branous substance appears chiefly under two elementary forms: viz. 1st, that of cellular texture; 2d, vascular texture.

divisions of the membrane, which forms these cells, are common to contiguous cells. The cellular system in animals contains the fat; in vegetables it is generally filled with resinous, oily, or saccharine juices; in some cases the cells contain air only. They are usually marked by small dots, (as at a, Fig. 116;) these are supposed to be apertures, through which fluids are transmitted from one cell to another.

The cellular texture composes most of the pith, parenchyma, and cotyledons of almost all vegetables. It is abundant in tuberous roots, pulpy and fleshy fruits, and the stems of grasses, and constitutes the principal part of mushrooms, and other cryptogamous plants. In the bark of plants the cellular texture is situated under the cuticle; it is filled with a juice which varies in colour in different species of plants, but is most commonly green; it gives its colour to the bark, as the same texture under the human cuticle gives colour to the skin. The green colour of leaves is caused by the cellular texture, which is enclosed on both sides by the cuticle. In the pith of young plants, the cells are filled with watery fluids, but in older plants they are empty, or only filled with air. The petals of flowers owe their beautiful hues to the presence of cellular texture, filled with juices, which refract and reflect the rays of light, in a peculiar manner.

Vasculart texture, consists of tubes, which, like the vessels of the animal frame, are capable of transmitting fluids. These tubes are open at both ends, and are protected by a coating of cellular integument; their sides are thick and almost opaque. These vessels extend throughout the whole plant, distributing air and other fluids necessary to vegetation. The vascular system of plants presents a variety in form, and also with respect to the functions which the different parts perform.

Some are entire vessels, or without any perforation, (Fig. 116, c;) these convey the proper juices of the plant, and generally contain oils and resinous juices.

Porous vessels exhibit many perforations, (Fig. 116,b;) they often separate and again unite, changing at length into cellular integu

ment.

* Dutrochet.

† The term vascular is derived from the Latin word vasculum, a little vessel.

Two kind of membranes-Cellular texture-how situated-Cause of the green colour of plants-of the hues of petals-What does the vascular texture consist of?Entire vessels-Porous vessels.

Spiral vessels are so called from their form, which resembles that of a screw, (Fig. 117, a;) they are sometimes termed trachea, from a supposed analogy to the trachea of insects, or their organs for breathing. These vessels are formed of a thread-like fibre turned spirally from right to left.

Annular vessels, (so called from the Latin annulus, a ring,) are so perforated as to make the tube appear to be composed of rings, (Fig. 117, b.)

Moniliform vessels (from monile, a necklace) resemble, in external appearance, a string of beads, (Fig. 117, c;) these serve to connect large vessels, and to convey sap from one set to another.

Mosses, fungi, and lichens, have no vascular system, but their tissue is all of the cellular kind. The solid substance of plants is all composed of some varieties of the two kinds of membranes we have now described. Roots and stems are made up of vascular fibres; these may easily be split longitudinally, as the vessels in this case are only separated, and the cellular texture easily yields; but in severing the roots and stems horizontally, greater resistance is to be overcome, since the tubes are to be cut across.

Vegetables, like animals, have a system of glands, or internal vessels, which are made subservient to the purpose of producing changes in the fluids of the plants;-thus the sap is converted into the proper juices; and from the same soil and nourishment plants of very different properties are produced.

Mirbel, by the aid or the microscope, succeeded in discovering a system of glands in the pores or cells, and on the borders of the spiral vessels. There are also external glands, which appear manifest to the naked eye; as the nectaries of flowers, which secrete or manufacture honey; and the stings of plants, which secrete an acrid substance, which, by penetrating the skin, causes a painful sensation. Fluid Parts of Vegetables.

The different fluids which are exhibited in the vegetable body may be considered under three general divisions: 1st, the sup, or ascending fluid; 2d, the cambium, or descending fluid; 3d, the proper juices.

