*79 
on the Descent of the Sap in Trees . 
every instance, precisely the same. It seems, therefore, that in 
the vine, the perspiratory vessels are confined to the under sur- 
face of the leaf: and these, like the cutaneous lymphatics of the 
animal economy, are probably capable of absorbing moisture, 
when the plant is in a state to require it. The upper surface 
seems, from the position it always assumes, either formed to 
absorb light, or to operate by the influence of that body ; and, 
if any thing exhale from it, it is probably vital air, or some 
other permanently elastic fluid. It nevertheless appears evident, 
in the experiments of Bonnet, that this surface of the leaves of 
many plants, when detached from the tree, readily absorbs 
moisture. 
Selecting two young shoots of the vine, growing perpendicu- 
larly against the back wall of my vinery, I bent them down- 
wards, nearly in a perpendicular line, and introduced their 
succulent ends, as layers, into two pots, without wounding the 
stems, or depriving them of any portion of their leaves. In this 
position, these shoots, which were about four feet long, and 
sprang out of the principal stem about three feet from the 
ground, grew freely, and, in the course of the summer, reached 
the top of the house. As soon as their wood became sufficiently 
solid to allow me to perform the operation with safety, I made 
two circular incisions through the bark of the depending part 
of each shoot, at a small distance from each other, near the 
surface of the mould in the pots ; and I wholly removed the 
bark between the incisions ; thus cutting off all communication, 
through the bark, between the layers and the parent stems. 
Had the subjects of this experiment now retained their natural 
position, much new wood and bark would have been formed at 
the upper lip of the wounds, and none at all at the lower, as I 
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