Eucalyptus Subgen. Symphyomyrtus

''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including ''Corymbia'', they are commonly known as eucalypts. Plants in the genus ''Eucalyptus'' have bark that is either smooth, fibrous, hard or stringy, leaves with oil glands, and sepals and petals that are fused to form a "cap" or operculum over the stamens. The fruit is a woody capsule commonly referred to as a "gumnut".

Most species of ''Eucalyptus'' are native to Australia, and every state and territory has representative species. About three-quarters of Australian forests are eucalypt forests. Wildfire is a feature of the Australian landscape and many eucalypt species are adapted to fire, and resprout after fire or have seeds which survive fire.

A few species are native to islands north of Australia and a smaller number are only found outside the continent. Eucalypts have been grown in plantations in many other countries because they are fast growing and have valuable timber, or can be used for pulpwood, for honey production or essential oils. In some countries, however, they have been removed because of the danger of forest fires due to their high inflammability.

Description

Size and habit

Eucalypts vary in size and habit from shrubs to tall trees. Trees usually have a single main stem or trunk but many eucalypts are mallees that are multistemmed from ground level and rarely taller than . There is no clear distinction between a mallee and a shrub but in eucalypts, a shrub is a mature plant less than tall and growing in an extreme environment. '' E. vernicosa'' in the Tasmanian highlands, '' E. yalatensis'' on the Nullarbor and '' E. surgens'' growing on coastal cliffs in Western Australia are examples of eucalypt shrubs.

The terms "mallet" and "marlock" are only applied to Western Australian eucalypts. A mallet is a tree with a single thin trunk with a steeply branching habit but lacks both a lignotuber and epicormic buds. '' E. astringens'' is an example of a mallet. A marlock is a shrub or small tree with a single, short trunk, that lacks a lignotuber and has spreading, densely leafy branches that often reach almost to the ground. '' E. platypus'' is an example of a marlock.

''Eucalyptus'' trees, including mallets and marlocks, are single-stemmed and include '' Eucalyptus regnans'', the tallest known flowering plant on Earth.

Tree sizes follow the convention of:
* Small: to in height
* Medium-sized:
* Tall:
* Very tall: over

Tasmania logging 08 Mighty tree.jpg, '' Eucalyptus regnans'', a forest tree, showing crown dimension, Tasmania
TocumwalTownBeach.jpg, '' E. camaldulensis'', immature woodland trees, showing collective crown habit, Murray River, Tocumwal, New South Wales
Eucalyptus cretata.jpg, '' E. cretata'', juvenile, showing low branching 'mallee' form, Melbourne, Victoria
Eucalyptus angustissima1.jpg, '' E. angustissima'', showing shrub form, Melbourne
Eucalyptus platypus.jpg, '' E. platypus'', showing ‘marlock’ form, Melbourne

Bark

All eucalypts add a layer of bark every year and the outermost layer dies. In about half of the species, the dead bark is shed exposing a new layer of fresh, living bark. The dead bark may be shed in large slabs, in ribbons or in small flakes. These species are known as "smooth barks" and include '' E. sheathiana'', '' E. diversicolor'', '' E. cosmophylla'' and '' E. cladocalyx''. The remaining species retain the dead bark which dries out and accumulates. In some of these species, the fibres in the bark are loosely intertwined (in stringybarks such as '' E. macrorhyncha'' or peppermints such as '' E. radiata'') or more tightly adherent (as in the "boxes" such as '' E. leptophleba''). In some species (the "ironbarks" such as '' E. crebra'' and '' E. jensenii'') the rough bark is infused with gum resin.

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''. In 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''.

'' E. globulus'' bark cells are able to photosynthesize in the absence of foliage, conferring an "increased capacity to re-fix internal CO₂ following partial defoliation". This allows the tree to grow in less-than-ideal climates, in addition to providing a better chance of recovery from damage sustained to its leaves in an event such as a fire.

Different commonly recognised types of bark include:
* Stringybark—consists of long fibres and can be pulled off in long pieces. It is usually thick with a spongy texture.
* Ironbark—is hard, rough, and deeply furrowed. It is impregnated with dried kino (a sap exuded by the tree) which gives a dark red or even black colour.
* Tessellated—bark is broken up into many distinct flakes. They are corkish and can flake off.
* Box—has short fibres. Some also show tessellation.
* Ribbon—has the bark coming off in long, thin pieces, but is still loosely attached in some places. They can be long ribbons, firmer strips, or twisted curls.

Apple box bark.jpg, Bark detail of '' E. angophoroides'', the apple-topped box
Eucalyptus deglupta-trees.jpg, Coloured bark of '' E. deglupta'' native to Southeast Asia
Eucalyptus bark.jpg, 'Box' bark of '' E. quadrangulata'', the white-topped box
Eucalyptus sideroxylon - bark.jpg, Dark, fissured ' ironbark' of '' E. sideroxylon''

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. Although mature ''Eucalyptus'' trees may be towering and fully leafed, their shade is characteristically patchy because the leaves usually hang downwards.

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 many exceptions to this pattern exist. 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, no definite transitional point occurs between the phases. The intermediate phase, when the largest leaves are often formed, links the juvenile and adult phases.Brooker & Kleinig (2001)

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

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