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Eucalyptus is a diverse genus of tree, the members of which dominate the tree flora of Australia. There are almost 600 species of eucalyptus native to Australia, and a much smaller number in New Guinea. Eucalyptus can be found in every part of the continent, adapted to all of Australia's climatic conditions. Eucalypts have been introduced to many parts of the world, notably California and Israel, where they are common, with several species being invasive and causing major problems for local ecology.

All eucalypts are evergreens. Eucalyptus leaves contain an essential oil which is a powerful natural disinfectant. The oil is readily distilled from the leaves and can be used for cleaning, deodorising, and in very small quantities in food supplements, especially cough drops and decongestants. Eucalyptus oil is fearsomely toxic in excessive quantities, but several of the marsupial herbivores, notably Koalas and some possums are immune to it. (In fact - while koalas and a few other species are more able to tolerate eucalyptols, their choice of trees is largely based upon sampling the foliage for low toxicity.)

On warm days vaporised eucalyptus oil rises above the bush to create the characteristic distant blue haze of the Australian landscape. Eucalyptus oil is highly flammable and bush fires can travel through the oil-rich air of the tree crowns with an explosive power that firefighters can do little about. Despite their vulnerability to fire, most Eucalypts are dependent on it for spread and regeneration: both from reserve buds under the bark, and from fire-germinated seeds sprouting in the ashes.

File:Euc.uk.250pix.jpg
Eucalyptus gunii in an English garden.
The lower part of this tree has become
clothed in ivy.
Larger version

File:Eucalypt-s.jpg
Typical woodland eucalypts.
Larger picture

Eucalypts originated between 35 and 50 million years ago, not long after Australia-New Guinea separated from Gondwana, their rise coinciding with an increase in fossil charcoal deposits (suggesting that fire was a factor even then), but they remained a minor component of the Tertiary rainforest until about 20 million years ago when the gradual drying of the continent and depletion of soil nutrients led to the development of a more open forest type, predominantly Casuarina and Acacia species. With the arrival of the first humans about 50 thousand years ago, fires became much more frequent and the fire-loving eucalypts soon came to account for roughly 70% of Australian forest.

Eucalypts were introduced to the West by Sir Joseph Banks, botanist on the Cook expedition in 1770. The name Eucalyptus means "well covered"; it describes the bud cap. A small genus of similar trees, Angophora, have also been known since the 18th century. In 1995 new evidence, largely, genetic, indicated that some prominent Eucalypt species were actually more closely related to the Angophoras than to the other eucalypts; they were split off into the new genus Corymbia. Although separate, the three groups are allied and it remains acceptable to refer to the members of all three genera, Angophora, Corymbia and Eucalyptus as "eucalpyts".

Specimens of the Australian mountain ash, Eucalyptus regnans, are among the tallest trees in the world at over 90 metres, making them the tallest of all flowering plants— the still taller coast redwood is a conifer.

The coolibah or coolabah tree of Waltzing Matilda is a eucalyptus, E. microtheca or E. coolabah.

See also:

• Angophora
• Corymbia
• Mountain ash