28

THE NATURE OF THEIR FOOD.

The natural food of plants consists of carbon in the state of carbonic acid, of nitrogen, certain earths and salts, and water. The latter, if distilled, has little power, by itself, of sustaining vegetable life but, as in nature it is universally mixed with various other substances, it conveys to the roots the nutritious matters that are required; and it furnishes, by its decomposition, a considerable supply of the oxygen consumed in the formation of carbonic acid, as well as much of the hydrogen that is assimilated by plants. It has been proved, experimentally, that plants cannot long exist upon pure water; but, if they are so circumstanced as to be able to obtain and decompose carbonic acid, they will grow in the absence of other matters. It is only, however, when the peculiar principles, whether earthy or saline, on which they naturally feed, are presented to them, that they become perfectly healthy: and especially when they have the means of obtaining nitrogen, which appears, from its great abundance in the youngest parts, to be indispensable to plants upon the first formation of their tissue.

The researches of chemists have shown that all rain-water contains ammonia, a compound of hydrogen and nitrogen, and thus the source of the nitrogen absorbed by plants was explained. But it has also been shown, especially by M. Barral, that other substances, upon which plants feed, are contained in rain-water to a much greater amount than was suspected. This observer was led, during six months of 1851, to examine minutely the water collected in the rain-gauges of the Observatory at Paris. His mode of investigation is declared by Messrs. Dumas, Boussingault, Gasparin, Regnault, and Arago, names foremost in French Science, to be free from all objection, and to bear the most severe counter trials to which they could expose it. M. Barral states, that although the quantities of the following substances varied in different months, yet the monthly average, from July to December inclusive, was as follows:

ROOTS FEED ON WATER, ETC.

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He did not ascertain whether all these substances are contained in rain-water collected at a distance from towns. But Dr. Bence Jones found at least nitric acid in rain-water collected in London, at Kingston in Surrey, at Melbury in Dorsetshire, and far from any town at Clonakelty in Ireland. If we assume that M. Barral's averages represent what occurs on an English acre, the quantity of such substances deposited on that extent of ground may be safely estimated as follows:

The average depth of rain which falls in the neighbourhood of London is well ascertained to be about twenty-four inches per annum. This is at the rate of 87,120 cubic feet, or 2466 cubic metres of rainwater per acre; and this, according to the proportions per cubic metre in the preceding table, would afford annually of—

Of these substances the three first are of the utmost importance, on account of their entering so largely into the indispensable constituents of the food by which vegetable life is sustained. The quantity of ammonia thus ascertained to exist is about what is expected in two hundred weight of Peruvian guano; and bountiful nature gives us, moreover, nearly one hundred and fifty pounds of nitrogenous matter, equally suited to the nutrition of our crops.

It has been confidently asserted that in addition to their feeding properties, roots are the organs by which plants rid themselves of the secreted matter which is either superfluous or deleterious to them. If you place a plant of Succory in water, it will be found that the roots will, by degrees, render the water bitter, as if opium had been mixed with it; a Spurge will render it acrid; and a leguminous plant mucilaginous. And, if you poison one half of the roots of any plant, the other half will throw the poison off again from the system. Hence it has been thought to follow that, if roots are so circumstanced that they cannot constantly advance into fresh soil, they will by degrees be surrounded by their own excrementitious secretions. More correct experiments have however shown

30

THEIR SUPPOSED EXCRETIONS.

that such results are only obtained when roots are lacerated, and that they have no greater power of excreting matter than other parts of the surface of a plant.

This theory of root excretions was sustained by Liebig, who regarded excretion as the necessary result of secretion. It is now abandoned. A correspondent of the Gardeners' Chronicle rightly observes, in answer to the question of what becomes of the inorganic matters which plants are constantly taking up? that in many instances, when taken up in large quantities, they are deposited in the tissues themselves so profusely as to obstruct the functions necessary to the life of the plant, and death is the consequence. Where these inorganic substances are not taken up in sufficient quantities to destroy the life of the plant, they are deposited in the tissues of the plant, either externally or internally, or both, according to its structure. Plants growing on the sea-shore, as Salsola and others, when exposed to the absorption of large quantities of sea-water, deposit in great abundance crystals of chloride of sodium in their tissues and upon their epidermis. He has examined Charas growing in pools, where the waters, from the presence of carbonic acid, hold in solution great quantities of carbonate of lime, and he has found this salt filling their large intercellular cavities, and forming a crop of beautiful crystals on their epidermis, whilst those of the same species, growing in ponds with a less quantity of carbonate of lime, have exhibited a comparative paucity of crystals. The colouring of wood, also, by introducing solutions of the metallic oxides into trees, is a good illustration of the mode in which superfluous inorganic matters are disposed of in the tissues of a plant.

As to the excretion of organic matter, there is no need to limit that function to roots, for nature assigns it to all parts of the surface, stems, leaves, flowers, and fruit, as is seen by such familiar facts as honeydew, glandular discharges like those of the Sweet-Briar Rose, nectarial emissions, &c.

In general, roots have no buds, and are, therefore, incapable of multiplying the plant to which they belong. But it constantly happens, in some species, that they have the power of forming what are called adventitious buds; and, in such cases, they may be employed for purposes of propagation. There is no rule by which the power of a plant to generate such buds by its roots can be judged of; experiment is therefore necessary, in all cases, to determine the point.

Exceptions to the common rule are found in the Moutan Pæony, in the Plum tree, or the Pyrus (Cydonia) japonica, which may be

ROOTS CAN GENERATE BUDS.

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increased with great facility by small bits of the root being inserted in a shady border and covered with a hand-glass; but in none of them does the power reside in the same degree as in the Japan Anemone. If a root of this plant be taken from the ground after flowering, it will be found to resemble brown cord, divided into a great number of ramifications, as is represented in the accompanying cut. Upon its surface will be perceived a multitude of white conical projections, sometimes growing singly, sometimes springing up in clusters, and

a

Fig. VI.-Root of Anemone japonica.

occasionally producing scales upon their sides. A magnified view of these bodies is shown at Fig. VI. a. They are young buds, every one of which, if cut from its parent, will grow and form a young plant in a few weeks. These buds are not confined to the main trunk of the root, but extend even towards its extremities; so that every fragment of the plant is reproductive. It is certain that vitality is stronger in the roots than in any other part of a plant. Live roots have been found in land many years after the trunks to which they belonged had been destroyed. I have myself seen live Whitethorn roots taken out of a field on the London clay where no one could recollect having seen a Whitethorn hedge. This fact was long since pointed out by Mr. Knight, who, in his experiments with fruit-trees, found continual evidence, as he has

32

ROOTS CAN GENERATE BUDS.

stated, of the roots of such trees possessing far more constitutional vigour than the branches. See his Physiological Papers, pp. 83, 325.

The following is another instance of the kind:-On the banks of the river Derwent stood a large Hawthorn hedge, which, being undermined by the water, fell in, and left the greater part of the roots in the bank, about one or two feet below the surface; the bank still wearing away has exposed them to the air for the length of three feet or more, and they are now in every respect similar to branches, developing buds, and consequently all the appendages of the axis; they appear anatomically the same as branches, excepting the pith, of which they are destitute. Now, it appears that roots when so circumstanced perform all the functions of the stem, confirming Knight's theory, that sap can at any time generate buds, without any previously-formed rudiment, when circumstances are favourable to their production.