Erect to drooping epiphytes with numerous roots and short fibrous leafy stems unbranched or sparsely branched from near the base. The thick, crowded leathery leaves are arranged in 2 ranks. Inflorescences are very short racemes, the main stalk usually thickened like a club. The flowers, which last a few hours, are produced in flushes at sporadic intervals. The flowers have a fleshy spurred labellum fixed to the base of the column. The column is short and lacks a column foot.
Significant Generic Characters
Epiphytic orchids; plants large, erect or drooping; roots numerous, moderately thick, often forming a buttress; stems short, fibrous, unbranched or sparsely branched from the base; leaves flat, crowded, arranged in 2 ranks , much longer than wide, thick, leathery; inflorescence an abbreviated raceme with a thickened rhachis; flowers produced in flushes at sporadic intervals; labellum firmly attached to the base of the column; lamina 3-lobed, with a basal spur; lateral lobes large; column short, lacking a foot; pollinia 2, sessile on a stipe.
Size and Distribution
A genus of about 7 species distributed in India, Malaysia, Polynesia, Philippines, Indonesia, New Guinea and Australia where there are 3 endemic species occurring in northeastern Queensland . Two species are restricted to the Iron Range (12º38’ S) and McIlwraith Range (13º34’ S) areas on Cape York Peninsula and the other species occurs between Cairns (16º55’ S) and Innisfail (17º32’ S). State occurrence: Queensland.
Ecology
The native species of Trachoma grow on trees in rainforest at low to moderate altitudes, usually in brightly lit situations. They often occur on emergent trees, such as Hoop Pines (Araucaria cunninghamiana) and grow along ridges where there is abundant air movement. The climate is tropical and the majority of rain falls during the summer wet season (December to March), with the remaining months much drier and having sporadic or intermittent rain.
Biology
Pollination: The flowers of Trachoma are produced at sporadic intervals in flushes, the scape lengthening slightly between each batch of flowers. Each flower lasts a few hours. They are probably pollinated by native bees.
Reproduction: Reproduction in Trachoma is solely from seed. Seed dispersal takes 10-12 months after pollination and the capsules develop in a porrect position. Apomixis is unknown in the genus.
Seasonal Growth: Trachoma plants grow mainly during the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: The native species of Trachoma flower sporadically between spring and autumn.
Hybrids: Natural hybrids involving the native species of Trachoma are unknown.
Derivation
Trachoma is derived from the Greek trachomos, roughness, in reference to the inflorescence which is roughened because of the remnants of the crowded bracts.
Botanical Description
Perennial, evergreen, epiphytic herbs, monopodial. Roots numerous, sometimes forming a buttress, moderately thick, elongate, much branched, mainly adherent. Plants small to moderately large, unbranched or sparsely branched from the base, erect, porrect or drooping. Stem relatively thick, short, fibrous, leafy towards the apex. Pseudobulbs absent. Trichomes absent. Leaves lasting several seasons, flat or convolute, distichous, sessile, crowded, spreading widely, much longer than wide, thick, coriaceous; base sheathing the stem, imbricate with its neighbour, persistent after leaf abscission; margins entire; apex unequally emarginate. Inflorescence lateral, an abbreviated raceme. Peduncle similar or shorter than the rhachis. Rhachis straight, thick, clavate, lengthening steadily and producing flowers at sporadic intervals. Floral bracts small, scarious, partly sheathing the base of the pedicel. Pedicel short, merging with the ovary. Ovary short, straight. Flowers resupinate, produced 1-several in spasms at sporadic intervals, stalked, lasting a few hours, cream, white or yellowish. Perianth segments relatively thin-textured, porrect to spreading. Dorsal sepal free, subsimilar to the lateral sepals. Lateral sepals free, subsimilar to the dorsal sepal, usually flanking the labellum. Petals free, narrower than the sepals. Labellum stiffly attached to the base of the column, markedly dissimilar in size and shape to the sepals and petals, calcarate. Labellum lamina 3-lobed, with a basal spur; spur short, cylindrical or conical, broad, blunt; lateral lobes large, erect, thin to thick and fleshy; midlobe short to long, fleshy or hollow. Spur basal (see labellum lamina). Callus obscure, consisting of fleshy ridges and protruberances. Nectar unknown. Column short, porrect from the apex of the ovary, lacking free filament and style, fleshy, straight. Column wings ventral, incurved. Column foot absent. Pseudospur absent. Anther terminal, incumbent, 2-celled, persistent, smooth, with a short rostrum. Pollinarium present. Pollinia 2, pale yellow to orange, hard, waxy, sessile. Stipe short to long, narrow to relatively broad. Viscidium relatively large, at an angle to the stipe. Rostellum short, bifid. Stigma entire, concave. Capsules dehiscent, elongate, glabrous, porrect; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Within the Vandeae, Trachoma is distinguished by fugacious flowers produced at spasmodic intervals, the inflorescence rhachis elongating between successive flushes of flowers; labellum fixed to the column base; lamina with a basal spur; column without a foot and, 2 sessile pollinia.
The genus is included by Tuberolabium Yamamoto by some authors (Wood 1990, Dockrill 1992) but the genera are readily separated by their floral ecology. In Tuberolabium the flowers last many days and open sequentially along the raceme with no elongation of the rhachis after the buds have formed.
Notes
Species of Trachoma have an interesting flowering habit that is influenced by environmental factors. The rhachis elongates between successive flushes of flowers and the buds develop to a stage and then become quiescent until promoted by an environmental stimulus after which they develop rapidly and all those at the correct stage of development open together.
Nomenclature
Trachoma Garay, Bot. Mus. Leafl. 23:, 207 (1972).
Type species: Dendrocolla rhopalorrhachis Rchb.f. [Trachoma rhopalorrhachis (Rchb.f.) Garay].
Infrageneric Taxa: No infrageneric taxa are currently recognised.
References
Christenson, E.A. (1999). Sarcanthinae Genera 25: Tuberolabium. Orchids, Dec;1999: 1218-1221.
Dockrill, A.W. (1967). Australasian Sarcanthinae. The Australasian Native Orchid Society, Sydney.
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.
Garay, L. (1972). On the systematics of the monopodial orchids. Bot. Mus. Leafl. Harv. Uni. 23(4): 149-212.
Wood, J.J. (1990). Notes on Trachoma, Tuberolabium and Parapteroceras (Orchidaceae). Nordic. J. Bot. 10: 481-486.