Eucalyptus is a diverse genus of trees (rarely shrubs), the members of which dominate the tree flora of Australia. There are more than 700 species of Eucalyptus, mostly native to Australia, with a very small number found in adjacent parts of New Guinea and Indonesia and one as far away as the Philippines. Eucalypts can be found in almost every part of the continent, adapted to all of Australia's climatic conditions; in fact, no other continent is so characterised by a single genus of tree as Australia is by eucalyptus. Many, but far from all, are known as gum trees; other names for various species include mallee, box, ironbark, stringybark, and ash.

Description

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The most readily recognisable characteristics of Eucalyptus species are the distinctive flowers and fruits. The name Eucalyptus means "well-covered"; it describes the bud cap (technically called an operculum). This cap forms from modified petals and falls off as the flower opens. Thus flowers have no petals, decorating themselves instead with many showy stamens. The woody fruits are roughly cone-shaped and have valves at the end which open to release the seeds.

Nearly all eucalypts are evergreen but some tropical species lose their leaves at the end of the dry season. As in other members of the Myrtle family, eucalypt leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Eucalypts also exhibit leaf dimorphism. When young, their leaves are opposite, oval to roundish, and occasionally without a petiole. When one to a few years old, the leaves become alternate, lanceolate to falcate (sickle-shaped), quite slender and pendulous with longer petioles. The adult leaves of most species, as well as the juvenile leaves of some, are the same on both sides, lacking the distinction between upper and lower surfaces shown by the leaves of most plants. Most species do not flower until adult foliage starts to appear; E. cinerea and E. perriniana are notable exceptions.

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