Mintbush Genus Limits

Victorian Christmas bush Prostanthera lasianthos, copyright Melburnian.

Despite (or perhaps because of) the severity of Australia's climate over much of the continent, the country has become famed for its wildflower displays. At the right time of year, the otherwise bleak landscape becomes a riot of form and colour. The display shown above belongs to a species of the genus Prostanthera, an assemblage of about 100 species of bushy shrubs (very rarely small trees) known as mintbushes, endemic to yet ubiquitous around Australia (new species continue to be described at fairly regular intervals). As suggested by their vernacular name, mintbushes belong to the mint family Lamiaceae, the same family as many well-known garden herbs such as sage, rosemary or thyme. Like these relatives, mintbushes have strongly aromatic foliage, due to the presence of glands secreting volatile oils on the leaves. However, the edibility of most species is unknown; I did find a couple of references to culinary uses of the round-leaved mintbush Prostanthera rotundifolia though it is not common (and a couple of comments in this thread suggest that it may be a bit pungent for regular use). Certainly, to push the pun in this post's title far further than it deserves, there is no evidence of mintbush gin.

Prostanthera species are most readily recognised by their flowers (Wilson et al. 2012). The calyx at the base of the flower has the sepals fused so that it is shaped as two lips, an upper and a lower. The corolla of petals has five lobes, two in the upper lip and three in the lower. There are four anthers, which often (though not always) have a distinct basal appendage; it is this appendage that gives the genus its name (from the same Greek word that gives us the term 'prosthetic'). Different species have flowers in a wide range of colours, and many Prostanthera species have become popular ornamentals.

Flower of scarlet mintbush Prostanthera aspalathoides, copyright Patrick Kavanagh.

Prostanthera is a member of a tribe of Australian Lamiaceae known as the Westringieae, members of which have a dry fruit splitting into four sections (Conn 1984). The two-lobed calyx of Prostanthera separates it from most other genera that have been recognised in the Westringieae except for a small genus called Wrixonia. The only significant difference between Wrixonia and Prostanthera is that whereas the latter retains four fertile anthers, the former has one pair of anthers sterile and reduced. A molecular phylogenetic analysis by Wilson et al. (2012) found that Wrixonia species were nested within Prostanthera, raising doubt about whether Wrixonia should be recognised as a separate genus. Also of interest was the relationship between the two sections into which Prostanthera has been divided: section Prostanthera and section Klanderia. These sections differ in characteristics of their flowers. Prostanthera section Prostanthera has flowers that are white, mauve or blue, with a corolla in which the central lower lobe is longer than the others so the overall appearance is similar to an orchid (such as in the P. lasianthos at the top of this post). In section Klanderia, the flowers are green, yellow or red, and the two upper lobes of the corolla are the longest so the appearance of the flower is more tubular (such as in the P. aspalathoides just above). Some authors have regarded the difference between two sections as enough to warrant recognised section Klanderia as a separate genus (in which case it becomes known as Cryphia, because botanical nomenclature is complicated like that). The two sections differ in flower morphology because they differ in pollinator type: flowers of section Prostanthera are pollinated by insects, whereas flowers of section Klanderia are pollinated by birds. Again, Wilson et al. (2012) found that the larger section Prostanthera, which retains the ancestral pollinator type, is paraphyletic with regard to the derived section Klanderia.

REFERENCES

Conn, B. J. 1984. A taxonomic revision of Prostanthera Labill. section Klanderia (F.v.Muell) Benth. (Labiatae). J. Adelaide Bot. Gard. 6 (3): 207–348.

Wilson, T. C., B. J. Conn & M. J. Henwood. 2012. Molecular phylogeny and systematics of Prostanthera (Lamiaceae). Australian Systematic Botany 25: 341–352.