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Fig. 91.

1st. Cellular tex. ture, (Fig. 91, a,) according to the opin. ion of Mirbel, is com.

posed of a mass of b

little hexagonalcells, resembling honey comb. Another French botanist* compares the appearance of the cel.

lular texture to the froth of fermenting liquor ; he considers that each cell is dis. connected with the others; while Mirbel believes that the 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 (See the dots at a, Fig. 91); 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 parts of mushrooms and other cryptogamous plants. In the bark of plants, the cellular tex. ture 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 tex. ture under the human cutiele gives colour to the skin. The green colour of leaves is caused by the cellular texture, which is inclosed 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 by air.

The petals of flowers owe their beautiful hues to the pres. ence 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 thick coat. ing of cellular integument ; their sides are thick and little trans

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* Dutrochet. + The term vascular is derived from the Latin word vasculum, a little vessel.

Cellular texture-how situated-Cause of the green colour of plants--Beautiful hues of petals—Vascular texture.

pa rent. These vessels extend throughout the whole plant, distributing air and other fluids necessary to vegetation. The vascular system of plants presents a variety as to form and with respect to the functions which the different vessels perform.

Some are entire vessels, or without any perforation (Fig. 91, c); these convey the proper juices of the plant, and are generally found containing oils and resinous juices.

Porous vessels have their sides pierced with many perfora. tions (Fig. 91, b); they often separate and again unite, changing at length into cellular integument. Fig. 92.

Spiral vessels are so called from their

form, resembling b

that of a screw (Fig. 92, a); they are sometimes termed trachea, from a supposed analogy to the trachea, or the organs of insects for breathing These vessels are formed of a thread-like fibre turned spirally from

right to left. Annular vessels (so called from annulus, a ring,) have perforations which make the tube appear as if composed of rings (Fig. 92, ).

Moniliform vessels (from monile, beads,) resemble, in exter. nal appearance, a string of beads (Fig. 92, 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 vessels are all of the cellular kind. The fibres of plants are all composed of some varieties of the two kinds of vessels we have now described. Roots and stems are composed of fibres; these may easily be split longitudinally, as the vessels in this case are only separated, and the cellular texture easily yields ; but in cutting the roots and stems horizontally, greater resistance is to be overcome, since the tubes are to be divided or cut across.

Vegetables, like animals, have a system of glands, or internal vessels, which are by the Author of nature, made subservient

Entire vessels-Porous vessels-Spiral vessels–Annular-Moniliform-All the fibres of plants composed of some of these vessels-Glands of vegetables

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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 appear plants of very

different

properties.

Mirbel, by the aid of the microscope, succeeded in discovering a system of glands, in the pores of cells, and on the borders of the spiral vessels. There are 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, that 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 sap, 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. You know that if the earth around the roots of plants is depriv. ed of moisture, they soon die. Moisture furnishes to the radi. cles, water holding in solution various substances; such as earths, salts, animal and vegetable matter. The radicles, by some unknown process, convert this fluid matter into sap, and then, by means of vessels which form what is called the sapwood 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 of the leaf, it enters into the vessels and cells which ex. tend 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 upon 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 Auids 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 to be too impatient to wait for supplies by means of the connecting sap vessels ; but if water is sprinkled upon them, they fail not to use their own power of absorption, and may, upon such an application, be seen to revive almost in. stantaneously.

Their use-Mirbel's discoveries External glands--I hree divisions of Auids Sap-Sap-wood--Ascent of the sap.

The sap

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, as you no doubt are aware, that incisions are made into the wood of maple trees, in order to procure sap for the manufacture of sugar. 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 developement 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 developement of buds, must be attributed to the ascension, and re. dundancy 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 developement 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 as. cent 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 as. serted that this phenomenon was owing to the contraction and dijalation of the air, and of the juices of the plant ; others have referred it to the action of heat; these two propositions, howev. er, 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.

This 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

* The term capillary, is taken from the Latin, capillus, a hair. What two circumstances cause the rapid ascent of the sap ?-Why are incisions made in maple trees in the spring, rather than any other period ?-Per. petual motion of sap—Cause of developement of buds—Vascular texture unlike the cellular in affording facilities for the ascension of sap—Explanations of the causes of the ascent of sap.

cases, much smaller, since few are visible to the naked eye. You have, in the study of Natural Philosophy, learned that ca. pillary tubes have the property of raising liquids, against the laws of gravitation, and with a force proportional to their small. ness of diameter: this law seems to explain, in some degree, the phenomenon we are considering. Yet we must realize that our researches here, as in every other case, terminate in mys. teries, impenetrable by our limited faculties.

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 with in 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 uníte 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, a change 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, the oxygen from the atmospheric air, is absorbed by the lungs; here, uniting to 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 has been said that a person exhales, in breathing twenty-four hours, 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, analagous to that in the animal

Exhalation of sap—Perspiration-Sap which remains after exhalation by means of the leaves--Sap compared to chyle Formation of carbonic gas,

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