FORMATION OF THE BARK OF TREES. 



249 



sively added to their points, many days before any albiirnous 

 substance is generated in them ; and in the succulent an- 

 nual shoot the formation of the bark long precedes that of 

 ttie alburnum. In the radicle the sap appears also evidently 

 to descend* through the cortical vesselsf, and in the succu- 

 lent annual shoot it as evidently passes up through the cen-> 

 tral vessels}, which surround the medulla. In both cases a 

 cellular substance, similar to that which was generated in 

 the preceding experiments, is first formed, and this cellular 

 substance in the same manner subsequently becomes vascu- 

 lar; whence it appears, that the true sap, or blood of the 

 plant, produces similar effects, and passes through similar 

 stages of organization, when it flows from different sources, 

 and that the power of generating a new bark, properly speak- 

 ing, belongs neither to the bark nor alburnum, but to a fluid, 

 which pervades alike the vessels of both, 



I shall, therefore, not attempt to decide on the merits of Bark not tran* 

 the theory of Malpighi, or of Hales, respecting the reproduc- mutpd intoaj- 

 tion of the interior bark ; but I cannot by any means admit 

 the hypothesis at Malpighi and other naturalists, relative to 

 the trasmutation of bark into alburnum; and I propose to 

 state my reasons for rejecting that hypothesis, in the ne$t 

 communication I have the honour to address to you. 



I am, my dear Sir, 



Your most obliged obedient Servant, 



FMon, Dec. 18, 18 Off. T. A. KNIGHT. 



* Phil. Trans, 1805 and 1806, or Philos. Journal, vol. XIII and XVL 



+ I wish it to be understood, that I exclude in fhese remarks, and in 

 those contained in my former Memoirs, all trees of the palm kiad, with 

 the organization of which I am almost wholly unaccquainted. 



% Phil. Trans. 1805. Mirbel has called the tubes, which I call the 

 central vessels, the " tissu tubuiaire" of the medulla. • 



If. 



