1 12 
Mr. Knight on the Formation 
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, inclined to believe, 
that, as the preceding facts seem to indicate, the matter, 
which composes the new bark, acquires an organisation cal- 
culated 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 the alburnum, in the 
ordinary course of the growth of the tree, it will be extremely 
difficult 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 previ- 
ously to the bark in contact with it, except when the cortical 
substance has been taken off, as in the preceding experi- 
ments. In the radicle of germinating seeds, the cortical ves- 
sels elongate, and new portions of bark are successively 
added to their points, many days 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 vessels,'!' and in the succulent 
annual shoot it as evidently passes up through the central 
* Phil. Trans. 1805 and 1806. 
f I wish it to be understood, that I exclude in these remarks, and in those con- 
tained in my former Memoirs, all trees of the palm kind, with the organisation of 
which I am almost wholly unacquainted. 
