ed;* but soon displays the most vigorous powers of vegetation. Its growth, inflorescence, and progress towards maturity are indicated by the decay and fall of the leaves which had hitherto protected it. The age of a palm-tree, or rather the number of times that it has fructified, or become crowned with fresh leaves, is calculated by the number of woody circles which are found marked on the stem. Its power of lasting seems to have no other limits than the resistance which the base offers to the weight it supports. In these colossal trees, a sensible diminution in the diameter of the stem is often perceptible towards the top, and in most of the species it is a fact equally well proved, that the fruit decreases in quantity when they have attained a certain epoch of their existence. In the cocoa-nut tree (lodicea cocus nucifera) the period of this decrease shows itself at about the age of thirty years, although this tree continues to bear for nearly a century.†

The leaves, the forms of which are so various, present however the greatest analogy in their organization: the green membranous substance of which they are almost entirely composed, is an extension of the parenchyma; the envelope which covers them answers to the epidermis.

It is in the leaves that the sap is subjected to the action of the atmosphere; it is there concentrated and peculiarly modified. According to the position of leaves upon the plant, their under sides, or those turned towards the ground, are distinguished from their upper sides which meet the light from above.

The upper side of the leaf is covered with a thick and frequently shining epidermis; this epidermis is sometimes endued with a substance rich in silicious matter, as in rushes. In the Steppes of South America I observed a tree, called Chapparal, the leaves of which are so highly silicious, that they are used for polishing metals. Generally speaking, the covering of the upper surface of leaves is a matter which is something of the nature of wax or resin. The epidermis which covers the lower surface is formed in most cases of a very thin, rough membrane, full of cavities and frequently covered with hairs or down.

The appearance and position of the leaves are not the same during the day and night. In the dark, simple leaves incline to fold up; in compound leaves, as in those of the acacia and sensitive plant, the same thing is still more marked; the effect can even be produced at will. If during the day a sensitive plant is placed in a dark room, the leaves immediately close; on lighting the room even with candles, they open again as if under the influence of the solar light; Linnæus, who first paid attention to this class of phenomena, admits that plants in the absence of light experience a sort of sleep.

* This bud, in certain species of palm-trees, is sought after as food, and is often spoken of as the cabbage of the palm-tree.

† Information communicated by Mr. Codazzi. The trunk of certain species of palmtrees shows an enlargement towards the middle of its height, as in the palma barrigona of Choco.

‡ Observation of M. de Candolle.

The flower is the forerunner of the fruit, the fruit is the medium in the heart of which the seed is developed. The organs which constitute the flower are the calyx and the corolla, destined to support, nourish, and protect the pistil and the stamina, which are the essential parts; the calyx is a green membrane which surrounds the corolla, and in certain flowers replaces it.

The corolla is monopetalous or polypetalous according as it is composed of one or of several pieces The stamens occupy the interior of the corolla; they are terminated by summits of a vascular texture; these are the anthers; the powder which covers and sticks slightly to them is designated under the name of pollen.

The pistil placed in the middle of the flower is composed of the ovary, the style, and the stigma.

The ovary encloses the germ, the embryo of the seed; but this embryo is only developed by the action of the pollen. The style is in some sort the tubular prolongation of the ovary; it supports the stigma, which is the glandular part that receives the fecundating influence of the pollen.

From what has now been said, the pistil may be considered as the female organ of the flower, the stamens as the male organs. Many flowers combine the organs of the two sexes. These flowers are hermaphrodites; those which only contain one organ, are called unisexual. Both male and female flowers are produced together on certain plants; in others, the flowers are all only of one sex, male or female. Polygamous plants are those which show a union of male and female flowers, or which have hermaphrodite flowers on the same stem.

