Walter Stiles 
i 74 
If P is the osmotic pressure of the liquid in the vacuole, P e the 
osmotic pressure of the external solution, T the inwardly directed 
pressure exerted by the cell wall, and if S is the net suction pressure, 
the relation between these quantities is given by 
S = P-P e -T , 
or, if the external liquid is pure water, so that P e = o, 
S = P-T. 
This relation was fully realised by de Vries (1884 a) who definitely 
states that the water attracting force of turgid tissue is not the same 
as that of the cell sap contained in its cells, but is smaller by a 
quantity corresponding to the elastic tension force of the protoplast 
and the cell wall. 
When a cell is in equilibrium with pure water the turgor pressure 
is at its highest possible value; replacing the water with an osmotically 
active solution, for example, that of a sugar or a salt, must result in 
the passage of water from the cell as the equilibrium is disturbed by 
the increase in the osmotic pressure of the solution outside the cell. 
Consequently the turgor pressure is less and the cell comes to occupy 
less volume. With progressive increase in the concentration of the 
external solution the protoplast will contract and the cell wall along 
with it until a point is reached in which the cell wall no longer con¬ 
tracts along with the protoplast, so that a space appears between 
cell wall and protoplasm. As it is assumed that the cell wall is com¬ 
pletely permeable both to water and dissolved substances, this space 
is filled with the external solution so that the unextended cell wall 
no longer exerts any pressure on the protoplast, the cell is flaccid 
and the turgor pressure is zero. The suction pressure in such plasmo- 
lysed cells is simply proportional to the difference between the os¬ 
motic pressures of internal and external solutions, and when equi¬ 
librium is attained the osmotic pressure of the cell sap is the same as 
that of the internal solution. 
It has already been noted that when the cell is in equilibrium 
with water the turgor pressure is at its highest possible value, and 
the suction pressure (at equilibrium) being zero, the turgor pressure 
is equal to the osmotic pressure of the cell. At the beginning of 
plasmolysis the turgor pressure has reached its minimum value, that 
is, zero. Between these limits, the turgor pressure possesses inter¬ 
mediate values depending on the quantity of water taken into the 
cell. It has generally been assumed that within the limits of elas¬ 
ticity of the cell wall the tension of the wall increases proportionately 
