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OF THE STEM AND THE ORIGIN OF WOOD.
The general purpose of the stem is, to bear the leaves and other appendages of the axis aloft in the air, so that they may be freely exposed to light and atmospheric action; to convey fluids from the root upwards, and from above downwards; and, if woody, to store up a certain portion of the secretions of the species either in the bark or in the heartwood.
Various notions have from time to time been entertained about the Pith. The functions of brain, lungs, stomach, nerves, spinal marrow, have by turns been ascribed to it. Some have thought it the seat of fecundity, and have believed that fruit trees deprived of pith became sterile; others supposed that it was the origin of all growth; and another class of writers have declared that it was the channel of the ascent of sap. It is, however, no part of the plan of this work to refute these and similar exploded speculations.
It is probable that its real and only use is, to serve, in the infancy of a plant, for the reception of the sap upon which the young and tender vessels that surround it are to feed when they are first formed; a time when they have no other means of support. Dutrochet considers it to act not only as a reservoir of nutriment for the young leaves, but also to be the place in which the globules which he calls nervous corpuscles are formed out of the elaborated sap (l! Agent Immediat, &c. p. 44. &c.); and Braschet imagines it and its processes to constitute the nervous system of plants!
The Medullary Sheath seems to perform an important part in the economy of plants; it diverges from the pith whenever a leaf is produced; and, passing through the petiole, ramifies among the cellular tissue of the blade, where it appears as veins: hence veins are always composed of bundles of woody tissue and spiral vessels. Thus situated, the veins are in the most favourable position that can be imagined for absorbing the fluid which, in the first instance, is conducted to the young pith, and which is subsequently impelled upwards through the woody tissue. So essential is the medullary sheath to vegetation in the early age of a branch, that, as is well known, although the pith and the bark, and even the young wood, may be destroyed, without the life of a young shoot being much affected; yet, if the medullary sheath be cut through, the pith, bark, or wood, being left, the part above the wound will perish. It may be supposed, considering the large proportion of oxygen it contains, that its office is in part to convey that gas to parts inaccessible to the external air, where it may combine with the carbon of such parts, and cause the production of carbonic acid; without a power of composing and decomposing which, no part exposed to light can long exist.
The Bark acts as a protection to die young and tender wood, guarding it from cold and external accidents. It is also the medium in which the proper juices of the plant, in their descent from the leaves, are finally elaborated, and brought to the state which is peculiar to the species. It is from the bark that they are horizontally communicated to the medullary rays, which deposit them in the tissue of the wood. Hence, the character of timber is almost wholly dependent upon the influence of the bark, as is apparent from a vertical section of a grafted tree, through the line of union of the stock and scion. This line will be sometimes found so exactly drawn, that the limits of the two are well defined even in old specimens: the woody tissue will be found uninterruptedly continuous through the one into the other, and the bark of the two indissolubly united; but the medullary rays emanating from the bark of each will be seen to remain as different as it was at the time when the stock and scion were distinct individuals. It is to be remarked, however, that bark has only a limited power of impelling secreted matter into the medullary rays; and that there are certain substances which, although abundant in bark, are scarcely found elsewhere; as, for instance, gum in a Cherry tree. This substance exists in the wood in so slight a degree as probably not to exceed in quantity what is to be found in most plants, whether they are obviously gummiferous or not. Are we from this to infer that the medullary rays have a power of rejecting certain substances? or, that their tissue is impermeable to fluids of a particular degree of density? or, that they only take up what settles down the bark through its cellular system, and that gum, descending by the woody system exclusively, is not in that kind of contact with the medullary rays which is required to enable the latter to take it up? or, that the latex, which flows exclusively through the cinenchyma, mingles but little with the medullary parenchyma?
As the bark, when young, is green like the leaves, and as the latter are manifestly a mere dilatation of the former, it is highly probable, as Knight believes, that the bark exercises an influence upon the fluids deposited in it wholly analogous to that exercised by the leaves, which will be hereafter explained. Hence it has been named, with much truth, the universal leaf of a vegetable. In fact, in succulent Cactaceae, Stapelias, and similar plants, there is no other part capable of performing the function of leaves.
