SECT. III.

Theory of Vegetation.

Vegetables may be regarded as the intermediate link in the great chain of creation, between animals and minerals. The latter grow by mere chymical affinity, and by additions, sometimes analogous and sometimes foreign from their own nature; while plants, like animals, have an organization that enables them to receive their food, digest and assimilate it to their own substance, reproduce their species, and maintain an existence of longer or shorter duration. Thus far the learned are agreed, but at the next step they differ.

What is this food that gives to plants their developement, and maturity, and powers of reproduction? Lord Bacon believed that water was the source of vegetable life, and that the earth was merely its home, its habitation, serving to keep plants upright, and to guard them against the extremes of heat and cold. Tull, on the other hand, (and after him Du Hamel) pronounced pulverized earth the only pabulum of plants, and on this opinion built his system of husbandry. Van Helmont and Boyle opposed this doctrine by experiments: the former planted and reared a cutting of willow in a bed of dry earth, carefully weighed and protected against accretion by a tin plate, so perforated as to admit only rain and distilled water, with which it was occasionally moistened. At the end of five years the plant was found to have increased one hundred and sixty-four pounds, and the bed of earth to have lost, of its original weight, only two ounces. Boyle pursued a similar process with gourds, and with a similar result. Notwithstanding the apparent conclusiveness of these experiments, their authority was shaken, if not subverted, by others made by Margraff, Bergman, Hales, Kirwan, &c. &c. The first of these showed, that the rain water employed by Van Helmont, was itself charged with saline and other earthy matter; Bergman demonstrated this by analysis, while Kirwan and Hales proved that the earth in which the willow cutting was planted, could absorb these matters through the pores of the wooden box which contained it, and that a glass case could alone have prevented such absorption. Hunter, finding that oil and salt entered into the composition of plants, concluded that these formed their principal food, and accordingly recommended, as the great desideratum in agriculture, an oil compost. Lord Kaimes attempted to revive the expiring creed of Lord Bacon, but finding from Hales' statics, that one third of the weight of a green pea was made up of carbonic acid, he added air to the watery aliment of the English philosopher—but entirely rejected oil and earth, as too gross to enter the mouths of plants, and salt as too acrid to afford them nourishment. Quackery, which at one time or other, has made its way into all arts and sciences, could not easily be excluded from agriculture. Hence it was, that the Abbe de Valemont's prolific liquor, and De Hare's and De Vallier's powders, &c. &c. were believed to be all that was necessary to vegetation, and found the more advocates, as they promised much and cost little. But before the march of modern chymistry, quackery could not long maintain itself; and from the labours of Bennet, Priestly, Saussure, Ingenhouz, Sennebier, Schæder, Chaptal, Davy, &c. &c. few doubts remain on this important subject.—These will be presented in the course of the following inquiry.

1st. Of earths, and their relation to vegetation.

Of six or eight substances, which chymists have denominated earths, four are widely and abundantly diffused, and form the crust of our globe. These are silica, alumina, lime, and magnesia.—The first is the basis of quartz, sand and gravel; the second, of clay; the third, of bones, river and marine shells, alabaster, marble, limestone and chalk; and the fourth, of that medicinal article known by the name of calcined magnesia.—In a pure or isolated state,[8] these earths are wholly unproductive; but when decomposed and mixed,[9] and to this mixture is added the residuum of dead animal or vegetable matter,[10] they become fertile, take the general name of soils, and are again specially denominated, after the earth that most abounds in their compositions respectively. If this be silica, they are called sandy; if alumina, argillaceous; if lime, calcareous; and if magnesia, magnesian. Their properties are well known: a sandy soil is loose, easily moved, little retentive of moisture, and subject to extreme dryness; an argillaceous soil is hard and compact when dry, tough and paste-like when wet, greedy and tenacious of moisture; turns up, when ploughed, into massive clods, and admits the entrance of roots with great difficulty. A calcareous soil is dry, friable and porous; water enters and leaves it with facility; roots penetrate it without difficulty, and (being already greatly divided) less labour is necessary for it than for clay. Magnesian, like calcareous earth, is light, porous and friable; but, like clay when wet, takes the consistency of paste, and is very tenaceous of water. It refuses to combine with oxygen, or with the alkalies; is generally found associated with granite, gneiss, and schiste, and is probably among the causes of their comparative barrenness.[11]

In these qualities are found the mechanical relations between earths and vegetables. To the divisibility of the former it is owing, that the latter are enabled to push their roots into the earth; to their density, that plants maintain themselves in an erect posture, rise into the air, and resist the action of the winds and rains; and to their power of absorbing and holding water, the advantage of a prolonged application of moisture, necessary or useful to vegetable life. But besides performing these important offices, there is reason to believe that they contribute to the food of vegetables. This opinion rests on the following considerations and experiments:

1. If earths do not contribute directly to the food of plants, then would be all soils alike productive; or in other words, if air and water exclusively supply this food, then would a soil of pure sand be as productive as one of the richest alluvion.

