Leptospermum and its Relatives - Background

Introduction

Species in the genus Leptospermum are commonly known as tea trees and they are probably some of the best known of all Australian native plants. Tea trees are members of the widespread myrtle family of plants (Myrtaceae) and within this family there are a number of genera which are regarded as having a close relationship with Leptospermum. The genera in this related group are known as the Leptospermum Alliance of the subfamily Leptospermoideae. Some of these related genera are also very well known, such as Callistemon (bottlebrushes) and Melaleuca (paperbarks and honey myrtles), and the Leptospermum alliance also includes some less well known genera such as Kunzea, Calothamnus, Beaufortia and Eremaea.

In the traditional characterisation of the myrtle family, there are about seven groupings or alliances within the subfamily Leptospermoideae. These include the Eucalyptus alliance and the Chamelaucium alliance. The former includes Angophora, Corymbia and Eucalyptus while the latter includes genera such as Chamelaucium (eg. Geraldton wax), Darwinia, Verticordia and Baeckea.

All members of the subfamily Leptospermoideae are characterised by having a dry fruit. In the Leptospermum alliance, the fruit consists of 3 or more cells each of which contain several (sometimes many) ovules, the unfertilized seed. The fruit is dehiscent (ie. the fruits open to release the seed, although this sometimes does not occur until the plant dies or the fruit is physically removed from the plant).

In recent years the traditional relationships within the myrtle family have been challenged and the somewhat neat arrangement of subfamilies and alliances within the family is now regarded as difficult to justify (see 'Myrtaceae revisited: a reassessment of infrafamilial groups' in Further Information for a technical discussion of the situation). Despite this, the plants within the traditional view of the Leptospermum alliance do form a consistent and recognisable grouping. For this reason , and for simplicity and convenience, the term "Leptospermum alliance" will be used here - the botany will sort itself out in due course.....

Callistemon and Melaleuca

One issue that has arisen recently that cannot be avoided is the relationship between the two genera Callistemon and Melaleuca. Traditionally, these two genera have been separated on the basis of the arrangement of the stamens. In Callistemon each stamen joins the floral tube independently of every other stamen (this is referred to as the stamens being "free"). In Melaleuca, on the other hand, the stamens are joined together into groups with each group joining the floral tube as a unit (this is referred to as the stamens being "united"). This can more easily understood by reference to the diagrams in the article "Callistemon and Melaleuca - Why are they Different?".

The problem with the classification of Callistemon and Melaleuca on the basis of the arrangement of the stamens is that this supposed difference is not clear cut - Callistemon tends to merge into Melaleuca rather than being unambiguously distinct. The well known Callistemon viminalis is one that has often been discussed as not easily fitting the accepted definition of Callistemon.

Over the years there have been suggestions that the differences between species of the two genera are not sufficient to warrant them being kept distinct. A paper by Lyn Craven of the Australian National Herbarium (Novon 16 468-475; December 2006 "New Combinations in Melaleuca for Australian Species of Callistemon (Myrtaceae)") argues that the differences between the two genera are insufficient to warrant them being retained separately and that they should be combined. As Melaleuca has precedence, adoption of Craven's work would transfer all species of Callistemon into Melaleuca but at this stage the re-classification has not been taken up by the Council of Heads of Australasian Herbaria (although the Queensland herbarium has accepted the change).

Craven's work is the basis of a forthcoming volume of the "Flora of Australia", so the possibilitity remains that Callistemon will eventually be subsumed into Melaleuca.

For the present ANPSA is accepting that Callistemon and Melaleuca are distinct but this classification will be kept under review.

Because these two genera are so well known and widely cultivated, separate sections of this web site have been developed to cater for each of them - see our profiles of Callistemon and Melaleuca.

Members of the Leptospermum Alliance

Table 1 lists the genera in the alliance as well as the number of species in each genus and the distribution of each genus by state. The list is based mainly on information from the Australian Plant Name Index, supplemented by information from the Western Australian Florabase data for Paragonis and Taxandria, recent classifications in that state for some species previously classified under Agonis. Some members of the Alliance are also found outside of Australia.


Table 1 - Genera in the Leptospermum Alliance

Genus No.of
Species*
Distribution Genus No.of
Species*
Distribution
Agonis
4
WA Angasomyrtus
1
WA
Asteromyrtus
7
WA, NT, Qld & Papua New Guinea Beaufortia
19
WA
Callistemon
36
All states & New Caledonia Calothamnus
37
WA
Conothamnus
3
WA Eremaea
14
WA
Homalospermum
1
WA Kunzea
48
All states
Lamarchea
2
WA, NT Leptospermum
82
All states & south-east Asia
Melaleuca
242
All states & south-east Asia Neofabricia
3
Qld
Paragonis
1
WA Pericalymma
4
WA
Phymatocarpus
2
WA Regelia
6
WA, NT
Taxandria
9
WA      
*Approximate number only; some genera contain numerous unnamed species and other genera are in need of botanical revision. Furthermore, within Agonis, some recent classifications have not yet been accepted by all authorities..

