310 State in which the true Sap (f Trees 



suspect that this substance i? generated, and deposited within 

 it, in the preceding summer and autumn. 



Du Haniel has remarked, and is evidently puzzled with 

 the circumstance, that frees perspire more in the month ot 

 August, when the leaves are full groA\n, and when the an- 

 nual shoots have ceased to elongate, than at anv earlier 

 period ; and we cannot suppose the powers ot" vegetation to 

 be thus actively employed, but in the execution of some 

 very important operation. Bulbous and tuberous roots are 

 almost w holly generated after the leaves and stems of the 

 plants, to which thev belong, have attained their full growth ; 

 and I have constantly found, in my practice as a farmer, 

 that th.e produce of my meadows has been immensely in- 

 creased when the herbage of the preceding year had re- 

 mained to perform its proper office till the end of the au- 

 tumn, on ground which had been mowed early in the sum- 

 mer. 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 Inids and 

 blossoms of the succeeding spring, and to enter mto the 

 composition of new organs of assimilation. 



If the preceding hypothesis be well founded, we may ex* 

 pect 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 anv given portion of winter-felled 

 wood will at the same time possess a greater degree of spe- 

 cific gravity, and yield a larsjer quantitv of extractive mat- 

 ter, than the same quantitv of wood which has been felled 

 in the spring or in the early part of the summer. To ascer- 

 tain these points I made the experiments, an account of 

 which T have now the honour to lav before you. 



As early in the last spring as the sap had risen in the 

 sycamore 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 in- 

 cision as much sap as I wanted. Ascertaining the specific 

 gravity of thfc sap of each tree, obtained at the different 

 elevations, I found that of the sap of the sycamore with 

 very little variation, in different trees, to be 1 .004 w hen 

 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 gravit)^, at greater elevation, 

 was comparatively the same. When extracted near the 

 ground the sap of botf) kinds was almost free from taste; 

 but when obtain-.'d at a greater height, it was sensibly sweet.- 



Th*!-' 



