Osmotic Pressure, Root Pressure, and Exudation 109 
equilibrium with water. The osmotic pressure of the solution with 
which the cell is in equilibrium gives a measure—in terms of osmotic 
pressure—of the absorbing power of the cell 1 (Ursprung and Blum, 
1916; Thoday, 1918). 
Accepting the differential permeability of the upper and lower 
membranes of the sporangiophore of Pilobolus it may be compared 
with an artificial cell A (fig. 1), closed below by a perfectly semi- 
permeable membrane B and above by a membrane C which is slightly 
leaky, i.e. allows some of the osmotic material to diffuse out slowly. 
If such a cell is placed with its lower end in water, fluid will pass in, 
Fig. 1 
and after a time the pressure exerted will equal the osmotic pressure 
of the contained solution. Water will then cease to enter, and the 
absorbing power of the cell will be zero. If now a drop of water be 
present above the upper membrane (C) osmotic substances will dif¬ 
fuse through the membrane and we shall have a weak solution above 
the cell. This solution will now have a higher absorbing power than 
the cell and so will draw water out of it. This loss will be immediately 
balanced by absorption through the lower membrane. The volume 
of the fluid at C will thus increase, the force with which water is 
drawn into it depending solely on the difference of the osmotic 
1 The absorbing power of a cell can be determined by finding the osmotic 
pressure of a solution which just does not alter the volume of the cell. This 
is the method used by Ursprung and Blum in their numerous measurements 
of the absorbing power of cells. 
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