294* JWr. Knight on the inverted Action 
Du Hamel which appears strongly to militate against my 
hypothesis ; and as that circumstance probably induced Hales 
to deny altogether the existence of circulation in plants, and 
Du Hamel to speak less decisively in favour of it than he pos- 
sibly might otherwise have done, I am anxious to reconcile the 
statements of these great naturalists, (which I acknowledge 
to be perfectly correct, ) with the statements and opinions I 
have on former occasions communicated to you. 
Both Hales and Du Hamel have proved, that when two 
circular incisions through the bark, round the stem of a tree, 
are made at a small distance from each other, and when the 
bark between these incisions is wholly taken away, that por- 
tion of the stem which is below the incisions through the 
bark continues to live, and in some degree to increase in size, 
though much more slowly than the parts above the incisions. 
They have also observed that a small elevated ridge (hour- 
velet ) is formed round the lower lip of the wound in the 
bark, which makes some slight advances to meet the bark 
and wood projected, in much large quantity, from the oppo- 
site, or upper lip of the wound. 
I have endeavoured, in a former Memoir,* to explain the 
cause why some portion of growth takes place below inci- 
sions through the bark, by supposing that a small part of the 
true sap, descending from the leaves, escapes downwards 
through the porous substance of the alburnum. Several facts 
stated by Hales seem favourable to this supposition ; and the 
existence of a power in the alburnum to carry the sap in 
different directions, is proved in the growth of inverted cut~ 
tings of different species of trees.-f But I have derived so 
* Phil, Trans, for 1803. f Ibid, for 1804. 
