Clumping epiphytes, lithophytes, rarely terrestrials, with thin roots. Plants can have small pseudobulbs with 1-3 terminal leaves or flattened stems with several leaves in 2 ranks. The leaves have a laterally flattened sheath and a flat or channelled blade. Thin erect to arching spikes or racemes, which arise from leaf axils or the base of the pseudobulbs, have numerous tiny white, cream, greenish, yellowish or brownish semi-tubular flowers, the lower parts of the segments overlapping with the tips usually spreading. The labellum, which is unlobed, is attached to the base of the column. The column is short, without a column foot.
Similar Genera
Significant Generic Characters
Epiphytic/lithophytic/terrestrial sympodial orchids; roots very thin; plants either with pseudobulbs or flattened stems; pseudobulbs small, with 1-3 terminal leaves; flattened stems with several leaves in 2 ranks; leaf base sheathing, laterally flattened; laminae thinly coriaceous, conduplicate, smooth; racemes from leaf axils or the base of pseudobulbs, erect to arcuate, multiflowered; flowers resupinate, semitubular, minute, lasting 2-4 days, white, cream, greenish, yellowish or brownish; sepals and petals free, overlapping in proximal half; labellum unlobed, stiffly attached, base shallowly saccate; column short, without a column foot; pollinia 8, in 2 groups of 4, all attached to a single common caudicle and viscidium.
Size and Distribution
A genus of about 20 species occurring in India, South-east Asia, Malaysia, Philippines, Indonesia, Polynesia, New Guinea and Australia where there is a single non-endemic species, Thelasis carinata, in northeastern Queensland and Thelasis capitata on Christmas Island (extending to Indonesia). State occurrence: Queensland, Western Australia (Christmas Island).
Ecology
Species of Thelasis 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, often close to streams and waterfalls.
Biology
Pollination: The flowers of Thelasis species are tiny and relatively short-lasting. Thelasis carinata is self-pollinating and Thelasis capitata appears to be insect-pollinated.
Reproduction: Reproduction in Thelasis is solely from seed. Seed dispersal takes 1-3 months from pollination and the capsules develop in a porrect position. Apomixis is unknown in the genus.
Seasonal Growth: Thelasis plants grow actively in summer and autumn and are relatively quiescent for the remainder of the year.
Flowering: Both Thelasis carinata and Thelasis capitata flower mainly April to June, but can flower sporadically at other times.
Hybrids: Natural hybrids are unknown in these orchids.
Derivation
The name Thelasis is derived from the Greek thele, nipple, possibly in reference to the extended rostellum which could be likened to a nipple.
Botanical Description
Perennial, evergreen, epiphytic, lithophytic or rarely herbs, sympodial. Plants glabrous, clumping or with a series of fan-like growths. Rhizome superficial or well-developed. Roots filamentous, thin. Stems either laterally flattened or pseudobulbous. Pseudobulbs small, multinoded, crowded, nearly naked or with overlapping scarious bracts. Trichomes absent. Leaves lasting several seasons, either 1-3 per shoot and terminal on the pseudobulbs or several leaves in 2 closely opposite ranks; base sheathing, laterally flattened, imbricate; lamina thinly coriaceous, flat or canaliculate, smooth; apex entire or emarginate. Venation unknown. Inflorescence spicate, multiflowered, erect or arcuate, arising from an axil or the base of a pseudobulb. Peduncle similar to or longer than the rhachis, thin, with scattered sterile bracts. Rhachis elongate or condensed, straight. Floral bracts narrow, closely sheathing, usually persistent after anthesis. Pedicels short or absent. Ovary short, often triangular, straight, smooth. Flowers resupinate, minute, tubular at the base, often with spreading tips, crowded, sometimes densely so, white, cream, greenish, yellowish or brownish, sessile to subsessile, opening sequentially in a spiral. Perianth segments short, relatively narrow, generally overlapping at least at the base. Dorsal sepal free, similar to the lateral sepals but slightly narrower, porrect. Lateral sepals free, similar to the dorsal sepal, porrect. Petals subsimilar to the sepals, usually shorter and narrower. Labellum stiffly attached to the base of the column, markedly dissimilar in size to the sepals and petals, ecalcarate or with a shallow basal spur. Labellum lamina unlobed, sessile, broadest at the base and narrowing distally, flat or with incurved margins. Spur absent or small and shallow. Callus obscure. Nectar absent. Column very short, lacking free filament and style. Column foot absent. Pseudospur absent. Column wings dentiform, apical. Anther terminal, incumbent, 2-celled, persistent, basifixed, rostrate. Pollinaria 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 long, narrow. Rostellum ventral, elongate, projecting. Stigma entire, concave. Capsules dehiscent, small, glabrous, porrect; peduncle not elongated in fruit; pedicels not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Thelasis as currently interpreted includes plants with pseudobulbous growths with few terminal leaves and others non-pseudobulbous with a fan-like habit, laterally flattened stems and several leaves in 2 opposite ranks. The genus requires a detailed morphological and molecular study to determine the significance of this variation in vegetative features.
Notes
Thelasis carinata has laterally flattened stems and several leaves per shoot arranged closely in 2 ranks. Thelasis capitata has pseudobulbs with 1-3 terminal leaves.
Nomenclature
Thelasis Blume, Bijdr. 385, t.75 (1825). Type species: Thelasis carinata Blume.
Infrageneric Taxa: No infrageneric treatment of Thelasis 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.
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.