652 Report of a Discussion on 
role does the imbibition or capillary attraction of the membrane play 
in this system? Once the steady state of supply and demand with 
regard to evaporation loss is established, it can play no active part. 
The attraction upon the water as much acts to retard evaporation as 
to elevate the sap. It is a purely static and undirected stress, and does 
no work. Without any more accurate knowledge of its physical nature 
than we at present possess we are safe in saying this much. 
In this system, therefore, we have an osmotic pressure keeping the 
membrane tense and the cell filled with water (and we assume, for the 
present, doing no more) ; an imbibition force located in this mem- 
brane keeping the membrane permeated with water; and finally, on 
the boundary of the system evaporation progressing. Clearly in such 
a system the only kinetic action effective in actually lifting the water 
— in fact the only directed action — is that derived from the directed 
energy of those water molecules which escape from the evaporating 
surface or meniscuses ; their loss being made good by molecular 
attractions and diffusion forces acting within the liquid. These are 
the actual lifting forces ; the work done in lifting the water against 
gravity and viscous resistance being but small compared to the 
normal work of evaporation. The energy expended by the liquid is 
finally restored by the inflow of heat at the evaporating surface. 
Again, from another point of view, the movement of water across 
the osmotic cell may be broadly referred to the difference of aqueous 
vapour pressure in the conduits and in the intercellular spaces. 
Of the two static forces — osmosis and imbibition — how are we to 
single out one as especially the upholding force ? We find osmotic 
pressure, indeed, in the drooping leaf actually playing an active part 
and functioning as a directed force. In the steady state we have not 
assumed it to do so any longer. Again, the imbibition force is 
located in a membrane owing its rigidity entirely, it may be said, to 
osmotic pressure. Are we justified in selecting the imbibition force 
as the long sought for source of the suction-force concerned in the 
elevation of the sap ? I see no reason for doing so. 
But this does not exhaust the question ; a matter of much interest 
remains over. The actions enumerated above do not in point of fact 
suffice for the phenomena concerned with the ascent of sap in the 
living leaf. It happens that it is possible to test one of the static 
forces concerned independently of the other — that of imbibition inde- 
pendently of osmosis. If a leafless stick of some considerable length 
