SAP OP TREES DURING WINTER. Ill 



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 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 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 different 

 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 extracted near the ground the sap of both kinds was 

 almost free from taste ; but when obtained at a greater height, it was 

 sensibly sweet. The shortness of the trunks of the sycamore trees, which 

 were the subjects of my experiments, did not permit me to extract the 

 sap at a greater elevation than seven feet, except in one instance, and in 

 that, at twelve feet from the ground, I obtained a very sweet fluid, whose 

 specific gravity was 1.012. 



I conceived it probable, that if the sap in the preceding cases derived 

 any considerable portion of its increased specific gravity from matter pre- 

 viously existing in the alburnum, I should find some diminution of its 

 weight, when it had continued to flow some days from the same incision, 

 because the alburnum in the vicinity of that incision would, under such 

 circumstances, have become in some degree exhausted : and on compar- 

 ing the specific gravity of the sap which had flowed from a recent and an 

 old incision, I found that from the old to be reduced to 1.002, and that 

 from the recent one to remain 1.004, as in the preceding cases, the incision 

 being made close to the ground. Wherever extracted, whether close to 

 the ground, or at some distance from it, the sap always appeared to con- 

 tain a large portion of air. 



In the experiments to discover the variation in the specific gravity of 

 the alburnum of trees at different seasons, some obstacles to the attain- 



