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 ouii. 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 followinfj : — If wood were reallv 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. Asjain — if a rinff 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, althoucrh it must have orisrinated in the 

 leaves of the tree which produces white wood. It is further 



