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OF THE LEAVES.
LEAVES are at once organs of respiration, digestion, and nutrition. They elaborate the crude sap impelled into them from the stem, decomposing its water, adding to it carbon, and exposing the whole to the action of air; and while they supply the necessary food to the young tissue that passes downwards from them and from the buds, in the form of alburnum and liber, they also furnish nutriment to all the parts immediately above and beneath them. There are many experiments to show that such is the purpose of the leaves. If a number of rings of bark are separated by spaces without bark, those which have leaves upon them will live much longer than those which are destitute of leaves. If leaves are stripped from a plant before the fruit has commenced ripening, the fruit will fall off and not.ripen. If a branch is deprived of leaves for a whole summer, it will either die or not increase in size perceptibly. The presence of cotyledons, or seminal leaves, at a time when no other leaves have been formed for nourishing the young plant, is considered a further proof of the nutritive purposes of leaves : if the cotyledons are cat off, the seed will either not vegetate at all, or slowly and with great difficulty; and if they are injured by old age, or any other circumstance, this produces a languor of habit which only ceases with the life of the plant, if it be an annual. This is the reason why gardeners prefer old melon and cucumber seeds to new ones : in the former the nutritive power of the seed-leaves is impaired, the young plant grows slowly, a languid circulation is induced from the beginning; by which excessive luxuriance is checked, and fruit formed rather than leaves or branches.
Nothing can be more admirable than the adaptation of leaves to such purposes as those just mentioned. It has been already shown, in speaking of the anatomy of a leaf, that in most cases it consists of a thin plate of cellular tissue pierced by air vessels and woody tissue, and enclosed within a hollow empty stratum of cells forming epidermis. Beneath the upper epidermis the component bladders of the parenchyma are compactly arranged perpendicular to the plane of the epidermis, and have but a small quantity of air cavities among them. Beneath the lower epidermis the parenchyma is loosely arranged parallel with it, and is full of air chambers communicating with the stomates. The epidermis prevents too rapid an evaporation beneath the solar rays, and thickens when it is especially necessary to control evaporation more powerfully than usual; thus in the Oleander, which has to exist beneath the fervid sun of Barbary, in a parched country, the epidermis is composed of not less than three layers of thick-sided cells. To furnish leaves with the means of parting with superfluous moisture, at periods when the epidermis offers too much resistance, there are stomates which act like valves, and open to permit its passage: or when, in dry weather, the stem does not supply fluid in sufficient quantity from the soil for the nourishment of the leaves, these same stomates open themselves at night, and allow the entrance of atmospheric moisture, closing when the cavities of the leaf are full. In submersed leaves, in which no variation can take place in the condition of the medium in which they float, both epidermis and stomates would be useless, and accordingly neither exists. For the purpose of exposing the fluids contained in leaves to the influence of air, the epidermis would frequently offer an insufficient degree of surface. In order, therefore, to increase the quantity of surface exposed, the tissue of the leaf is cavernous, each stomate opening into a cavity beneath it, which is connected with multitudes of intercellular passages. But, as too much fluid might be lost by evaporation in parts exposed to the sun, we find that the cells of the upper stratum of parenchyma only expose their ends to the epidermis, and interpose a barrier between the direct rays of the sun and the more lax respiring portion forming the under stratum. It is not improbable, moreover, that those cells which form the upper stratum perform a function analogous to that of the stomach in animals, digesting the crude matter they receive from the stem; and that the lower stratum takes up the matter so altered, and submits it to the action of the atmosphere, which must enter the leaf purely by means of the stomates. Nor are the stomates and the cavernous parenchyma of the leaf the only means provided for the regulation of its functions. Hairs, no doubt, perform no mean office in their economy. In some cases these processes seem destined only for protection against cold, as in those plants in which they only clothe the buds and youngest leaves, falling away as soon as the tender parts have become hardened; but it can hardly be doubted that in many others they are absorbent organs, intended to collect humidity from the atmosphere. In succulent plants, or in such as grow naturally in shady places, where moisture already exists in abundance, they are usually wanting; but in hot, dry, exposed places, where it is necessary that the leaf should avail itself of every means of collecting its food, there they abound, lifting up their points and separating at the approach of the evening dews, but again falling down, and forming a layer of minute cavities above the epidermis, as soon as the heat of the sun begins to be perceived.
