Chamelaucium and its Relatives - Background

Introduction

Chamelaucium is a well known genus in the myrtle family (Myrtaceae) through the very commonly cultivated species Chamelaucium uncinatum, the Geraldton wax. Within the myrtle family there are a number of genera which are regarded as having a close relationship with Chamelaucium, the genera in this related group being known as the Chamelaucium Alliance of the subfamily Leptospermoideae. All members of the subfamily are characterised by having a dry fruit but the factors that distinguish the Chamelaucium alliance from other members of the subfamily are not easily explained in simple terms.

Initially, the Chamelaucium Alliance was regarded as including those genera that had a a one-celled ovary (rarely two-celled) containing a single ovule (rarely two) in each cell, as shown in Figure 1 (the ovules are the unfertilized seed). A later classification extended the alliance to include several genera related to Baeckea. These genera have 2 or 3-celled ovaries which may contain two or more ovules, as shown in Figure 2.

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 Leptospermum alliance. The former includes Angophora, Corymbia and Eucalyptus while the latter includes a number of species and genera that are well known and in widespread culivation (eg Callistemon, Leptospermum, Melaleuca).

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 Chamelaucium Alliance do form a consistent and recognisable grouping. For this reason, and for simplicity and convenience, the term "Chamelaucium Alliance" will be used here - the botany will sort itself out in due course.....

One-celled ovary    Two and multi-celled ovaries
Figure 1    Figure 2
Adapted from the article Shrubby Myrtles by Rodger Elliot

Taxonomy changes in Baeckea

The genus Baeckea is one of the better known members of the Chamelaucium Alliance. However, in recent years there has been been considerable reclassification of species in that genus with the result that many species that were formerly classified as Baeckea are now classified under several new or re-instated genera including Babingtonia, Euryomyrtus, Harmogia, Kardomia, Ochrosperma, Sannantha and Triplarina. This means that Baeckea itself has been much reduced in size.

   Babingtonia similis
   Sannantha similis, previously classified as Babingtonia similis and before that as a form of Baeckea virgata.
Photo: Brian Walters

One very well known species that has been affected by these changes is Baeckea virgata, a small to large shrub which has been widely cultivated. In several papers published in the 1990s, Tony Bean of the Queensland herbarium revised the genus Baeckea and one result was the change of Baeckea virgata to Babingtonia virgata. However, a second consequence of Bean's work was that he regarded Babingtonia virgata as being restricted to New Caledonia - i.e. under Bean's revision, Babingtonia (Baeckea) virgata is not an Australian native plant. This meant that all Australian plants previously allocated to Baeckea virgata required new names and Bean described 8 new Babingtonia species to cater for this Baeckea virgata complex. The article "When is a Baeckea not a Baeckea" in the December 2001 issue of Australian Plants online explains the situation in more detail.

However....

While Bean's work and classifications were adopted by most authorities, research into Baeckea and closely related genera, particularly Babingtonia, continued. In a paper Re-evaluation of the genus Babingtonia (Myrtaceae) in eastern Australia and New Caledonia (Wilson P G, Heslewood, M M and Quinn, C J; Australian Systematic Botany 20, 302-318, September 2007), the authors concluded that the genus Babingtonia should be considered restricted to Western Australia where it encompasses a relatively small group of species. They described two new genera, Sannantha and Kardomia, and reinstated a third genus, Harmogia, to cater for the other species of Babingtonia previously included under Bean's classification.

This new work has been adopted by both the NSW and Queensland herbaria, where the majority of the affected species occur.

Members of the Chamelaucium Alliance

As already noted, the best known species in the Chamelaucium Alliance is the Geraldton Wax (Chamelaucium uncinatum). This is extremely popular in cultivation for its long lasting, waxy flowers which are excellent for cutting. Numerous cultivars of this species have been developed both in Australia and overseas. Other members of the alliance which are commonly (or occasionally) seen in cultivation include Baeckea, Sannantha, Thryptomene, Verticordia, Homoranthus and Darwinia.

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. Note that there is still active research being undertaken within this group of plants and the number of species within each genus can only be approximate.

The list in Table 1 is based mainly on information from the Australian Plant Name Index, supplemented by information from the Western Australian Florabase data for Enekbatus, a recent classification in that state for some species previously classified under Baeckea. It also acknowledges the recent reclassification of Babingtonia, previously mentioned.


Table 1 - Genera in the Chamelaucium Alliance

Genus No.of
Species*
Distribution Genus No.of
Species*
Distribution
Single-celled ovary (rarely 2-celled) containing a single ovule (rarely 2)
Actinodium
2
WA Aluta
5
WA
Calytrix
70
All states Chamelaucium
14
WA
Corynanthera
1
WA Darwinia
50
All mainland states
Homalocalyx
11
Qld, NT, WA Homoranthus
15
Qld, NSW, SA
Malleostemon
6
WA Micromyrtus
36
All mainland states
Pileanthus
8
WA Thryptomene
33
All states
Verticordia
101
WA and NT  
Two or three--celled ovary containing a two or more ovules
Anticoryne
1
WA Astartea
6
WA
Astus
4
WA Babingtonia
2
WA
Baeckea
55
All states except SA Balaustion
2
WA
Cyathostemon
1
WA Enekbatus
11
WA
Euryomyrtus
7
All states except Qld Harmogia
1
Qld, NSW
Hypocalymma
26
WA Kardomia
6
Qld, NSW
Ochrosperma
6
NT, Qld, NSW Rinzia
12
WA
Sannantha
15
Qld, NSW, Vic, New Caledonia Scholtzia
13
WA
Triplarina
7
Qld, NSW         
*Approximate number only; some genera contain numerous unnamed species and other genera are in need of, or are undergoing, botanical revision.

