140 ON THE FORMATION OF THE BARK OF TREES. 



down the bark, where part of it is employed in generating the new sub- 

 stances 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 maturity to perform its office of carry- 

 ing up the sap, I took off the bark between these incisions ; and I abraded 

 the surface of the alburnum to prevent a reproduction of it. The alburnum 

 in the decorticated spaces soon became externally dry and lifeless ; and 

 several incisions were then made longitudinally through it. The incisions 

 commenced a little above, and extended below the decorticated spaces, so 

 that, if the sap of the central vessels generated a cellular substance (as I 

 concluded it would), that substance might come into contact and form a 

 union with the substance of the same kind emitted by the bark above and 

 below. 



The experiment succeeded perfectly, and the cellular substances gene- 

 rated by the central vessels, and the bark, soon united, and a perfect 

 vascular bark was subsequently formed beneath the alburnum, and 

 appeared perfectly to execute the office of that which had been taken off; 

 the medulla appeared to be wholly inactive. 



I have already observed, that the vessels, which were generated in the 

 cellular substance on the surface of the alburnum of the sycamore and the 

 apple-tree, traversed that substance in almost every direction ; and the 

 same .thing appears to occur beneath the old bark, when united to the 

 alburnum. For having attentively examined, through every part of the 

 spring and summer, the formation of the internal bark, and alburnous 

 layer beneath it, round the basis of regenerated buds, which I had made 



