sixth. Thus, in all these combinations, whether it be from the distance of the systems, or of the part, or from the size of the leaves, which diminish as they ascend, we soon find that all leaves are so arranged as to enjoy the free action of the solar light.” ae most plants, when the leaves have ‘cacitssins their physio- — logical functions, they fall, even in the year which witnesses their — birth. But there are others which are not detached till the following year, while others still remain for many years attached to the stem. The leaves of most of the conifers, those of the Box : 4 (Buxus), the Holly (Ilex), of the Orange-trees (Citrus), do not

3 shed them in the year in which they are developed, but are met 4e by a continual growth of new leaves. These plants are never seen. ; naked; they constitute the plants commonly known as evergreens.

In the first of these states the plant is said to be deciduous, from dz, down, and cado, I fall, when the leaves fall before the next

spring. They are marcescent, from marcesco, withering, when they wither before falling, as in the Oak and Beech. They are persistent, from persistens, remaining, standing, when they remain longer than a year. In some plants, as the Cactus, the leaves are shed almost as soon as they appear. These are said to be caduccans, from cado, I fall. The leaf thus dies like all created beings when the purpose for _ which it was created is accomplished. The immediate cause of death seems to be this: the cells of which it is composed have become encrusted with foreign matter, deposited during the processes of digestion and evaporation carried on by the organ, which then becomes incapable of further action.

On the subject of the distribution of evergreen trees, Auguste de St. Hilaire makes the following remarks:—“ As we retire from the tropics, the number of evergreen trees goes on diminishing in rapid succession. At Porto Allegra, near latitude 30° south, I found in the coldest season that the trees nearly all changed their leaves. At San Francisco de Paula, near the Rio Grande, in 34°, nearly one-third of the ligneous vegetation had lost their leaves; and finally, at two degrees farther south, a tenth of the trees only preserved their foliage. At Montpellier, the fields in winter are hot yet deprived of verdure; and Lisbon, Madeira, and Teneriffe

present a still more considerable number of trees always green. It must not be supposed, however, that in the tropics all the trees are evergreens. Even in the vast forests which occupy the Brazilian coast, and where vegetation is maintained in continual activity by its two principal agents, heat and moisture, there exist trees, such as some of the Begnoniacex, which lose every year, like - our trees, all their leaves at once, but immediately after they are covered with flowers, and in a very short time these are succeeded by new foliage. I speak here of woods growing in equinoxial regions, where, as with us, rain and drought have no determinate period. In countries, on the other hand, where six months’ continual rain is succeeded by uninterrupted dry weather, there are woods which every year remain for a considerable time destitute of verdure, and the traveller who traverses them 18 scorched by the ardent blaze of the equinoxial zone, while he has before his eyes the leafless image of European forests during winter. We have even seen this excessive drought continue during two years, and the trees remain for two years without their foliage.”

But evergreen trees are only exceptional in the vegetable world. Most trees, shrubs, and herbaceous plants are without their leaves during one half the year. When the leaves have performed their functions, when the fruits have appeared, matured, and ripened, vegetation has entered into a new phase; the leaves lose their brilliant green and assume their autumnal tint, sometimes clothed, however, in colours of accidental, though transitory, brightness. e green, when it is persistent, is more grave and sober in its hue; it becomes brown in the Walnut, it takes a whitish tone ™ the Honeysuckle. The leaves of other plants, as the Ivy, the Sumach (Rhus Cotinus), the Dog-wood tree (Cornus), become clothed in a reddish tint; they become yellow in the Maple (Acer campestre), and many others of our forest trees. But whatever may be the variety of shades which leaves take in their decay, a certain air of sadness pervades these ornaments of our fields, which proclaims their approaching dissolution, and betrays the imminence of the cold season. Oold and humidity will soon arrest the SP and disorganise the petiole;,the leaves, withered and deformed, will soon cumber the ground, to be blown hither and thither by: the wind. It is the season of the fall of the leaf with all 1° —

melancholy associations which the yes have depicted in so many well-known stanzas. Nevertheless, leaves when separated from the vegetable which has given birth to and matured them, are not lost to the earth which receives them. Everything in nature has its use, and leaves have their uses also in the continuous circle of vegetable reproduction. The leaves which strewed the ground at the foot of the trees, or which have been disseminated by the autumn winds over the naked country, perish slowly upon the soil, where they are transformed. into the /umus, or vegetable mould, indispensable to the life of plants. Thus the dédris of vegetables prepares for the coming and formation of a new vegetation. Death prepares for new life; the first and the last give their hands, so to speak, in _ vegetable nature, and form the mysterious circle of organic life which has neither beginning nor end.

