CHAP. V. 
ORIGIN OF WOOD. 
313 
previous season. Duhamel believed that it was deposited by 
the secretion already spoken of as existing between the bark 
and wood, and called cambium : he was of opinion that this 
cambium was formed in the bark, and became converted into 
both cellular and woody tissue ; and he demonstrated the 
fallacy of those theories according to which new wood is pro- 
duced by the wood of a preceding year. He removed a por- 
tion 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 waiting 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. Many similar experiments 
were 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 were formed by old wood, it would be subsequently 
found pushed outwards, and continuing to occupy the same 
situation ; but that, if new wood were 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 enclosed 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 that 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 
