Roots are either annual, lasting one year, as the Poppy, Barley, &c.; biennial, when produced in one year, and flowering the next, as Wheat, Canterbury Bell, &c.; or perennial, when they last many years, as the Rose, Trees, &c. There are various kinds of roots, some are bulbous, as the Tulip and Onion; tuberose, as the Potatoe and Turnip; fusiform, as the Carrot and Radish; or fibrous, as in Trees and Grasses. The root generally consists of two parts; the body and the fibres: the latter is the part which imbibes nourishment from the carth for the support of the plant. The seed of a plant committed to the ground, swells by the moisture it imbibes, and, in a few days, throws out two shoots; the first strikes downward into the soil, and forms the root or radical; and the other forces its way into the air. As soon as the young plant feeds from the soil, it requires the assistance of leaves, which are the organs by which the plant throws off its superabundant fluid. Vegetation is then essentially injured by destroying the leaves of a plant. It not only diminishes the transpiration, but also the absorption by the roots; for the quantity of sap absorbed is always in proportion to the quantity of fluid thrown off by transpiration.

The Trunk, or Stem, is that part of a plant which produces the leaves and flowers, and serves to elevate them above the ground. It consists, 1. Of the Epidermis, cuticle, or exterior thin membranous covering, furnished with pores, which transmit or throw off the excretory products of vegetation; answering to the skin of animals. 2. The Cortex, or outer bark, consists of vesicles and utricles, (small membranous cavities or cells to receive the sap,) so very numerous, and close together, as to form a continued coating. It is among these glands that the work of digestion appears to be performed; and the product of this elaboration is afterwards conveyed through the whole vegetable, by vessels propagated through all its parts; these conduits even passing through the body of the tree, crossing the ligneous strata. In this net-work it is that the colouring matter of vegetables is decomposed: the light which penetrates the Epidermis concurs in enlivening the colour:

the decomposition of water and the carbonic acid. 3. The Liber, or inner bark, is the part in which the vital principle of a plant is chiefly seated; its parts are easily detached from each other; their laminæ, (fibrous scales or layers,) are not extended lengthwise along the stem, but are curved in various directions; and leave openings, or meshes between them, which are filled by the cellular matter itself, and it is from their resemblance to the leaves of a book, that they have been called liber. It is this part of the bark on which the ancients wrote, before the invention of parchment, or paper. In proportion as these coatings approach the ligneous body, or wood, they become hard; and at length form the external softer part of the wood, which workmen call the sap. The wood or ligneous part of the plant, is a compact fibrous substance, formed by new layers, which are added, every year, from the innermost part of the bark; so that the age of a tree, or shrub, may be ascertained by the number of ligneous circles which appear upon cutting the stem near the root.

The bark is the most essential part of the vegetable, by means of which the principal functions of life, such as nutrition, digesting the secretions, &c. are performed. The bark of a tree is only a congeries of the roots of the individual buds of the plant. These roots spread themselves over the last year's bark, making a new bark over the old one, and thence descending, cover with a new bark the old roots also.

The wood, or ligneous part, is not essential, many plants being without it; such as grasses, reeds, and all plants that are hollow within.

The hollow Oak-trees, and Willows, are often seen with the whole wood decayed and gone, and yet the few remaining branches flourish with vigour.

Grasses, properly speaking, have only the cortical part. The thin outside cover of the bark is of great consequence to them: it is of great strength, and appears to be constituted of a sort of glassy net-work, which is chiefly siliceous earth, as has been lately ascertained. This is the case in the Wheat, Oat, and in different plants.

The Stalks are those parts which branch out from the

B

straw in grasses; the flower stalks, leaf stalks, &c.

The Pith is a tolerably firm juicy substance, which is diffused through the inner part of the stalk, to give energy and vigour to the whole; it is abundant in young plants, diminishes as they grow up, and at length totally disappears.

The Sap is the fluid which nourishes the plant. The warmth of the spring dilates the vessels of plants, producing a kind of vacuum, into which the sap rises; but, when the cold weather returns, the fibres and vessels contract, the sap is forced down into the root; the leaves wither, and are no longer able to perform their offices of transpiration; the secretions stop, the roots cease to absorb sap from the soil, and if the plant be annual, its life then terminates; if not, it remains in a state of torpor during the winter. The basis of this juice, which the roots suck up from the soil, is water.

Heat promotes vegetation; it excites the activity of plants; it increases the disposition of some of their constituent parts for new attraction and combination, to obtain such substances as may be requisite and proper for new growth; it likewise causes them to reject such matters as would be hurtful to them; it hastens the dissolution or digestion, the formation and secretion of their different products. It enables them to dispose of their superabundant portion of fluids, by promoting perspiration and evaporation. Yet the heat must not be too great, or continued for too long a time, as it occasions a too rapid digestion, and perspiration of their nourishment, and consequently an exhaustion.

Plants have an independent heat of their own. But, if it be difficult to account for the spontaneous production of heat in animal bodies, as all physiologists have found, it must be much more so to account for the generation of vegetable heat. Light, and atmospheric air, at least, are known to be essential to the vital functions of both.

In all decomposition, caloric, or heat, is disengaged, and may not the chemical process which takes place within the plant, supply it with the heat which they are acknowledged to possess, and which, it is asserted, tempers the cold of the atmosphere; while the evaporation which takes place through the whole plant, continually moderates the scorching heat of the sun?

Dr. Hunter observed, upon this independent heat, that

or healthy tree, it constantly indicated a temperature several degrees above that of the atmosphere, when it was below the fifty-sixth division of Fahrenheit; whereas the vegetable heat in hotter weather was always several degrees below that of the atmosphere. The same philosopher has likewise observed, that the sap which, out of the tree, would freeze at 32°, did not freeze in the tree unless the cold were augmented 15° more.

