314 
PHYSIOLOGY. 
BOOK II. 
part of a branch be deprived of leaves, the branch will die 
down to the point where leaves have been left, and below 
that will flourish. Hence an inference is drawn, that the 
wood is not formed out of the bark as a mere deposit from it; 
but that it is produced from matter elaborated in the leaves 
and sent downwards, either through the vessels of the inner 
bark, along with the matter for forming the liber, by which 
it is subsequently parted with ; or that it and the liber are 
transmitted distinct from one another, the one adhering to 
the alburnum, the other to the bark. I know of no proof of 
the former supposition ; of the latter there is every reason to 
believe the truth. Knight is of opinion that two distinct sets 
of vessels are sent down, one belonging to the liber, the other 
to the alburnum; and if a branch of any young tree, the 
wood of which is formed quickly, be examined when it is 
first bursting into leaf, these two sets may be distinctly seen 
and traced. Take, for instance, a branch of Lilac in the be- 
ginning of April, and strip off its bark : the new wood will be 
distinctly seen to have passed downwards from the base of 
each leaf, diverging from its perpendicular course, so as to 
avoid the bundle of vessels passing into the leaf beneath it ; 
and, if the junction of a new branch with that of the previous 
year be examined, it will be found that the wood already 
seen proceeding from the base of the leaves, having arrived 
at this point, has not stopped ‘there, but has passed rapidly 
downwards, adding to the branch an even layer of young 
ligneous matter, and turning off at every projection which 
impedes it, just as the water of a steady but rapid current 
would be diverted from its course by obstacles in its stream. 
Again, in Guaiacum wood, the descending tubes of pleuren- 
chyma cross and interlace each other, in a manner that is un- 
intelligible upon the supposition of wood being formed by 
the mere deposit of secreted matter. If the new wood were 
a mere deposit of the bark, the latter, as it is applied to every 
part of the old wood, would deposit the new wood equally 
over the whole surface of the latter, and the deviation of the 
fibres from obstacles in their downward course would scarcely 
occur. This, therefore, in my mind, places the question as 
to the origin of the wood beyond all further doubt. Or, if 
