Walter Stiles 
198 
pressure. The indirect plasmolytic method consists in (1) measuring 
the osmotic pressure of the cell by plasmolysis as already described, 
and (2) in estimating the wall pressure. This is done by assuming 
that a linear relationship holds between wall pressure and cell volume 
between the extreme limits when the wall pressure is zero at plasmo¬ 
lysis and when the wall pressure is a maximum and equal to the 
osmotic pressure of the cell sap, when the cell is in equilibrium with 
pure water. It has already been pointed out that such an assumption 
can only be regarded as an approximation. 
This method, it will be observed, involves the measurement of 
the original volume of the cell, its volume at plasmolysis, and its 
volume when fully swelled in distilled water. It is also necessary to 
determine the osmotic concentration of the cell. 
Equilibrium method. The principle of this method was clearly 
enunciated by de Vries (1884 a) in reference to the determination of 
^the suction pressure of transpiring shoots, but there are apparently 
no records of any measurements made by him. It consists simply in 
determining that concentration of a solution of a substance to which 
the protoplast is impermeable in which the cell undergoes no change 
in volume. In this case the net suction pressure is zero, so that if 
P e is the osmotic pressure of the external solution, P the osmotic 
pressure of the cell sap and T the wall pressure, 
o = P-P e -T, 
or P e = P — T .(1). 
When the cell is placed in distilled water, if S is the full suction 
pressure which is then exerted, 
S = P — T .(2), 
so that, as T is the same in equations (1) and (2), 
S = P e . 
Hence the full suction pressure of a cell is equal to the osmotic pres¬ 
sure of a solution of a non-penetrating substance in which the cell 
undergoes no change in volume. When the suction pressure of tissues 
is in question it will generally be found simpler to use weight as a 
criterion instead of volume. 
Whichever method of determination is used solutions of sucrose 
are likely to be the most generally useful external media. It is also 
to be noted that the method neglects any external forces which may 
be operative such as tissue tensions. If there should be acting on a 
cell in the intact organ an external pressure of the same order of 
magnitude as the suction pressure found for the cell isolated from 
