90 Mr. Knight concerning the State in which 
year had remained to perform its proper office till the end of 
the autumn, on ground which had been mowed early in the 
summer. Whence I have been led to imagine, that the leaves, 
both of trees and herbaceous plants, are alike employed, during 
the latter part of the summer, in the preparation of matter 
calculated to afford food to the expanding buds and blossoms 
of the succeeding spring, and to enter into the composition of 
new organs of assimilation. 
If the preceding hypothesis be well founded, we may expect 
to find that some change will gradually take place in the 
qualities of the aqueous sap of trees during its ascent in the 
spring ; and that any given portion of winter-felled wood will 
at the same time possess a greater degree of specific gravity, 
and yield a larger quantity of extractive matter, than the same 
quantity of wood which has been felled in the spring or in the 
early part of the summer. To ascertain these points I made 
the experiments, an account of which I have now the honour 
to lay before you. 
As early in the last spring as the sap had risen in the syca- 
camore and birch, I made incisions into the trunks of those 
trees, some close to the ground, and others at the elevation of 
seven feet, and I readily obtained from each incision as much 
sap as I wanted. Ascertaining the specific gravity of the sap 
of each tree, obtained at the different elevations, I found that 
of the sap of the sycamore with very little variation, in dif- 
ferent trees, to be 1.004 when extracted close to the ground, 
and 1.008 at the height of seven feet. The sap of the birch 
was somewhat lighter ; but the increase of its specific gravity, 
at greater elevation, was comparatively the same. When ex- 
tracted near the ground the sap of both kinds was almost free 
