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(or the tree, if the main trunk have been operated on) perishes in a longer or shorter time according to circumstances. If the ascending current were impeded, it is obvious the accumulation which causes the tumor, must take place on, or below the lower lip of the incision. This descending sap, or proper juice,-whose chemical composition appears to be water and carbon, and which itself principally in the form of gum, is capable of being, by very slight modifications, transformed into fecula (starch), sugar, and lignine, quits the leaves during the night, and traversing the bark and pith in exogenous, and the wood in endogenous plants, reaches the roots. In its progress it deposits nutritious matter, which, more or less mixed in the woody portions with the ascending sap, or absorbed with the water which is taken up through the medullary rays by the cellular envelop, is imbibed by and elaborated in the cells. It meets in its course and especially in the bark, glands and glandular cells, which imbibe it and form in their cavities peculiar secretions (51), most of them incapable of nourishing the plant, and destined to be rejected or carried into the substance of the tissue.

The water which rises from the roots to the foliage is almost as pure when it reaches it, as at its entrance into the plant, if its course has been rapid through the older wood,* where the particles are slightly soluble; that on the contrary which has traversed those younger portions in which there is much cellular tissue filled with nutritive particles, slackens its course, mixes with and dissolves them,

*It has been proved by coloring the water with cochineal, that the ascent of the sap certainly takes place through the ligneous system, though the particular channels may be doubtful.

and arrives at the higher parts of the plant loaded with nourishment. The cells appear to be the true organs of nutrition, in which the decomposition and assimilation of the juices take place. In each cell ligneous matter is deposited which coats its walls, and the inequalities of this deposit in many cases appear to have given rise to the idea that the cells were perforated-the thinner portions being so transparent, that under the microscope they have the appearance of pores.

It is

evident from the above detail that there is no circulation in plants strictly similar to that of animals, but that there is an alternate ascent and descent of

the sap.

45. It will be gathered from the account of the course of vegetable nutrition just given that the oxygen, hydrogen, carbon, and nitrogen, of which plants are chemically composed (1), are thus derived. The oxygen is abundantly furnished by the decomposition of carbonic acid, by the surrounding atmosphere, and by the water taken up into the system. The carbon, which constitutes so large a part of the texture of plants that it retains the form and character of the species when the other portions have been separated from it, and it alone remains as charcoal,* is also mainly derived from the decomposition of carbonic acid. The hydrogen is partly obtained from the water the plant takes up by its roots and leaves, and also from the same source as the nitrogen, which although so abundant in our atmosphere as to constitute four-fifths of its whole composition, does not appear to be thence imbibed in its simple form by plants, but to be supplied to

* A remarkable instance of this may be noticed in the triangular pith of the alder used in the manufacture of gunpowder.

them combined with hydrogen in the form of ammonia, the great ingredient in those animal manures so important in agriculture. "It appears," says Dr. Carpenter, "from recent inquiries, that the organized tissues of plants, that is, their cells, fibres, vessels, &c., freed from their contents, are composed of a substance which every where possesses the same composition; and that this consists of 24 carbon, 20 hydrogen, and 10 oxygen, without any nitrogen;" on the other hand the substances into whose composition nitrogen enters, though very generally diffused through the tissues of the plant, do not seem to undergo organization, but to form part of the contents of the cells, vessels, &c., of which these tissues are composed. It is curious to remark that precisely the reverse is the case with animals; their tissues being composed of a substance containing nitrogen, and substances which are destitute of it being never found in their bodies in an organized state, but only existing there in the cavities of their cells, tubes," &c. (Veg. Physiology, p. 117, § 163.)

46. It is, obvious from the nature of the nourishment which plants require, that the condition of the soil in which they are grown is a matter of great importance. This subject has already been noticed in the "Introduction to Organic Chemistry," which forms the fourth number of these "Small Books," § 27, &c. There is scarcely perhaps a stronger proof in the history of human progress, of the light which Truth sheds on everything within its influence, than the improvement that modern agriculture has derived from the science of Chemistry. The earth has been in some sort cultivated from the time when Adam was sent forth to

till it, yet not until the last half century,* had the advantages the husbandman may derive from an acquaintance with the composition of the soil of his fields, been known, and little could the landowners of the days in which the alchemist, half empiric and half enthusiast, was preparing the way by his toilsome and blind gropings for the more enlightened researches of his successors, imagine that the time would come when chemistry should, at least metaphorically, teach him how to turn earth into gold. The subject is worthy of all attention, not merely from the pecuniary advantage the scientific cultivator may reasonably expect to gain, but from the mental exercise which he may thus obtain, while laboring in his proper calling. The words of Sir Humphrey Davy, in concluding his volume on this subject, are admirable: "The same energy of character, the same extent of resources which have always distinguished the people of the British Islands, and made them excel in arms, commerce, letters, and philosophy, apply with the happiest effect to the improvement of the cultivation of the earth. Nothing is impossible to labor, aided by ingenuity. The true objects of the agriculturist are likewise those of the patriot. Men value most what they have gained with effort; a just confidence in their own powers results from success; they love their country better, because they have seen it improved by their own talents and industry; and they identify with their interests, the existence of those

* Sir Humphrey Davy, in his first lecture before the Board of Agriculture, delivered in the year 1802, says, 66 Agricultural Chemistry has not yet received a regular and systematic form. It has been pursued by competent experimenters but for a short time only; the doctrines have not as yet been collected into any elementary treatise," &c.

institutions which have afforded them security, independence, and the multiplied enjoyments of civilized life."

CHAPTER III.

GROWTH AND SECRETIONS.

47. THE progress of the growth of a plant, and the annual course of vegetation remain to be considered, but it must be borne in mind, to use the words of Professor Henslow, that " of the precise manner in which the assimilation of nutriment takes place we know nothing, and the first steps towards the formation and development of any organized being are entirely concealed from us." New cells, fibres, and vessels are most undoubtedly formed, or the leaf buds must remain for ever undeveloped, but we are ignorant of the immediate cause, and of the first commencement of the effect: for when we say that the vital action is excited (whether in the growth and nourishment of a plant or an animal), what do we more than state a fact, whose course we may indeed follow when we have once observed it, but whose origin is, in the present state of our knowledge, beyond our reach ?—The course of growth, however, as far as we can trace it, seems to be the following. When a leaf bud begins to be developed, it is seen to be formed of a short axis surrounded by many leafy folds or scales. This axis begins to lengthen; the ascending sap is consumed by the developing leaves, which separate from each other by nearly equal distances, proving that the shoot in

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