40) A TEXTBOOK OF GENERAL BOTANY 
The force exerted by diffusion is called osmotic pressure. This 
force may be demonstrated by the use of a semipermeable mem- 
brane. A semipermeable membrane is a membrane through which 
the solvent can diffuse readily, but through which at least some 
of the substances which may be in solution cannot pass readily, 
if at all. Good examples of semipermeable membranes are ani- 
mal bladders and the thin membrane around 
the white of an egg. When two solutions of 
unequal concentration are separated by a semi- 
permeable membrane, diffusion still tends to 
produce an equal distribution of the dissolved 
substance throughout the solvent, and this 
results in the diffusion of the solvent from the 
less concentrated to the more concentrated solu- 
tion. The solvent from the less concentrated 
solution accumulates in the more concentrated 
solution and dilutes it, and the tendency is for 
this process to continue until the two solutions 
have the same concentration. 
The operation of a semipermeable membrane 
can be illustrated in*the following manner: 
When a dilute solution of sugar in water is 
separated by a semipermeable membrane from 
a more concentrated solution of sugar in water, 
and the semipermeable membrane is such that 
water can pass through it readily while sugar 
cannot, the water from the dilute solution will 
diffuse into the concentrated solution, the tendency being for 
this diffusion to continue until the solutions on both sides of the 
membrane are of equal concentration. The operation of osmotic 
pressure through a semipermeable membrane may be expressed 
in general by saying that if two solutions are separated by a 
semipermeable membrane, the solvent from the less concentrated ~ 
solution will tend to accumulate in the more concentrated one 
until the two solutions have the same molecular concentration, 
Diffusion through a semipermeable membrane is called osmosis, 








Fig. 32. Appara- 
tus for demonstrat- 
ing osmosis 
