detached leaves of plants. 293 
num ; which current feeds the young succulent shoots and 
growing leaves. Subjecting the alburnum to a slight degree 
of pressure at this period, I found that a considerable quan- 
tity of liquid, being apparently the true sap of the tree, issued 
out laterally through the medullary processes, as well as 
longitudinally through the cellular substance of the alburnum: 
but the tubes of it continued empty, and their position was 
marked by depressions of the surface of the extravasated 
fluid. I endeavoured to ascertain, what proportion of water 
a given quantity of the alburnum of such oak trees contained 
at this period ; and I found that 1000 parts lost by drying 
only 371 parts : which is not more than the weight of the 
water that the cellular substance appears capable of containing, 
entirely independent of the tubes. That the tubes, never- 
theless, are not always empty, but that they act at other 
periods of the year as reservoirs for the sap, I have given an 
opinion in a former communication ; and I am now in pos- 
session of facts which prove them to perform this office, even 
in the heart wood, to a much greater extent than I had ever at 
any former period suspected; and which incline me to believe, 
that the durability of the heart wood, as well as of the albur- 
num of the oak, will be found to depend to a great extent 
upon the period in which the tree is felled: but I propose to 
make my observations upon these points the subject of a 
future communication. 
I am, my dear Sir, &c. 
T. A. KNIGHT.. 
Q<l 2 
