1 T 2 
D. Thoday. 
equilibrium between two cells a and b we have p a =p b) or 
P a —T a =P b —T b . If P a —T a is greater than P b —T b , then cell a 
will absorb water from cell b, irrespective of the absolute values 
of P a and P b . In fact we may theoretically have cell b with the 
greater osmotic pressure, yet because its turgor pressure is also 
greater it may not only be unable to absorb water from cell b but 
may yield water to it. 
(2) Osmotic pressure and suction force. It is most important to 
realise that a fully turgid cell has no power to absorb xvater. It 
follows from this that the suction force of a transpiring shoot 
depends on its cells not being completely turgid , l and increases 
rapidly from zero as turgor is lost and the “saturation deficit” 2 
increases. The suction force of a shoot corresponds to the water 
absorbing power of its cells, not in general to the osmotic pressure 
of their sap. 
(3) Automatic regulation of transpiration. Any consideration 
of the physics of the generation and transmission of suction force in 
a transpiring shoot clearly involves the same principle. For 
simplicity we may imagine a single cell in contact with a vessel. 
Water is lost by evaporation from the surface of the cell wall, an 
imbibition force is thus developed and water is withdrawn from the 
cell sap until the water absorbing power of the compressed proto¬ 
plast is sufficient to counterbalance the imbibition force of the wall. 
The “ saturation deficit ” necessarily involves a lowering of the 
vapour pressure at the evaporating surface, and a consequent 
diminution of the rate of water loss, 3 which will, however, continue 
so long as this vapour pressure is greater than the tension of water 
vapour in the air bathing the surface. 
At the other end of the cell, water will in the the same way be 
withdrawn from the cell wall and in turn from the cavity of the vessel. 
The condition for static equilibrium is that every component of 
the system should have the same water absorbing power, or 
conversely, the same power of resisting the withdrawal of water. 
The factor corresponding to this in the cavity of the vessel is the 
tension in the water postulated by the Cohesion Theory. Putting 
C for this cohesion tension, and I for the imbibition force in the 
1 “ Jede Zelle, die iiberhaupt transpiriert und saugt, vermag nur infolge 
eines Sattigungsdefizits zu saugen. Auch in den Blattern normal transpirier- 
ender, gat mit Wasser versorgter Pflanzen bleibt die Sattigung immer unter 
dem Maximum.” Renner, Flora, 103, 1911, p. 246 
a Renner’s “ Sattigungsdefizit,” loc. cit. On the relation between suction 
force and saturation deficit, cf. also Livingston and Brown, Bot. Gaz., 53, 
1911, p. 311 et seq. 
s It is not suggested that the rate is determined by this factor alone. 
The behaviour of a colloidal membrane is complicated by other factors, as for 
instance an increasing resistanse to flow as the water content diminishes. 
