THE 
NEW PHYTOLOGIST 
March 22, 1922 
Vol. XXI, No. 1 
PERMEABILITY 
By WALTER STILES 
CHAPTER VI 
OSMOTIC PRESSURE 
he diffusion of a liquid through a membrane is termed osmosis. 
As long ago as 1748 it was observed by Nollet that when a 
bladder filled with alcohol was immersed in water the bladder swelled, 
while when a bladder full of wa ter was immersed in alcohol the bladder 
shrank owing in each case to the water passing through the membrane 
more rapidly than alchohol. 
Similarly, if an aqueous solution of, say, sucrose is contained in 
a bladder impermeable to the solute but permeable to the solvent, 
on immersing the bladder in water, the latter alone passes through 
the membrane into the bladder; which is in consequence distended, 
and may indeed ultimately burst if the solution inside is sufficiently 
concentrated. Thus when a pure solvent and solution are separated 
by a semi-permeable membrane, the pure solvent tends to pass 
through the membrane so as to dilute the solution, for this is the 
only way in which diffusion can take place so as to bring about 
equalisation of concentration throughout the system. Consequently 
when solution and pure solvent are separated by a semi-permeable 
membrane a hydrostatic pressure is produced which forces water 
through the membrane from solvent to solution. 
The osmotic pressure of a solution is defined as “the equivalent 
of the hydrostatic pressure produced when the solution and solvent 
are separated by a perfectly semi-permeable membrane; or as the 
equivalent of the excess pressure which must be imposed on a solution 
in order to prevent the passage into it of solvent through a perfectly 
semi-permeable membrane” (Findlay, 1913). 
Phyt. xxi. 1. 1 
