Epiphytes or lithophytes with very thin roots and fan-shaped growths that occur singly or in small groups. The main stem is difficult to discern, usually swollen, fleshy and with overlapping leaf bases. The leaves, which are thick and fleshy, are arranged in 2 ranks. Thin erect to arching spikes are borne in the leaf axils. They have numerous tiny white, cream or greenish cup-shaped flowers. Labellum tiny, mostly simple with flat or erect margins, stiffly attached to the apex of the column foot. Column tiny, with a relatively long column foot.
Similar Genera
Significant Generic Characters
Epiphytic/lithophytic monopodial orchids; roots very thin; stems difficult to discern, swollen, fleshy, with imbricate leaf bases; leaves distichous, thick, fleshy; racemes axillary, erect to arcuate, multiflowered; flowers tiny, lasting 2-4 days, white, cream or greenish; sepals and petals relatively broad, porrect to spreading; labellum with a narrow basal hypochile and a broad simple lamina, stiffly attached to the apex of the column foot; column short, with a relatively long foot; pollinia 8, in 2 groups of 4, each group forming a separate hemipollinarium.
Size and Distribution
A genus of about 20 species occurring in South-east Asia, Malaysia, Philippines, Indonesia, Polynesia, New Guinea and Australia where there is a single species, Plexaure crassiuscula endemic in northeastern Queensland, Plexaure limenophylax on Norfolk Island and Plexaure listeri on Christmas Island. The Australian mainland species is distributed between the Big Tableland (about 15°40' S), south of Cooktown, and Paluma (19°00' S). State occurrence: Queensland, New South Wales (Norfolk Island), Western Australian (Christmas Island).
Ecology
Species of Plexaure grow on trees and rocks in moist to wet forests, especially rainforest. Plants commonly grow in shady humid situations where there is free and abundant air movement.
Biology
Pollination: The flowers of Plexaure species are tiny and relatively short-lasting. Little is known about their pollination syndrome. Some species are probably self-pollinating and others are possibly pollinated by rain splash.
Reproduction: Reproduction in Plexaure is solely from seed. Seed dispersal takes 1-2 months from pollination and the capsules develop in a porrect position. Apomixis is unknown in the genus.
Seasonal Growth: Plexaure plants grow actively in summer and autumn and are relatively quiescent for the remainder of the year.
Flowering: Plexaure crassiuscula and Plexaure limenophylax flower mainly January to April; Plexaure listeri flowers September and October.
Hybrids: Natural hybrids are unknown in these orchids.
Derivation
The name Plexaure is derived from the Latin plexus, braided and auris, ear, possibly in reference to the braid-like inflorescence.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs, monopodial. Plants glabrous, somewhat fan-like, consisting of single growths or a small cluster of growths. Roots filamentous, thin. Stems reduced, condensed, swollen, fleshy. Pseudobulbs absent. Trichomes absent. Leaves lasting several seasons, distichous, thick and fleshy; base sheathing, persistent, fleshy, imbricate; lamina fleshy, canaliculate; apex entire. Venation unknown. Inflorescence spicate, multiflowered, erect or arcuate, axillary in a leaf base. Peduncle much shorter than the rhachis, thin to relatively thick, with scattered sterile bracts. Rhachis thin to relatively thick, straight or curved. Floral bracts narrow, closely sheathing. Pedicels short or absent. Ovary short, straight, smooth. Flowers non-resupinate, minute, cupulate, crowded, white, cream or greenish, sessile to subsessile, opening sequentially in a spiral. Perianth segments short, relatively broad. Dorsal sepal free, similar to the lateral sepals but slightly narrower, porrect to incurved. Lateral sepals fused at the base to the margins of the column foot to form a mentum, similar to the dorsal sepal, porrect to spreading. Petals subsimilar to the sepals, usually shorter and narrower. Labellum stiffly attached to the apex of the column foot, markedly dissimilar in size to the sepals and petals, ecalcarate. Labellum lamina divided into a hypochile and epichile; hypochile short, flat; lamina broad, flat or with incurved margins; apex broadly obtuse, truncate or emarginate. Spur absent. Callus obscure. Nectar absent. Column very short, lacking free filament and style. Column foot relatively long, at about 90° to the column, apex sharply recurved. Pseudospur absent. Column wings short, dentiform. Anther terminal, transverse, incumbent, 2-celled, persistent, basifixed. Pollinaria 2 (hemipollinaria). Pollinia 8 in 2 groups of 4, clavate, orange, stalked. Viscidium small. Rostellum ventral, often small. Stigma entire, concave. Capsules dehiscent, small, glabrous, porrect; peduncle not elongated in fruit; pedicels not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Plexaure has been included in Phreatia but the plants lack pseudobulbs, have a monopodial growth habit, fleshy leaves and the column has a foot.
Notes
The Australian species, which has been included in Phreatia for many years, was recently transferred to Plexaure (Jones and Clements 2004).
Nomenclature
Plexaure Endl., Prodr. Fl. Norfl. 30 (1833). Type species: Plexaure limenophylax Endl.
Infrageneric Taxa: No infrageneric treatment of Plexaure is available.
References
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.
Jones, D.L. and Clements, M.A. (2004). Miscellaneous new species, new genera, reinstated genera and new combinations in Australian Orchidaceae. Orchadian 14(8): Scientific Supplement xiv.