On the Inconvertibility of Bark into Alburnum. 137 



for which the disciples of Malpighi contend ; and if the 

 fibres and vessels of the bark really became those of the al- 

 burnum, a very great degree of similarity ought to be found 

 in the organization of those substances. No such similarity, 

 however, exists; and not any thing at all corresponding 

 with the circular row of large tubes in the alburnum of the 

 oak is discoverable in the bark of that tree. These tubes are 

 also generated within the interior surface of the bark, which 

 is well defined ; and during their 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 ob- 

 servation : nor does the organization of the bark in other 

 instances, in any degree indicate the character of the wood 

 that is generated beneath it : the bark of the wych elm 

 {idmus 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 an- 

 other, and scarcely presents a fibrous texture ; yet the albur- 

 num of these trees is not very dissimilar, and the one is 

 often substituted for the other in the construction of agricul- 

 tural instruments. 



Mirbel has endeavoured to account for the dissimilar or<- 

 ganization of the bark, and of the wood-into which he con-? 

 ceives it to be converted, by supposing that the cellular sub- 

 stance 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 {tUsu tubulaire) of the liber, or in- 

 terior 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 alhurnum. 



But if this theory (which I believe I have accurately 

 stated, though I am not quite certain that I fully compre- 

 hend its author*) were well founded, the texture qf the al- 



* Chap, iit article 5, Traiic d'Atiatqwi? it de P'rjtiologie Vegetale. 



burn urn 



