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ple, the root of the plant forces itself into the earth, the branches shoot out on each side, the leaves expose their superior surface to the open air, and their inferior surface to the earth, or the inner part of the plant. If a seed be sown the contrary way, the radicle and the little stalk will each bend backward; the former, in order to penetrate into the earth, and the latter to gain the air. If a young stalk be kept inclined, its extremity, notwithstanding, will grow upright. Bend the branches of all sorts of plants; cause the inferior surfaces of their leaves to turn toward the sky; you will soon perceive that all these leaves will resume their former position.
Many great naturalists have supposed a circulation of the sap in plants; but Dr. Hales has demonstrated, that the sap does not circulate, but that it ascends and descends. In order to understand the motion of the sap, according to his principles, it is to be considered, that during the heat of a summer's day, all plants perspire freely from the pores of their leaves and bark. At that time, their juices are highly rarefied. The diameters of the tracheæ, or air-vessels, are enlarged, so as to
press upon and straiten the vessels that carry the sap; in consequence of which, their juices, not being able to escape by the roots, are pressed upward where there is the least resistance, and perspire off the excrementitious parts by the leaves and branches, in the form of vapour. When the solar heat declines, the trachea are contracted, the sap vessels are enlarged, and the sap sinks down in the manner of the spirits of a thermometer. In consequence of this change, the capillary vessels of the leaves and top branches become empty. Being surrounded with the humid vapours of the evening, they fill themselves, from the known laws of attraction, and send down the new ac
more elaborated. As soon as the sun has altered the temperature of the air, the trachea become again distended, and the sap-vessels are straitened. The same cause always produces the same effect; and this alternate ascent and descent through the same system of vessels, continues as long as the plant survives. The irregular motion of the stem and branches is another cause that contributes to the ascent of the sap. Whenever these parts are agitated by the air, they are made to assume a variety of angles, whereby the sap-vessels are suddenly straitened. The contained juices, consequently receive reiterated impulses, similar to what happens to the blood of animals from the contraction of the heart. These observations convey a general idea of the motion of the sap, which varies according to the temperature of the weather, which is seldom the same in any succeeding moment; and, therefore, the sap must sometimes move more quickly, and sometimes slow: it may rise and fall many times in the day, pushed forward by sudden heats, and falling by sudden cold. Thus the juices are blended, and the secretiens forwarded.
An ingenious female naturalist, Mrs. Agnes Ibbetson, has published an interesting series of Essays on Vegetable Physiology in the Philosophical Magazine.' In a few particulars she assigns different functions to certain portions of plants, to those pointed out in these Papers; though in the main she agrees. In her comparison between animal and vegetable life, she proves, that the vegetable frame is a mere muscular creature, having life only, but no sensation; possessing alone the property of the muscles, irritability. The comparison proves also, that as the only part in the human body that possesses selfmotion, independent of all other parts, is the
muscle; so the only part in the plant that possesses the same vis insita, is the spiral wire, which therefore operates as the muscle of the vegetable. (See the Philosophical Magazine for August, 1815.)
THE PROCESS OF NATURE IN THE VEGETATION OF PLANTS.
Nor only through the lenient air this change,
Hos natura modos primum dedit: his genus omne
Thus Nature did ordain,
THE theory of the ascent and descent of the sap in plants, with which I concluded my former paper, is beautifully adopted by Thomson, in a noble apostrophe to the God of Seasons:
Hail, Source of Being! Universal Soul
Of heaven and earth! Essential Presence hail!
By Thee disposed into congenial soils,
By wintry winds; that now in fluent dance,
I shall now consider the very curious and admirable process of nature in the vegetation of plants, which is described to this effect by the excellent Malpighi and others: The seed of the plant, after it has dropped from the pod or husk, may be considered as an impregnated egg, within which the embryo plant is securely lodged. In a few days after it has been committed to the earth, may be observed the rudiments of the future plant, every part of which appears to exist in miniature. The nutricious juices of the soil insinuate themselves between the original particles of the plant, and bring about an extension of its parts. This is called the growth of the vegetable body; and, with regard to this increase by addition and extension, there seems to be a great analogy between the animal and vegetable kingdoms, on which, however, I shall not enlarge.
To illustrate the subject of vegetation, let us take a view of what happens to a bean, after it has been committed to the earth. In a few days, sooner or later, according to the temperature of the weather and the disposition of the soil, the external coverings open at one end, and discover to the naked eye part of the placenta, or body of the grain. This substance consists of two lobes, or divisions, between which the seminal plant is securely lodged. Soon after the opening of the membranes, a sharp-pointed body appears, which is the root. By a kind of principle, which seems to bear some appearance of instinct, it seeks a
passage downward, and fixes itself into the soil. At this period, the root is a smooth and polished body, and has, perhaps, but little power to absorb any thing from the earth, for the nutriment of the germ or sprout. The two lobes now begin to separate, and the germ, with its leaves, may be plainly discovered. As the germ increases in size, the two lobes are further separated, and the tender leaves, being closely joined, push themselves forward in the form of a wedge. These leaves take a contrary direction to the root. They seek a passage upward, which having obtained, they lay aside their wedge-like form, and spread themselves in a horizontal direction, as being the best adapted for receiving the rain and dew. The root, increasing every hour in size and vigour, pushes itself deeper into the earth, from which it now draws some nutritive particles. At the same time, the leaves of the germ, being of a succulent nature, assist the plant, by attracting from the atmosphere such particles as the tender vessels are fit to convey. These particles, however, are of a watery kind, and have not in their own nature a sufficiency of nutriment for the increasing plant. Vegetables, as well as animals, during their tender state, require a large share of balmy nourishment. As soon as an animal is brought into life, the milk of its mother is supplied in a liberal stream; while the tender germ seems to have only the crude and watery juices of the earth for its support. This, however, is not the case: for the vegetable lives upon a similar fluid, although differently supplied. For its use, the farinaceous lobes are melted down into a milky juice, which, as long as it lasts, is conveyed to the tender plant by innumerable small vessels, which are spread through the substance of the lobes. These vessels, uniting into one common trunk, enter the body of