INCONVERTIBILITY OF BARK INTO ALBURNUKT. ^ 



progressive changes could not, I think, possibly have escaped 

 my observation. Nor does the organization of the bark in Barks of wych 

 other instances in any degree indicate the character of the differ'essen- 

 wood, that is generated beneath it : the bark of the wych tklly, 

 elm fubnus niontanaj is extremely rough 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 ffrax'mus excel- 

 sior) 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 the 

 alburnum of these trees is not very dissimilar, and the one but not the al- 

 ls often substituted for the other in the construcaon of agri- ^"'■"^™» ^"^ 

 cultural instruments. 



Mirbel has endeavoured to account for the dissimilar or- f^iirbers the^ 

 ganizatlon of the bark, and of the wood into which he ' " 

 conceives it to be converted, by supposing, that the cellu- 

 lar substance of the bark is always springing from the al- 

 burnum, v/hile the tree is growing; and that it carries with 

 it part of the tubular substance ftissu tuhulairej of the 11- 

 her, or interior bark. These parts of the interior bark, which 

 are thus removed from contact with the alburnum, he con- 

 ceives to constitute the external bark or cortex, while the 

 interior part of the liber pi'ogressively changes into albur- 

 num. 



But if this theory (which I believe I have accurately Objections to 

 stated, though I am not quite certain, that I fully compre- ^^'^ '^^'^^y* 

 hend its author*) were well founded, the texture of the 

 alburnum must surely be much more intricate and inter- 

 woven than it is, and its tubes would lie less accurately pa- 

 rallel with each other than they do : and were the fibrous 

 substance of the bark progressively changing into alburnum, 

 tlje bark must of necessity be firmly attached to the albur- 

 num 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 fiom the 

 alburauai ; to which it appears to be attached by a substance 

 that is apparently rather gelatinous than fibrous or vascular : 



* Chap. Ill, Article 5, TVaiU d'Anatomie etde Physlologie Ve'g^tale. 



and 



