264 
PHYSIOLOGY. 
BOOK II. 
This opinion would probably have been more generally 
received if it had not been too much mixed up with hypothe- 
tical statements, to the reception of which there are in the 
minds of many persons strong objections ; as, for example, that 
mentioned in the last paragraph. But it is remarkable that 
the antagonists of Du Petit Thouars have been from a class 
of naturalists of whom it may be said, that they are better 
known in consequence of the celebrity of the object of their 
attack than for any reputation of their own. To this how- 
ever, there are some exceptions, as, for instance, Mirbel and 
Desfontaines, two of the most learned botanists of France. 
The theory, nevertheless, seems the only one that is adapted 
at once to explain the real cause of the many anomalous 
forms of exogenous stems which must be familiar to the re- 
collection of all botanists, and that, at the same time, is equally 
applicable to the exogenous and endogenous modes of growth ; 
a condition which, it will be readily admitted, is indispensable 
to any theory of the formation of wood that may be proposed. 
It also offers the simplest explanation of the phenomena that 
are constantly occuring in the operations of gardening. 
The most important of the objections that have been taken 
to it are the following : — 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 demarcation 
separates the wood of one from the other, and the shoots 
emitted from the stock, by wood said to have been generated 
by the leaves of the scion, are in all respects of the nature of 
the stock. Again — if a rinjj 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, althouofh it must have originated in the 
leaves of the tree which produces white wood. It is further 
