The term "eucalypt" in its broad sense refers to certain taxa within the family Myrtaceae. These taxa include species in the genera Eucalyptus and Angophora. They also include species in the bloodwood groups Corymbia and Blakella which are often included as sub-generic groups within Eucalyptus (e.g. Pryor and Johnson 1971, Brooker and Slee 1996), but have also been described as a separate genus, Corymbia (Hill and Johnson 1995).
The following botanical descriptions of the eucalypts are based on the genera Eucalyptus (including the bloodwoods) and Angophora, and have been derived primarily from Chippendale (1988), Brooker and Slee (1996) and Mabberley (1997). Technical terminology has been defined using non-technical language wherever possible, and technical terms have been included in square brackets.
Eucalyptus L'Her. (including Corymbia K.Hill & L. Johnson)
Evergreen trees, shrubs or mallees. Bark smooth or rough, including fibrous, stringy or in squarish patches [tessellated]. Leaves of seedling and young plant distinct from adult leaves [heterophyllous] in shape, position and colour; adult leaf phases occurring in most species, the latter phases sometimes not achieved [neoteny] so that some species are reproductively mature while crown is in the juvenile leaf phase; oil glands present in most species. Adult leaves mostly borne singly at different levels along a stem [alternate], usually with a leaf stalk [petiolate], ovate to broadest in the lower half and tapering to the tip [lanceolate] or curved [falcate], dull or glossy, smooth and hairless [glabrous], pendulous, rarely erect, with a distinct midvein and pinnately-arranged veins [penninerved], or with parallel veins. Inflorescences borne in the angle formed by the leaf with its branch [axillary], but in some species on leafless shoots in axils of leaves at ends of branchlets [terminal pseudopanicles]. Flower buds single, or in 3s, 7s, 11s etc.; sepals and petals absent, replaced by one or two covering caps [operculae] which are shed at flower opening [anthesis]; stamens numerous, often brightly coloured; sterile stamens [staminodes] present in some species; ovary sunk into the floral tube [hypanthium], below the level of attachment of other flora parts [inferior] or partly above the level of attachment [superior], with 3 to 8 chambers [locules]; ovules many, partly inverted [hemitropous] or fully inverted [anatropous]. Fruit a capsule with a usually woody floral tube [hypanthium]; releasing contents [dehiscing] by valves which are either exserted, level with capsule rim, or inserted; seeds several to many, ellipsoid, cuboid or pyramidal but variously flattened or distorted, variously coloured; with terminal or ventral separation scar [hilum]; numerous chaff grains [ovulodes] usually included with seeds.
The most characteristic genus of the Australian landscape; includes more than 700 species, all but a few endemic to Australia; varies from dwarf shrubs to some of the tallest hardwood trees known. Main groups recognisable by their bark: gums (smooth and deciduous), boxes (rough but fibrous), peppermints (finely fibrous), stringybarks (long fibrous), ironbarks (hard, rough fissured, dark) and bloodwoods and ghost gums (included in Corymbia) (tessellated).
Over 200 species introduced elsewhere; the most important dicotyledon plantation trees worldwide; widely planted in other countries for timber, oils, tannins, lowering watertables etc; dominate scenery in parts of some countries e.g. California, East Africa, Sri Lanka, Portugal; aggressive and invasive in some parts.
Evergreen trees or shrubs. Bark rough or fibrous or smooth. Leaves of 2 different kinds [dimorphic]; lateral veins very close, straight and parallel; juvenile leaves borne on opposite sides of stem at the same level [opposite], heart-shaped [cordate] at base, without a stalk [sessile], often rough with stiff, bristly hairs [hispid], with raised oil glands; adult leaves opposite, lanceolate, rarely curved [falcate], stalked [petiolate], usually smooth and hairless [glabrous], with upper and lower surfaces of a different colour [discolorous]. Inflorescence a terminal aggregate of flowers [compound] comprising clusters [umbels] of 3-7 flowers borne on primary or secondary branches of the main axis [paniculate]. Flowers creamy-white to creamy-yellow, stalked [pedicellate], calyx-lobes [sepals] 4 or 5, free, reduced to persistent projections on rim of floral tube [hypanthium]; petals 4 or 5, flat and more or less circular [orbicular], overlapping [imbricate], broadly attached at the base, deciduous; stamens numerous in several whorls, free, anthers bilobed, opening by two longitudinal slits, swinging freely around the point of attachment to the filament [versatile], filaments creamy yellow, rarely pink, smooth and hairless [glabrous]; floral tube [hypanthium] extending beyond ovary which is below the level of attachment of other flora parts [inferior], usually with 3 chambers [locules], ovules partly inverted [hemitropous]. Fruit a hard, woody, longitudinally-ribbed capsule enclosed mostly by the persistent floral tube [hypanthium], releasing contents [dehiscing] by terminal valves. Seeds 1 per chamber [locule], broadly elliptic, flat, dark brown, leaves of the embryo [cotyledons] folded; numerous chaff grains [ovulodes] included with seeds.
Includes thirteen species, all endemic to eastern Australia. Closely related to Eucalyptus but distinguished by the petals and the bristle-like emergent oil glands that are interspersed with several-celled white hairs.