Whether or not leaves have the power of absorbing atmospheric fluid, independently of their hairs, is a matter of doubt. By some it is believed that they do possess such a power, and that absorption takes place indifferently by either the upper or under surface of the leaf, but that some plants absorb more powerfully by one surface than by the other. Bonnet found that, while the leaves of Arum, the Kidneybean, the Lilac, the Cabbage, and others, retained their verdure equally long whichever side was deprived of the power of absorption, the Plantago, some Verbascums, the Marvel of Peru, and others, lost their life soonest when the upper surface was prevented from absorbing; and that, in a number of trees and shrubs, the leaves were killed very quickly by preventing absorption by the lower surface. But others contend that Bonnet's experiments merely produced a hindrance of evaporation in some cases, and of respiration in others; and that leaves have, in fact, no power of attracting fluid. In proof of this it is urged, that, if leaves are made to float on coloured infusions, no colouring matter enters them. Considering, however, the thinness of the epidermis of many plants, and the great permeability of vegetable membrane in general, it can hardly be doubted that they do possess the power of absorption which Bonnet contends for. This seems to be proved by the effects obviously produced by a shower of rain in the summer, or by syringing the fading plants in a hothouse.
In their growth, leaves usually increase in length by addition to the base, the apex altering but little after it is originally formed. Upon this subject there is a paper by Steinheil which deserves to be consulted (Ann. Sc., n. S., viii. 257.). This is only what theory would necessarily lead to, when it is considered that a leaf is an expansion of the epiphlæum (see p. 89.) and mesophlæum, its apex representing the external or ungrowing part of those cortical layers, and its base their interior or growing part,
Leaves usually are so placed upon the stem that their upper surface is turned towards the heavens, their lower towards the earth ; but this position varies occasionally. In some plants they are imbricated, so as to be almost parallel with the stem; in others they are deflexed till the lower surface becomes almost parallel with the stem, and the upper surface is far removed from opposition to the heavens, A few plants, moreover, invert the usual position of the leaves by twisting the petiole half round, so that either the two margins become opposed to earth and sky, or the lower surface becomes uppermost: the former is especially the case with plants bearing phyllodia, or spurious leaves.
At night a phenomenon occurs in plants which is called their sleep: it consists in the leaves folding up and drooping, as those of the Sensitive Plant when touched. This scarcely happens perceptibly except in compound leaves, in which the leaflets are articulated with the petiole, and the petiole with the stem: it is supposed to be caused by the absence of light, and will be farther spoken of under the head of Irritability.
After the leaves have performed their functions, they fall off: this happens at extremely unequal periods in different species. In some they all wither and fall off by the end of a single season; in others, as the Beech and Hornbeam, they wither in the autumn, but do not fall off till the succeeding spring; and, in a third class, they neither wither nor fall off the first season, but retain their verdure during the winter, and till long after the commencement of another year's growth : these last are our evergreens. Mirbel distinguishes leaves into three kinds, as characterised by their periods of falling:
1. Fugacious, or caducous, which fall shortly after their appearance; as in Cactus.
2. Deciduous, or annual, which fall off in the autumn; as the Apple.
3. Persistent, evergreen, or perennial, which remain perfect upon the plant beyond a single season; as Holly, common Laurel, &c.
With regard to the cause of the fall of the leaf a number of explanations have been given, which may be found in Willdenow's Principles of Botany, p. 336. There are, however, only two much worth recording; those of Du Petit Thouars and De Candolle.
If you watch the progress of a tree, of the Elder, for example, says the former writer, you will perceive that the lowest leaves upon the branches fall long before those at the extremities. The cause of this may be, perhaps, explained upon the following principle:- In the first instance, the base of every leaf reposes upon the pith of the branch, to the sheath of which it is attached. But, as the branch increases in diameter by the acquisition of new wood, the space between the base of the leaf and the pith becomes sensibly augmented. It has, therefore, been necessary that the fibres by which the leaf is connected with the pith should lengthen, in order to admit the deposition of wood between the bark and the pith. Now how does this elongation take place? As the bundles of fibres which run from the pith into the leaf-stalk are at first composed only of spiral vessels, it is easy to conceive that they may be susceptible of elongation by unrolling. And in this seems to lie the mystery of the fall of the leaf; for the moment will come when the spiral vessels are entirely unrolled, and incapable of any further elongation: they will, therefore, by the force of vegetation, be stretched until they snap, when