142 ON THE FORMATION OF THE BARK OF TREES. 



Instead of the reproduced buds of the preceding experiment, buds were 

 inserted in the foregoing summer, or attached by grafting in the spring ; 

 and, when these succeeded, though they were in many instances taken 

 from trees of different species, and even of different genera, no sensible 

 difference existed in the vessels, which appeared to diverge into the bark 

 of the stock, from these buds and from those reproduced in the preceding 

 experiments. 



It appears, therefore, probable, that a pulpous organisable mass first 

 derives its matter either from the bark or the alburnum, and that this 

 matter subsequently forms the new layer of bark ; for, if the vessels had 

 proceeded, as radicles *, from the inserted buds, or grafts, such vessels 

 would have been, in some degree, different from the natural vessels of 

 the bark of the stocks ; and it does not appear probable, even without 

 referring to the preceding facts, that vessels should be extended, in a few 

 days, by parts successively added to their extremities, from the leaves to 

 the extremities of the roots ; which are, in many instances, more than 

 200 feet distant from each other. I am, therefore, inclined to believe, 

 that, as the preceding facts seem to indicate, the matter which composes 

 the new bark acquires an organisation calculated to transmit the true 

 sap towards the roots, as that fluid progressively descends from the 

 leaves in the spring ; but whether the matter which enters into the 

 composition of the new bark, be derived from the bark or alburnum, in 

 the ordinary course of the growth of the tree, it will be extremely dim- 

 cult to ascertain. 



It is, however, no difficult task to prove, that the bark does not, 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 previously to the bark in contact with it, except when 

 the cortical substance has been taken off, as in the preceding experiments. 

 In the radicle of germinating seeds, the cortical vessels elongate, and 

 new portions of bark are successively added to their points, many days 

 before any alburnous substance is generated in them ; and in the succu- 

 lent annual shoot the formation of the bark long precedes that of the 

 alburnum. In the radicle the sap appears also evidently to descend f 

 through the cortical vessels j, and in the succulent annual shoot it as 



* Darwin's Phytologia. f See above, Nos. V. and VII. 



J I wish it to be understood, that I exclude in these remarks, and in those contained in ray 

 former memoirs, all trees of the palm kind, with the organisation of which I am almost wholly 

 unacquainted. 



