Permeability 
23 
Tradescantia (. Rhceo) discolor. Similar cells which may be supposed 
to have the same osmotic concentration were plasmolysed in solu¬ 
tions of a salt of different concentrations. If the salt enters the cells, 
the latter deplasmolyse. Now a stronger solution will produce a 
greater degree of plasmolysis than a weaker solution, so that as cells 
that have been plasmolysed in the stronger solution deplasmolyse, 
a stage will be reached in which the degree of plasmolysis is the same 
as that originally produced in the weaker solution. During the time 
that elapses between the commencement of deplasmolysis and the 
reaching this less degree of plasmolysis, a quantity of salt must have 
entered the cells sufficient to increase the concentration of the salt 
in the cells by the difference in concentration between the weaker 
and stronger solutions. The degree of plasmolysis is measured by a 
rough estimate of the proportion of cells in the preparation which 
are plasmolysed, as, for example, one-half, or three-quarters. As 
there is no great precision in this mode of determining the degree of 
plasmolysis, Fitting’s method cannot be regarded as an exact one. 
It also does not take account of exosmosis from the cells, which Fitting 
supposes is eliminated or rendered negligible by a preliminary treat¬ 
ment with water, a conclusion which is probably not correct. Results 
obtained by its means by Fitting and Trondle will be dealt with in the 
next chapter. 
HofleP s plasmometric method. The principle of the method used 
by Hofler (1918 a, b, 1919) is the same as that of the preceding, but 
the degree of plasmolysis is determined by actual measurements of 
the cells examined, so that the rate of deplasmolysis is followed in 
individual cells. The cell is plasmolysed by a decidedly hypertonic 
solution, the degree of plasmolysis being p x . After the lapse of a time 
t the degree of plasmolysis is p 2 . Then if the concentration of the 
external solution is C, and the osmotic concentrations of the cell 
corresponding to the two degrees of plasmolysis are respectively C 1 
and C 2 , we have 
and 
whence 
C 2 C 1 — C (pz — pp. 
Since the change in osmotic concentration of the cell in unit time is 
a measure of the rate of intake of the dissolved substance, the rate 
of intake can be determined from the concentration of the external 
solution and the change in the degree of plasmolysis. 
Hbfler’s method, like those of Lepeschkin and Fitting, neglects 
the possible effects of exosmosis. Apart from this and the assumption 
