ends, will much more readily generate a new bark than the albumum
does under similar circumstances. A similar fluid exudes, and the
same phenomena are observable, in both cases. The cellular sub-
stance, however, which is thus generated is for some time very imperfectly organized, since the vessels which carry the true sap are at
first wanting; but Mr. Knight has been enabled to trace their progress, and, by appropriate management, to direct the course of their
growth laterally or downwards with a greater or less degree of inclination; but obServes, that their natural tendency is perpendicularly downwards, and that they cannot be made to extend themselves upwards excepting to a very short distance.
Mr. Knight having thus satisfied himself that both the albumum and the bark of trees are capable of generating new bark, or at least of transmitting a fluid to which the bark owes its existence, his attention was next directed to discover the sources from which this fluid is derived.
Both the bark and the alburnum, he observes, are composed prim cipally of two substances, one of which consists of long tubes, the other is cellular; and the cellular substance of the bark is in contact with a similar substance in the albumum. Mr. Knight has shown, in former communications, that the true sap of trees acquires those properties which distinguish it from the fluid recently absorbed, by circulating through the leaf; that it descends down the bark, where part of it is employed in generating the new substance annually added to the tree; and that the remainder, not thus expended, passes into the albnmum, and there joins the ascending current of sap.
In the preceding experiments the cellular substance, both of the bark and albumum, has been proved to be capable of affording the sap a passage through it; and it therefore appears not very improbable that it executes an office similar to that of anastomosing vessels in the animal economy, when the cellular surfaces of the bark and albumum are in contact with each other; and hence the passing fluid is capable of exuding from both surfaces when they are detached; because almost all the vessels of trees are capable of an inverted action in giving motion to the fluids they contain.
Since the power of generating new bark appeared to reside alike in the sap of the bark and in that of the alburnum, Mr. Knight endeavoured to ascertain whether the fluid which ascends through the central vessels of the succulent annual shoot possesses similar power. For this purpose he removed the bark between two circular incisions round several annual shoots of the vine; and having taken can to prevent the formation of new bark on the surface of the albumum, he, after a day or two, made longitudinal incisions through the dry and lifeless albumum. These incisions commenced a little above and extended a little below the decorticated spaces. A cellular substance was in consequence generated through the whole length of the incision, and a perfect vascular bark was subsequently formed. and appeared to execute the oflice' of that which had been removed, although placed beneath the alburnum.
