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system. We will consider the sap as bearing a resemblance to the animal chyle, and the leaves to the animal lungs. These vegetable lungs are furnished with pores, by which they, too, inhale gases; but here our comparison fails, since, instead of ox. ygen, the plant inhales carbonic acid ; this it decomposes; and converting to its own use the carbon, which is an important element of vegetable compounds, it exhales the oxygen necessary for the support of animal life. Light, however, is necessary for this process of respiration in the plant; deprived of this agent, vegetables absorb instead of giving off oxygen.

The carbon, which is deposited in the sap, seems, in order to be fitted for the nourishment of the plant, to require the far. ther agency

of

oxygen, to convert it into carbonic acid ; this is done by means of the oxygen, which, during the light, is ab. sorbed by the leaves. At the appearance of light, carbonic acid is again decomposed and oxygen evolved. Besides the oxygen which the plant separates from the carbonic acid inhaled by its leaves, it is undoubtedly furnished with this gas by the decomposition of water* and other substances which are ab. sorbed by the root.

The cambium is the sap elaborated by the chemical process carried on in the leaves, and rendered fit for the nourishment of the plant.

In tracing the descent of the cambium or returning sap, we shall not find it passing through the same vessels by which it ascended; it is chiefly conveyed by a system of vessels between the liber or inner layer of bark, and the alburnum or young wood; here it contributes both to the formation of new wood and new bark, and extending from the extremity of the roots, to the upper extremity of the plant, it furnishes materials for the formation of new buds and radicles.

If a ring is cut through the bark of a tree, the cambium will be arrested in its course, and accumulating around the upper edge of the bark, will cause a ridge or an annulart protuberance. This vegetable blood being thus prevented from hav. ing access to the lower part of the plant, the roots cease to grow,

the
sap ascends but feebly, and the tree dies in two or

If the incision is not made too deep, the wound will soon heal by the union of the disconnected bark, and the circulation of the cambium proceeds as before. This experi

three years.

* Water consists of oxygen in union with hydrogen. + From the Latin word annulus, a ring. In what respect does the comparison between the respiration of plants and animals fail ?-What is needed in order to fit the carbon for the nourishment of the plant ?-Cambium, or descending sap-How conveyed—Importance of this fluid-What is the effect of cutting a ring through the bark of a tree ?

ment proves the importance of this fluid to the existence of the plant.

The Proper Juices of Vegetables. This division compre. hends all the fluids furnished by the plant except the sap

and cambium; as oils, gums, &c. These are the product of the cambium, as in the animal, tears are secreted from blood. The secretions, carried on by the vegetable glands from the cambium, are of two kinds; 1st, such as are destined to remain in the plant, as milk, resins, gums, essential and fixed oils; 2d, such as are destined to be conveyed out of the plant, as useless or incongruous; these consist chiefly of vapours and gases exhaled from flowers, and are sometimes called excre. tions.

LECTURE XVIII.

Bark, Wood and Pith. We have exhibited to your view the minute discoveries made by the help of the microscope in the solid parts of vegetable substances; and we have also noticed those important fluids, the circulation of which appears to constitute the life, and produce the growth of plants. We have now to consider the solid parts already described, as composing the body of the vegetable, and collected under three forms of Bark, Wood, and Pith.

Bark. The bark consists of the epidermis, cellular integument and cortex.

1st. Epidermis is the skin or membrane which extends over the surface of every vegetable. It is also called the cuticle, a name which anatomists have given to the external covering of the animal body. There is a striking analogy between animal and vegetable cuticle or skin. In the animal it varies in thickness from the delicate film which covers the eye, to the hard skin of the hand or foot, the coarser covering of the ox, or the hard shell of the tortoise. In the vegetable it is exquisitely delicate, as in the covering of a rose leaf, and hard and coarse in the rugged coats of the elm and oak. In the birch you may see the cuticle or outer bark peeling off in circular pieces. This seems not to be endowed with the vital principle, and in this respect it differs from all other parts of the plant. The epidermis or cuticle serves for protection from external injuries, and regulates the proportion of absorption and perspiration through its pores. It is transparent as well as porous, so as to admit to the cellular integument, the free access of light

Proper juices of vegetables Secretions from the cambium-Of what are the bark, wood and pith composed ?-Divisions of the bark-Describe the epidermis. and air, while it excludes every substance which would be injurious.

It is to the cuticle of wheat, oats, rye, and some of the grasses, that we are indebted for straw and imitation Leghorn hats. In their manufacture, the straws are scraped, so that nothing remains but the cuticle. It has been ascertained that the outer bark of many of the grasses contains silex, or flint; in the scouring rush, (Equisetum,) the quantity of silex is such, that housekeepers find it an excellent substitute for sand, in scour. ing wood or metals. A singular property of the cuticle is, that it does not seem to be subject to the same changes as the other parts of bodies; it is, of all substances found upon ani. mal or vegetable matter, the least indestructible. The cuticle is sometimes like the skin of animals, clothed with wool or down, and it then becomes an important security against the effects of heat and cold. The leaf of the mullein has its cuti. cle covered with a kind of wool; the pericarp of the peach has a downy cuticle.

