« AnteriorContinuar »
best “ give God thanks, and make no boast of it ; and as for their writing, let that appear when there is no need of such vanity.”
Professor Draper's large and handsome quarto volume is a work of higher pretensions. It is addressed to the learned, and purports to contain important discoveries in the domain of science.
“ The time has now arrived,” he announces in his Preface, “ when both vegetable and animal physiology are to have their foundations laid on chemistry and natural philosophy, the only basis which can elevate them from their present deplorable position to that of true sciences. It is with this impression, that the explanations which I have given in this book, of the mode by which light acts in determining organization, and of the mechanical causes by which such organized matter is transmitted from point to point of living systems, (for these are the leading facts which this work is designed to illustrate,) are offered to the attention of chemical philosophers."
The italics are our own. The preface bears the date of October 1, 1944. It is, of course, incumbent
upon us to give our readers some general account of these discoveries. The problem which Dr. Draper has undertaken to investigate is a very interesting and fundamental one ; and towards its elucidation much has already been done, and more may be expected from chemical and physical research. So imposing a volume of original researches demands our special attention. But as our author, writing much in the manner of a popular lecturer, and taking nothing for granted, although ostensibly addressing “ chemical philosophers,” seldom distinguishes between that which is to be deemed original, and that which mere sucklings in science might be supposed to know, it may be worth while, in the first place, to favor our unlearned readers with a sketch of those general conclusions, which the vegetable physiologists and chemists had already obtained. This we shall attempt to do, after our own fashion, taking the vegetable kingdom as our startingpoint, and pursuing the course which, in our view, leads to the most natural and clear exposition of the general phenomena and results of vegetation, considered under a chemico-physiological aspect.
Of what, it may be asked, do plants essentially consist ? Whence do they obtain the matter which enters into their
composition, that is, their food? And what changes does the plant impress upon the materials it feeds on? The answer to these questions brings to view the reciprocal relations and influences of the mineral, vegetable, and animal kingdoms, and enables us to comprehend the office of plants in the general economy of the world.
The food and the elementary composition of plants stand in a necessary relation to each other. Since it is absurd to suppose that vegetables possess the power of creating any element, whatever they contain must have been derived from without. Their composition, therefore, indicates their food. If we have learned the chemical composition of a vegetable, and also what it gives back to the soil and the air, we know what it must have derived from without ; that is, its food. Or, if we have ascertained what the plant takes from the soil and air, and what it returns to them, we have learned its chemical composition, — the difference between these two. And when we compare the nature and condition of the materials which the plant takes from the soil and the air with what it gives back to them, we may comprehend the influences of vegetation upon the mineral kingdom.
If we consider, then, the materials of which plants are composed, we shall learn what their food must necessarily contain. These materials are of two kinds ; the difference between which may readily be made apparent. Every one knows that water, that universal solvent, as it percolates the soil, dissolves a portion of the various earthy matters it meets with. These, being drawn into the plant with the water which the roots imbibe, are at length deposited either in the wood or the leaves, from which the greater part of the water at length escapes by evaporation ; they form the ashes which are left on burning a leaf or a piece of wood. Now, although it is true, that these earthy or mineral matters are often turned to account by the plant, and although some of them are necessary in the formation of certain products of certain plants, — as, the finty matter which gives needful firmness to the stalks of wheat, and the phosphate of magnesia, the iron, &c., which always exist in the grain, yet none of them are essential to simple vegetation, which may, and often does, proceed as well without them. These materials, whose presence is in some sort accidental, may be called the earthy, mineral, or inorganic constituents of plants.
But if we analyze the newly formed tissue of plants, or any vegetable substance, such as a piece of wood, after this incrusting mineral matter has been chemically removed, we find it invariably composed of either three or four elements ; elements which are indispensable to vegetation, and which make up, at least, from eighty-eight to ninety-nine per cent. of every vegetable substance. These are the universal, organic constituents of plants.
These we now consider, leaving the mineral materials entirely out of view for the present. The four essential organic elements of plants are carbon, hydrogen, oxygen, and nitrogen. The proper vegetable structure, that is, the substance of the cells and vessels, uniformly consists of only three of these simple elements, namely, carbon, hydrogen, and oxygen. These are absolutely essential and universal ; while the fourth, nitrogen, though perhaps always present, is only requisite in the formation of certain very important products.
These four elements must be furnished by the food upon which the vegetable lives; they must be drawn from the soil or the air. Since the leaves, as ordinarily situated, can imbibe air or vapor only, and the hygrometric tissue of the rootlets is specially adapted to the absorption of liquids, refusing admission to solid particles, the food of the plant must be received in the gaseous or liquid form, chiefly in the latter. But in whatever form imbibed, the great vehicle of the plant's nourishment is evidently water, which as a liquid bathes its roots, and in the state of vapor continually surrounds its leaves. The question, therefore, naturally arises, Is water itself the food of plants ?
