1905. | Mechanics of the Ascent of Sap in Trees. 461 
through the porous partitions between them; provided there is an upward 
osmotic gradient, «¢.,if the dissolved substances are maintained in greater 
concentration in the higher vessels.* This difference of density must be 
ereat enough, between adjacent vessels, to introduce osmotic pressure in 
excess of that required to balance the head of fluid in the length of the 
upper one, into which the water has to force its way. Thus, in comparing 
vessels at different levels, the sap must be more concentrated in the upper 
ones by amounts corresponding to osmotic pressure more than counter- 
acting the total head due to difference of levels, in order that it may be 
able to rise. As osmotic pressure is comparable with gaseous pressure for 
the same density of the molecules of the dissolved substance, the concen- 
tration required on this view is considerable, though not very great. 
Such a steady gradient of concentration could apparently, on the whole, 
become self-adjusting, through assistance from the vital stimuli of the 
plant.; for concentration in the upper vessels is promoted by evaporation. 
Yet pressures in excess or defect of the normal atmospheric amount might 
at times accumulate locally, the latter giving rise to the bubbles observed 
in the vessels, through release of dissolved gases. 
It may be that this assumes too much concentration of dissolved material 
in the sap, as 2 exists inside the vessels of the stem, to agree with fact. In 
that case the capillary suction exerted from the nearest leaf surface might 
be brought into requisition, after the manner of Dixon and Joly, to assist in 
drawing off the excess of water from the vessels. The aim proposed in this 
note is not to explain how things happen, which is a matter for observation 
and experiment, but merely to support the position that nothing abnormal 
from the passive mechanical point of view need be involved in this or 
other vital phenomena. 
* Thus in an ordinary osmotic experiment with a (U-tube, the percolation of water 
through the plug gradually produces a difference of hydrostatic pressure on its two faces, 
which is sustained by the fixity of the plug itself, but would be at once neutralised if the 
plug were free to slide in the tube. This increase of volume of the salt-solution, by the 
percolation of pure water into it, is on the van’t Hoff analogy correlated with the free 
expansion of the molecules constituting a gas. It goes on with diminished speed under 
opposing. pressure, until a definite neutralising pressure is reached, inaptly called the 
osmotic pressure of the molecules of the solute, which just stops it, while higher pressures 
would reverse it. The stoppage is due to the establishment of a balance between the 
amounts of water percolating one way under osmotic attraction, and the opposite way 
under hydrostatic pressure. The pressure established, e.g., in an organic cell immersed in 
salt-solution, is thus really the reaction which is set up against the osmotic process. That 
process itself is perhaps more directly and intelligibly described as the play of osmotic 
affinity or attraction, even though it must be counted as of the same nature as the affinity 
of a gas for a vacuum. Cf. ‘Proc. Camb. Phil. Soc.,’ January, 1897, or Whetham’s 
“Theory of Solution,” p. 109. 
