Family Myrtaceae > Subfamily Myrtoideae > Tribe Eucalypteae.
‘Eucalypt’ and ‘Gum’ are common generic names for plants in the Tribe.
The terms particularly apply to the Eucalyptus, Angophora and Corymbia genera.
‘Gums’ is better reserved for those species whose bark leaks a gum when damaged.
Most of the around 800 species are in the Eucalyptus genus, 115 in Corymbia and 13 in Angophora with 7 species in minor genera.
The C.S.I.R.O. recognises 934 eucalypt species and Plantnet over 900.
Angophora species have opposite adult leaves and buds with no operculum.
Eucalyptus and Corymbia have alternate adult leaves and an operculum.
Corymbia species are said to have flowers arranged in a corymb#, Eucalyptus do not.
Eucalyptus is by far the largest with about 800 species.
# See note under the Corymbia genus.
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Classification.
There have been a number of classifications suggested including a recent one that puts all Eucalypt species in one Eucalyptus genus with the old genera reduced to subgenera. There has been robust debate about this and it has not been accepted in Australia including by Mabberley or by Kew or World Flora Online. I will use the old classification with Eucalyptus, Corymbia and Angophora genera.
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Eucalypt and Gum features. (Not the Eucalyptus genus.)
All but 16 of the over 800 species of Eucalypts are native to Australia.
Most are trees including some of the tallest in the world.
Eucalyptus diversicolor, the Karri trees of Western Australia can be 60 m high and some others can be 100 m.
There are some small multi-stemmed Mallee trees and a few shrubs.
Mallees have a bulbous mostly underground root system with up to 8 stems, all around the same length, growing from it.
Eucalypt twigs can be round, 4-angled or winged and some have a waxy coating.
Eucalypts produce a new layer of bark every year and the old bark may remain or some or all of it fall off.
There are many different bark types with some given common or generic names.
Some of these are listed below but there are other types and gradations between some.
1. Eucalypts with deciduous bark.
Smoothbarks are the many Eucalypts that shed all the old bark in long narrow strips, large plates or smaller pieces. The smooth new layer often shows many-coloured patterns.
On Ribbon Gums the old bark falls off in long narrow strips which may hang from the trunk or be caught on lower branches.
Scribbly Gums have scribble-like tracks caused by insect larvae.
Halfbarks or Blackbuts shed most of the old bark but some remains on the base or lower part of the trunk.
2. Dead bark retained.
Stringybarks have old bark that has long fibres and can be pulled off in thick pieces.
Boxes have short tighter fibres that cling to the trunk but may eventually peel off.
Ironbarks have hard rough deeply furrowed old bark. Dried sap makes the old bark dark red or black.
Tessellated bark is fissured into small pieces.
In the Bloodwoods it is the timber that has pockets of dark red gum.
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The Queensland Government’s pamphlet ‘Key to the Eucalypts in the Greater Brisbane area’ uses bark to divide them into 5 groups.
1. Fibrous and stringy bark group has 11 Eucalyptus species with rough stringy bark on the trunk that can be pulled off in strips.
2. Rough bark on the trunk but smooth branches is seen in 2 Eucalyptus and 2 Corymbia species.
3. Smooth bark with most of the old shed in patches or strips but some may remain at the base.
This group has 1 Angophora, 3 Corymbia and 10 Eucalyptus species.
4. Bloodwood and rough barked apple-like trees have soft, rough bark that comes off in short oblong pieces.
This group has 2 Angophora and 4 Corymbia species.
5. Ironbarks are the 5 Eucalyptus species with persistent hard rough bark that is dark grey or black and furrowed.
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The Moreton Bay Regional Council also has a small 32 page booklet on its website.
www.moretonbay.qld.gov.au This uses bark and it has drawings of the buds and gumnuts.
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Other important Eucalypt and Gum features.
Leaves commonly have a petiole up to around 2 or 3 cm long but some species have none.
