659 
the Ascent of Water in Trees. 
the root-action, we can regard the whole phenomenon of the ascent of 
sap to depend broadly upon a difference of vapour pressure in the 
atmosphere surrounding the roots and in the atmosphere surrounding 
the leaves 1 . 
In reference to the doubt suggested by Mr. Darwin (p. 641) as to 
the re-solution of air by root-pressure sap, we would say that the 
onus probandi would rather lie with those who question such action. 
The root-sap contains only a small quantity of dissolved air, and, 
except the laws of absorption of gasses by liquids differ inside the tree 
from elsewhere in the world, we expect that it will be willing to take 
up more. Here too we may recall what Strasburger quotes 2 from 
Volkens and Vesque as to the appearance and disappearance of air 
bubbles in the conduits of herbaceous plants. 
In regard to the general considerations of the stable properties of 
a dust-free liquid under tensile stress, it is interesting to notice the 
analogy with the behaviour of a dust-free vapour under pressure. In 
the first case an enormous stress is required to initiate a bubble of 
vapour, for the surface tension of this bubble will be the greater 
proportionately as its radius of curvature is smaller. In fact without 
actual fracture of the liquid, only a dry particle (exposing already 
a free surface of appreciable radius of curvature) can initiate the 
bubble. In the case of the vapour it requires a high pressure to 
initiate the precipitation or formation of drops, because the vapour 
pressure of these will be the greater as the radius of curvature is 
smaller. Here, too, we require a particle to initiate a wet surface of 
appreciably great radius of curvature. The increased vapour pressure 
at sharply convex surfaces may not be without influence in the 
evaporation at the surface of the minute mesophyll-cells, when these 
present a wetted surface to the intercellular space. I may remind 
you, in connexion with these remarks, of Aitkin’s experiments on the 
counting of dust particles, and Maxwell’s remarks on how small drops 
of rain are unstable in the presence of big ones 3 . 
Reviewing the whole matter in a few words : — 
We have in the tracheal system of the plant a water-way which is 
freely open to water-movement, while closed to the movement of 
free gas. Every bordered pit is an open door to the sap and a closed 
1 Ascent of Sap, Phil. Trans., pp. 574-75. 
2 Leitungsbahnen, pp. 696-97. 
3 See Tait’s Properties of Matter, Chapter on Capillarity. 
