Permeability 
21 
in a certain time. Pieces of tissue assumed to be similar are 
placed in solutions of the substance of various concentrations, and 
the tissue examined at different times from the commencement of 
the experiment. The concentration of the substance in which plasmo- 
lysis is just visible is noted at each time. Then if after the lapse of 
io minutes this concentration is xN and after the lapse of a further 
10 minutes yN, it is assumed that a quantity of salt has entered the 
cells of the tissue sufficient to raise the concentration of the substance 
in the cell sap by a quantity (y — x) N. This method, like other work 
of the same author, assumes that the external concentration of the 
substance has no influence on the rate at which it is absorbed, which 
indeed Trondle emphasizes is the case in salt absorption, an assump¬ 
tion the evidence in favour of which is very doubtful, and the evi¬ 
dence against which is very strong. 
Lepeschkin’s Method. An attempt to obtain data with regard to 
the entrance of glycerol into cells of Spirogyra was made by Lepesch¬ 
kin (1908 a) in the following way. Cells were plasmolysed in a solution 
of sucrose, which is assumed not to enter the cells, and left in the 
solution for an hour, when their volume was measured. The sugar 
solution was then replaced by a solution of glycerol and the volume 
again measured after 0*5 hour and again after a further two hours in 
the glycerol solution. Then if the volume of the cell after the first 
0*5 hour is V 2 and after a further two a hours V 3 , the quantity of 
glycerol which enters the cell in two hours is 
C (Js z YA 
1000 
gm.-mols., 
where C is the concentration of the glycerol solution in gm.-mols. 
and the volume is measured in c.c. 
Lepeschkin defines the permeability as the ratio of the number 
of gram-molecules of the substance which pass through unit area of 
the cell membrane in an hour to the difference in concentration of 
the substance on the two sides of the membrane. If then the mean 
surface of the cells is S, the permeability is 
Vo - V 9 
1000 S j~ i — 
(V s ~ V 2 ) (F 3+4 F 2 ) 
8 V 2 V s 
]' 
for the mean difference in concentration between the external and 
internal concentrations of the substance (glycerol) is 
r I r(Vs - v 2 ) c (V 3 ~ V 2 ) q 
C 2l 4 f 2 + v 3 J* 
