Walter Stiles 
24 
that the substance diffuses into the cells and there undergoes no 
reaction with the cell contents, the method seems to be a sound one 
for the determination of the absorption of dissolved substances. 
Hofler, however, like so many other observers, confuses the rate of 
intake with permeability. Thus, he defines permeability as the quan¬ 
tity of substance entering the cell in unit time, thus neglecting the 
fact that the rate at which the substance enters the cell depends not 
only on the permeability of the cell membrane, but also on the differ¬ 
ence in concentration of the substance in the external solution and 
in the cell sap. This does not matter so much, as Hofler makes it per¬ 
fectly clear what he is measuring, so that there is no obscurity. His 
results will be dealt with later. 
11. The Determination of Permeability by Changes 
in Weight or Volume of Turgid Tissues 
The principle of this method is much the same as that of the 
plasmolytic method. If turgid tissues are immersed in a solution of 
a substance strong enough to produce a contraction in the volume 
of the cells, but not concentrated enough to produce plasmolysis, the 
penetration of the dissolved substance into the cells of the tissue will 
bring about a gradual increase in volume (and consequently in weight) 
of the tissues. This method has been employed to test the entrance 
of salts and other substances into animal tissues and cells, e.g. blood 
corpuscles, by Koeppe (1895 a, b), Kozawa (1913, 1914) and others. 
For work with plant tissues the method has been developed by 
Lundegardh (1911) who used it to study the entrance of salts into 
roots of Vida Fab a. For this purpose the roots were placed hori¬ 
zontally in a small cell under the microscope and changes in length 
of the root measured. The roots were first treated with a solution of 
the salt under investigation which produced a contraction in length. 
As the salt entered the tissue the root gradually increased in length. 
The time taken for the root to increase in length from 25 per cent, to 
75 per cent, of the total increase it underwent was taken as a rough 
measure of the rate of entrance of the salts. 
12. Tissue Tension Method 
A tissue tension method for studying permeability to dissolved 
substances has been described by Brooks (1916 c). The experimental 
procedure is as follows. Similar strips of the peduncle or midrib of 
dandelion (Taraxacum offidnale, Weber) were fixed at one end between 
