Tiny epiphytes or lithophytes with very thin roots, short stems and short fleshy leaves in 2 ranks that are jointed to the apex of a short sheathing leaf base. Thin erect spikes are borne in the leaf axils. They have numerous tiny white, cream, yellowish or greenish widely opening flowers with the petals much smaller than the sepals. The tiny labellum is unlobed and stiffly attached to the base of the column and the tiny column lacks a column foot.
Similar Genera
Significant Generic Characters
Tiny epiphytic/lithophytic monopodial orchids; roots very thin; stems difficult to discern, with short, imbricate leaf bases; leaves distichous; lamina articulate on the apex of the leaf base, thick, fleshy; racemes thin, axillary, multiflowered; flowers tiny, lasting 2-4 days, white, cream, yellowish or greenish; sepals and petals free, the petals much smaller than the sepals; labellum unlobed, sessile, stiffly attached to the base of the column; column short, without a foot; pollinia 8, in 2 groups of 4, all attached to a single common caudicle and viscidium.
Size and Distribution
A genus of 20-35 species occurring in Sri Lanka, South-east Asia, Malaysia, Philippines, Indonesia, Polynesia, New Guinea and Australia where there is a single species, Octarrhena pusilla, endemic in northeastern Queensland, between the Big Tableland (about 15°40' S), south of Cooktown, and Paluma (19°00' S). State occurrence: Queensland.
Ecology
Species of Octarrhena grow on trees and rocks in moist to wet forests, especially rainforest. The native species is restricted to the ranges and tablelands above 600 m alt., extending to high altitudes where clouds and mists are frequent. The plants grow in a range of situations from shade to bright light, always where there is free and abundant air movement.
Biology
Pollination: The flowers of Octarrhena 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 Octarrhena 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: Octarrhena plants grow actively in summer and autumn and are relatively quiescent for the remainder of the year.
Flowering: Octarrhena pusilla flowers sporadically throughout the year.
Hybrids: Natural hybrids are unknown in these orchids.
Derivation
The name Octarrhena is derived from the Greek octa, eightfold and arrhen, male, stamen, in reference to the 8 free pollinia.
Botanical Description
Tiny, perennial, evergreen, epiphytic or lithophytic herbs, monopodial. Plants glabrous, consisting of single growths or a small cluster of growths. Roots filamentous, thin. Stems reduced, condensed, fleshy or fibrous. Pseudobulbs absent. Trichomes absent. Leaves lasting several seasons, distichous; base short, sheathing, persistent, imbricate; lamina articulate on the apex of the sheath, cylindrical or laterally flattened, thick, fleshy; apex entire. Venation unknown. Inflorescence racemose, multiflowered, erect, axillary in a leaf base. Peduncle shorter or longer than the rhachis, thin, with scattered sterile bracts. Rhachis thin, straight. Floral bracts narrow, closely sheathing or spreading, persistent after anthesis. Pedicels short. Ovary short, straight, smooth. Flowers resupinate or non-resupinate, minute, semitubular or cupulate, crowded, white, cream, yellowish or greenish, shortly stalked, opening simultaneously or sequentially in a spiral. Perianth segments short, relatively broad. Dorsal sepal free, similar to the lateral sepals, porrect to spreading. Lateral sepals free, similar to the dorsal sepal, porrect to spreading. Petals smaller than the sepals. Labellum stiffly attached to the base of the column, markedly dissimilar in size to the sepals and petals, ecalcarate. Labellum lamina unlobed, sessile, flat, concave or hooked, thin or fleshy. Spur absent. Callus obscure. Nectar absent. Column very short, lacking free filament and style. Column foot absent. Pseudospur absent. Column wings obscure. Anther terminal, incumbent, 2-celled, persistent, basifixed. Pollinarium present. Pollinia 8 in 2 groups of 4, clavate, orange, stalked, the individual caudicles of each pollinium fused distally to form a common caudicle attached to a single terminal viscidium. Viscidium small. Rostellum obscure, toothed. Stigma entire, deeply concave. Capsules dehiscent, small, glabrous, porrect; peduncle not elongated in fruit; pedicels not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Octarrhena has been included in Phreatia but the plants are very small, lack pseudobulbs, have a monopodial growth habit, fleshy leaves, tiny flowers and the column lacks a foot.
Notes
The Australian species had been included in Phreatia for many years (Clements and Jones 1992).
Nomenclature
Octarrhena Thwaites., Enum. Pl. Zeyl. 4: 305 (1861). Type species: Octarrhena parvula Thwaites.
Infrageneric Taxa: No infrageneric treatment of Octarrhena is available.
References
Clements, M.A. and Jones, D.L. (1992). New combinations in Appendicula Blume and Octarrhena Thwaites (Orchidaceae) in Australia. Phytologia 73(2): 117-118.
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.