The absence of light is favourable to the germination of seeds for light acts upon plants in such a manner as to take away oxygen by the decomposition of carbonic acid gas, and to deposite carbon, now this is just the reverse of the process required in germination, where the carbon must be evolved and the oxygen in excess.

A certain degree of heat is necessary to germination. Seeds planted in winter, will remain in a torpid state; but as soon as the warmth of spring is felt, the embryo emerges into life. By increasing heat, the vegetating process of seeds may be hastened; thus the same seed, which with a moderate degree of heat would germinate in nine hours, may be brought to this state in six hours, by an increase of temperature. Too great heat destroys the vital principle; thus corn which has been roasted cannot be made to vegetate. The process of malting consists in submitting some kind of grain, (barley is most commonly used,) to a process which causes an incipient state of germination; this is done by moistening the grain, and exposing it to a suitable degree of warmth; as soon as germination commences, the process is stopped by increasing the heat. taste of the grain is then found to have become sweetish. The term malt is given to grain which has been submitted to this process. When mixed with water it forms a sweet liquor; and the fermentation of this liquor produces beer.

The

There is a great difference in plants as to their term of germinating; some seeds begin to vegetate before they are separated from the pericarp.*

In the greater number of vegetables, however, there is no germination until after the opening of the pericarp and the fall of the seed. The time at which different species of seeds, after being committed to the earth, begin to vegetate, varies from one day to some years. The seeds of grasses, and the grain-like plants, as rye, wheat, corn, &c., germinate within two days. The cruciform plants, such as radish and mustard, the leguminous, as the pea and bean, require a little more time. The peach, walnut, and peony, remain in the earth a year, before they vegetate.

All kinds of plants germinate sooner, if they are sown immediately after being separated from their pericarps. Many vegetables preserve their vital principle for years; some lose it as soon as they are detached from their pericarps. This is said to be the case with respect to coffee and tea. The seeds of some of the grasses, as wheat, &c. are said to retain their vital principle even for centuries. It is asserted that mosses, kept for near two hundred years in the herbariums of botanists, have revived by being soaked in water. An American writert says, that "seeds, if imbedded in stone or dry earth, and removed from the influence of air or moisture, might be made to retain their vegetative quality or principle of life for a thousand years." But he very rationally adds, "life is a property which we do not understand; yet life, however feeble and obscure, is always life, and between it and death, there is a distance as great as existence and non-existence."

* In the month of January, on observing the seeds of a very juicy apple, which had been kept in a warm cellar, I saw that they were swollen, and the outward coat hau burst; examining one seed, by removing the tegument and separating the cotyledons, I saw, by the help of a microscope, the embryo as if in a germinating state; the radicle was like a little beak; in the upper part or plume was plainly to be seen the tuft of leaves and the stem.

† P. Barton.

The absence of light favourable to the germination of plants-Heat-Effects of too great heat exemplified in the process of malting-Malt--Season of germinating-Time ef germinating varies-Vital principle of fruits.

The subjects upon which, in this lecture, we have been engaged, properly come under the head of vegetable physiology, a departnient of botany highly interesting, but too complicated in its nature to be, to any great extent, presented to the mind of the youthful investigator. The physician finds in the vegetable organization striking analogies to the internal structure of the animal frame; to him the language of physiological botany is familiar, because it is bor rowed from his own science. On the other hand, the botanical student, in learning the names and offices of the various internal organs of plants, is making no inconsiderable improvement in the knowledge of the animal economy, and stupid must be that mind which is not, by the consideration of the one, led to reflect upon the organization of the other.

LECTURE XVIII.

PHYSIOLOGICAL VIEWS-SOlid and fluiD PARTS OF VEGETABLES.

THE careless observer of nature may consider the trunk of a tree, a leaf, or a stem of an herb, as very simple in its structure, presenting little more than a homogeneous mass; but the botanical philosopher looks with a far different eye upon the vegetable being. He has learned that plants, like animals, are formed of vessels of dif ferent kinds, variously fitted to carry on the operations of imbibing nourishment, of making a chemical analysis of the same, and of appropriating to themselves such elements as are necessary to promote their health and vigour, and of rejecting such as are useless. In short, that they have parts which are analogous to skin, bones, flesh, and blood: that they are living, organized beings, composed of solid and fluid parts; and, like animals, the subjects of life and death.

Plants differ from animals in being destitute of the organs of sense. They can neither see, hear, taste, smell, nor touch. Some vegetables, however, seem to have a kind of sensibility like that derived from the organs of touch; they tremble and shrink back upon coming in contact with other substances; some turn themselves round to the sun, as if enjoying its rays. There is a mystery in these circumstances which we cannot penetrate; it is not yet fully known at what point in the scale of existence animal life ends, and vegetable life commences. Some beings, like the sponge and corals, seem almost destitute of any kind of sensation, and yet they are ranked among animal substances. The subject of the distinctions and analogies between plants and animals, we shall consider more extensively hereafter.

Solid parts of Vegetables.

We shall now treat of the solid portions of the vegetable organization; these are all composed of a membranous substance, which exists in every part of the plant, forming by its various modifications, the different textures which the plant exhibits. This mem

Vegetable Physiology-Its language borrowed from animal physiology-Different aspects of vegetables to the careless observer and the philosopher-Difficult to deter mine where vegetable life commences-Solid parts of plants.

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 co lour of plants-of the hues of petals-What does the vascular texture consist of?Entire vessels-Porous vessels.

Fig. 117.

с

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 ¡uices; 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 Mirbel-External glands-Three kinds of fluids-What is the sap, and how formed ?—What 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.