Abaxianthus convexus

A small genus with only one species occurring in Australia, growing mainly on mangroves in swamps and around the mouth of large rivers and estuaries. An epiphyte with small hard cone-to egg-shaped pseudobulbs that are ellipsoid in cross-section, each on a single stem. A single long and thick leaf with a notched apex grows from the apex of the pseudobulb. A single flower grows from the apex of the pseudobulb behind the leaf lasting for only 8 to 10 hours. Flowers are 1 to 1.5 cm wide with the dorsal sepal and petals smaller than the lateral sepals. Flowers are mainly green and have a reddish-pink labellum with a yellow apex. The labellum is distinctly three-lobed and the midlobe has 3 straight ridges running along it and 2 large accessory lobes at its apex. In Australia Abaxianthus occurs only in northern Queensland along the coast and flowers sporadically throughout the year.

Similar Genera

Flickingeria

Significant Generic Characters

Epiphytic orchids; rhizomes long, thin; roots unbranched; pseudobulbs stalked, single-noded; leaves lasting several seasons, solitary on a pseudobulb, petiolate, thick-textured, longer than wide, flat; inflorescence short, perennial, occurring behind the leaf, each meristem flowering once only; flower single, lasting 8-10 hours; perianth segments thin-textured, flat; dorsal sepal smaller than the lateral sepals; lateral sepal bases partly fused with each other and with the column foot; petals smaller than the sepals; labellum firmly attached to the apex of the column foot; lamina three-lobed; lateral lobes small; midlobe with two large apical accessory lobes; callus with low ridges.

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Size and Distribution

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A small genus of about 4 species occurring in Malaysia, Philippines, Indonesia, New Guinea and northern Australia. The native species, Abaxianthus convexus, which is not endemic, occurs in north-eastern Queensland between the McIlwraith Range (1346' S) on Cape York Peninsula and the Daintree River (1617' S). State occurrence: Queensland.

Ecology

In Australia, Abaxianthus convexus, grows mainly on mangroves in swamps and around the mouths of large rivers and estuaries. The climate where it grows is tropical, usually very hot and humid, with the majority of rain falling during the summer wet season (December to March) and the remaining months being much drier with sporadic rainfall.

Biology

Pollination: The flowers of Abaxianthus convexus last for 8-10 hours and are insect-pollinated but the vector is unknown. 

Reproduction: Reproduction in Abaxianthus is solely from seed. Seed dispersal takes 4-6 months after pollination and the capsules develop in a nodding position. Apomixis is unknown in the genus.

Seasonal Growth: The plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year. They form a network of rhizomes around the trunk of the host tree.

Flowering: Flowering occurs sporadically throughout the year.

Hybrids: Natural hybrids involving species of Abaxianthus are unknown.

Derivation

Abaxianthus is derived from the Latin abaxialis, away from the axis and anthus, flower, in reference to the flowers arising from the apex of the pseudobulb behind the leaf, not in the leaf axil.

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Botanical Description

Perennial, evergreen, epiphytic herbs, sympodial. Roots elongate, unbranched, filamentous, produced from nodes along the rhizome. Rhizome well-developed, thin, brittle, branched. Pseudobulbs well-developed, widely-spaced, 1-noded, each supported on a short stem, hard, relatively short, mostly obloid, when young covered by brown scarious bracts. Trichomes absent. Aerial growths absent. Leaves lasting several seasons, solitary on a pseudobulb, petiolate, longer than wide, thick textured, coriaceous, smooth, flat, not grooved or channelled and with no obvious midrib; base not sheathing the pseudobulb; margins entire; apex unequally emarginate. Inflorescence single-flowered, arising from a meristem situated at the apex of a pseudobulb on the abaxial side of a leaf; each meristem consists of a small cavity with a sheathing bract and is capable of producing a single flower only. Peduncle absent.  Floral bracts scarious, sheathing the base of the pedicel. Pedicel very short, thin, merging with the ovary. Ovary short, straight or curved, porrect from the pedicel. Flowers non-resupinate, stalked, lasting 8-10 hours, white or cream with yellow in the labellum; labellum with contrasting colours. Perianth segments thin-textured, widely spreading, entire, flat. Dorsal sepal free, smaller than the lateral sepals, straight; apex entire, flat.  Lateral sepals larger than the dorsal sepal, straight, partly fused with each other and attached by their broad bases to the column foot; apex entire. Petals free, smaller than the sepals; apex entire. Labellum stiffly attached to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina more or less oblong, relatively thick-textured, strongly three-lobed; lateral lobes relatively small, erect, flanking the column, entire; midlobe well-developed, porrect; apex with two large accessory lobes that extend forwards in line with the midlobe. Spur absent. Callus consisting of three low, parallel ridges or keels, the outer two extending to the apex of the midlobe. Nectar present. Column lacking free filament and style, fleshy, shorter than the perianth segments, nearly straight. Column wings present, reduced, ventral and with short tooth-like apical stelidia. Column foot well developed, longer than the column and more or less in line with it, shallowly curved. Pseudospur present, formed by the union of the anterior basal margin of the lateral sepals and the bases of the lateral sepals with the margins of the column foot. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, rostrate; apex smooth. Pollinarium absent. Pollinia 4 in 2 pairs, straight or slightly falcate, yellow or orange, hard, waxy. Viscidium absent. Rostellum ventral, short, transverse. Stigma entire, transverse, concave. Capsules dehiscent, glabrous, cernuous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.

Taxonomy

Abaxianthus is a recently described segregate from Flickingeria (Jones & Clements 2002).

Nomenclature

Abaxianthus M.A.Clem. & D.L.Jones, Orchadian 13(11): 485 (2002). Type species: Desmotrichum convexa Blume [Abaxianthus convexus (Blume.) M.A.Clem. & D.L.Jones].

Infrageneric Taxa: No infrageneric taxa are currently known.

References

Clements, M.A. and Jones, D.L. (2002). Nomenclatural changes in the Dendrobieae (Orchidaceae). 1: The Australasian region. Orchadian 13(11): 485-497.

Dockrill, A.W. (1969). Australian Indigenous Orchids. Volume 1. The Society for Growing Australian Plants, Halstead Press, Sydney.

Dockrill, A.W. (1992). Australian Indigenous Orchids. Volume 1 & 2. Surrey Beatty & Sons in association with The Society for Growing Australian Plants, Chipping Norton, NSW.

Schlechter, R. (1982). The Orchidaceae of German New Guinea (English translation by R.S. Rogers, H.J. Katz and J.T. Simmons). Australian Orchid Foundation, Melbourne.

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