But the most remarkable instance of heat in plants upon record, is what is related of the Arum maculatum. Lamarck says, in his Flore Française, "that when the flowers are in a certain state of perfection, the spadix is so hot as to seem burning, and not at all of the same temperature as the surrounding bodies.”

Mr. Senebier noticed that this heat began when the sheath was about to open, and the spadirt just peeping forth, and that it was perceptible from three or four o'clock in the afternoon till midnight.

The Leaves consist of an immense number of fibres or nerves, divided into two sets, one belonging to each surface. The surface of the leaf is full of minute pores, through which it imbibes the dew, air, &c. necessary to the growth of the plant, so as to enable it, in some degree, to dispense with supplies from the root; as we see in plants which live in the water, or swim in that element, which serves them for food; they have no roots, but receive the fluid at all their pores, and, by decomposition, the hydrogen gas of the water, which constitutes the chief aliment of the plant, is separated, and becomes a principle of the vegetable; while the oxygen gas, the other constituent part of water, is thrown off by the vital forces, escaping by the pores, where the action of light causes its disengagement.-[See note 1st, upon Gas.]

Air is also necessary to the growth of a plant. But the air which it requires is not the same appropriated to the use of man.

Drs. Priestley and Ingenhousz have proved that it is the

Sheath, or Spathe-a kind of calyx that opens lengthwise, and puts forth a flower-stalk or spadix, as in the palm arum, &c. It is also applied to the calyx of some flowers which have no spadiz, as of the narcissus, crocus, iris, &c. A membrane investing a stem or branch, as in grasses.

Spadix-the receptacle proceeding from a spathe, as in the palm, and some other plants, so called from being produced within a spatha, or sheath.

that it had the smell of common Tansy, and ultimately proved it to be that plant, which had been so changed by growing in the dark. Indeed it was recollected, that some soil had been taken into the drain from a neighbouring garden, some time before it was found so altered.

It has been ascertained, by experiment, that the green colour of vegetables may be produced by the light of a lamp, in the absence of the more perfect light of the sun; as discovered by the Abbe Tessier.

Leaves give out moisture by their under surface, in proportion to the intensity of light, and not of heat; so that there is scarcely any evaporation during the night. The water which exhales from vegetables is not pure, but serves as the vehicle of the aroma; it is equal to the third part of their weight every twenty-four hours, in healthy plants.

Leaves also expose the sap which they receive from the wood, to the action of the air, and return it again to the bark by its fibres or vessels. They also serve to nourish and prepare the buds of the future shoots, which are always formed at the base of the leaf stalk, and to shade them, as well as the fruit, from the too powerful heat of the sun. Hence it is, that in tropical countries the tree is never divested of the leaf.

Water is the only aliment which the root draws from the earth; and a plant can live and propagate itself, without any other assistance than the contact of water and air; as may be seen every day, in the Hyacinth, and other bulbous plants, which adorn our mantlepieces, as well as gramineous, or grassy plants, such as wheat, &c. raised in saucers or bottles, containing mere water.

In vegetables, hydrogen is the principle which fixes itself, while oxygen gas (the other constituent part of water) makes its escape.

But although pure water is more proper for vegetation, than water charged with salts, yet water may be disposed in a more favourable manner to the developement of vegetables, by charging it with the remains of vegetable and animal decomposition: the plant then receives juices already assimilated to its nature. Independent of those juices already formed, the nitrogen gas (which has already been mentioned, as constituting one of the nutritive principles of plants) is abundantly afforded by the alteration of vegetables and animals, and must facilitate their developement.

Although it has been proved, by various experiments, that pure water is sufficient to the support of plants, we must not, therefore, consider the earth as of no use; it imbibes and retains water; it is the reservoir destined by nature to preserve the elementary juice which the plant continually requires; and to furnish that fluid in proportion to its wants, without exposing it to the equally fatal alternatives of being either inundated, or dried up.

The nature of the soil must be varied accordingly as the plant requires a more or less considerable quantity of water, in a given time; and accordingly as its roots extend to a greater or less distance. Every kind of earth is not suitable for every plant; and, consequently, a slip cannot be grafted, indifferently, upon every species.

A proper soil, is one which affords a sufficiently firm support to prevent the plant from being shaken; which permits the roots to extend themselves to a distance with ease; which becomes impregnated with humidity, and retains the water sufficiently, that the plant may not be without it when wanted.

To answer these several conditions, it is necessary to make a proper mixture of the primitive earths, for none of them in particular possesses them.

Siliceous (white sand) and Culcareous (limy) may be considered as hot and drying; the Argillaceous (clayey), moist and cold; and the Magnesian, (a primitive earth, having for its base a metallic substance, called Magnesium, generally found in combination with other substances,) as possessing intermediate properties. Each, in particular, has its faults, which render it unfit for culture: clay absorbs water, but does not communicate it; calcareous earth receives and gives it out quickly; but the properties of these earths are so happily opposed, that they correct each other by mixture. Accordingly we find, that, by adding lime to an argillaceous earth, this last is divided; and the drying property of the lime is mitigated, at the same time that the stiffness of the clay is diminished.

Saline substances have been supposed of importance in vegetation by some, but they do not appear essential to the growth of any sort of plant except the Marine; such matters may, however, be of use to vegetation, though not essential to it. That of common salt may operate upon plants as it does upon the human body, by assisting to digest the food, without furnishing nutriment itself.

It is upon this principle, I presume, that common table