34,0 Mr. Knight’s Experiments on 
wood. I should not venture to state an opinion in opposition 
to his ; but I believe he has not any where distinguished 
those I call the central vessels, from the common tubes of the 
wood. 
Having hitherto found that all advancing fluids appeared to 
pass either along the tubes of the alburnum, or along the cen- 
tral vessels, I had little doubt that the fruit was fed through the 
latter; but my efforts to ascertain this, in the autumn of 1 799, 
were not successful. In the last spring, I was more fortunate. 
Placing small branches of the apple, the pear, and the vine, 
with blossoms not yet expanded, in a decoction of logwood, I 
found that the colouring matter readily passed up the central 
tubes of the fruit-stalks of all ; and, in the apple and pear, I easily 
traced it, through the future fruit, to the base of the stamina. 
The office of the tubes in the bark did not appear in this expe- 
riment ; but, as I have reason to believe the motion of the sap 
in the bark to be always retrograde, I am disposed to conclude 
that it is so here, and that, through the bark of the stalk, any 
superfluous humours existing in the fruit, from excessive humi- 
dity of weather, or other cause, is carried back, and absorbed 
by the tree. I have, however, very frequently repeated an ex- 
periment on the vine, which, I think, evidently proves that the 
fluid returned (if any) is essentially different from that which 
is derived from the leaf. In the culture of this fruit, I have fre- 
quently pinched off the young shoot, immediately above a bunch, 
as soon as the latter became visible in the spring, letting the 
leaf opposite the bunch remain. In this case, the wood below 
the upper leaf acquired nearly its proper length and substance. 
But, when I have taken off that leaf, the wood between the 
bunch and the next leaf below, has ceased to elongate ; and has 
