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posed bark. And it was evident, that in each instance a new layer, 

 both of cortex and of alburnum, was generated. 



Mr. Knight's attention was next directed to the progressive for- 

 mation of alburnum in the young shoots of an oak coppice ; but he 

 could discover nothing like transmutation of bark into alburnum, 

 although the commencement of alburnous layers in this tree is pecu- 

 liarly conspicuous, by a circular row of very large tubes. These 

 tubes he found, at their first formation, passing through a soft gela- 

 tinous substance, much less tenacious than the surrounding pre- 

 existent bark ; and there was nothing in the bark at all correspond- 

 ing to the circular row of tubes contained in the alburnum. The 

 interior surface of the bark is at the same time well defined, and its 

 own peculiar vessels are distinctly visible, and by no means exhibit 

 any appearance of progressive transmutation. 



Mr. Knight remarks also, that the qualities of different kinds of 

 wood are not in any degree indicated by the bark which covers them. 

 He instances the wych-elm and the ash, the woods of which, for 

 agricultural implements, are frequently substituted for each other, 

 although the textures of their barks are extremely dissimilar ; inas- 

 much as one is brittle, and the other so tough as frequently to be 

 used for ropes. 



Another circumstance, very unfavourable to the theory of conver- 

 sion, is the firm adhesion which subsists between the layers of bark 

 to each other, in comparison to their adhesion to their alburnum. 



Two experiments of Dil Hamel are, however, cited by Mirbel in 

 support of that theory. 



In the first, pieces of silver wire, inserted into the bark, were fre- 

 quently found in the alburnum ; but the evidence is defective, as it 

 was not rightly ascertained that the pieces of wire did not, at their 

 first insertion, pass between the bark and alburnum, and thus be 

 liable to be covered by a new deposition of either one or the other. 



In the second experiment, the bud of a peach-tree, with a piece 

 of bark attached to it, was inserted into a plum-stock : a layer of 

 wood was afterwards found beneath the inserted bark, perfectly si- 

 milar to the peach ; but it is easier to conceive a layer of alburnum, 

 generated by deposition from fluids that have circulated through the 

 inserted bud, than that a part of its bark should be converted into a 

 layer of alburnum more than twice as thick as the inserted bark. 



Mr. Knight also remarks, that when the bud is destroyed, the bark 

 deposits no alburnum ; but, being small, it becomes ultimately co- 

 vered by the successive alburnous layers of the stock, and may be 

 found many years afterwards to have made no progress towards con- 

 version into wood. 



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