Water is composed of oxygen and hydrogen. It may, therefore, furnish two of the plant's essential elements. It must furnish the principal part of the hydrogen; for this element does not exist, in a separate state, either in the soil or in the air, nor in any form, (with an exception that may be left out of the general view,) unless as one of the components of water. And reasons, which will hereafter become apparent, also show that nearly all the oxygen which actually enters into the composition of the plant must be derived from the water which it so freely imbibes.
But water can never furnish what it does not itself contain. It has no carbon in its composition, no nitrogen.
Therefore, pure water, though indispensable, is insufficient for the nourishment of plants. Yet if the water, which so copiously filtrates through the leafy plant, actually contains even a minute quantity of the two remaining ingredients, carbon and nitrogen, in such a form that they may be detained in the plant when the superfluous water is exhaled, abundant provision for the food of the vegetable would thereby be afforded ; for, however diluted, the plant may condense and accumulate it to the requisite extent, just as the extremely minute portion of earthy matter, which the water holds, accumulates in the leaves and wood. The question, then, recurs in this form : Does the water, which the plant actually imbibes, contain a quantity of these remaining elements ? Though pure water cannot, yet may not rain-water furnish the needful carbon and nitrogen ?
Nearly seventy-nine per cent. of the atmosphere consists of nitrogen gas, in an uncombined state ; that is, merely mingled with oxygen. Now, as this gas is to a certain extent absorbed by water, every rain-drop that falls through the air must bring with it a minute portion of nitrogen ; and this is necessarily imbibed with the water which the roots take in. Accordingly, free nitrogen gas is always found in growing plants. There is, besides, a compound of nitrogen, which is always formed when any animal, and almost any vegetable, substance decays, and which is found to be especially adapted to the nourishment of plants, namely, ammonia, or hartshorn, - a compound of hydrogen and nitrogen. We know that ammonia exists in the air, though in quantities so minute as to be inappreciable to our senses ; since, being very volatile, it must continually rise from decaying animal and vegetable substances. We readily apprehend why it does not accumulate, in the progress of time, so as to become perceptible ; for it is extremely soluble in water, as well as all its compounds, and is therefore greedily absorbed by aqueous vapor, and thus washed out of the atmosphere, as it were, and brought down to the surface by rain. The chemists can accordingly detect its presence in all rain-water. In this form, as ammonia and its volatile compounds, nitrogen must continually be presented to the roots of plants. That the roots actually absorb it may be inferred from the familiar fact, that plants grow most luxuriantly when the soil is supplied with those substances which yield much ammonia,
such as most manures. And finally, ammonia, as such, may be detected in the juices of almost all plants.
Rain-water, therefore, contains the third element of vegetation, nitrogen, — drawn from the air, both in a separate form, and in that of ammonia. But what is the source of the remaining constituent, carbon, of which plants must have a copious supply, since it forms by far the largest portion of their bulk ? If we obtain the carbon of a leaf or a piece of wood, separate from the other organic elements, - which is done by charring, that is, by heating it out of contact with the air, so as to drive off the oxygen, hydrogen, and nitrogen, — although we necessarily lose a part of the carbon in the operation, yet what is left perfectly preserves the shape and bulk of the original material, even of its most delicate cells and vessels. From forty to sixty per cent., by weight, of the original body remains in the form of carbon or charcoal.
Now, all the other elements of plants are gases in their natural state, actually existing in the air, from which they are drawn and presented to the roots in the form of rain-water ; while carbon is a solid, absolutely insoluble in water. In what form can it be employed by the plant, which imbibes fluids only? The chief, if not the only, form in which a fluid compound of carbon is naturally presented to the plant is that of carbonic acid gas; which consists of carbon and oxygen. This gas nakes up, on an average, one two-thousandth of the bulk of the atmosphere ; from which it may be directly absorbed by the leaves. But it is freely soluble in water, up to a given extent, and so must be carried down by the rain, and imbibed by the roots. The carbonic acid of the atmosphere is clearly the great source of carbon for vegetation.
The mineral world is made up of a large number of elementary bodies, and their compounds are multiplied almost to infinity ; but only three or four of these are concerned in the general phenomena of vegetation. The air, water, and carbonic acid are the few materials upon which organic nature universally operates. The atmosphere that surrounds us — considering water in the state of vapor to form a component part of it - contains all the essential elements of plants, and contains them in the best, if not the only available, form ; namely, in a Auid state. It furnishes water, which