Permeability 13 
vantage of this method of studying exosmosis is that the quantities 
of diffusing substances may be too small for determination. 
The change in colour of a solution of a dye external to the cells 
or tissue can similarly be used to determine the absorption of the 
dye, and by colorimetric estimation the method may be made quanti¬ 
tative. The chief objection that can be levelled against this means of 
determining intake is that a substance diffusing out from the tissue 
might react with the dye to produce a colourless or differently 
coloured substance, so that a decrease in the intensity of the colour 
of the external solution might not be due actually to an absorption 
of the dye by the tissue. That this may happen in some cases appears 
to be suggested by an observation of Miss Redfern (1922 b) to the effect 
that a solution of congo-red deepens in colour after disks of carrot root 
are immersed in it for some time. As such a deepening of colour does 
not occur in a solution of the dye to which no tissue has been added, 
the only possible explanation appears to be that the colour is altered 
by the exosmosis of some substance from the tissue. A more serious 
difficulty arises from the fact that the dye may be absorbed by the 
cell walls and never reach the protoplasm or vacuole.. While this 
appears to be the case sometimes, it is not always so, and clearly 
observations by this method are free from objection if supported by 
others following the first method described in this chapter. 
5. Determination of Permeability by Chemical Analysis 
of the Medium External to the Tissue 
This method is similar to the one previously described, but as a 
quantitative method of determining absorption it is as a rule more 
refined. It has found considerable use as a method of investigation 
of cell permeability and intake or excretion. A decrease in the con¬ 
centration of a substance in the external solution indicates absorption 
of the substance by the cells or tissue while increase in its concentra¬ 
tion indicates exosmosis. As a qualitative test of permeability and a 
quantitative measurement of absorption and excretion the method is 
open to the same objections as that just described. It cannot be used 
quantitatively for measuring permeability any more than other 
methods described previously in this chapter. 
Information relating to absorption of salts has been obtained by 
this method by a number of workers, both entire plants and pieces 
of tissue having been used. Among those who have worked with the 
former may be mentioned particularly Demoussy (1900) who deter- 
