C *37 3 
Vill. the office of the heart wood of trees. By I\ A. Knight, 
Esq. F.R.S, In a Letter addressed to the Rt. Hon. Sir 
Joseph Banks, Bart . G. C. B. P. R. S. 
Read February 5, 1818. 
MY DEAR SIR, 
Trees of every species, that afford timber, live many years 
before any portion of their alburnum becomes converted into 
heart wood ; and vegetation proceeds with as much vigour 
previously to the existence of that substance, as subsequently. 
In the oak it is rarely seen till the seedling tree becomes 
nearly twenty years old ; when it is readily distinguishable 
from the alburnum by a deeper colour, higher specific gra- 
vity, and greater hardness. The tubes also, which extend 
through the tree longitudinally, and are always open in the 
alburnum, so as freely to permit the passage of air or water, 
are closed in the heart wood ; and the cellular substance of it 
has appeared, in every experiment that has come under my 
observation, to be incapable of conveying the ascending fluid. 
It does not therefore appear to execute any very important 
office in the vegetable economy ; farther than that it obvi- 
ously gives, as I have remarked in a former communication, 
much additional strength to the stem and branches, when 
these, particularly the latter, become more subject to receive 
injury, both from the influence of winds and gravitation, on 
account of the increased distance of their foliage from the 
points of suspension. Its mode of operation in this case 
mdcccxviii. T 
