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ends, will much more readily generate a new bark than the alburnum 

 does under similar circumstances . A similar fluid exudes, and the 

 same phenomena are observable, in both case,s. The cellular sub- 

 stance, however, which is thus generated is for some time very im- 

 perfectly organized, since the vessels which carry the true sap are at 

 first wanting ; but Mr. Knight has been enabled to trace their pro- 

 gress, and, by appropriate management, to direct the course of their 

 growth laterally or downwards with a greater or less degree of in- 

 clination ; but observes, that their natural tendency is perpendicu- 

 larly downwards, and that they cannot be made to extend them- 

 selves upwards excepting to a very short distance. 



Mr. Knight having thus satisfied himself that both the alburnum 

 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 at- 

 tention was next directed to discover the sources from which this 

 fluid is derived. 



Both the bark and the alburnum, he observes, are composed prin- 

 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 alburnum. 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 alburnum, and there joins the ascending current of sap. 



In the preceding experiments the cellular substance, both of the 

 bark and alburnum, has been proved to be capable of affording the 

 sap a passage through it ; and it therefore appears not very impro- 

 bable that it executes an office similar to that of anastomosing ves- 

 sels in the animal economy, when the cellular surfaces of the bark 

 and alburnum 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 ac- 

 tion 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 en- 

 deavoured 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 care to 

 prevent the formation of new bark on the surface of the alburnum, 

 he, after a day or two, made longitudinal incisions through the dry 

 and lifeless alburnum. 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 in- 

 cision, and a perfect vascular bark was subsequently formed, and 

 appeared to execute the office of that which had been removed, al- 

 though placed beneath the alburnum. 

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