Fig. 45.—Transverse section of a bud, showing two leaves arranged in a supervolute manner.
The arrangement of the leaves in the bud is termed vernation or prefoliation. In considering vernation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated in figs. 36 to 45, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and called reclinate or replicate; or rolled up in a circular manner from apex to base, as in ferns (fig. 36), and called circinate; or folded laterally, conduplicate (fig. 37), as in oak; or it has several folds like a fan, plicate or plaited (fig. 38), as in vine and sycamore, and in leaves with radiating vernation, where the ribs mark the foldings; or it is rolled upon itself, convolute (fig. 39), as in banana and apricot; or its edges are rolled inwards, involute (fig. 40), as in violet; or outwards, revolute (fig. 41), as in rosemary. The different divisions of a cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of vernation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of vernation are produced. Sometimes they are nearly in a circle at the same level, remaining flat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise to valvate vernation. At other times they are at different levels, and are applied over each other, so as to be imbricated, as in lilac, and in the outer scales of sycamore; and occasionally the margin of one leaf overlaps that of another, while it in its turn is overlapped by a third, so as to be twisted, spiral or contortive. When leaves are applied to each other face to face, without being folded or rolled together, they are appressed. When the leaves are more completely folded they either touch at their extremities and are accumbent or opposite (fig. 42), or are folded inwards by their margin and become induplicate; or a conduplicate leaf covers another similarly folded, which in turn covers a third, and thus the vernation is equitant (fig. 43), as in privet; or conduplicate leaves are placed so that the half of the one covers the half of another, and thus they become half-equitant or obvolute (fig. 44), as in sage. When in the case of convolute leaves one leaf is rolled up within the other, it is supervolute (fig. 45). The scales of a bud sometimes exhibit one kind of vernation and the leaves another. The same modes of arrangement occur in the flower-buds.
Leaves, after performing 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 oxidation 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. Most of the trees of Great Britain have deciduous 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 in 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 intervals, 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. The fall is directly caused by the formation of a layer of tissue across the base of the leaf-stalk; the cells of this layer separate from one another and the leaf remains attached only by the fibres of the veins until it becomes finally detached by the wind or frost. Before its fall the leaf has become dry owing to loss of water and the removal of the protoplasm and food substances to the stem for use next season; the red and yellow colouring matters are products of decomposition of the chlorophyll. Inorganic and other waste matters are stored in the leaf-tissue and thus got rid of by the plant. The leaf scar is protected by a corky change (suberization) in the walls of the exposed cells.
(A. B. R.)
LEAF-INSECT, the name given to orthopterous insects of the family Phasmidae, referred to the single genus Phyllium and characterized by the presence of lateral laminae upon the legs and abdomen, which, in association with an abundance of green colouring-matter, impart a broad and leaf-like appearance to the whole insect. In the female this deceptive resemblance is enhanced by the large size and foliaceous form of the front wings which, when at rest edge to edge on the abdomen, forcibly suggest in their neuration the midrib and costae of an ordinary leaf. In this sex the posterior wings are reduced and functionless so far as flight is concerned; in the male they are ample, membranous and functional, while the anterior wings are small and not leaf-like. The freshly hatched young are reddish in colour; but turn green after feeding for a short time upon leaves. Before death a specimen has been observed to pass through the various hues of a decaying leaf, and the spectrum of the green colouring matter does not differ from that of the chlorophyll of living leaves. Since leaf-insects are purely vegetable feeders and not predaceous like mantids, it is probable that their resemblance to leaves is solely for purposes of concealment from enemies. Their egg capsules are similarly protected by their likeness to various seeds. Leaf-insects range from India to the Seychelles on the one side, and to the Fiji Islands on the other.
(R. I. P.)
LEAGUE. 1. (Through Fr. ligue, Ital. liga, from Lat. ligare, to bind), an agreement entered into by two or more parties for mutual protection or joint attack, or for the furtherance of some common object, also the body thus joined or “leagued” together. The name has been given to numerous confederations, such as the Achaean League (q.v.), the confederation of the ancient cities of Achaia, and especially to the various holy leagues (ligues saintes), of which the better known are those formed by Pope Julius II. against Venice in 1508, often known as the League of Cambrai, and against France in 1511. “The League,” in French history, is that of the Catholics headed by the Guises to preserve the Catholic religion against the Huguenots and prevent the accession of Henry of Navarre to the throne (see [France]: History). “The Solemn League and Covenant” was the agreement for the establishment of Presbyterianism in both countries entered into by England and Scotland in 1643 (see [Covenanters]). Of commercial leagues the most famous is that of the Hanse towns, known as the Hanseatic League (q.v.). The word has been adopted by political associations, such as the Anti-Corn Law League, the Irish Land League, the Primrose League and the United Irish League, and by numerous social organizations. “League” has also been applied to a special form of competition in athletics, especially in Association football. In this system clubs “league” together in a competition, each playing every other member of the association twice, and the order of merit is decided by the points gained during the season, a win counting two and a draw one.