“Lo! the oak that hath so long a nourishing
From the time that it ’ginneth first to spring,
And hath so long a life, as we may see,
Yet at the last wasted is the tree.”
Chaucer.
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 vegetable tissue compressed into a given space, in the first construction of the tree, but also on the quantity and quality of the secretions ultimately lodged in it. The same species of tree will present a difference in this respect, according to the country or situation where it grew, and also according to the character of the seasons during the time it flourished. According to the nature of the tree, if placed in favourable circumstances in reference to soil and weather, it invariably prepares and lodges in the stem those principles which it was designed to elaborate—the oak preparing tannin—the sugar-maple preparing its saccharine juice. That the primary object of these was some advantage to the tree itself can scarcely be doubted, but the secondary applications of which they are capable, give reason to suppose that these also were contemplated in their formation. The consideration of the means by which they are formed, and the direct consequences of their formation on the air, by abstracting certain elements from it, and supplying others, belong to the subject of leaves; it is the object of the present paper to view them as formed, and to show their amazing utility.
The mechanical properties of the stems of trees, both exogenous and endogenous, render them extremely serviceable to mankind. The uses to which a single species of plant may be put are numerous and important, of which the reed (arundo phragmites) is an example, for after the root has assisted in binding and consolidating the soil, the stem is susceptible of the most varied applications[A].
In a low state of civilization the palm, or a palm-like grass, supplies all that man requires; of the former of which, the Mauritia flexuosa, or sago-palm of the Oronooko, and still more the cocos nucifera, or cocoa-nut palm; and of the latter, the bamboo (bambusa arundinacea, and other species) are proofs. The bamboo suffices for all the needs of the humbler Chinese; even their paper, as well as their abodes, are made of it; and from the materials furnished by the cocoa-nut tree, not merely food, as shall be afterwards noticed, but larger and more elegant houses, with all their appurtenances, are constructed at Goa and other places. The obligations of the Guaraons to the Mauritia flexuosa cannot be expressed[B]. In proportion as man rises in civilization, the importance of timber becomes greater, being a material for which no adequate substitute can be found. It combines lightness with strength, elasticity with firmness, and possesses in many instances a durability rivalling, or even surpassing, that of the rocks yielded to us by the solid substance of the globe. The adaptation of timber to the numerous wants of civil life is too familiar to require exposition; but in addition to all the ends it serves in these points, we have an interesting view presented to us in considering what a vast quantity of timber is required for the construction of our shipping, from the countless boats and small craft employed in our coasting trade up to the larger ships, which are so many floating towns or communities. These conduce to the accomplishment of objects of the most momentous nature. Were it not for our shipping we should still be in the condition described by the Romans, as Britons cut off from the rest of the world.—But by their means we now visit without restraint,
“Earth’s farthest verge, and ocean’s wildest shore,”[C]
and though, in times past, they have been too often used as engines fraught with destruction, directed by man against his fellow man, let us hope that they may be required in future only to convey in amicable interchange the produce of one country to another, or to bear to his destination the missionary bent on extending the blessings of that religion whose spirit is “peace on earth, good will among the children of men[D].”
As a means of supplying fuel, without which man must remain constantly in the savage state, wood is of inestimable value. In the process of combustion, the elements of the trees enter into new combinations, evolving both light and heat, which at once maintain life and render it a state of enjoyment and usefulness. For this purpose in Britain, we chiefly employ fossil fuel, stored up in the secret places of the earth, and, therefore, we attach less importance to recent wood; but other parts of the world are not so favourably situated, and to the inhabitants of these places fresh, or but lately felled, wood is necessary for their existence. Even in France, though partially possessed of coal, it is estimated that the quantity of wood employed to supply heat, whether for comfort, cooking, or in manufactures which require a high temperature, amounts to seven-tenths of the entire consumption. The superiority of wood fuel, whether fossil or recent, over every other material resorted to with a like intention, shall be shown in a subsequent part of this paper. I therefore pass on at present to demonstrate the utility of vegetable substances in affording the means of subsistence to man and animals.
In the observations I am about to make, it is impossible to avoid anticipating some of the remarks which belong to the subject of fruits and seeds as articles of food, since the same principles of nutriment are found in the stems of certain plants as are deposited in the fruits or seeds of others.
Though man is omnivorous, and can subsist either on animal or vegetable food—an arrangement which fits him to dwell in any part of the habitable globe,—yet he is subject, with regard to the actual material of his diet, in a remarkable manner, to the influence of climate, since a particular kind of aliment, which is very appropriate in one country is improper in another; thus, as we advance from the equator towards the poles, the necessity for animal food becomes greater, till, in the very north, it is the sole article of subsistence. Animal food, from containing nitrogen, is more stimulating, and, therefore, less suitable for hot climates, where, on the contrary, saccharine, mucilaginous, and starchy materials are preferred; hence, in the zone of the tropics, we find produced in abundance rice, maize, millet, sago, salep, arrowroot, potatoes, the bread-fruit, banana, and other watery, or mucilaginous fruits. Quitting this zone, we enter that which produces wheat, and here, where the temperature is lower, providence has united with the starch of this grain a peculiar principle (gluten), possessing all the properties of animal matter, and yielding nitrogen and ammonia in its decomposition[E]. Thus, by a gradual and almost insensible transition, nature furnishes to man the food which is most appropriate for him in each region. In the subtropical zone vegetable diet is still preferred, but, in chemical constitution, the favourite articles approximate animal substances. This holds also in the temperate zone, not only in respect of wheat, but also in the chesnut, which is almost the sole means of subsistence in some of the mountainous regions of France, Italy, and Spain, though, instead of the gluten of wheat, this seed contains albumen, the relation of which to animal food is even closer than that of gluten. In reviewing the geographical distribution of the cereal grains[F], we find that starch nearly pure is produced in the greatest abundance in the hottest parts of the world, particularly in rice and maize; it becomes associated in the subtropical regions with an equivalent for animal food; and in still colder regions, where wheat fails, oats and barley take its place. These, though possessed of less gluten than wheat, are, nevertheless, more heating, and, therefore, better calculated for northern latitudes. The inhabitants of Scotland and Lapland, with their oaten and barley or rye bread, are thus as thoroughly provided with the best food, as the Hindoo with his rice or Indian corn[G].