320 
PHYSIOLOGY. 
BOOK II. 
to prove his statements, which M. Gaudichaud is said to have 
produced in the Memoir laid before the Academy of Sciences 
in 1834. 
The most important of the objections which have been 
taken to the opinion now under consideration are the fol- 
lowing : — If wood were really organised matter emanating 
from the leaves, it must necessarily happen that in grafted 
plants the stock would in time acquire the nature of the 
scion, because its wood would be formed entirely by the 
addition of new matter, said to be furnished by the leaves 
of the scion. So far is this, however, from being the fact, 
that it is well known that, in the oldest grafted trees, there 
is no action whatever exercised by the scion upon the stock ; 
but that, on the contrary, a distinct line of organic de- 
marcation separates the wood of one from the other, and the 
shoots emitted from the stock, by wood said to have been ge- 
nerated by the leaves of the scion, are in all respects of the 
nature of the stock. Again, if a ring of bark from a red- 
wooded tree is made to grow in the room of a similar ring of 
bark of a white-wooded tree, as it easily may be made, the 
trunk will increase in diameter, but all the wood beneath the 
ring of red bark will be red, although it must have originated 
in the leaves of the tree which produces white wood. It is 
further urged, that, in grafted plants, the scion often over- 
grows the stock, increasing much the more rapidly in diameter ; 
or that the reverse takes place, as when Pavia lutea is grafted 
upon the common horsechestnut ; and that these circum- 
stances are inconsistent with the supposition that wood is or- 
ganic matter engendered by leaves. To these statements 
there is nothing to object as mere facts, for they are true ; 
but they certainly do not warrant the conclusions which have 
been drawn from them. One most important point is over- 
looked by those who employ such arguments, namely, that in 
all plants there are two distinct simultaneous systems of 
growth, the cellular and the fibro-vascular, of which the 
former is horizontal, and the latter vertical. The cellular 
gives origin to the pith, the medullary rays, and the principal 
part of the cortical integument; the fibro-vascular, to the 
wood and a portion of the bark : so that the axis of a plant 
