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 show 

 that the descent of the fluids in vegetables is unnecessary and never 

 takes place in ordinary circumstances. I 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 fluids, or to effect those changes 



