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Introduction to the Eucalypts

Prepared by Andrew Lyne al@anbg.gov.au 27 March 1992

Introduction

The genus Eucalyptus is in the family Myrtaceae. Worldwide, Myrtaceae comprises some 140 genera and 3000 species. In Australia Mrytaceae is represented by some 75 genera and 1400 species. Myrtaceae is found mainly in Australia, South America and Malesia.

The first botanical collections of Eucalyptus were made by Joseph Banks and Daniel Solander in 1770. They collected specimens of what are now known as E. gummifera from Botany Bay, New South Wales and E. platyphylla from Endeavour River, north Queensland. It was not until 1788 however that the first species of Eucalyptus, E. obliqua was named by L'Heritier, a French botanist who was working in London at the time.

Classification

Eucalyptus (as traditionally recognized) can be distinguished from other Myrtaceae by their flower buds being covered with a calyptra (or "cap") formed from the fused perianth segments.

In 1971, L.D. Pryor and L.A.S. Johnson proposed a classification that would split Eucalyptus into a number of genera. These genera are Corymbia (red and yellow bloodwoods, c. 80 spp.), Blakella (ghostgums, c. 20 spp.), Eudesmia (c. 20 spp.), Gaubea (c. 2 spp.), Idiogenes (1 sp.), Nothocalyptus (tallowood, 1 sp.), Symphyomyrtus (c. 450 spp.), and Monocalyptus (c. 140 spp.).

See here for a breakdown of the major groups of Eucalyptus and the occurrence in New South Wales and the Australian Capital Territory..

Habit

Eucalypts exhibit a variety of habits. They can be shrubs (eg. Eucalyptus vernicosa from western Tasmania grows to about 1m and may sometimes be prostrate), mallees (characterized by having several stems from a common lignotuber) or trees (the tallest is Eucalyptus regnans from Victoria and Tasmania which can grow to about 100m). Most however occur as forest trees of 30-50m height or as woodland trees of 10-25m height.

Lignotuber

Lignotubers occur in some but not other species. They are a woody swelling, partly or wholly underground and have been shown to contain a mass of vegetative buds and substantial energy reserves. Eucalypts that possess lignotubers are often those tolerant to fire, drought and defoliation. Those species that do not produce lignotubers are usually prolific seed producers and survive such disasters as fire with massive seedling regeneration.

Bark

Bark types fall into two catagories. There is the persistant type, the "rough barks" (eg. stringybarks, boxes, peppermints and ironbarks) or partly or wholly regularly shedding type, the "smooth barks" (e.g. scribbly gums).

Leaves

Eucalypts show a remarkable dimorphism between their juvenile leaves and their adult leaves. In the majority of cases, juvenile leaves are opposite whilst the adult leaves are usually disjunct opposite. Oil glands are also usually present. The leaves may sometimes be glaucous (covered in a white wax) which causes the leaf to appear whitish, bluish or blue-grey. Coppice growth usually has the form of the juvenile foliage. Some species rarely produce mature foliage and will flower with juvenile foliage present (e.g. E. cinerea).

Inflorescences

The inflorescence forms an umbellaster. These are usually simple and axillary (eg. E. pauciflora), compound and axillary (eg. E. fastigata) or compound and terminal (eg. E. polyanthemos).

Buds, flowers and fruit

The calyx and/or corolla fuse to form a calyptra which covers the stamens and style. There may be one or two calyptra, depending upon the species. The shape of the calyptra varies considerably between species. The stamens are numerous, usually in several continuous whorls and the anthers can be either versatile or adnate and dehisce by separate or confluent slits or pores. The ovary can be inferior or half-inferior with 2 - 7 loculi. The fruit is a capsule that becomes woody in most species. The valves may be sunken, at rim level or exerted. The disc may be level, ascending or descending. The shape of the fruit is also very variable between species.

Identification

The Australian National Botanic Gardens' offers a plant identification service. Eucalyptus can sometimes be tricky to identify, so any specimen should have at least mature buds, mature fruits, adult leaves, locality information, a description of its habit and a description of the bark. Examples of how bark can be described are - bark rough, bark rough on trunk and smooth on branches, bark smooth or bark smooth with a persistant stocking of rough bark. Also of value in identification are flowers and juvenile or coppice leaves.

References

Andrew Lyne ( al@anbg.gov.au ) 27 March 1992