Some of these characters may be considered either relatively “primitive” or “advanced” (ie derived) in terms of whether they are associated with taxa that are basal or more recent in the eucalypt phylogenetic tree. Examples of “primitive” and “advanced” characters of the eucalypts are listed in Table 2
Table 2 Examples of “primitive” and “advanced” characters in eucalypts
- leaf blades horizontal, the upper leaf surface [adaxial surface] usually different in colour from the lower leaf surface [abaxial surface]
- leaf blades vertical and hanging via leaf stalk [petiole]
- flowers at tips of stems [terminal inflorescences]
- flowers in the angle between the upper surface of a leaf and the stem which bears it [axillary inflorescences]
- small numbers of buds in each flowering shoot [or unit inflorescence]
- larger numbers of buds in each flowering shoot [or unit inflorescence]
- green corolla parts [sepals] distinct from coloured corolla parts [petals] in the flower
- a single covering cap [operculum] or two covering caps replacing corolla parts [sepals and petals] in the flower
- anther attached to the filament of the stamen by a small area on its upper [dorsal] side so that it can turn freely [versatile anther]
- anther attached to the filament of the stamen along its entire length so that it is fixed in position [adnate anther]
- developing seeds [ovules] in odd number of rows
- developing seeds [ovules] in even number of rows
- ovules partly inverted [hemitropous]
- ovules fully inverted [anatropous]
The taxonomy of the eucalypts remains not fully understood despite considerable work over the past two centuries. Indeed, eucalypt taxonomy generally has been viewed as difficult (see Ladiges 1997). A number of taxonomic classifications for the eucalypts, and for Eucalyptus in particular, have been proposed at different times. Some have recognised the existence of separate taxon groups within Eucalyptus (e.g. Bentham 1867, Blakely 1934, Pryor and Johnson 1971, Chippendale 1988). Others have proposed a split of the genus into two or more genera on the basis of these groups (e.g. two genera, Carr and Carr 1962; three genera, Hill and Johnson 1995; five genera, Andrews 1913). The most common classification of the eucalypts has been into two genera, Angophora and Eucalyptus (e.g. Pryor and Johnson 1971, Chippendale 1988, Brooker and Slee 1996). In general, the comprehensive classification by Pryor and Johnson (1971) which recognises these two genera has been the most widely accepted and used.
In addition to recognising Angophora and Eucalyptus, Pryor and Johnson (1971) delineated seven groups within Eucalyptus which they informally recognised as subgenera. These groups included Blakella, Corymbia, Eudesmia, Gaubaea, Idiogenes, Monocalyptus and Symphyomyrtus. Although they considered Angophora as equal to the other seven groups, they chose not to include it at this stage. It has been suggested subsequently that these informal sub-generic groups may warrant full generic status (e.g. Pryor and Johnson 1981). This view is based on the possibility that at least some of the groups may have had separate origins from closely-related ancestral stocks. In this regard, two of the subgeneric groups, Corymbia and Blakella, the bloodwoods and ghost gums respectively, have recently been separated from Eucalyptus and described as a single new genus, Corymbia, by Hill and Johnson (1995). Even when Eucalyptus is treated in this narrower sense, there remain relatively distinct subgeneric groups such as Symphyomyrtus and Monocalyptus recognizable within the genus.
Notwithstanding the different views on the taxonomic ranking of the eucalypt groups such as Angophora, Corymbia and Eucalyptus sensu stricto (in the narrow sense), there is a general, strong consensus among eucalypt taxonomists that the entire stock has developed from a single lineage.
Reviews of evidence for the eucalypts, including molecular and morphological data, also support the concept of a single phylogenetic tree for Angophora, Corymbia and Eucalyptussensu stricto (Ladiges et al. 1995, Ladiges 1997). Two major branches (clades) or lineages are evident within this monophyletic group. One clade is further differentiated into Angophora and the bloodwoods and ghost gums represented by Corymbia and Blakella; the other includes taxa associated with Eucalyptus sensu stricto (e.g. Ladiges 1997, Figure 2.2, p. 20).
A phylogenetic summary tree that includes both the eucalypts and closely-related taxa within the Myrtaceae, based on the latest published and unpublished morphological and molecular evidence including nuclear and chloroplast DNA, further supports the interpretation of a monophyletic lineage for the eucalypts (Ladiges et al., unpublished). This latest tree demonstrates three major branches for the eucalypts and a close relative. One branch corresponds to Eucalyptus, a second is differentiated into Angophora and the bloodwoods Corymbia and Blakella, and the third is associated with the putative closest living relative of the eucalypts, the genus Arillastrum, which is extant only in New Caledonia. This evidence for three groups of eucalypts, including Eucalyptus, Angophora and the bloodwoods, is broadly consistent with the recent classification by Hill and Johnson (1995).
The existence of major groups among the eucalypts appears to be strongly supported by the evidence, and has gained general acceptance. A central issue within eucalypt taxonomy that remains to be resolved is the appropriate level or rank at which these groups should be recognised. Irrespective of how they are ranked taxonomically, it is clear that the eucalypts comprise a monophyletic stock which is exceptional in the way in which it has diversified to dominate the vegetation of an entire continent and has influenced the evolutionary moulding of an entire continental flora.