Sometimes the juvenile leaves start with no petiole but older leaves have one.
Most species have adult leaves that are alternately arranged in a spiral.
Many species have opposite juvenile leaves and alternate adult ones.
Sickle-shaped leaves are common but they can be almost any shape.
This includes linear (a few mms wide), lance-shaped, elliptic, ovate or oblong.
Others are round or heart-shaped.
The leathery leaves may be equally green on both surfaces or paler underneath.
Some leaves are blue-green either just as juveniles or as adult leaves.
There are oil glands in the leaves and sometimes in other parts.
Most leaves have 1 midvein with lateral veins that are close or well separated.
Some species only have 3 longitudinal veins.
Axillary or terminal inflorescences can be a single flower.
Most are an umbel or corymb and these can be in a branched cluster.
In an umbel all the flowers are attached to the tip of the peduncle.
In a corymb flowers are attached down the peduncle but all end at the same level.
Each group can have up to 7 or more flowers.
Some species have 2 inflorescences in each leaf axil.
In most species flowers are on a pedicel but some are directly attached to the peduncle.
Most species have no bracts or bracteoles.
Any bracts fall very early and any bracteoles are small and may fall early.
The bud shape and size is important in identifying a plant.
Buds can be round or conical, long or short and some are angular.
All flowers have a tough hypanthium.
This is formed by fusion of the bases of the sepals and stamen filaments plus any petals.
Most species have no petals.
The hypanthium is variously shaped including urn-like, oblong, cylindrical, ovoid, conical, hemispherical or angled.
It may be smooth, ribbed or warty and may have oil glands.
The 4 or 5 sepal lobes on the rim of the hypanthium may be free or fused.
When fused they form a hood or operculum.
Sometimes called a calyptra it protects the developing reproductive parts.
Where petals are present the lobes are fused into an inner operculum.
Exceptions are the Angophora species that have no operculum and have tiny free petals.
The operculum is often a wide or narrow cone with a blunt or sharp tip.
They can also be hemispherical or cylindrical.
The ratio of operculum to hypanthium lengths can aid identification.
The operculum is shed as the stamens mature.
It usually comes off cleanly but bits may remain on the hypanthium.
It often leaves a scar on the rim of the hypanthium.
All flowers have numerous white, cream, pink or red stamens.
Their free filament ends are attached to the top of the hypanthium.
They may be curved inwards, outwards, be erect or randomly arranged.
They can be in 1 or more whorls and be free or have their bases fused.
Typically they are all fertile but the outer ones may be infertile.
Some older classifications were based on the features of the anthers.
Variously shaped and dorsifixed or almost basifixed they have 2 pollen sacs.
They can open through slits (separate or joined) or small pores.
The ovary is inferior or almost completely inferior and lies in the hypanthium.
It can be fused to the hypanthium or free and attached just at the base.
Its flat or domed upper surface, the ‘disc’ is attached to the hypanthium at or below the rim.
There are 2 or more vertical rows of ovules.
The stout style extends past the stamens.
A lot of nectar is produced from nectiferous cells.
These lie on the top of the disc and, when the disc is recessed, inside the hypanthium.
The fruit are typically a woody loculicidal capsule known as a ‘gumnut’.
From a few mms to a few cms long they can be bell, funnel or cup-shaped.
Others are spherical, ovate, cylindrical, obovate or angular sometimes with ribs.
The ovaries have 5 (2, 3, 4, 6) locules.
When the seeds are mature the capsule splits vertically into each chamber (loculicidal).
The individual sections are called ‘valves’.
The large or small valve tips are variously shaped including blunt or pointed.
The tips can lie within the hypanthium or project past it.
There are a few to numerous black, grey, brown, yellow or red seeds.
Seed shapes include linear, angled, flattened, spherical, ovoid and some have a wing.
They may be mixed with infertile seeds known as chaff.
J.F.
