248 FORMATION OF THE BARK OF TREES. 



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 th$ 

 vessels, which appeared to diverge into the bark of the 

 stock, from these buds and from those reproduced in the 

 preceding experiments. 

 Theory. It appears, therefore, probable, that a pulpous organizable 



mass first derives its matter either from the bark, or the al- 

 burnum ; 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 two hundred 

 feet distant from each other. I am, therefore, incline^ to 

 believe, that, as the preceding facts seem to indicate, the 

 matter, which composes the new bark, acquires an organiza- 

 tion 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 compo- 

 sition of the new bark, be derived from the bark or the albur- 

 num, in the ordinary course of the growth of the tree, it will 

 be extremely difficult to ascertain. 

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



times exists no f j n a |j cas€S spring from the alburnum ; for many cases 

 previous to the ' ,, ,..",.., ' . , 



alburnum. may be adduced, in which it is always generated previously to 



the existence of the alburnum beneath it; but none, I be- 

 lieve 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 cor- 

 tical vessels elongate, and new portions of bark are succes- 



• Darwin's Phytologia. 



sive)y 