The business of the Medullary Rays is, no doubt, exclusively to maintain a communication between the bark, in which the secretions receive their final elaboration, and the centre of the trunk, in which they are at last deposited. This is apparent from tangental sections of dicotyledonous wood manifesting an evident exudation of liquid matter from the wounded medullary rays, although no such exudation is elsewhere visible. In endogenous plants, in which there appears less necessity for maintaining a communication between the centre and circumference, there are no special medullary rays. These rays also serve to bind firmly together the whole of the internal and external parts of a stem, and they give the peculiar character by which the wood of neighbouring species may be distinguished. If plants had no medullary rays, their wood would probably be, in nearly allied species, undistinguishable; for we are scarcely aware of any appreciable difference in the appearance of woody or vascular tissue; but the medullary rays (the silver grain of carpenters), differing in abundance, in size, and in other respects, impress characters upon the wood which are extremely well marked. Thus, in the cultivated Cherry, the plates of the medullary rays are thin, the adhesions of them to the bark are slight, and hence a section of the wood of that plant has a pale, smooth, homogeneous appearance; but in the wild Cherry the medullary plates are much thicker, they adhere to the bark by deep broad spaces, and are arranged with great irregularity, so that a section of the wood of that variety has a deeper colour, and a twisted, knotty, very uneven appearance. In Quercus sessiliflora the medullary rays are thin, and so distant from each other that the plates of wood between them do not readily break laterally into each other, if a wedge is driven into the end of the trunk in the direction of its cleavage: on the contrary, the medullary rays of Quercus pedunculata are hard, and so close together that the wood may be rent longitudinally without difficulty; hence the wood of the latter is the only kind that is fit for application to park paling.
As the medullary rays develope in a horizontal direction only, when two trees in which they are different are grafted or budded together, the wood of the stock will continue to preserve its own peculiarity of grain, notwithstanding its being formed by the woody matter sent down by the scion; for it is the horizontal developement that gives its character to the "grain of wood," and not the perpendicular pleurenchyma encased in it.
The Wood is at once the support of all the deciduous organs of respiration, digestion, and fertilisation; the depositary of the secretions peculiar to individual species; and also the reservoir from which newly forming parts derive their sustenance, until they can establish a communication with the soil.
Regarding the precise manner in which it is created, there has been great diversity of opinion. Linnteus thought it was produced by the pith; Grew, that the liber and wood were deposited at the same time in a single mass which afterwards divided in two, the one half adhering to the centre, the other to the circumference. Malpighi conceived that the wood of one year was produced by an alteration of the liber of the previous season. Duhamel believed that it was deposited by the secretion already spoken of as existing between the bark and wood, and called cambium: he was of opinion that this cambium was formed in the bark, and became converted into both cellular and woody tissue; and he demonstrated the fallacy of those theories according to which new wood is produced by the wood of a preceding year. He removed a portion of bark from a Plum tree; he replaced this with a similar portion of a Peach tree, having a bud upon it. In a short time a union took place between the two. After waiting a sufficient time to allow for the formation of new wood, he examined the point of junction, and found that a thin layer of wood had been formed by the Peach bud, but none by the wood of the Plum, to which it had been tightly applied. Hence he concluded that alburnum derives its origin from the bark, and not from the wood. Many similar experiments were instituted with the same object in view, and they were followed by similar results. Among others, a plate of silver was inserted between the bark and the wood of a tree, at the beginning of the growing season. It was said, that, if new wood were formed by old wood, it would be subsequently found pushed outwards, and continuing to occupy the same situation; but that, if new wood were deposited by the bark, the silver plate would in time be found buried beneath new layers of wood. In course of time the plate was examined, and was found enclosed in wood.
Hence the question as to the origin of the wood seemed settled; and there is no doubt that the experiments of Duhamel are perfectly accurate, and satisfactory as far as they go. It soon, however, appeared, that, although they certainly proved that new wood is not produced by old wood, it was not equally clear that it originated from the bark. Accordingly a new set of experiments was instituted by Knight, for the purpose of throwing a still clearer light upon the production of the wood. Having removed a ring of bark from above and below a portion of the bark furnished with a leaf, he remarked that no increase took place in the wood above the leaf, while a sensible augmentation was observable in the wood below the leaf. It was also found, that, if the upper