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Eucalyptus is a genus of tree very commonly found in Australia. There are close to 600 species of eucalyptus native to Australia, and a much smaller number in New Guinea. Eucalyptus can be found in every part of the country, adapted to virtually all of Australia's climatic conditions. Some species are eaten by koalas. Eucalypts have become naturalized in California and Israel.
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. Koalas are immune to it.
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, 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.
Eucalypts originated between 35 and 50 milion years ago, their rise coinciding with an increase in fossil charcoal deposits (suggesting that fire was a factor even then), but 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 Eucalypt soon came to account for roughly seventy percent of Australian forest. Hence the quandary, human development in forests that rely on bush fires.
The plant was introduced to the West by Joseph Banks, a botanist on the Cook expedition in 1768.
Specimens of the Australian Mountain Ash, Eucalyptus regnans, are among the tallest trees in the world at over 90m. This makes them the tallest flowering plants - the taller coast redwood is a conifer.