Permeability 183 
A great advantage of the plasmolytic method lies in the fact 
that it allows the investigation of the osmotic pressure of individual 
cells; when the mean value of all the cells of a piece of tissue is 
required the concentration isotonic with the mean cell sap may be 
reckoned as that in which half the cells examined show definite 
plasm oly sis. 
Plasmometric method. This method is due to Hofler (1917); it is 
essentially a variant of the plasmolytic method, but in some cases, 
at any rate, is easier in execution. The essentials of the method are 
as follows. If a non-turgid but unplasmolysed cell is plasmolysed in 
a decidedly hypertonic solution, its volume, when plasmolysis is com¬ 
plete, is reduced by a definite fraction of the original volume, let us 
say by 1 /«. Assuming complete semi-permeability as in the case of 
the plasmolytic method, the concentration of the cell sap must have 
increased to n/n — 1 of its original value. Then if V P is the volume 
of the plasmolysed protoplast and vacuole, V z the original internal 
volume of the cell, P e the osmotic pressure of the plasmolysing 
solution, the osmotic pressure of the cell is given by 
P * = P e r . (I) ' 
y Z 
and similarly if the cell was originally in a turgid condition with a 
volume V, the osmotic pressure is given by 
P = P e £ .( 2 )- 
The ratio VjV s is the degree of turgor stretching (cf. p. 175) and so 
the last equation can be written 
where G is the degree of turgor stretching. 
If it is assumed that the protoplast does not take part in the 
plasmolytic contraction the first and third of these equations become 
modified respectively to 
cC 
II 
aT 
V r 
y z 
I -p 
. (4), 
p = p * 
1 1 
W 
.(5), 
where p is the proportion of the whole volume of the unplasmolysed 
but non-turgid cell occupied by the protoplast. Should the proto- 
