Eucalyptus (Gr. ev well covered), a genus of myrtaceous trees, mostly natives of Australia and the Indian archipelago. The calyx tube is turbinate or campanulate ; sepals united in a calyptra or cover, which separates in flowering by a fissure at the base, carrying the petals with it; stamens indefinite, free, with threadlike filaments; anthers versatile, opening longitudinally; ovary 3-4-celled ; style filiform, stigma small; seeds usually angular, with a membraneous testa. While in many species the flowers are small and inconspicuous, they are large in E. macrocarpa, and rendered showy by the carmine color of the numerous stamens. Sometimes the eucalypti are found in greenhouse collections, but in pot culture they are kept as mere shrubs. E. Preissiana and others are used in Europe for this purpose. The eucalypti are lofty trees, often attaining a height of nearly 250 ft. and a circumference of 70 ft. at the base. The leaves on young trees are opposite, on old usually alternate, coriaceous, entire, and feather-veined. About 100 species are recognized, mostly belonging to Australia, where they form 99 per cent. of the forest vegetation, and afford large supplies of honey to bees.
E. gigantea, the "stringy bark," is a valuable timber tree, and the E. globulus or bluegum is even better. The wood is hard and heavy, and well adapted to cabinet work. The latter has been raised in southern Europe, and has been largely planted in Algeria and in some parts of British India. It succeeds remarkably in California, where its growth is exceedingly rapid. E. resinifera produces a gum much resembling kino, and called Botany Bay kino. E. robusta has cavities in the stem between the annual rings, which contain a brilliant red or vermilion-colored gum. From E. mannifera and E. dumosa, during the dry season only, a kind of manna is obtained resembling the officinal manna in its properties, but of a more agreeable taste. E. 'piperita yields an essential oil sold as a medicine in the eastern bazaars. E. Gunnii when wounded gives a sap cool and refreshing and slightly aperient; the Tasmanians allow this to ferment, when a pleasant beer results. Almost all the species contain much tannin in the bark, which has been profitably extracted in Australia and sent to England. - A new interest has been given to the genus by the discovery of properties in the leaves and bark of several species resembling those of the cinchona or Peruvian bark, and much more abundant.
The E. globulus (bluegum) is the species supposed to be most efficacious in marsh and other fevers, and is known in Spain as the "fever tree." Vauquelin obtained in analysis an essential oil containing eucalyptal or eucalypt camphor, and a resin closely resembling resin of cinchona. This extract yielded a substance capable of neutralizing the strong acids, and forming crystalline salts. The sulphate crystallizes in star-shaped crystals like the sulphate of quinia or cinchona, and presents the green coloration on the action of chlorine and ammonia supposed to be peculiar to that salt. The part of the tree most used in medicine is the leaves dried and powdered, although the bark and even the wood have produced favorable results.