Eucalyptus (From Greek, meaning "well covered") is a diverse genus of trees (and a few shrubs), the members of which dominate the tree flora of Australia. There are more than seven hundred species of Eucalyptus, mostly native to Australia,
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 in reference to the habit of many species to exude copious sap from any break in the bark.
Eucalyptus, outside of their natural ranges eucalypts are both lauded for its beneficial economic impact on poor populations and derided for being an invasive water-sucker, leading to controversy over its total impact and future. 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 malaria risk.
About
A Eucalyptus may be mature as a low shrub or as a very large tree. There are three main habit and four size categories that species can be divided into.
As a generalisation "forest trees" are single-stemmed and have a crown forming a minor proportion of the whole tree height. "Woodland trees" are single-stemmed although they may branch at a short distance above ground level.
Tree sizes follow the convention of:
• Small - to 10 metres in height
• Medium sized - 10 to 30 metres in height
• Tall - 30 to 60 metres in height
• Very Tall - over 60 metres in height
Eucalyptuses 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.
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.
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.
The most readily recognisable characteristics of Eucalyptus species are the distinctive flowers and fruit (capsules or "gumnuts"). Flowers have numerous fluffy stamens which may be white, cream, yellow, pink or red; in bud the stamens are enclosed in a cap known as an operculum which is composed of the fused sepals or petals or both. Thus flowers have no petals, decorating themselves instead with the many showy stamens. As the stamens expand the operculum is forced off, splitting away from the cup-like base of the flower; this is one of the features that unites the genus. The name Eucalyptus, from the Greek words eu-, well, and kaluptos, cover, meaning "well-covered", describes the operculum. The woody fruits or capsules, are roughly cone-shaped and have valves at the end which open to release the seeds. Most species do not flower until adult foliage starts to appear; Eucalyptus cinerea and Eucalyptus perriniana are notable exceptions.
The appearance of Eucalyptus bark will vary with the age of the plant, the manner of bark shed, the length of the bark fibres, the degree of furrowing, the thickness, the hardness and the colour. All mature eucalypts put on an annual layer of bark, which contributes to the increasing diameter of the stems. In some species the outermost layer dies and is annually deciduous either in long strips (as in Eucalyptus sheathiana) or in variably sized flakes (E. diversicolor, E. cosmophylla or E. cladocalyx). These are the gums or smooth-barked species. The gum bark may be dull, shiny or satiny (as in E. ornata) or matt (E. cosmophylla). In many species the dead bark is retained. Its outermost layer gradually fragments with weathering and sheds without altering the essentially rough barked nature of the trunks or stems - for example E. marginata, E. jacksonii, E. obliqua and E. porosa.
Many species are ‘half-barks’ or ‘blackbutts’ in which the dead bark is retained in the lower half of the trunks or stems - for example, E. brachycalyx, E. ochrophloia and E. occidentalis - or only in a thick, black accumulation at the base, as in E. clelandii. Some species in this category - for example - E. youngiana and E. viminalis - the rough basal bark is very ribbony at the top, where it gives way to the smooth upper stems. The smooth upper bark of the half barks and that of the completely smooth-barked trees and mallees can produce remarkable colour and interest, for example E. deglupta.
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