While the actions which lead to the various effects on the external appearance of a tree, described in the former paper, are going on, many important changes occur in the internal parts, producing alterations not less admirable, whether in respect of the tree itself, or of the ends to which it may be rendered subservient. The base of an exogenous tree is not merely widened by the superposition of annual layers of wood over the first shoot, by which it gains greater mechanical power to support the extending head of wide-spreading branches, but the central portion is, in most cases, progressively rendered more and more solid by the deposition in it of various secretions prepared by the leaves, and transmitted from them through the medullary rays into this part as their ultimate resting-place. The fibres descending from the developing buds on the stem, and passing between the plates of cellular tissue, which constitute the medullary rays, and the cells of which have a horizontal direction, are but the basis of the vegetable fabric. The stem of an exogenous plant has been compared to a piece of linen, of which the weft is composed of cellular tissue, and the warp of fibrous and vascular tissue--crossing each other. Now, after the portion is once formed, which is woven every year by the wondrous machinery set to work for this purpose, it receives no fresh texture, yet each fibre remains a conducting tube to transmit the sap upwards, or, in the course of time, becomes charged with various principles, prepared, as already stated, by the leaves, and returned to the central part by that apparatus or system of canals for their transit inwards, the medullary rays, and at last are obstructed, so that no passage of fluid is effected through the inner layers of wood. But for every layer that is thus blocked up, a new one, which will continue pervious, is formed exterior to those already existing, so that a constant provision is made for carrying on the vital processes; to accomplish which, a free channel from the points of the roots to the surface of the leaves is absolutely necessary. The outer strata, produced by a tree of considerable age, are observed to be thinner than those formed at an earlier period, and become successively thinner and thinner, so that ultimately, if accident should not have previously caused it, the death of the tree is inevitable. The portions which are obstructed constitute the _duramen_ or heartwood, the pervious portion the _alburnum_ or sapwood. The original tissue is colourless; but according to the nature of the secretions deposited in it, the heartwood is either of a deeper colour, sometimes party-coloured, or at least of a much greater specific gravity than the sapwood. The removal of the juices by any solvent restores the wood to its primitive hue, and renders it again light. The difference of weight of a cubic foot of wood depends not merely on the different quantity of