ioB 
Mr. Knight on the 
though I am not quite certain that I fully comprehend its au- 
thor* ) were well founded, the texture of the alburnum must 
surely be much more intricate and interwoven than it is, and its 
tubes would lie less accurately parallel with each other than 
they do : and were the fibrous substance of the bark progres- 
sively 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 substances. 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 corti- 
cal 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 Mal- 
pighi, as with that of Mirbel. 
Many of the experiments of Du hamel are, however, appa- 
rently favourable to the theory of Malpighi, respecting the 
conversion of bark into alburnum ; and Mirbel has cited two, 
which he appears to think conclusive. -f In the first of these, 
Duhamel shews that pieces of silver wire, inserted in the 
bark of trees, were subsequently found in their alburnum ; 
but Duhamel himself has shewn, with his usual acuteness and 
candour, that the evidence afforded by this experiment is ex- 
tremely defective ; and he declares himself to be uncertain 
that the pieces of wire did not, at their first insertion, pass be- 
tween the bark and the alburnum ; in which case they would 
* Chap. III. Article 5, Traite d’Anatomie et de Physiologie Vegetale. 
f Chap. III. Article 5. 
