LECT. VII.] ANATOMY OF STEMS. 343 



sufficiently satisfactory, so far as regarded the 

 fact that the presence of the bark is necessary for 

 the formation of the new zone of wood ; but he 

 denied their conclusions ; and it was still essential 

 to ascertain, whether the wood is transmuted liber ? 

 or, if not, whether the matter of which it is 

 formed be a secretion from the bark, or supplied 

 by some other of the vegetable organs ? That it is 

 not transmuted liber is evident from the dissimi- 

 larity of the liber and the alburnum. Thus, accord- 

 ing to Mr. Knight *, the commencement of the 

 alburnous layers in the Oak is distinguished by a 

 circular row of very large tubes, which appear in 

 spring, arranged in ridges in a gelatinous mass, 

 beneath the cortical vessels ; but such tubes are 

 not found in the bark of the tree, which would be 

 the case were the alburnum a transmutation of the 

 liber. The bark of the Wych Elm (Ulmus monta- 

 na) also is so fibrous and tough that it may be 

 formed into cords, while that of the Ash (Faxinus 

 excelsior) is very fragile and not at all fibrous; ne- 

 vertheless the wood of both these trees> and conse- 

 quently the alburnum, is nearly alike-f-. As con- 

 vincing a proof also is the simple fact, that the 

 layer of alburnum is often more than twice the 

 thickness of the bark. But the question was set- 

 tled by the following experiment of Mr. Knight. 



* Phi Intophical Transactions, 1805. f Ibid. 1808. 



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