Characteristics of Leptospermum and its Relatives

Plants in the Leptospermum alliance vary in habit from small, sometimes prostrate shrubs to medium sized trees. Like other members of the myrtle family, the leaves contain aromatic oils which can be smelled by crushing the leaves between the fingers. In some cases the aroma can be citrus-like, as in Leptospermum petersonii. The flowers are superficially quite variable. All have five-petalled flowers but in some (eg. Callistemon and Melaleuca) the petals are small or deciduous and the stamens form the conspicuous parts of the flowers. In others (eg. Leptospermum and Agonis) the petals are the most conspicuous feature. In others again (eg Kunzea) some species have conspicuous petals (eg. K.baxteri) and others have conspicuous stamens (eg. K.affinis) - botany was never easy!

Many of the plants in the Leptospermum alliance have flowers that produce large amounts of nectar. These include Callistemon, Melaleuca, Beaufortia and Regelia. These tend to be pollinated mainly by birds, including honeyeaters and lorikeets. Insects appear to be the main pollination vectors for other members of the alliance such as Leptospermum, Agonis and Neofabrica. Kunzea, is an interesting genus in that it has some species that are mainly bird pollinated and others that are insect pollinated.

The majority of species in this group of plants are found in heath, woodland or open forest of mainly temperate areas. They are absent in rainforest and arid areas although many species do occur in the tropics.

Table 2 lists some of the general features of each genus. The table is not intended to be a botanical 'key' as differences between the genera may not always be apparent to the casual observer, requiring botanical knowledge and a high magnification viewer.


Table 2 - Some Characteristics of Leptospermum and its Relatives

Genus Characteristics
Agonis Medium shrubs to medium trees; flowers "tea tree-like" with petals that tend to remain attached to the floral tube for some time after flowering is finished.
Angasomyrtus Monotypic genus (Angasomyrtus salina). Small to medium shrub with characteristics similar to Leptospermum but it has opposite foliage (alternate in Leptospermum). It is also similar to Kunzea but has stamens that are shorter than the petals.
Asteromyrtus Medium shrubs to medium trees with flowers in clusters similar to Melaleuca, one difference being that the seeds are 'winged' in Asteromyrtus.
Beaufortia Small to medium shrubs with flowers in clusters similar to Melaleuca, a distinguishing feature being that the anthers are basifixed in Beaufortia. It is closely related to Calothamnus, Eremaea, Phymatocarpus and Regelia) but differs in the structure of the anthers. Flowers may be grouped in 'bottlebrush' or globular clusters.
Callistemon Small and medium shrubs to medium trees with flowers grouped in 'bottlebrush' clusters. It differs from Melaleuca in having stamens that are free (but see discussion above).
Calothamnus Small to medium shrubs with flowers in clusters similar to Melaleuca, a distinguishing feature being that the anthers are basifixed in Calothamnus. It is closely related to Beaufortia, Eremaea, Phymatocarpus and Regelia) but differs in the structure of the anthers. Flowers grouped in one-sided 'bottlebrush' clusters
Conothamnus Small shrubs with flowers in clusters similar to Melaleuca, The main difference being that the 3-celled ovary contains only one ovule in Conothamnus.
Eremaea Small to medium shrubs with flowers in clusters similar to Melaleuca, a distinguishing feature being that the anthers are basifixed in Eremaea. It is closely related to Beaufortia, Calothamnus, Phymatocarpus and Regelia) but differs in the structure of the anthers. Flowers tend to be grouped in cone-shaped clusters.
Homalospermum Monotypic genus (Homalospermum firmum). Medium shrub to about 2.5 metres high with flowers that are "tea tree-like". Closely related to Pericalymma but differing in the structure of the ovary.
Kunzea Small to large shrubs which have flowering characteristics between Callistemon and Leptospermum (ie. some have "tea tree-like" flowers with prominent stamens while others tend to have flowers in 'bottlebrush' clusters.
Lamarchea Small and medium shrubs to small trees with flowers that are superficially similar to Calothamnus but which occur singly rather than in clusters. However, Lamarchea is more closely related to Melaleuca based in the characteristics of the anthers.
Leptospermum Small and medium shrubs to small trees having flowers with petals that may be deciduous or persistent. The stamens occur in a single row around the floral tube (a difference to Neofabricia) and they are shorter than the petals (a difference to Kunzea). The leaves are alternate (a difference to Angasomyrtus.
Melaleuca Small and medium shrubs to large trees with flowers usually grouped in either 'bottlebrush' or globular clusters. It differs from Callistemon in having stamens that are united (but see discussion above). It differers from Beaufortia, Calothamnus, Eremaea, Phymatocarpus and Regelia in the characteristics of the anthers and from Conothamnus in the structure of the ovary.
Neofabricia Small to medium shrubs and small trees similar to Leptospermum but having stamens in arranged in several rows.
Paragonis Small to medium shrubs previously included in Agonis and similar to that genus.
Pericalymma Small to medium shrubs with flowers that are "tea tree-like". Closely related to Homalospermum but differing in the structure of the ovary.
Phymatocarpus Small to medium shrubs with flowers in clusters similar to Melaleuca, a distinguishing feature being that the anthers are basifixed in Phymatocarpus. It is closely related to Beaufortia, Calothamnus, Eremaea and Regelia) but differs in the structure of the anthers. Flowers are grouped in globular clusters.
Regelia Small to medium shrubs with flowers in clusters similar to Melaleuca, a distinguishing feature being that the anthers are basifixed in Regelia. It is closely related to Beaufortia, Calothamnus, Eremaea and Phymatocarpus) but differs in the structure of the anthers. Flowers are grouped in globular clusters.
Taxandria Small to medium shrubs previously included in Agonis and similar to that genus.

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