Turgesce?ice and the Absorption of Water. 109 
kind, for it has subtle ramifications which often vitiate discussion 
of the osmotic relations of transpiring leaves, of the passage of 
water through tissues, and of other similar questions. 
It has therefore seemed desirable to attempt a brief elemen¬ 
tary exposition of the conditions which govern the equilibrium of a 
cell with a watery solution and with other cells ; and to illustrate 
the consequences by applying them in some important cases. 
Osmotic equilibrium in turgescent cells. 
In all applications of physical and chemical principles to the 
analysis of complex physiological phenomena a clear understanding 
of their action in simple cases is an essential preliminary to the 
consideration of observed complications or discrepancies. The 
old explanation of turgescence associated with the names 
of de Vries and Pfeffer is essentially a simplified approximate 
description of the phenomena, and in this sense will always hold 
good. Whatever the ultimate explanation of osmotic pressure, per¬ 
meability, imbibition and other factors that may have to be reckoned 
with in an exhaustive analysis, there can be no doubt that osmotic 
pressure, acting against an elastic cell wall, plays the chief part 
in developing and maintaining the turgor so characteristic of 
vacuolated plant cells and tissues. In the first instance we may 
therefore ignore the swelling of the colloidal constituents of the cell. 
If an isolated cell is placed in water, water enters it and 
distends the cell-wall. The wall is elastic, in the sense that it 
resists extension (though it is not inextensible) 1 and within certain 
limits does not suffer permanent extension but is capable of 
returning to its original dimensions on removal of the stretching 
force. 2 The distended wall compresses the protoplasm and sap, 
in which the consequent hydrostatic pressure (called by Lepeschkin 
“ Turgordruck,” or turgor pressure) is equal and opposite at 
any moment to the inward component of the tensions in the 
wall. This turgor pressure tends to force water out of the cell. 
When swelling is complete and equilibrium reached the turgor 
pressure balances the osmotic pressure of the sap. In this 
condition the cell is fully turgid; it is in equilibrium with water ; 
and it is incapable of absorbing more water so long as neither the 
osmotic pressure of the sap nor the tensions in the wall 3 suffer 
1 Stiles and Jorgensen have confused elasticity with extensibility— 
l.c., p. 420. 
1 How far this is strictly true still requires critical investigation. It is 
probable that hysteroesis is shown, and even some degree of plasticity, by thin 
cellulose walls, but hitherto the time factor has been left out of account in 
investigations on the elasticity of plant tissues. 
3 E." , by growth (intussusception) or owing to some degree of plasticity 
under prolonged strains 
