that the thorns of plants are abortive branches; the scales of buds have, in like manner, been regarded as transformed leaves; and it has lately been conjectured by M. de Candolle, that their petals are not special organs, but stamens in an abortive or transformed state.* Plants are also possessed of cutaneous secernents or perspiratory vessels; and in many plants the quantity of fluid thrown off by this emunctory is very considerable. Keill, by a very acurate set of experiments, ascertained that in his own person he perspired 31 ounces in twenty-four hours. Hales, by experiments equally accurate, determined that a sun-flower, of the weight of three pounds only, throws off 22 ounces in the same period of time, or nearly half its own weight. To support this enormous expenditure, it is necessary that plants should be supplied with a much larger proportion of nutriment than animals; and such is actually the fact. Keill ate and drank 4 lb. 10 oz. in the twenty-four hours. Seventeen times more nourishment was taken in from the roots of the sun-flower than was taken in by the man. Plants, nevertheless, do not appear to have the smallest basis for sensation, admitting that sensation is the result of a nervous system; and we are not acquainted with any other source from which it can proceed: notwithstanding that Percival and Darwin, as already observed, have not only endowed them with sensation, but with consciousness also; and the latter, indeed, with a brain, and the various * Mém. de la Société d'Arcueil, tom. iii. passions and some of the senses to which this organ gives birth.* Yet, though the vessels of plants do not appear to possess any muscular fibres, we have evident proofs of the existence of a contractile and irritable power from some other principle; and a variety of facts concur in making it highly probable that it is by the exercise of such a principle that the different fluids are propelled through their respective vessels : nor is there any other method by which such propulsion can be reasonably accounted for. Grew ascribed the ascent of the sap to its levity, as though acting with the force of a vapour; Malpighi, to an alternate contraction and dilatation of the air contained in what he erroneously conceived to be air-vessels; Perrault to fermentation; Hales and Tournefort, to capillary attraction: not one of which theories, however, will better explain the fact than another, as Dr. Thomson has ably established; as he has also the probability of a contractile power in the different sets of vessels distributed so wonderfully over the vegetable frame.+ That a contractile power may exist independently of muscular fibres, we have abundant proofs even in the animal system itself. We see it in the human cutis or skin, which, though totally destitute of such fibres, is almost for ever contracting or relaxing upon the application of a variety of other powers; powers external and internal, and totally different in their mode of operation. Thus, austere preparations and severe degrees of cold contract it very sensibly; * Willdenow, Princip. of Botany, § 226. : heat, on the contrary, and oleaginous preparations, as sensibly relax it. The passions of the mind exercise a still more powerful effect over it for while it becomes corrugated by fear and horror, it is smoothed and lubricated by pleasure, and violently agitated and convulsed by rage or anger. Yet, could it even be proved that the vessels of plants are incapable of being made to contract by any power whatever, still we should have no great difficulty in conceiving a circulatory system in animals or vegetables without any such cause, while we reflect that one half of the circulation of the blood in man himself is accomplished without such a contrivance; and this, too, the more difficult half, since the veins, through the greater extent of their course, have to oppose the attraction of gravitation, instead of being able to take advantage of it. It is in the present day, however, a well-known fact, and has been sufficiently ascertained by the late Dr. Parry of Bath, and on the Continent by Professor Dollinger, that the contractile power of the muscular fibres is not called into action even by the arteries in the course of the ordinary circulation of the blood, since, as we shall have occasion to observe, no increase of size or change of bulk of any kind takes place in arteries either in the contraction or dilatation of the heart's ventricles in a state of health, unless where they are pressed upon by the finger or some other cause of resistance. In what part of a plant the vital principle chiefly exists, or to what quarter it retires during the winter, we know not; but we are just as ignorant in respect to animal life. In both it operates towards every point: it consists in the whole, and resides in the whole; and its proof of existence is drawn from its exercising almost every one of its functions and effecting its combinations in direct opposition to the laws of chemical affinity, which would otherwise as much control it as they control the mineral world, and which constantly assume an authority as soon as ever the vegetable is dead. Hence the plant thrives and increases in its bulk; puts forth annually a new progeny of buds, and becomes clothed with a beautiful foliage of lungs (every leaf being a distinct lung in itself *) for the respiration of the rising brood; and, with an harmonious circle of action, that can never be too much admired, furnishes a perpetual supply of nutriment, in every diversified form, for the growth and perfection of animal life; while it receives in rich abundance, from the waste and diminution, and even decomposition, of the same, the means of new births, new buds, and new harvests. In fine, every thing is formed for every thing; and subsists by the kind intercourse of giving and receiving benefits. The electric fire, that so alarms us by its thunder, and by the awful effects of its flash, purifies the stagnant atmosphere above us; and fuses, when it rushes beneath us, a thousand mineral veins into metals of incalculable utility. New islands are perpetually rising from the unfathomable gulfs of the ocean, and enlarging the boundaries of organised life; sometimes thrown up all of a sudden by the dread agency of volcanoes, * On the leafing of trees, there is a curious and valuable paper in the Swedish Amœnitates Academicæ, vol. iii, art. 46. by H. Barck, 1753, entitled Vernatio Arborum. . and sometimes reared imperceptibly by the busy efforts of corals and madrepores. Liverworts and mosses first cover the bare and rugged surface, when not a vegetable of any other kind is capable of subsisting there. They flourish, bear fruit, and decay, and the mould they produce forms an appropriate bed for higher orders of plant-seeds, which are floating on the wings of the breeze, or swimming on the billows of the deep. Birds next alight on the new-formed rock, and sow, with interest, the seeds of the berries, or the eggs of the worms and insects on which they have fed, and which pass through them without injury; and an occasional swell of the sea floats into the rising island a mixed mass of sand, shells, drifted sea-weed, skins of the casuarina, and shells of the cocoa-nut. Thus the vegetable mould becomes enriched with animal materials; and the whole surface is progressively covered with herbage, shaded by forests of cocoa and other trees, and rendered a proper habitation for man and the domestic animals that attend upon him. The tide that makes a desolating inroad on one side of a coast, throws up vast masses of sand on the opposite the lygeum, or sea-mat weed, that will grow on no other soil, thrives here and fixes it, and prevents it from being washed back or blown away; to which the lime-grass *, couch-grass +, sandreed‡, and various species of willow, lend their aid. Thus fresh lands are formed, fresh banks upraised, and the boisterous sea repelled by its own agency. *Elymus arenarius. Arundo arenaria. + Triticum repens. |