Characteristics of Chamelaucium and its Relatives

Plants in the Chamelaucium Alliance are usually small to medium sized shrubs often with small (sometimes minute) leaves. 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 quite 'sweet', as in Chamelaucium uncinatum, or citrus-like, as in Darwinia citriodora. The flowers are quite variable; many have five-petalled flowers superficially similar to those of the 'tea tree' (Leptospermum sp.) which can be quite large, as in the case of Geraldton wax or tiny, as in some Micromyrtus and Thryptomene species. In other cases, the individual flowers may be quite small and inconspicuous but are clustered together and surrounded by large, colourful petal-like bracts (eg. some Darwinia species). Still others have conspicuous sepals with conspicuous hairs on their margins (Verticordia species).

Insects appear to be the main pollination vectors for the Chamelaucium Alliance. However, birds are also attracted to some species, particularly in the genus Darwinia. This is especially noticeable with the bell-shaped inflorescence of some members of that genus (eg. D. leiostyla, D. macrostegia, D. meedoldii)

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 some 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 Chamelaucium and its Relatives

Genus Characteristics
Actinodium Small shrubs; inflorescences terminal (occurring at the ends of the branches) and 'daisy-like' in appearance.
Aluta Small shrubs; genus split off from Thryptomene in 2000 and very similar in appearance to that genus. Five-petalled flowers occurring in the leaf axils (axilliary) with petals persistent.
Anticoryne Small to medium shrubs previously included in Baeckea and similar to that genus.
Astartea Small shrubs with five-petalled flowers in the leaf axils; similar to Baeckea but differing in the arrangement of the stamens.
Astus Small to medium shrubs previously included in Baeckea and similar to that genus.
Babingtonia Small to medium shrubs previously included in Baeckea and similar to that genus.
Baeckea Small to large shrubs. Small, five-petalled flowers in the leaf axils. Baeckea also occurs outside of Australia.
Balaustion Small genus of 2 species. Small shrubs with flowers occurring singly in the leaf axils. Base of the flower is distinctly inflated.
Calytrix Small to medium shrubs. Most Calytrix have long hairs or awns on the tips of the calyx and the flowers nearly always occur in terminal clusters at the ends of the branches. Calytrix includes species previously classified as Lhotzkya and Calythropsis and differs from Homalocalyx in having persistent (not deciduous) sepals.
Chamelaucium Small to large shrubs; waxy, five-petalled flowers occurring in the leaf axils and often larger than other members of the alliance. Some species are commercially exploited for cut flowers.
Corynanthera Monotypic genus. Corynanthera flava is a small shrub similar to Micromyrtus but having inflorescences in terminal, elongated spikes.
Cyathostemon Small to medium shrubs previously included in Astartea and similar to that genus.
Darwinia Small shrubs; small flowers with short petals and an elongated calyx tube terminating in 5 small petal-like lobes. Flowers in pairs or up to 30 in a compound head, sometimes surrounded by leafy bracts.
Enekbatus Small to medium shrubs previously included in Baeckea and similar to that genus. This genus is yet to be formerly published but the classification has been adopted in Western Australia.
Euryomyrtus Small to medium shrubs previously included in Baeckea and similar to that genus.
Harmogia Small to medium shrubs previously included in Babingtonia and similar to that genus.
Homalocalyx Small shrubs similar to Calytrix but having petal-like, deciduous sepals - includes all species previously included in Wehlia.
Homoranthus Small shrubs similar to Darwinia but calyx lobes feature single or multiple hair-like awns.
Hypocalymma Small to medium shrubs. Closely related to Astartea and Cyathostemon.
Kardomia Small to medium shrubs previously included in Babingtonia and similar to that genus.
Malleostemon Small shrubs. Genus split off from Micromyrtus and Thryptomene with characteristics similar to both of those genera.
Micromyrtus Small shrubs similar to Thryptomene. Five-petalled flowers occurring in the leaf axils (axilliary) with petals usually deciduous.
Ochrosperma Small to medium shrubs previously included in Baeckea and similar to that genus.
Pileanthus Small to medium shrubs similar to Chamelaucium but having 20 stamens rather than 10. Flowers occur in corymbs at the tips of the branches.
Rinzia Small to medium shrubs previously included in Baeckea and similar to that genus.
Sannantha Small to medium shrubs previously included in Babingtonia and similar to that genus. Sannantha also occurs outside of Australia.
Scholtzia Small to medium shrubs with five-petalled flowers similar to Baeckea.
Thryptomene Small shrubs with five-petalled flowers occurring in the leaf axils (axilliary) and having persistent petals.
Triplarina Small to medium shrubs previously included in Baeckea and similar to that genus.
Verticordia Small shrubs similar to Darwinia and Homoranthus but calyx lobes have feather-like hairs, giving rise to the common name "feather flowers".

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