In some flowers, the sexual organs at the period of fecundation acquire the property of motion, so as to facilitate this grand act. The stamens, for example, are seen in certain plants to approach the stigma, to deposite their pollen on it, and then to withdraw. It occasionally happens again that stamens, which are at first naturally in a position inclined with reference to the pistil, become suddenly straightened in such a way as to cast their pollen on the female organ, after which they resume their original position. In certain flowers a very considerable evolution of caloric has been perceived on the approach of the period of fecundation. In some arums, for example, the temperature has been observed to rise to 40° and even 50° cent. (104° to 122° Fahr.) It is probable that this phenomenon is quite general, and that it only varies in point of intensity.

Fecundation accomplished, the office of the flower is at an end. It collapses, withers, and dies. But the impregnated ovary enlarges by degrees, until it has attained maturity, when it presents two distinct parts, which by their union compose the fruit: these parts are the pericarp, and the seed-the husk or shell and the grain. The pericarp always surrounds the seed; but it sometimes happens that it is so thin and delicate that it blends with the seed.

The germination of seeds, the evolution of new plants, is only accomplished under certain physical conditions which demand our consideration.

We have already said incidentally, that in order that a seed may germinate, it must be in contact with moisture, have communication with the air, and be under the influence of a certain temperature. The same conditions continue to be indispensable after the seed has sprung, and the plant has been organized; and in addition the access of light is now imperative.

Roots seek in the soil the moisture which is requisite to vivify the whole vegetable. These organs are terminated by hair-like fibres of extreme delicacy, and having spongioles at their extremities : it is by these spongioles that absorption is effected. The following experiment is sufficient to prove that this is the case: let such a plant as a turnip be placed with the hair-like extremities of its root plunged in water, and the plant will continue to live, although almost the whole body of the root is in the air; let things be now so arranged that the hair-like filaments of the root are not in the water, but that the bulb or body of the plant is so the leaves will soon droop and wither.

The force which brings into play the suction power of the roots, resides in almost every part of the plant: thus a root deprived of its spongioles, a stem, a branch, and a leaf, exert this suction power when plunged in water. But the absorption effected in this way has a limit, and we soon discover the necessity of making fresh sections of the extremities, which have no power of renovation like the filaments furnished with spongioles, which terminate a root.

We are still ignorant of the cause which produces the ascent of liquids in vegetables, and which carries them to the remotest leaves, in spite as it were of the laws of hydrostatics. We readily conceive* how the spongioles of the roots, surrounded by earth abundantly charged with moisture, should imbibe by the simple effect of porosity. We can also understand how, after having been modified by the spongioles, the water and the principles contained in it should be transformed into sap; but the porosity of the extremities of the roots, and the chemical modification effected by the spongioles upon the fluid imbibed, give no kind of explanation of the rapid ascent of the sap. The force which occasions this rise is very considerable, as was demonstrated by Dr. Stephen Hales in a series of ingenious experiments more than a century ago.

Hales adapted a tube bent at a right angle and filled with water, to the extremity of the root of a pear-tree, the point of which had been cut off; the extremity of the tube opposite to that which was connected with the root dipped into a bath of mercury. In a few minutes a portion of the water contained in the tube was absorbed, and the mercury rose above the surface of the bath to the extent of eight inches. In the beginning of April, Hales cut off a vine stem at the distance of thirty-three inches from the ground. The stem had no lateral branches, and its cut surface, which was nearly circular, had a diameter of ths of an inch. To this section, he adapted a reversed syphon: and things being so disposed, he poured in a quantity of mercury, which after a time, and from the effect of the pressure exerted by the sap as it escaped, rose in one of the arms

of the syphon, and remained stationary at the height of thirty-eight inches above its original level. This column of mercury, it is obvious, represents a pressure very much greater than that of our atmosphere.