The sap is a limpid, inodorous liquid, the elements of which are imbibed from the earth by pores in the radicles of the root. Every one knows, that if the earth around the roots of plants is destitute of moisture, they soon die. Water holding in solution various substances, such as earths, salts, animal and vegetable matter, is absorbed by the radicles; by some unknown process, they convert this fluid matter into sap, and then, by means of vessels which form what is called the sap-wood, or alburnum, this sap ascends through the stems to the branches; passing through the woody part of the petioles, and those minute branches of the petiole which form the ribs and veins

Spiral vessels-Annular-Moniliform-All the solid substance of plants composed of some of these vessels-The use of glands-Glands discovered by MirbelExternal glands-Three kinds of fluids-What is the sap, and how formed ?--What is the use of the sap-wood?

of the leaf, it enters into the vessels and cells which extend throughout its substance.

The ascending sap is always in circulation, but its energy varies with the season, and the age of the plant. Heat has an important influence in quickening the ascent of the sap; yet, during a dry and hot season, it often appears to ascend but slowly. This is because the absorption of fluids from the earth is checked by the dryness of the soil. The plant, by a little stretch of the imagination, may be considered as thirsty, and thus man may seem not only provident, but humane, in administering to its roots refreshing draughts of water. Even the leaves, at such a period, seem too impatient to wait for supplies by means of the connecting sap-vessels; for if water is sprinkled upon them, they fail not to use their own power of absorption, and upon such an application, may be seen to revive almost instantaneously.

When the moisture of the earth coincides with elevation of temperature, the sap ascends with the greatest rapidity; this is the case in spring. It is at this period, that incisions are made into the wood of maple-trees, in order to procure sap for the manufacture of sugar. The sap may at this time be seen flowing almost in a stream. It has been thought that the circulation of sap was wholly suspended during winter; this, however, seems not to be the case; for we may observe during this season a gradual development of some parts of the plant; we see many plants preserving the freshness and verdure of their foliage, and mosses putting forth their flowers. We must then believe, that the sap is in perpetual motion, susceptible of being accelerated or retarded by changes of temperature, and humidity, or dryness of the earth. The development of buds must be attributed to the ascension and redundancy of the sap, which dilates and nourishes their parts. In spring, when the ascent of the sap is accelerated, the buds enlarge rapidly, and their complete development is soon perfected.

The vascular texture appears by its tubes and channels to afford great facilities for the ascension of the sap. In imperfect plants, such as mushrooms and lichens, which are wholly composed of cellular texture, it is not known that there is any ascent of sap, but they seem to be nourished by fluids absorbed from the air.

The question naturally arises, by what force is the sap made to ascend, contrary to the laws of gravitation? Some have asserted, that this phenomenon was owing to the contraction and dilatation of the air, and of the juices of the plant; others have referred it to the action of heat; these two propositions, however, amount to the same thing, since heat is the cause of the contraction and dilatation referred to. Some ascribe the ascent of the sap to the irritability of the vessels, and the energy of vital power.

The latter is but a vague and unsatisfactory explanation, since we know neither the cause of this irritability, nor in what this vital power consists. There is no doubt but the ascent of the sap is, in a degree, owing to capillary attraction, assisted by heat. You will recollect that the vessels containing this fluid, were described as very small tubes, no larger than a hair, and, in most cases, much smaller, since few are visible to the naked eye. Those who understand something of Natural Philosophy, know that capillary tubes have the property What effect has drought upon the plant ?-What two circumstances cause the rapid ascent of the sap ?-Why are incisions made in maple-trees in the spring, rather than at any other period ?-Perpetual motion of sap-Cause of development of beds-Vascular texture unlike the cellular in affording facilities for the ascension of sap-Ex planations of the pauses of the ascent of the sap.

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 Indian 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 whether 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-Perspiration of plants-What is the nature of the sap which remains after exhalation ?-Sap compared to animal chyle-Formation of carbonic gas -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?

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 consider 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.

Cambium, or descending sap-How conveyed-Importance of this fluid-What is the effect of cutting a ring through the bark of a tree ?-What are the proper juices of vegetables? Of what three parts is the body of the vegetable composed ?-Divisions of the bark-Describe the epidermis.