i i o D. Tlioday. 
change. If we represent the turgor pressure by T, and the osmotic 
pressure by P, we can express this condition by the equations 
P=T or P—T=0.' 
The water-absorbing power of a cell. 
We have seen that a fully turgid cell is incapable of absorbing 
more water. If, however, the cell be removed from water and left 
for a time exposed to the air, it will lose water by evaporation and 
become less turgid: in other words, as the volume diminishes the cell 
wall is less distended and the turgor pressure correspondingly falls. 
At the same time the cell becomes once more capable of absorbing 
water if it should be replaced in water. In this condition 
T < P and P—T>0. 
The expression P—T represents a quantity of very great 
importance in any consideration of the water relations of plants, 
but one for which there is no recognised term. It is that part of 
the osmotic pressure of the cell sap which is not balanced—and so 
neutralised from the point of view of water absorption—by the 
turgor pressure. It is thus the net force tending to bring about 
the entry of water. We may for the present call it simply the 
water-absorbing poiver of the cell to distinguish it from the full 
osmotic pressure of the cell sap. If we represent it by p, we have 
P— T=p as a general expression for the condition of a cell, viewed 
as a simple osmotic system. 
That this expression requires further elaboration is obvious; 
but whatever complications may enter into the further analysis will 
not invalidate it. It must always remain true that the inward 
pressure of the distended cell wall is balanced by the hydrostatic 
resistence of the compressed fluid within ; and that the force with 
which water tends to enter, whatever the explanation of it, is pro¬ 
gressively antagonised as the turgor pressure increases. 
We may now consider the case of a cell immersed in a 
solution. In this case the water-absorbing power of the cell is 
opposed by the osmotic pressure of the surrounding medium. If 
no flow of water occurs in either direction the cell is already in a 
condition of equilibrium with the solution. Clearly in this case we 
may represent the equilibrium by the equation 
P'=p—P—T 
where P' is the osmotic pressure of the solution outside. Thus, 
the osmotic pressure of this solution is a measure of the water¬ 
absorbing power of the cell. Moreover, the solution will obviously 
be isotonic with the cell sap only when T=0, i.e., when there is 
1 Expressing magnitude only, not direction. 
