io8 Mr. Knight on the Formation 
substance, both of the bark and alburnum ; and on the sur- 
face of these substances alone, in many instances, the new 
bark was reproduced in small detached pieces. 
I have endeavoured to prove in former communications,* 
that the true sap of trees acquires those properties which dis- 
tinguish it from the fluid recently absorbed, by circulating 
through the leaf ; and that it descends down the bark, where 
part of it is employed in generating the new substances 
annually added to the tree ; and that the remainder, not thus 
expended, passes into the alburnum, and there joins the 
ascending current of sap. The cellular substance, both of 
the bark and alburnum, has been proved, in the preceding 
experiments, to be capable of affording the sap a passage 
through it ; and therefore it appears not very improbable, 
that it executes an office similar to that of the anastomosing 
vessels of the animal economy, when the cellular surfaces of 
the bark and alburnum are in contact with each other ; and, 
when detached, it may be inferred, that the passing fluid will 
exude from both surfaces : because almost all the vessels of 
trees appear to be capable of an inverted action in giving 
motion to the fluids which they carry. 
As the power of generating a new bark appeared in the 
preceding cases to exist alike in the sap of the bark and of 
the alburnum, I was anxious to discover how far the fluid, 
which ascends through the central vessels of the succulent 
annual shoot, is endued with similar powers. Having there- 
fore made two circular incisions through the bark, round 
the stems of several annual shoots of the vine, as early in the 
summer as the alburnum within them had acquired sufficient 
* Phil, Trans. 1801, 1805, and 1806. 
