42 On the Formation of the Bark of Trees. 



matter, which com})oses the new bark, acquires an organi- 

 sation calculated to transmit the true sap towards the roots, 

 as that fluid progressiveiv descends from the leaves in the 

 spring: but whether the matter, which enters into the com- 

 position of the new bark, be derived from the bark or the 

 alburnum, in the ordinary course of the growth of the tree, 

 it will be extreinelv difficult to ascertain. 



It is, however, no difficult task to prove, that the bark does 

 rot, in all cases, spring from the alburnum ; for many cases 

 may be adduced in which it is always generated previously to 

 the existence of the alburnum beneath it: but none, I believe, 

 in which the external surface of the alburnum exists previ- 

 ously to the bark in contact with it, except when the cor- 

 tical substance has been taken off, as in the preceding ex- 

 periments. In the radicle of germinating seeds, the cortical 

 vessels elongate, and new portions of bark are successively 

 added to their points, many davs before any alburnous sub- 

 stance is generated in them ; and in the succulent annual 

 shoot the formation of the bark long precedes that of the 

 alburnum. In the radicle the sap appears also evidently to 

 descend* through the cortical vesselsf, and in the succulent 

 annual shoot it as evidently passes up through the central 

 vessels|, which surround the medulla. lu both cases a cel- 

 lular substance, similar to that which was generated in the 

 preceding experiments, is first formed, and this cellular sub- 

 stance in thii same manner subsequently becomes vascular ; 

 whence it appears, that the true sap, or blood of the plant, 

 produces similar effects, and passes through similar stages of 

 organisation, when it flows froin diflerent sources, and that 

 the power of generating a ticw bark, properly speakino;, 

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

 which pervades r.like the vessels of both. 



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

 the theory of Malpighi, or of Hales, respecting the repro- 



• Phil. Trans. 180.3 and 1806. 



■)• I wish it to be understood, that I exclude in these remarks, and in those 

 contained in my former memoirs, all trees of the palm kind, M-ith the orga- 

 ijrsation of which I am almost wholly unacquainted. 



\ Phil. Trans. 180.'. Mirbel has called the tubes, which I call the central: 

 vessels, the " tissu tubulaire" of the moJulh. 



ductioi^ 



