Erect to pendulous epiphytes, lithophytes or terrestrials with coarse roots and relatively thick fibrous leafy stems, the crowded, leathery leaves arranged in 2 ranks. Inflorescences have a long basal stalk with the bracts and flowers crowded on a short apical section. The flowers develop a few at a time in spasms throughout the year, each flower lasting less than one day. They have thin-textured segments and a stiffly attached pouched labellum with a broad internal structure that extends forwards from the rear wall. The column is short with a broad column foot.
Significant Generic Characters
Epiphytic/lithophytic/terrestrial orchids; plants small to large; stems fibrous; leaves crowded, in 2 ranks, much longer than wide, leathery, unequally notched; flowers small to relatively large, lasting less than a day; perianth segments thin-textured; sepals subsimilar; petals slightly smaller than the sepals; labellum firmly attached to the apex of the column foot; lamina saccate throughout, 3-lobed; spur basal, saccate, with a broad, thick callus structure on the lower wall extending towards the labellum apex; column short, with a short broad foot; pollinia 4, sessile on a stipe, in 2 very unequal pairs; stipe vestigial or short and broad.
Size and Distribution
A genus of about 100 species distributed in India, South-east Asia, Malesia, Melanesia, including New Guinea, and some islands of the Pacific. Two non-endemic species, Thrixspermum congestum and T. platystachys, occur in northeastern Queensland between Cape York (10º41' S) and Townsville (19º16' S), T. congestum also extending to the Northern Territory. Another species occurs on Christmas Island. State occurrence: Queensland, Northern Territory, Western Australia (Christmas Island).
Ecology
These orchids generally grow on rainforest trees in lowland situations and sometimes colonise mango trees and relict trees in paddocks. Some species favour coastal habitats such as estuaries and mangroves. 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 Thrixspermum species last a few hours and are probably pollinated by native bees.
Reproduction: Reproduction in Thrixspermum is solely from seed. Seed dispersal takes 10-12 months after pollination and the capsules develop in a porrect or pendulous position. Apomixis is unknown in the genus.
Seasonal Growth: Plants of Thrixspermum species grow mainly during the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: Thrixspermum species flowers sporadically throughout the year, with an increased frequency over the summer months.
Hybrids: Natural hybrids involving species of Thrixspermum are unknown in Australia.
Derivation
Thrixspermum, which is derived from the Greek thrix, hair and sperma, seed, refers to the hair-like seeds.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs (rarely terrestrial), monopodial. Roots thick, elongate, aerial or adherent. Plants small to large, erect (sometimes semi-climbing), porrect or pendulous. Stem relatively thick, short to long, fibrous, unbranched or sparsely branched. Pseudobulbs absent. Trichomes present on the labellum. Leaves lasting several seasons, distichous, sessile, much longer than wide, flat or channelled, thick, coriaceous, smooth; base sheathing the stem, usually imbricate with its neighbour, persistent after leaf abscission; margins entire; apex unequally emarginate. Inflorescence lateral, spicate, erect, arcuate or pendulous, arising from a distal axil, multiflowered. Peduncle much longer than the rhachis, with few imbricate scarious bracts. Rhachis straight, condensed, terete or laterally flattened. Floral bracts scarious, crowded, either laterally flattened and in 2 ranks, or congested and spirally arranged, sheathing the base of the pedicel. Pedicel short to long, thin, merging with the ovary. Ovary short, mostly straight. Flowers resupinate, small to relatively large, stalked, opening sporadically, lasting one day, cream, white or yellowish. Perianth segments short and broad or long and narrow, thin-textured, spreading. Dorsal sepal free, subsimilar to the lateral sepals. Lateral sepals free, subsimilar to the dorsal sepal, flanking the labellum. Petals free, slightly shorter than the sepals. Labellum stiffly attached to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, calcarate. Labellum lamina pouched, with a basal spur, glabrous or hairy, 3-lobed; spur saccate, with a broad, thick callus structure on the lower wall extending towards the labellum apex; lateral lobes short, blunt; midlobe short or long, porrect, thick, fleshy. Spur (see labellum). Callus (see labellum). Nectar unknown. Column short, porrect from the apex of the ovary, lacking free filament and style, fleshy, nearly straight. Column wings small, ventral. Column foot short, broad. Pseudospur absent. Anther terminal, incumbent, 2-celled, persistent, smooth, with a narrow curved or hooked rostrum. Pollinarium present. Pollinia 4, in 2 appressed pairs, very unequal, orange, hard, waxy, sessile. Stipe vestigial or short, broad. Viscidium present, small. Rostellum ventral, bifid. Stigma entire, concave. Capsules dehiscent, glabrous, porrect; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Within the Vandeae, Thrixspermum is recognised by its inflorescence consisting of a long peduncle and condensed rhachis, spasmodic flowering habit, crowded fugacious flowers, saccate spur with a broad, thick callus structure on the lower wall extending towards the labellum apex and, 4 sessile pollinia in 2 very unequal pairs.
Notes
These orchids have a multiflowered inflorescence but the flowers develop sequentially with only 1-2 flowers open on an inflorescence at any time. The buds develop to a distinctly inflated stage and then remain quiescent as if awaiting a stimulus before developing rapidly and opening. The flowers open very early in the morning and are often finished by mid-afternoon.
Nomenclature
Thrixspermum Lour. Fl. Cochinch. 2: 519 (1790). Type species: Thrixspermum centipeda Lour.
Infrageneric Taxa: A genus of 2 sections;
1. Thrixspermum section Thrixspermum with laterally flattened rhachis and distichous floral bracts and flowers. Type species: Thrixspermum centipeda Lour.
2. Thrixspermum section Dendrocolla (Blume) J.J.Smith with terete rhachis and spirally arranged floral bracts and flowers. Type species: Type species: Thrixspermum hystrix (Blume) J.J.Smith.
References
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.