VIII. On the office of the heart wood of trees. By T. A. Knight, Esq. F .R . S. In a letter addressed to the Rt. Hon. Sir Joseph Banks, Bart. G. C. B. P. R. S

Abstract

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 gravity, 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 obviously 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 appears, however, to be purely mechanical, and not to be in any degree dependent upon the vital power of the tree; and some writers on vegetable physiology have regarded it as a wholly lifeless substance. This opinion I have always rejected, though I was unable to adduce any decisive evidence in opposition to it; but I have now reason to believe that the heart wood becomes, during winter, in common with the alburnum and bark, a reservoir of the organizable matter which the tree expends in germination in the spring; and that the fluid sap passes abundantly into it laterally, though it does not ascend through it. I had long previously been perfectly satisfied that every species of tree, and perennial plant, contains within itself, during winter, all the organizable matter which it employs in the formation of its first foliage, and succulent shoots, in the spring; and that it is owing to the presence or absence of such reservoir, that the lives of plants become annual, biennial, and perennial. The annual plant exhausts itself wholly in feeding its flowers and seeds: it forms no reservoir, and consequently perishes. Its vital powers are not expended, for detached parts of the same plant, and obviously possessing the same life, become perennial, if planted in such season that they cannot exhaust themselves by the production of flowers before winter. A biennial plant (the common turnip affords a good and familiar example) fills its reservoir in one season, and wholly expends it in the following, when it consequently dies, like the annual plant. In the tree, as in the biennial plant, a part of the reserved sap descends early in the spring to form new roots, whilst another portion ascends to feed its buds; but the tree also discharges laterally a large quantity of organizable matter to generated new layer of bark over the whole surface of its alburnum, and it thus adapts its organization to its more permanent state of existence.