2d. Cellular Integument, is situated beneath the epidermis or outer skin of the bark; it is filled with a resinous substance, which is usually green in young plants. This cellular layer possesses glands, which, when submitted to the action of light, carry on the process of decomposing carbonic acid gas, by retaining the carbon and evolving the oxygen gas. The cellular integument envelopes branches, as well as trunks of trees, and herbaceous stems; it extends into roots, but there it neither remains its green colour nor decomposes carbonic acid gas. It is the seat of colour, and in this respect analogous to the cutis, or true skin of animals, which is the substance situated under the cuticle, and is black in the Negro, red in the Indian, and pale in the American. In the leaves of vegetables, the cellular integument occupies the spaces comprised between the nerves, and is of a green colour; in flowers and fruits it is of various colours. The cellular substance of some aquatic plants is filled with air; in the pine, sumach, &c. it is filled with the proper juices of the plant. This herbaceous envelope of the trunks of trees, after a time dries, appearing on the sura face in the form of the cuticle, and often cleaves off. It is re. newed internally from the cambium.

The petals of flowers are almost entirely composed of cellular texture, the cells of which are filled with juices fitted to refract and reflect the rays of light, so as to produce the bril. liant and delicate tints which constitute so great a portion of

Uses of the epidermis or cuticle-Cellular integument--Glands of the cellular integument-Cellular integument in roots-The seat of colour-Cellular integument in leaves, &c.-In aquatic plants-How renewed in the trunks of troes-Found in the petals of flowers, &c.

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their beauty. The fuci, a species of sea weed, and some other plants, appear to be altogether composed of cellular texture.

3d. Cortex. Immediately under the cellular integument, we find the true bark, which in plants that are only one year old, consists of one simple layer; but in trunks of older trees, it consists of as many layers as the tree has numbered years. The cortex is formed of bundles of longitudinal fibres called cortical vessels.

The peculiar virtues or qualities of plants chiefly reside in the bark. Here we find the resin of the fir, the astringent principle of the oak, and the aromatic oil of the cinnamon.

The inner cortical layer is called the liber ; it is here only, that the essential vital functions are carried on ; this integu. ment is called liber, from its fine and thin plates, which are thought to bear some resemblance to the leaves of a book, which in Latin is liber. This substance, by its developement, produces new roots, branches, leaves, flowers and fruits. It is composed of a kind of net work, which has been compared to cloth; the elongated fibres representing the warp, and the cellular texture the filling up. It has been observed that the cambium descends between the liber and the wood, and that a layer of new liber is every year made from that liquid ; as the new layer is formed the old one is pushed outward, and at length, losing its vital principle, it becomes a lifeless crust. The natives of Otaheite manufacture garments from the liber of the paper mulberry. The liber of flax is by a more refined process converted into fine linen. This part of the bark is important to the life of vegetables ; the outer bark may be peeled off without injury to them, but the destruction of the liber is generally fatal.

The operation of girdling trees, which is often practised in new countries, consists in making, with an axe, one or more complete circles through the outer bark and the liber of the trunk. Trees seldom survive this operation, especially if it have been performed early in the spring, before the first flow. of the sap from the root towards the extremities.

During the repose of vegetation, that part of the liber most recently organized, and which of course retains its vital pow. er, remains inactive between the wood and the outer layers of the bark, until the warmth of spring causes the ascent of the sap. After promoting the developement of buds, and the growth of new radicles, the liber hardens and becomes lifeless like that of the preceding year..

Cortex, or true bark-Liber--Annually renewed--Girdling-What ultimately becomes of the liber?

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bdca Fig. 93, at A, represents a young dicotyledonous stem, cut transversely; the inner circle surrounds the pith ; the wood extends to the bark, which at a appears darkly shaded.

At B is a section of the same stem magnified ; a b, is the bark; b i, is the wood, and i k, the pith.

The divisions of the bark may be seen as follows: a c, represents the cuticle, or the dry, disorganized part; at c d, is the cellular integument; at d b, the cortex, the extreme part of which, as at b, is the liber.

Wood. The wood (lignum) consists of two parts, alburnum or sap-wood, and perfect wood

The alburnum is so called from albus, white, on account of the paleness of its colour. This is the most newly formed wood, and constitutes the outer part of the woody substance of the plant. It is at first soft and tender, and in this state, appears to be active with the principle of life. As the liber is formed annually from the cambium or descending sap, new layers of alburnum are supposed to have the same origin, and to be formed during the same intervals of time. Most of the sap ascends through the alburnum, though some passes through the perfect wood. The sap which nourishes the buds, passes through the centre of the stem, and from thence is conveyed in appropriate vessels to the buds.

The perfect wood, is sometimes called the heart; its colour is usually darker than that of the sap wood, and its texture is firmer and more compact; it is also more durable for timber. It is formed by the gradual concentration and hardening of the alburnum. The wood constitutes the greater part of the bulk

Describe a dicotyledonous or exogenous stem-Wood—Alburnum-Perfect wood.

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