260 
PHYSIOLOGY. 
BOOK II. 
both cellular and woody tissue ; and he demonstrated the 
fallacy of those theories according to which new wood is 
produced by the wood of a preceding year. He removed a 
portion of bark from a plum tree ; he replaced this with a 
similar portion of a peach tree, having a bud upon it. In a 
short time a union took place between the two. After wait- 
ing a sufficient time to allow for the formation of new wood, he 
examined the point of junction, and found that a thin layer of 
wood had been formed by the peach bud, but none by the 
wood of the plum, to which it had been tightly applied. 
Hence he concluded that alburnum derives its origin from the 
bark, and not from the wood. A variety of similar experi- 
ments w^as instituted with the same object in view, and they 
were followed by similar results. Among others, a plate of 
silver was inserted between the bark and the wood of a tree 
at the beginning of the growing season. It was said, that if 
new wood was formed by old wood, it would be subsequently 
found pushed outwards, and continuing to occupy the same 
situation ; but that if new wood was deposited by the bark, 
the silver plate would in time be found buried beneath new 
layers of wood. In course of time the plate was examined, 
and was found inclosed in wood. 
Hence the question as to the origin of the wood seemed 
settled ; and there is no doubt that the experiments of Du- 
hamel are perfectly accurate and satisfactory as far as they 
go. It soon, however, appeared that, although they certainly 
proved that new wood is not produced by old wood, it was 
not equally clear tliat it originated from the bark. Accord- 
ingly a new set of experiments was instituted by Knight, 
for the purpose of throwing a still clearer light upon the pro- 
duction of the wood. Having removed a ring of bark from 
above and below a portion of the bark furnished with a leaf, 
he remarked that no increase took place in the wood 
above the leaf, while a sensible augmentation was observable 
in the wood below the leaf. It was also found that if the 
upper part of a branch is deprived of leaves, the branch will 
die down to the point where leaves have been left, and below 
that will flourish. Hence an inference is drawn that the 
wood is not formed out of the bark as a mere deposit from it ; 
