Tree Spider Orchids
Epiphytes with very short almost indiscernible rhizomes anchored by thin roots that arise from nodes at the base of the pseudobulbs. The unusually shaped, hard pseudobulbs have a very narrow attenuate basal section and a thick main section that is distinctly 4-sided. The thin leaves are confined to the pseudobulb apex and the racemes arise from perennial meristems, mostly on the apical nodes, sometimes from leafless side nodes. The flowers, which are borne on short thin racemes, last several days and are relatively large and spidery. The perianth segments are narrow and the bases of the lateral sepals fused with the column foot. The labellum, which is distinctly 3-lobed, has large lateral lobes and a broad, pointed midlobe. The callus consists of 3 narrow ridges that do not extend to the midlobe.
Significant Generic Characters
Epiphytic/lithophytic orchids; rhizomes almost indiscernible; pseudobulbs with a thin attenuate basal section supporting a broader tetragonal part, hard, multinoded; leaves 1-5 per shoot, apical and subapical, thin, flat, not basally sheathing; racemes mainly from the apical nodes, sometimes from leafless lateral nodes, 1-5-flowered; flowers lasting several days, large, stellate, thin-textured, scented; perianth segments narrow, flat or slightly twisted; lateral sepal bases fused with the column foot; petals narrower than the sepals; labellum stiffly hinged to the apex of the column foot, 3-lobed; lateral lobes large, rounded; midlobe broad, pointed; callus of 3 ridges not extending onto the midlobe.
Size and Distribution
A genus of 4 species endemic in Australia between the Iron Range (12°38’S) on Cape York Peninsula in northeastern Queensland and near Tomerong (35º03'S) in southern New South Wales. State occurrence: Queensland, New South Wales.
Ecology
Species of Tetrabaculum range from lowland situations to about 1200 m alt. in the ranges and tablelands and occur in tropical, subtropical and temperate regions. They grow mainly on trees, rarely rocks, in shady, humid positions where there is free air movement. Habitats include rainforest, woodland and open forest, often near streams.
Biology
Pollination: The flowers Tetrabaculum species last several days and are pollinated by native bees.
Reproduction: Reproduction in Tetrabaculum is solely from seed. Seed dispersal takes 4-6 months after pollination and the capsules develop in a pendulous position. Apomixis is unknown in the genus.
Seasonal Growth: Tetrabaculum plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: Flowering mainly occurs in late winter-spring, but sporadic flowering can occur at other times.
Hybrids: Natural hybrids involving species of Tetrabaculum are unknown.
Derivation
Tetrabaculum, which is derived from the Greek tetra, four and baculum, cane or stem, refers to the distinctive 4-sided pseudobulbs.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs, sympodial. Plants glabrous or pubescent on the labellum. Roots elongate, branched, produced from nodes on the base of the pseudobulb. Rhizome short, condensed, branched. Pseudobulbs well-developed, multinoded, crowded, hard, elongate, base very narrow and attenuate, main part expanded, tetragonal, when young covered by scarious bracts. Trichomes sometimes present on the labellum and callus. Aerial growths occasional. Leaves few (1-5 per shoot), distichous, apical and subapical, sessile, nearly as wide as long, thin-textured, coriaceous, smooth, flat, not grooved or channelled; base not sheathing the pseudobulb; margins entire or undulate; apex entire or unequally emarginate. Inflorescence relatively short, racemose, arising mainly from perennial apical nodes covered with persistent sheaths, sometimes from leafless lateral nodes on the pseudobulb, 1-5-flowered. Peduncle shorter than the rhachis, thin, sometimes much reduced, with scattered scarious bracts. Floral bracts small, scarious, sheathing the base of the pedicel. Pedicel relatively long, thin, merging with the ovary. Ovary short, straight, porrect from the pedicel. Flowers resupinate, stalked, stellate, lasting several days, cream, green or yellowish, often with marginal bands and blotches, scented; labellum often with distinct purplish striae. Perianth segments narrow, thin, widely spreading, entire, flat or sometimes slightly twisted, straight. Dorsal sepal free, subsimilar to the lateral sepals, straight; apex entire, flat. Lateral sepals subsimilar to the dorsal sepal, falcate, attached by their bases to the column foot; apex entire. Petals free, smaller than the sepals; apex entire. Labellum stiffly hinged to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina more or less elliptical, relatively thin-textured, strongly 3-lobed, glabrous or pubescent; lateral lobes large, erect, broadly rounded, flanking the column, entire; mid-lobe narrow to broadly flared, porrect to recurved; apex acuminate. Callus consisting of 3 narrow median parallel ridges, sometimes irregularly sinuate distally, not extending to the midlobe. Nectar absent. Spur absent. Column at an angle to the ovary, lacking free filament and style, fleshy, shorter than the perianth segments, nearly straight. Column foot longer than the column, at right angles to the column, straight or curved. Pseudospur absent. Column wings reduced, ventral, with short tooth-like apical stelidia. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, with a deflexed rostrum; apex smooth. Pollinarium absent. Pollinia 4 in 2 unequal pairs, straight or falcate, orange, hard, waxy. Viscidium absent. Rostellum ventral, small, transverse. Stigma entire, concave. Capsules dehiscent, glabrous, pendulous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Tetrabaculum has been included in Dendrobium but can be immediately distinguished by the unusual, hard, 4-sided pseudobulbs that have a very narrow attenuate base and widen distally, apical and subapical non-sheathing leaves lasting many seasons, inflorescences mainly arising from apical nodes, stellate flowers and the labellum distinctly 3-lobed.
Nomenclature
Tetrabaculum M.A.Clem. and D.L.Jones, Orchadian 13(11): 490 (2002).
Type species: Dendrobium tetragonum A.Cunn. [Tetrabaculum tetragonum (A.Cunn.) M.A.Clem. and D.L.Jones].
Infrageneric taxa: No infrageneric taxa are currently recognised.
References
Clements, M.A. and Jones, D.L. (2002). Nomenclatural changes in the Dendrobieae (Orchidaceae) 1: the Australasian region. Orchadian 13(11): 485-497.
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.