of the Bark of Trees . i j g 
vessels,* which surround the medulla. In both cases a cel- 
lular substance, similar to that which was generated in the 
preceding experiments, is first formed, and this cellular sub- 
stance in the same manner subsequently becomes vascular ; 
wdrence it appears, that the true sap, or blood of the plant, 
produces similar effects, and passes through similar stages of 
organisation, when it flows from different sources, and that 
the power of generating a new bark, properly speaking, 
belongs neither to the bark nor alburnum, but to a fluid, 
which pervades alike the vessels of both. 
I shall, therefore, not attempt to decide on the merits of 
the theory of Malpighi, or of Hales, respecting the repro- 
duction of the interior bark ; but I cannot by any means 
admit the hypothesis of Malpighi and other naturalists, rela- 
tive to the transmutation of bark into alburnum ; and I propose 
to state my reasons for rejecting that hypothesis, in the next 
communication I have the honour to address to you. 
I am, my dear Sir, 
Your much obliged obedient Servant, 
T. A. KNIGHT. 
Elton , Dec. 18, 180b. 
* Phil. Trans. 1805. Mirbel has called the tubes, which I call the central ves- 
sels, the ** tissu tubulaire” of the medulla. 
MDCCCVII . 
Q 
