Eucalyptus Essential Oil
Scientific Specifications
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Kingdom
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Plantae
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(unranked)
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Angiosperms
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(unranked)
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Eudicots
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(unranked)
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Rosids
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Order
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Myrtales
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Family
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Myrtaceae
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Subfamily
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Myrtoideae
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Tribe
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Eucalypteae
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Genus
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Eucalyptus
L'HÃ'r.
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Species
Eucalyptus (pronounced /ju/is a diverse genus of flowering trees (and a few shrubs) 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 parts of New Guinea and Indonesia and one as far north as the Philippines islands. 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, England, 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 name eucalyptus comes from the Greek:, meaning "well covered".
Eucalyptus has attracted attention from global development researchers and environmentalists. It is a fast-growing source of wood, its oil can be used for cleaning and functions as a natural insecticide, and it is sometimes 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 and derided for being invasive water-suckers, leading to controversy over their total impact.
Leaves
Nearly all Eucalyptus are evergreen but some tropical species lose their leaves at the end of the dry season. As in other members of the Myrtle family, Eucalyptus leaves are covered with oil glands. The copious oils produced are an important feature of the genus.
Leaf and flowerbud cluster of Eucalyptus angophoroides
Eucalyptus tetragona, showing glaucous leaves & stems
The leaves on a mature Eucalyptus plant are commonly lanceolate, petiolate, apparently alternate and waxy or glossy green. In contrast, the leaves of seedlings are often opposite, sessile and glaucous. But there are many exceptions to this pattern. Many species such as E. melanophloia and E. setosa retain the juvenile leaf form even when the plant is reproductively mature. Some species, such as E. macrocarpa, E. rhodantha and E. crucis, are sought-after ornamentals due to this lifelong juvenile leaf form. A few species, such as E. petraea, E. dundasii and E. lansdowneana, have shiny green leaves throughout their life cycle. E. caesia exhibits the opposite pattern of leaf development to most Eucalyptus, with shiny green leaves in the seedling stage and dull, glaucous leaves in mature crowns. The contrast between juvenile and adult leaf phases is valuable in field identification.
Four leaf phases are recognised in the development of a Eucalyptus plant: the seedling, "juvenile", "intermediate" and "adult" phases. However there is no definite transitional point between the phases. The intermediate phase, when the largest leaves are often formed, links the juvenile and adult phases.[7] In all except a few species, the leaves form in pairs on opposite sides of a square stem, consecutive pairs being at right angles to each other (decussate). In some narrow-leaved species, for example E. oleosa, the seedling leaves after the second leaf pair are often clustered in a detectable spiral arrangement about a five-sided stem. After the spiral phase, which may last from several to many nodes, the arrangement reverts to decussate by the absorption of some of the leaf-bearing faces of the stem. In those species with opposite adult foliage the leaf pairs, which have been formed opposite at the stem apex, become separated at their bases by unequal elongation of the stem to produce the apparently alternate adult leaves
An essential oil extracted from Eucalyptus leaves contains compounds that are powerful natural disinfectants and can be toxic in large quantities. Several marsupial herbivores, notably koalas and some possums, are relatively tolerant of it. The close correlation of these oils with other more potent toxins called formylated phloroglucinol compounds allows koalas and other marsupial species to make food choices based on the smell of the leaves. For koalas, these compounds are the most important factor in leaf choice.
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