100 Circulation in Vegetables. 
is exhaled ; the remaining sap passes into a different sytem of vessels 
which commence near the surface of the leaves and pass out through 
the mid rib into the liber. It now receives the name of cambium. 
In its progress downwards, it adds a new layer to the alburnum and 
also to the liber. ‘These deposits constitute the annual ring, so con- 
spicuous in the oak and fir. ‘The object of the ascent of sap to the 
leaves is to effect changes in it, by exposing it to light air and oxygen. 
The ascending sap is considered analogous to the chyme and the de- 
scending to the chyle of animals. Mr. Knight adduces the following 
experiments in support of his views. 1. When he removed a ring 
of bark from the limb of a tree, he found the deposits of new matter 
to be made principally on the upper edge of the ring. He explains 
the result of the experiment by saying, that the descending sap or 
cambium could not reach the lower edge of the ring, in consequence 
of the removal of a portion of continuous matter. Mr. Knight’s 
statement of the result of this experiment is certainly remarkable. 
He does not say that the deposite is entirely on the upper edge but 
mostly. From an examination of a great many trees which have 
been girdled, or which have lost large patches of bark, I am satisfied 
that there is a deposit at both edges of the ring. He admits that if a 
leaf is left growing near the lower edge, the thickness of the deposit: 
is augmented. 2. If two parallel rings of bark are removed, leaving 
between them a leaf, it dies in consequence, it is said, of cutting off 
the supply of cambium from above. But is not the supply of sap 
equally cut off from the root; the leaf it is evident, is as much insulated 
from the supply below as from above. 3. If a branch be stripped of its 
leaves it dies ; “ the organs,” as Mr. Knight would say “ which elabo- 
rate the cambium are destroyed.” But may it not be said that the pow- 
er which assists in the elevation of the sap is removed, and that the 
limb dies of starvation. ‘These are the experiments of Mr. Knight 
to prove that the cambium descends, and that the office of the leaves 
is to effect a change in the sap, they acting as media through which 
the sap is exposed to the light, atmosphere, &c. similar in fact to what 
takes place in the lungs of animals. 
In the farther discussion of the subject it will be my object to Sibi 
that the descent of the fluids in vegetables is unnecessary and never 
takes place in ordinary circumstances. 1 do not undertake to prove 
that the sap cannot descend at all, but that the sap in a vegetable de- 
riving its nourishment from the roots only ascends, and that it is not 
a function of the leaf to elaborate the fiuids, or to effect those changes 