The ascent of the sap in trees takes place by the woody layers. It is easy to obtain conviction of this by making a plant absorb a watery solution of cochineal. By making sections in the stem at different heights, we can readily trace the colored liquid in its progress; it is undoubtedly the course which the natural sap would have taken. We see no indication of the coloring matter in the pith nor in the bark, the woody tissue alone is colored, sometimes entirely, but more generally in its younger parts only. The dyeing which results from this injection of the wood is in lines, and parallel with the axis of the trunk, like the woody fibres themselves; but in some cases the sap may deviate from the rectilinear course. Hales showed this by the following experiment: upon a tree he made four notches, one above the other; each notch occupied one quarter of the trunk and reached to its centre. In this way the whole of the woody fibres were cut through at different heights, so that to continue its ascent the sap must necessarily experience a series of lateral deviation, which in fact took place.

Thus

The ascending sap of vegetables, as it has hitherto been procured for examination, is an extremely watery fluid, holding in solution very small quantities of divers saline and organic substances. Having attained the leaves, the sap there undergoes modification, and becomes concentrated by losing water. It at the same time experiences, through the agency of the atmospheric air, under the influence of light, a great modification in its constitution. elaborated, the sap takes a descending course; following the liber, it retrogrades towards the soil, and therefore performs a kind of circulation in its passage through the plant. The descending course of the sap is demonstrated by throwing a ligature round the trunk of a tree; after a time there is formed, above the part that is tied, an enlargement which is owing to the accumulation of the principles of the sap; but below it the tree experiences no increase. The descending course of the elaborated sap is no effect of simple gravity; because, if the ligature be thrown around a pendent branch, the enlargement still forms between the ligature and the free extremity of the branch. The descending sap passing through the cortical layers must necessarily contribute to their formation; and it is almost certain, as appears from the capital experiment of Duhamel, that it is the cambium which is changed into liber, and so concurs in the growth of trees. The concentration of the ascending sap, which occurs during its passage through the leaves, by the simple effect of evaporation, is the phenomenon which is spoken of under the name of the exhalation of plants: this exhalation of plants, it is easily understood, is favored by a high temperature, dryness, and motion of the air. In favorable circumstances, the water escapes in the state of vapor. Hales compared the watery exhalation of plants to the perspiration of animals, and made many experiments

to ascertain the quantity of watery vapor which they usually throw off.

Hales planted a sun-flower in an air-tight vessel, the top of which was sealed hermetically by a leaden cover. This cover was pierced by two holes one for the passage of the stem of the plant, the other for the introduction of the water necessary to its growth. For a fortnight the apparatus was regularly weighed, and our ingenious experimenter found that the green parts of the sun-flower threw off on an average about twenty ounces of water in twelve hours of the day. The evaporation was always increased during dry and warm weather; moist air lessened it; during the night season, the evaporation was sometimes no more than three ounces, and it occasionally happened that it was nil.

Vegetable life appears to be intimately connected with the phenomenon of evaporation. From the inquiries which I have myself undertaken on this subject, so well deserving the attention of observers, it would appear that a plant grows only when it transpires, and that in hindering this transpiration, we in fact arrest vegetation.

We now associate with the phenomenon of exhalation the source or accumulation of certain substances which are met with in considerable quantity in the organization of plants, although scarcely a trace of them can be detected in the water with which they are supplied. The water evaporating, leaves these substances behind; and as the mass of liquid imbibed by the roots and exhaled by the green parts is very considerable, it is easy to conceive how they should finally come to be present in rather large quantity.

A portion of the water which a plant in full vigor absorbs, must necessarily enter into its constitution; the water exhaled by the leaves, therefore, cannot equal the whole of that which has been absorbed by the roots. Sennebier endeavored to ascertain the relation which exists between the absorption and the exhalation, and he found in the particular case which he observed, that about of the water absorbed was fixed, and became a constituent part of the vegetable.

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§ II. CHEMICAL PHENOMENA OF VEGETATION. THE chemical phenomena of vegetation are accomplished by the concurrence of the elements of the atmosphere, of water, and of certain organic substances which exist as constituents of the soil.

The action of the atmosphere upon plants presents two phases perfectly distinct from one another; germination, and vegetation properly so called, which last comprises the development, the growth, and the multiplication of species.