus is made up of masses of protoplasm 
(the cells) imbedded in a gelatinous intercellular substance. From 
this intercellular substance, in which the salts are present in the 
same concentration as in sea water, the salts will diffuse out intoa 
surrounding medium, and will alter its conductivity if it be other 
than that of sea water. From the protoplasm, also, a similar 
diffusion may take place, which for convenience may be designated 
as “exosmosis.”’ 
That there is actual passage of salts through the tissue was 
shown by the fact that the conductivity of the more dilute solution 
always increased, while that of the more concentrated solution 
decreased to a corresponding degree. There was no appreciable 
change in the volume of either solution even during experiments 
whose duration was greater than 24 hours. The relative amounts 
of increase in the upper cell and of decrease in the lower cell, as 
found in the experiments, were in fair agreement with those calcu- 
lated. If a given amount of salt passes from one salt solution to 
an equal volume of another solution having half the concentration 
of the first, the percentage of increase in the concentration of the 
latter will be double the percentage of decrease in the former. If 
there be a difference in volume between the two solutions, the 
change caused by the addition or removal of a given amount of 
salt will be inversely proportional to the volume. Thus, in one 
experiment the increase in concentration in the upper cell was 
I per cent per hour, while the decrease in the lower cell was 0.26 per 
cent per hour. Since the volume in the upper cell was 5.4 cc. as 
compared with 12.5 cc. in the lower, and the concentrations were 
as 1:2, the expected ratio between the changes in the two cells 
would serge SAP a4. 6; while the observed ratio was ao 3. 8. 
The premise was reasonably satisfactory, and it therefore 
could be assumed that changes in the concentration of the solutions 
in the upper cells would be nearly proportional to the amount of 
salt passing through the diaphragm. Two modifications of the 
method, however, were sufficient to eliminate entirely the errors 
due to both diffusion and exosmosis. The error due to diffusion 
of salts from the intercellular substance was eliminated by filling 