But let us return to the general study of leaves. We have still to note a last and most important phenomena in the variety of their functions. We speak of the spontaneous movements executed. by leaves under many circumstances.

Leaves almost always assume the horizontal position. They have an upper surface turned towards the heavens, and a lower surface looking on the earth. -This position is so natural, and hence so necessary, that leaves take it of themselves during. day and night when from any accidental cause that position has been lost. If we place a plant in an apartment lighted by a single window, it is soon observed that all the leaves direct their upper surface towards the light. This is an experiment which our readers can readily repeat for themselves. But leaves perform other movements, equally remarkable, on which we must pause an instant. The study of these movements has been, as we shall see, the subject of some curious and interesting experiments.

Dutrochet, having placed a young pea in a chamber lighted on one side only, soon observed that the leaf inclined itself towards _ the light, and directed its petiole towards the heavens, or rather inclined it towards the dark part of the chamber. The tendril was now nearly straight, now curved and arched, presenting very irregular motions. Dutrochet placed certain fixed indicators both over the tendril and over the petiole, and at the insertion of the __

two folioles. He was thus able to state what direction it took in retiring from the fixed indicators. He soon observed that the summit of the petiole described an ellipsoide curve in the air while the tendril which terminated it had various motions. He soon observed that the. internode of this leaf participated in this movement of revolution, and that it was even the principal agent in it. The merethale or internode and the leaf then produced: by their general movements a sort of cone, whose summit occupied the lower part of the metherale, and whose base was the curve described in the air by the summit of the petiole. The tendril, during the movement of revolution, constantly described its point towards the bottom of the chamber, thus shunning the light, turning itself when the movement of its revolution in leading its point near the window tended in the direction of the light.

‘This revolution was effected in a period varying with the terhperature and the age of the leaf. It lasted from an hour to twenty minutes at a temperature of 24°; from seven to eleven hours when the temperature was lowered to 5° or 6°. The extent of the revolutions diminished in proportion as the temperature decreased. ‘ What is the cause of this revolving movement a asks Dutrochet. ‘It is not revealed to our eyes. It is some vital and internal exciting cause. Not only does the light contribute nothing towards the production of the movement, but it operates against it, and when unusually vivid it seems to stop it.”

Dutrochet observed the movement of revolution in the tendril of the Bryone, and also in the cultivated Cucumber. In the Bryone - the tendril moved in very varied directions, sometimes moving horizontally, sometimes upwards, sometimes downwards, and some- — times directing its points towards the heavens, then taking any curve whatever in order to take immediately a curve in the contrary direction. The tendrils of the cucumber moved like the hands of a watch placed on the dial plate, directing its points successively towards every point of the compass, sometimes to the right, sometimes to the left. But it defied all the sagacity, all the quick and deliberate observation of Dutrochet, to discover the slow and obscure movements of which we speak, or their origin.

The spontaneous movements which we have now to note In certain vegetables, are much more apparent. Let us speak first. -

the movement of the plant known as the Desmodium gyrans, Fig. 121. This plant belongs to the family of Leguminacee. It was discovered in Bengal, in the neighbourhood of Daca, by an English lady, named Morison, whose devotion to natural history had led her to undertake the Indiah voyage, and who died on one of her botanical excursions. The leaves of the Desmodium are composed of three folioles. The terminal foliole is very large, and the laterals very small, but these last are almost always in motion. They execute little jerks somewhat analagous to the movements of the seconds of a watch. One of the folioles rises and the other descends at the same time, and with a corresponding force. When the first begins to descend, the other begins to rise. The large foliole moves // also, inclining itself now We. 1G Dapaodlien gynnen: to the right, now to the left, but by a continuous and very slow movement when compar with that of the lateral folioles. This singular mechanism endures throughout the life of the plant. It exercises itself day and night, through drought and humidity. The warmer and more humid is the day, the more lively are its movements. In India the plant has been known to make sixty jerks in the minute.

This curious plant, which was introduced into Europe for the first time in 1777, is cultivated in the Museum of Natural History of Paris. The auditors of M. Brougmart’s course of lectures have frequentlyhad their attention directed to the strange phenomenon of which it is the subject. Its movements occur spontaneously and without any apparent cause. But there are movements in other plants which are produced by external causes. Such are those of the Catchfly andthe Sensitive plant.

The Dionea muscipula, Fig. 122, is originally from South America. Its leaves, which are spread out on the soil near the roots,

are composed of two parts—the one elongated, which may be considered as a sort of petiole, the other larger and broader and nearly circular, formed like two trap-nets, which are united at the base by a nervure, fashioned like a hinge, and furnished round the edge with rough hairy cils. In the upper surface these plates are furnished with certain small glands, whence exudes a viscous —

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