462 On the Mechames of the Ascent of Sap in Trees. 
As regards estimating the amount of flow, at first sight it may not appear 
obvious, @ priori, that the transpiration through a porous partition or 
membrane, due to osmotic gradient, is equal or even comparable in amount to 
what would be produced, with pure water, by a hydrostatic pressure-head equal 
to the difference of the osmotic pressures on the two faces of the partition. 
But more exact consideration shows that on the contrary osmotic pressure 
is defined by this very equality ;* it is that pressure-difference which would 
produce such an opposite percolation of water as would just balance the 
direct percolation due to the osmotic attraction of the salt solution. 
It would, however, appear that the great resistance to flow offered by what 
botanists call Jamin-tubes, viz., thin liquid columns containing and carrying 
along numerous broad air-bubbles, is conditioned mainly by the viscosity of 
the fluid, and involves only indirectly the surface-tension of the bubbles. In 
fact the resistance to flow may be expected to remain much the same if each 
bubble were replaced by a flat solid disc, nearly but not quite fitting the 
tube. Its high value arises from the circumstance that the mass of liquid 
between two discs moves on nearly as a solid block when the flow is steady, 
so that the viscous sliding has to take place in a thin layer close to the wall 
of the tube, and is on that account the more intense, and the friction against 
the tube the greater. The increased curvature of the upper capillary meniscus 
of the bubble is thus merely a gauge of the greater intensity of the viscous 
resistance instead of its cause, and modification of the surface-tension cannot 
be involved as a propelling power. The experimental numbers given by 
Dr. Ewart show that, even where the vessels are largelv occupied by bubbles, 
the greater part of the resistance to active transpiration still resides in the 
partitions between them. 
If the osmotic gradient, assisted possibly by capillary pull at the leaf- 
orifices, is Insufficient to direct a current of transpiration upward, capillary 
alterations inside the vessels, arising from vitally controlled emission and 
absorption of material from the walls, cannot be invoked to assist : rather it 
must be osmotic alterations from one vessel to the next, of, so to speak, a 
peristaltic character, that might thus come into play. But any such alteration 
(of either kind) will involve local supply of energy. Is there a sufficient fund 
of energy, latent in the stem, to provide permanently the motive power for 
the elevation of the sap? In what form could this energy get transported 
there? The energies of the plant-economy come from the sunlight absorbed 
by the leaves. The natural view would appear to be that the work required 
to lift the sap is exerted at the place where the energy is received, and that 
it operates through extrusion of water by evaporative processes working 
* See preceding footnote. 