2. Though plants may be made to grow in pounded glass, or in metallic oxides, yet is the growth, in these, neither healthy nor vigorous; and,

3. All plants, on analysis, yield an earthy product;[12] and this product is found to partake most of the earth that predominates in the soil producing the analyzed plant; if silica be the dominant earth, then is the product obtained from the plant silicious; if lime prevail, then is the product calcareous, &c. &c. This important fact is proved by De Saussure.

1st Experiment.

Two plants (the pinus abies) were selected, the one from a calcareous, the other from a granitic soil, the ashes of which gave the following products;

GraniticCalcareous
soil.soil.
Potash3 6015
Alk. and mu. sul4 2415
Carbonate of lime46 3463
Carbonate of magnesia6 7700
Silica13 4900
Alumina14 8616
Metallic oxides10 5200

2d Experiment.

Two Rhododendrons were taken, one from the calcareous soil of Mount de la Salle, the other from the granitic soils of Mount Bevern. Of a hundred parts, the former gave fifty-seven of carbonate of lime and five of silica; the latter, thirty of carbonate of lime, and fourteen of silica.

3d Experiment.

This was made to determine whether vegetables, the product of a soil having in it no silica, would, notwithstanding, partake of that earth.—Plants were accordingly taken from Reculey de Thoiry, (a soil altogether calcareous) and the result was a very small portion of silica.

These experiments, says Chaptal, leave little if any doubt, but that vegetables derive the earthy matter they contain from the soil in which they grow.[13]

2. Of water, as an agent in vegetation.

Seeds placed in the earth, and in a temperature above the freezing point, and watered, will develope; that is, their lobes[14] will swell, their roots descend into the earth, and their stems rise into the air. But without humidity, they will not germinate; or deprived of humidity after germinating, they will perish. When germination is complete, and the plant formed, its roots and leaves are so organized as to absorb water. The experiments of Hales prove, that the weight of plants is increased in wet and diminished in dry weather; and that in the latter, they draw from the atmosphere (by means of their leaves)[15] the moisture necessary to their well being.—Du Hamel (and after him Sennebier) has shown, that the filaments that surround the roots of plants, and which has been called their hair, perform for them in the earth, the office that leaves perform in the atmosphere, and that if deprived of these filaments the plants die.

It would be easy, but useless, to multiply facts of this kind tending to establish a doctrine not contested, but which after all does not assert, that water makes part of the food of plants. On this point two opinions exist—the one, that this liquid is a solvent and conductor of alimentary juices: the other, that is itself an aliment and purveyor of vegetable food at the same time. The first opinion is abundantly established. Water when charged with oxygen, supplies to germinating seeds the want of atmospheric air, and saturated with animal or vegetable matter in a state of decomposition, or slightly impregnated with carbonic acid, very perceptibly quickens and invigorates vegetation. The second opinion is favoured by some of De Saussure's experiments. On these, Chaptal makes the following remark, which expresses very distinctly an approbation of the doctrine they suggest:—"The enormous quantity of hydrogen (which makes so large a part of vegetable matter) cannot be accounted for but by admitting (in the process of vegetation) the decomposition of water, of which hydrogen is the principal constituent; and that though there is nothing in the present state of our experience that directly establishes this doctrine, yet that its truth ought to be presumed, from the analysis of plants and the necessary and well-known action of water on vegetation.

(To be continued.)

Correction.—In copying the second section, page 55, an error escaped in relation to the Tuscan plough; the passage should have read thus—"The plough of the north of Europe, like that of this country, has the power of a wedge, and acts horizontally—that that of Tuscany has the same direction, but very different form. With the outline of a shovel, it consists of two inclined planes, sloping from the centre, and forms a gutter and two ridges.