of raising liquids against the laws of gravitation, and with a force proportional to their smallness of diameter;-this law seems to explain, in some degree, the phenomenon we are considering.

But it is necessary for us now to trace the progress of the sap, after it has ascended to the leaves and extremities of the plant. A considerable portion of it is, by pores in the leaf, exhaled in the form of almost pure water, while the particles of various kinds, which the sap held in solution, are deposited within the substance of the leaf. This process is sometimes termed the perspiration of plants; it is visible in some grass-like plants, particularly upon the leaves of Inlian corn. If these are examined before sunrise, the perspiration appears in the form of a drop at the extremity of the leaf; the ribs of the leaf unite at this point, and a minute aperture furnished for the passage of the fluid, may be discovered.

The sap which remains, after the exhalation by means of the leaves, is supposed to consist of about one third of that originally absorbed by the root; this remainder possesses all the nutritive particles which had, before, been divided through the whole of the sap. At this period, an important change in its nature takes place, and one which has its analogy in the animal economy.

We have compared the sap to the blood of animals, but it is, in reality, more like the animal substance, chyle, which is a milk-like liquor, separated by digestion, from the food taken into the stomach. A considerable part of this chyle is converted into blood, which passing first into the arteries and then into the veins, are by the latter conveyed to the heart; the heart, by its contractions, sends the blood to the lungs. At each inspiration of the breath, oxygen from the atmospheric air is absorbed by the lungs; here uniting with the carbon of the blood, it forms carbonic gas, which is thrown off at every expiration of the breath. Thus the carbon, which, in the animal system, is accumulated by feeding on vegetables, and which requires to be diminished, is carried off; it is said that a person in breathing twenty-four hours, expires almost one pound of carbon, or the basis of charcoal!

We will now return to the sap in the leaves of plants, and see w nether a change takes place, analogous to that in the animal 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 oxygen, 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, in order to be fitted for the nourishment of the plant, seems to require the further agency of oxygen, to convert it into carbonic acid; this is effected by means of the oxygen, which, during the night, is absorbed 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 fur

Exhalation of sap-Perspiratiofutic plants What is the nature of the sap which re mains after exhalation ?-Sap ed to animal chyle-Formation of carbonic gas -In what respect does the comr between the respiration of plants and animals fail ? What is needed in order nts-e carbon for the nourishment of the plant?

in r

nished with this gas by the decomposition of water* and other substances which are absorbed 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 the bark, and the alburnum or young wood; here it contributes both to the formation of an outward layer of new wood and an inward layer of new bark; extending also 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 annular protuberance. This vegetable blood being thus prevented from having access to the lower part of the plant, the roots cease to grow, the sap ascends but feebly, and in two or three years the tree dies. 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 experiment proves the importance of this fluid to the existence of the plant.

The Proper Juices of Vegetables. This division comprehends 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 system, 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; these consist chiefly of vapours and gases exhaled from flowers, and may, perhaps, more properly be called excretions than secretions.

LECTURE XIX.

PHYSIOLOGICAL VIEWS-BARK, WOOD, AND PITH-GROWTH OF A DICOTYLEDONOUS PLANT-GROWTH OF A MONOCOTYLEDONOUS PLANT.

We have exhibited to your view the minute discoveries made by the help of the microscope in the solid parts of the vegetable substances; 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 nsider the solid parts already described, as composing the body of the vegetable, and collected under the three forms of Bark, Wood, and Pith.

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

cortex.

1st. Epidermist is the skin of the membrane which extends over

*Water consists of oxygen in union with hydrogen.

+ The word eperdimis is from epi, upon, and derma, the skin.

the

e-mportance of this fluid-What is

Cambium, or descending sap-How conve of ise -What are the proper juices of vegetables? Of what three parts is the boarisol of the bark-Describe the epidermis.

o fit th

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 thick 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, or hard and coarse, as 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; it seems not to be endowed with the vital principle, and in this respect differs from all other parts of the plant. The 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 and air, while it excludes every substance which would be injurious.

It is to the cuticle of wheat, oat, rye, and some of the grasses, that we are indebted for straw and Leghorn hats. In their manufacture the cellular texture is scraped away, so that nothing remains but the cuticle. It has been ascertained that the outer bark of many of the grasses contains silex, qr flint;-in the scouring rush, (Equisetum,) the quantity of silex is such, that housekeepers find it an excellent substitute for sand, in scouring wood or metals. A peculiar property of the cuticle is, that it is not subject to the same changes as the other parts of bodies; it is, of all substances found upon animal or vegetable matter, the most 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 cuticle covered with a kind of wool; the pericarp of the peach has a downy cuticle.

2d. Cellular Texture, 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 envelops branches, as well as trunks of trees, and herbaceous stems; it extends into roots, but there it neither retains 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 surface in the form of a cuticle, and often cleaves off. It is renewed 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 brilliant and delicate teints which constitute so great a portion of their beauty. The fuci,

Uses of the eperdimis, or cuticle--Cellular texture-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 trees-Found in the petals of flowers, &c.

1

a species of sea-weed, and some other succulent plants, appear to be altogether composed of cellular texture.

3d. Cortex. Immediately under the cellular integument, we fina 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 layer of the bark is called the liber; it is here only, that the essential, vital functions, are carried on; this integument is so called from liber, a book, on account of its fine and thin plates, which are thought to bear some resemblance to the leaves of a book. This substance, by its development, 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, and of new wood, are every year formed from that liquid; as the new layer of bark 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 gar ments 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 be performed early in the spring, before the first flow of the sap from the root towards the extremities.

During the repose of veget tion, that part of the liber most recently organized, and which of course retains its vital power, 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 development of buds, and the growth of new wood and bark, the liber hardens and loses its vital energy, like that of the preceding year.

Fig. 118, 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; ab, is the bark, bi, 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; atd b, is the cortex, the extreme part of which, at b, is the .iber.

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 newly formed wood, and consti

What is said of the cortex ?-Liber-Annually renewed-Girdling-What ultimately becomes of the liber?-Describe a dicotyledonous or exogenous stem-Of how many parts does the wood consist ?-Alburnum

tutes 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 conveved in appropriate vessels to the buds.

Fig. 118.

b de

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 of trees and shrubs; when cut across, it is found to consist of numerous concentric layers. It is supposed that one of these circular layers is formed every year. To prove that the wood is deposited externally from the cambium, pieces of metal have been introduced under the bark of trees that were growing, and the wounds carefully bound up; after some years, on cutting them across, as many layers of new wood have been found on the outside of the metal, as years had elapsed since its insertion.

The strength and hardness of wood, is owing to woody fibres extending longitudinally; these fibres are chiefly of vascular texture, and contain sap, and the various secreted juices; some contain only air.

For illustration of the formation of wood, see Fig. 118, B, which represents a section of a woody stem of three years' growth; ih, next the pith, is a layer of the first year's growth, and the hardest part of the wood; h g, is a layer of the second year's growth; and g b, of the third; the last is the sap-wood recently formed from the cambium.

Pith. The pith (see Fig. 118, B, k i) is situated in the centre of the trunk and branches of plants, and is a soft, spongy substance, analogous to the marrow of animals. It is composed of cellular texture. The cells, which are very large in the elder and some other

Perfect wood-How has it been proved that wood is deposited externally?-Strength and hardness of wood-Illustrate the formation of wood by a reference to Fig. 118