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. Leaves which are articulated with the stem, as in the walnut and horse-chestnut, fall and leave a scar, while those which are con tinuous 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 the 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 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. Be fore its fall the leaf has become dry owing to loss of water; cer tain of the materials of the leaf pass back into the stem and are available for further growth. The leaf scar is protected by a change of a corky nature (suberization) in the walls of the exposed cells. (A. B. R. ; V. H. B.) LEAF-CUTTER BEE, the name applied to solitary bees of the genus Megachile, which line their nests with portions of leaves or flower petals. These they cut out with their mandibles
and fit together into thimble shaped cells one within the other, each one containing an egg and some pollen and honey paste. The best known species, on both sides of the Atlantic, is M. centunctulus, which utilizes rose leaves. (See BEE ; HYMENOPTERA.) LEAF-INSECT, the name given to orthopterous insects of the family Phasmidae, referred to the group Phyllinae and charac terized by the presence of lateral laminae upon the legs and abdomen, which, in association with green colouring-matter, im part a leaf-like appearance to the whole insect. In the female this deceptive resemblance is enhanced by the large size and foli aceous form of the front wings which, when at rest, edge to edge on the abdomen, forcibly suggest in their veining the mid rib and veins 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. Since leaf-insects are purely vegetable feeders it is probable that their resemblance to leaves is for concealment from enemies. Their egg capsules are likewise protected by their similarity to various seeds. Leaf-insects range from India to the Seychelles on the one side, and to the Fiji Islands on the other.