io8 
V. H. Blackman 
with which Lepeschkin surrounds his discussion of exudation that 
has protected him from the criticism that might be expected. The 
only other criticism that has been met with is that of Hofler (1920), 
who remarks in a footnote: “Lepeschkin's Ausfuhrungen wonach 
der osmotische Druck der Zellen von deren Permeabilitat abhangt 
scheint von theoretischen Standpunkt einer Revision bedurften.” 
Thoday (1918) has pointed out in this Journal how unsatisfactory 
is the usual presentation of the botanical aspects of osmotic pressure. 
For this the writers of text-books of physics and physical chemistry 
would seem in part responsible, for usually they do not sufficiently 
insist that osmotic pressure is the result of the tendency towards 
equilibrium between water and the solution, the absence of equilibrium 
being shown by the lower vapour tension of the solution as com¬ 
pared with that of water. The osmotic pressure of a solution is thus 
an equilibrium pressure, i.e. the pressure which must be applied to 
the solution to bring it into equilibrium with water 1 . 
To ascribe to the osmotic pressure of a given solution the responsi¬ 
bility for the exudation of water or of a weaker solution from that 
solution is certainly most unsatisfactory. Other pressures can be 
superimposed on osmotic pressure which may increase, decrease or 
even reverse it, but once osmotic pressure is recognised as an equili¬ 
brium pressure it is clear that it must always exhibit itself as a 
tendency to draw water into the solution, i.e. to bring about the 
absorption of water. Substances may pass out of the solution by 
diffusion if the membrane is permeable and so bring other forces 
into play outside the solution, or there may be other forces at work 
(such as electrical ones) which drive the solution out through the 
membrane, but such processes cannot be ascribed to the action of 
the osmotic pressure of the enclosed solution. 
Since osmotic pressure is an equilibrium pressure it is obvious 
that by subjecting a solution to varying pressure it can be brought 
into equilibrium with— i.e. will neither give water to nor take water 
from—a solution of any concentration lower than that of the solution 
itself. It is easy then to appreciate that as a flaccid cell swells and 
the pressure of the cell wall increases the cell contents come into 
equilibrium with solutions of lower and lower concentration or osmotic 
pressure, and finally when the pressure exerted by the membrane 
is equal to the osmotic pressure of the cell contents the cell is in 
1 A very satisfactory treatment of osmotic pressure is that by E. W. 
Washburn, An Introduction to the Principles of Physical Chemistry. New York. 
1915- 
