Osmotic Pressure, Root Pressure, and Exudation 113 
that of the osmotic pressure of the solution above B absorption 
through B will stop, since, owing to the pressure, the solution on 
the one side of the membrane B will be in equilibrium with the water 
on the other side. At A however there will be no equilibrium at 
the two faces of the membrane owing to the higher concentration 
of the solution and water will continue to enter. As a result the 
pressure in the tube will go on increasing and there will no longer 
be equilibrium at B, but water will be forced out and the solution 
in that arm will become more concentrated. If B were not in water 
but exposed to the air water would appear at its surface. A trans¬ 
ference of water through the tube has occurred and so work has 
been done, but there is no contravention of the second law of thermo¬ 
dynamics, for the process will not continue indefinitely as the two 
solutions will gradually mix; the work has been done at the expense 
of the energy of diffusion. If the process is to continue in the living 
cell work will have to be done to keep the two solutions at different 
concentration in different parts of the cell 1 . An advantage of a 
theory of this kind is that exudation under high pressure could be 
obtained, for the exudation pressure would be the difference between 
the osmotic pressure of the two solutions. If the membrane in contact 
with the solution of lower concentration is slightly permeable the 
1 Rowell (1918) following Pfeffer has a somewhat similar model but it 
would seem to be unworkable since the two solutions of different concentration 
are separated by semi-permeable membranes. 
3—11 
