Eucalyptus is a diverse genus of flowering trees and shrubs (including a distinct group with a multiple-stem mallee growth habit) in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia. There are more than 700 species of Eucalyptus, mostly native to Australia, and a very small number are found in adjacent areas of New Guinea and Indonesia and one, Eucalyptus deglupta, ranges north to the Philippines. Only 15 species occur outside Australia, and only 9 do not occur in Australia. Species of Eucalyptus are cultivated throughout the tropics and subtropics including the Americas, Europe, Africa, the Mediterranean Basin, the Middle East, China and the Indian Subcontinent.
Eucalyptus is one of three similar genera that are commonly referred to as "eucalypts," the others being Corymbia and Angophora. Many, but far from all, are known as gum trees because many species exude copious sap from any break in the bark (e.g. Scribbly Gum). The generic name is derived from the Greek words ευ (eu) "well" and καλυπτος (kalyptos) "covered," referring to the operculum on the calyx that initially conceals the flower.
Some Eucalyptus species have attracted attention from global development researchers and environmentalists. Such species have desirable traits such as being fast-growing sources of wood, producing oil that can be used for cleaning and functions as a natural insecticide, or an ability to be used to drain swamps and thereby reduce the risk of malaria. Outside their natural ranges, eucalypts are both lauded for their beneficial economic impact on poor populations:22 and criticised for being "invasive water-suckers", leading to controversy over their total impact.
On warm days Eucalyptus forests are sometimes shrouded in a smog-like mist of vaporised volatile organic compounds (terpenoids); the Australian Blue Mountains take their name from the haze.
