of the Bark of Trees , 105 
matter, which enters into the composition of this cellular 
substance, is evidently derived from the alburnum. 
These facts are therefore extremely favourable to the 
theory of Hales ; but other facts may be adduced which 
are scarcely consistent with that theory. 
The internal surface of pieces of bark, when detached from 
contact with the alburnum, provided they remain united to the 
tree at their upper ends, much more readily generate a new 
bark, than the alburnum does under similar circumstances : a 
similar fluid exudes from the surfaces of both, and the same 
phenomena are observable in both cases. The cellular sub- 
stance, however, which is thus generated, though it presents 
every external appearance of a perfect bark, is internally very 
imperfectly organized ; and the vessels which contain the 
true sap in the bark, are still wanting; and I have found, 
that these may be made, by appropriate management, to 
traverse the new cellular substance in almost any direction. 
When I cut off all communication above, and on one side, 
between the old bark and that substance, I observed, that the 
vessels proceeded across it, from the old bark on the other 
side, taking always in a greater or less degree an inclination 
downwards ; and when the cellular substance remained united 
to the bark at its upper end only, the vessels descended 
nearly perpendicularly down it; but they did not readily 
ascend into it, when it was connected with the hark at its lower 
extremity only ; the result of similar experiments, when made 
on different species of trees, was, however, subject to some 
variations. 
Pieces of bark of the walnut-tree, which were two inches 
MDCCCVII, P 
