146 ON THE INCONVERTIBILITY 



formation the vessels of the bark are distinctly visible, as different organs; 

 and had the one been transmuted into the other, their progressive changes 

 could not, I think, possibly have escaped my observation : nor does the 

 organisation of the bark in other instances in any degree indicate tho 

 character of the wood that is generated beneath it : the bark of the wych 

 elm ( Ulmus montana) is extremely tough and fibrous ; and it is often 

 taken from branches of six or eight years old, to be used instead of 

 cords ; that of the ash (Fraxinus excelsior), on the contrary, when taken 

 from branches of the same age, breaks almost as readily in any one 

 direction as in another, and scarcely presents a fibrous texture ; yet tho 

 alburnum of these trees is not very dissimilar, and the one is often substi- 

 tuted for the other in the construction of agricultural instruments. 



Mirbel has endeavoured to account for the dissimilar organisation of 

 the bark, and of the wood into which he conceives it to be converted, by 

 supposing that the cellular substance of the bark is always springing from 

 the alburnum, whilst the tree is growing, and that it carries with it part 

 of the tubular substance (tissu tubulaire) of the liber, or interior bark. 

 These parts of the interior bark, which are thus removed from contact 

 with the alburnum, he conceives to constitute the external bark or cortex, 

 whilst the interior part of the liber progressively changes into alburnum. 



But if this theory (which I believe I have accurately stated, though I 

 am not quite certain that I fully comprehend its author*) were well 

 founded, the texture of the alburnum must surely be much more intricate 

 and interwoven than it is, and its ti :bes would lie less accurately parallel 

 with each other than they do : and were the fibrous substance of the 

 bark progressively changing into alburnum, the bark must of necessity be 

 firmly attached to the alburnum during the spring and summer by the 

 continuity, and indeed identity, of the vessels and fibres of both these sub- 

 stances. This, however, is not in any degree the case, and the bark is 

 in those seasons very easily separated from the alburnum ; to which it 

 appears to be attached by a substance that is apparently rather gelatinous 

 than fibrous or vascular : and the obvious fact, that the adhesion of the 

 cortical vessels and fibres to each other is much more strong than the 

 adhesion of the bark to the alburnum, affords another circumstance 

 almost as inconsistent with the theory of Malpighi, as with that of 

 Mirbel. 



Many of the experiments of Duhamel are, however, apparently favour- 

 able to the theory of Malpighi, respecting the conversion of bark into 

 alburnum ; and Mirbel has cited two, which he appears to think conclu- 



* Traitc d' Anatomic et de Physiologic Vege"tale, Chap. iii. Article 5. 



