C 88 3 
IV. Concerning the State in which the true Sap of Trees is depo- 
sited during Winter. In a Letter from Thomas Andrew 
Knight, Esq. to the Right Hon. Sir Joseph Banks, Bart. K. B. 
P.R.S. 
Read January 24, 1805. 
MY DEAR SIR, 
It is well known that the fluid, generally called the Sap in 
trees, ascends in the spring and summer from their roots, 
and that in the autumn and winter it is not, in any considerable 
quantity, found in them ; and I have observed in a former 
Paper, that this fluid rises wholly through the alburnum, or 
sap-wood. But Du Hamel and subsequent naturalists have 
proved, that trees contain another kind of sap, which they 
have called the true, or peculiar juice, or sap of the plant. 
Whence this fluid originates does not appear to have been 
agreed by naturalists ; but I have offered some facts to prove 
that it is generated by the leaf ; * and that it differs from the 
common aqueous sap owing to changes it has undergone in its 
circulation through that organ : and I have contended that from 
this fluid ( which Du Hamel has called the sue propre, and 
which I will call the true sap,) the whole substance, which is 
annually added to the tree, is derived. I shall endeavour in 
the present Paper to prove that this fluid, in an inspissated 
state, or some concrete matter deposited by it, exists during 
* See Phil. Trans, of 1801, page 336. 
