Page images
PDF

position. The direct rays of the sun are necessary to the effect: no other light, however strong, will suffice. The course of the phenomena connected with the respiration of plants appears to be the following. The water which enters the plant by the roots contains carbonic acid, which is carried with it into the green parts; it is there decomposed under the influence of the sun's rays—the carbon is fixed in the plant, and the oxygen escapes. The carbonic acid which is formed from the oxygen of the air, in all those portions of the plant which are not green, is partly dispersed in the atmosphere, partly dissolved in water, which water at last reaches the plant again, and thus is ultimately absorbed by the roots, drawn up to the leafy parts and there decomposed. The water taken up by the roots holds, besides its carbonic acid, a certain quantity of soluble matter containing carbon: this carbon is also carried with the sap into the green parts; it combines during the night with the oxygen which had been previously absorbed by them, and the following day such of this carbonic acid thus formed in the leaves as has not been given out during the night, is decomposed by the solar light, as if the carbon could not be usefully deposited in the nutritive juices unless it proceed from the decomposition of carbonic acid gas. Thus the whole of this important function, i.e., vegetable respiration, appears to have for its object the fixing carbon in the plant, while the result of animal respiration is to diminish its quantity in the body, or, in other words, to supply animal heat by its combustion.* It is well remarked by Mr. Hunt, that “the animal kingdom is constantly producing carbonic acid, water in the state of vapor, nitrogen, and, in combina

* See Introduction to Practical Organic Chemistry, p. 61.

tion with hydrogen, ammonia. The vegetable kingdom, continually consumes ammonia, nitrogen, water, and carbonic acid. The one is constantly pouring into the air what the other is as constantly drawing from it, and thus is the equilibrium of the elements

maintained.

“Plants may be regarded as compounds of carbon, vapor, oxygen, hydrogen, and nitrogen gases, consolidated by the all-powerful, all-pervading influences of the solar ray; and all these elements are the produce of the living animal, the conditions of whose existence are also greatly under the influence of those beams, which are poured in unceasing flow from the centre of our system. Can anything more completely display a system of the loftiest design and most perfect order than these phenomena?”

44. It has been shown that the watery juices, pumped up as it were, by the roots, have been drawn to the leafy parts; a large part of the water is there evaporated, green matter is formed, and the decomposition of carbonic acid, ammonia, and water, fixes carbon, nitrogen, and hydrogen in the residuum. From these changes, to which the term assimilation has been given, results the formation of a new and descending juice whose existence is perhaps less palpable than that of the ascending sap, but concerning which there can be no doubt.. If a circular incision be made in the bark of an exogenous tree, a tumor will in a short time appear above the wound; this tumor increases, and if the cut be very narrow, it soon reaches the lower lip of the wound, the communication is restored, and the tree lives on as usual, but if the wound be too wide to admit of this junction, the tumor continues to increase till the branch

* Researches on Light, p. 200.

(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 everywhere 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

« PreviousContinue »