Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/129

LEAVES.] 13 T A N Y 119 and light. The fluids so exposed are elaborated, aud thus fitted for the formation of the various vegetable tissues and secretions. For the proper performance of this func tion the structure of the leaves, and their arrangement on the stem and branches, render them well adapted. The cells in the lower side of a leaf where stomata exist are chiefly concerned iu the aeration of the sap, whilst other assimilative processes go on in the upper cells. The ela boration of fluids in the leaves necessarily implies inter change of their constituents with those of the surrounding atmosphere ; hence two processes are inevitable a passing inwards into the leaf of the atmospheric elements by a process of absorption, and an outward current of the com ponents of the plant-juices by a process of exhalation. The absorption of carbonic acid, water, and other fluids is carried on by the leaves, chiefly through their stomata, and most rapidly by the under surface of ordinary leaves in which the cuticle is thinnest, the cellular tissue least con densed, and stomata most abundant ; the upper surface of the leaf, which usually presents a polished and dense epidermis with few stomata, taking little part in such a process. An exhalation of both liquids and gases also takes place from the leaves, regulated by the number and the size of stomata as well as by the nature of the epidermis. The process of transpiration of fluids imparts moisture to the atmosphere, and hence the difference between the air of a wooded country and that of a country deprived of forests. Thus leaves have an important influence upon the climate of a country. In darkness little or no trans piration takes place, and in diffuse daylight it is less than in the sun s rays. The exhalation of gases constitutes the process of respiration. The nature and amount of the gases respired depends both on the circumstances in which the leaves are placed and on the condition of the plant. But normally at all times there seems to be a respiration of carbonic acid, which, under the influence of light, is at once decomposed by the green parts of the plant, the carbon being fixed and the oxygen set free ; consequently, in darkness no oxygen is eliminated. Leaves, after per forming their functions for a certain time, wither and die. In doing so they frequently change colour, and hence arise the beautiful and varied tints of the autumnal foliage. This change of colour is chiefly occasioned by the diminished circulation in the leaves, and the higher degree of oxida tion to which their chlorophyll has been submitted. Leaves which are articulated with the stem, as in the Walnut and Horse-chestnut, fall and leave a scar, while those which are continuous with it remain attached for some time after they have lost their vitality, as in the Beech. Most of the trees of Great Britain have decidu ous leaves, their duration not extending over more than a few months, while in trees of warm climates the leaves often remain for two or more years. In tropical countries, however, many trees lose their leaves iu the dry season. The period of defoliation varies in different countries according to the nature of their climate. Trees which are called evergreen, as Pines and Evergreen-oak, are always deprived of a certain number of leaves at inter vals, sufficient being left, however, to preserve their green appearance. The cause of the fall of the leaf in cold climates seems to be deficiency of light and heat in winter, which causes a cessation in the functions of the cells of the leaf ; its fluids disappear by evaporation ; its cells and vessels become contracted and diminished in their calibre ; various inorganic matters accumulate in the textures; the whole leaf becomes dry ; its parts lose their adherence ; a process of disjunction takes place by a folding inwards of the tissue at the point where the leaf joins the stem or branch which gradually extends until complete separation takes place, and the leaf either falls by its own weight or is detached by the wind. In warm climates the dry season gives rise to similar phenomena. II. ORGANS OF REPRODUCTION. We now proceed to pass in review the reproductive organs of plants. In Phanerogamous plants, as already mentioned, these organs are conspicuous, and constitute what is known as the flower ; in Cryptogamous plants they are inconspicuous. All Dicotyledonous and Mouocoty- ledonous plants are included in the former ; Acotyledonous and Thallogenous plants compose the latter. The structures which go to form these organs are not, however, formations of a new type, but are merely modifications of those struc tures which we have already considered under the nutritive organs. For example, the various parts of the flower in Phanerogams are really phyllomes, the supporting structures of the flower are caulomes, the spore-bearing sac of many Cryptogams is a trichome ; and in this way a morphological equivalency may be traced betwixt the two series of organs. Further, the difference betwixt the reproductive organs of Phanerogams and those of Cryptogams is one more of degree of differentiation than of actual morphological difference. In Phanerogams the flowers or floral axes are produced from flower-buds, just as leaf-shoots arise from leaf-buds. These two kinds of buds have a resemblance to each other as regards the arrangement and the development of their parts ; and it sometimes happens, from injury aud other causes, that the part of the axis which, in ordinary cases, would produce a leaf-bud, gives origin to a flower- bud. A flower-bud has not in ordinary circumstances any power of extension by the development of its central cel lular portion. In this respect it differs from a leaf-bud. In some cases, however, of monstrosity, especially seen in the Rose (fig. 145) and Geum, the central part is prolonged, and bears leaves or flowers. In such cases the flowers, so far as their functional capa bilities are concerned, arc usually abortive. Flower-buds, like leaf- buds, are produced in the axil of leaves, which are called floral leaves, bracts, or hypsophyllary leaves. The term bract is pro perly applied to the leaf from which the primary floral axis, whether simple or branched, arises, while the leaves which arise on the axis between the bract an d the outer en velope of the flower are bracteolei or bractlets. Bracts sometimes do not differ from the ordinary leaves, and are then called leafy, as iu Veronica hederi- folia, Vinca, Anagallis, and Ajuga. Like leaves they are entire or divided. In general as regards their form and appearance, they differ from ordinary leaves, the difference being greater in the upper than in the lower branches of an inflorescence. They are distinguished by their position at the base of the flower or flower-stalk. Their phyllotaxis is similar to that of the leaf. When the flower is sessile the bracts arc often Bracts. Fig. 145. Proliferous or monstrous Rose, showing the prolongation of the axis beyond the flower. C, calyx transformed into leaves; P, petals multiplied at the ex pense of the stamens, which are reduced in number; F, coloured leaves repre senting abortive carpels; A, axis pro longed, bearing an imperfect flower at

its apex.