Elbow Orchids
Deciduous terrestrial orchids that reproduce from seed and also form clonal colonies. Plants are dimorphic. Sterile plants consist solely of a rosette of leaves. Fertile plants consist of a multiflowered inflorescence that, late in flowering or after flowering, develops a leafy rosette from a lateral growth at the base, rarely the rosette surrounding the base of the peduncle (A. rosulatus). The small flowers are upside-down and dull-coloured (usually greenish). Perianth segments free and inconspicuous. The dorsal sepal is held close to the column and the lateral sepals and petals reflex back against the ovary, leaving the labellum exposed. Labellum delicately hinged at the end of a moderately long narrow column foot, dangled like a fishing lure. The labellum itself is unlobed and reduced to a simple strap-shaped structure, but it is dominated by a large shiny, colourful insect-like callus. Column elongate, incurved, with 2 unequal pairs of projecting wings, the upper pair smaller and extending above the anther.
Similar Genera
Significant Generic Characters
Deciduous terrestrial; plants dimorphic; rosette leaves longer than wide; racemes multiflowered; flowers small, non-resupinate, greenish; sepals and petals inconspicuous; lateral sepals and petals reflexed; sepal bases and column foot papillate; labellum lamina simple, ligulate, with a peltately attached supporting stalk; callus intricately insectiform and dominating the labellum lamina; labellum structure delicately hinged to the base of the column foot and dangled like a fishing lure; column with two unequal pairs of projecting column wings and a foot; pollinia deeply bilobed, not attached to a viscidium.
Size and Distribution
A genus of c. 10 species distributed in northern Australia, including Torres Strait Islands, and on the east coast extending from Cape York in Queensland to central New South Wales. The genus possibly also occurs in south-eastern parts of Papua New Guinea. Latitudinally species of Arthrochilus are distributed from about 10° S on Moa Island in Torres Strait to about 33°50’ S in the northern suburbs of Sydney. State occurrence: Queensland (including Moa and Thursday Island), New South Wales, Northern Territory.
Ecology
Arthrochilus is a small highly specialised genus that reaches its best development in tropical areas of northern and eastern Australia. Growth in all of these species is closely tied in with the monsoonal wet season. Most species are distributed in lowland regions with Arthrochilus oreophilus being the exception, occurring as it does in ranges and tablelands at 900-1200 m alt. Habitats include open forest, woodland, heathland and Epacrid Scrub. These orchids commonly grow in sandy or gravelly soils that may be either well-drained or more frequently are seasonally wet.
Biology
Pollination: Species of Arthrochilus are pollinated by male thynnine wasps that are attracted to the flowers by pheromone-like odours. The wasp grasps the labellum and attempts to fly off with it, causing the labellum to pivot on its hinge trapping the insect between it and the protruding column wings whih have a hooked apex. Pollen deposition or transference is achieved as the wasp struggles to free itself. These encounters can be quite violent and frequently result in the labellum being torn from the flower. Arthrochilus irritabilis is pollinated by the wasp Arthrothynnus rufiabdominalis and Arthrochilus latipes by an undescribed species of Arthrothynnus (Brown 1996, Bower 2001).
Reproduction: Species of Arthrochilus reproduce vegetatively to form clonal colonies. Daughter tubers are produced on the end of thin stolonoid roots that can sometimes be surprisingly long. Seeds are dispersed rapidly in the tropical species, usually within 6-10 weeks of pollination.
Seasonal Growth: In tropical regions species of Arthrochilus respond strongly to the wet season, with the tubers sprouting rapidly after the first soaking rains, usually late November to January. Inflorescences appear first and these are followed by rosettes of leaves produced from non-flowering plants. In the subtropics and temperate regions the plants generally appear a few weeks later than in the tropics. Plants die back rapidly in the tropics as the season dries out and they survive 6-8 months of the dry season as dormant tubers. Plants in subtropical and temperate regions are above ground for longer than those in the tropics.
Flowering: Flowering in the tropics occurs mainly in December-January whereas in subtropical and especially temperate regions, plants can flower as late as April.
All of these colony-forming species flower and grow actively over summer.
Hybrids: Natural hybrids involving species of Arthrochilus are unknown.
Fire: Fires are frequent in the areas where these orchids grow but any effects of the fires on the orchids are unknown.
Derivation
The generic name Arthrochilus, which is derived from the Greek arthros, jointed, and cheilos, lip, refers to the jointed labellum stalk of these orchids.
Botanical Description
Perennial geophytic herbs, sympodial. Plants glabrous. Roots both filamentous and stolonoid. Tubers present, ovoid, solitary, fleshy, naked; replacement tubers present or absent; daughter tubers formed on the end of slender stolonoid roots. Stem erect, short, unbranched, with membranous cataphylls at each node. Trichomes absent or reduced to papillae. Sterile and flowering plants dimorphic. Sterile plants consist of a rosette of leaves. Fertile plants consist of an inflorescence from which leaves develop in a rosette on the peduncle late in anthesis or after flowering, rarely the rosette surrounding the base of the peduncle (Arthrochilus rosulatus). Rosette usually absent at anthesis. Leaves 2-6 in a rosette arising on a lateral growth from the base of the peduncle; petiole short or nearly absent, grooved adaxially. Leaf lamina mostly longer than wide, convolute in bud, hypostomatic; margins entire. Venation anastomosing. Inflorescence racemose, few-many-flowered, erect, terminal. Peduncle wiry, with many sterile bracts. Floral bracts reduced, semi-sheathing. Ovary elongate, ribbed, glabrous. Flowers non-resupinate, facing outwards, dull coloured (basically greenish with coloured calli), shortly pedicellate; small columnar papillae massed on the bases of the perianth segments and anterior surface of the column. Dorsal sepal free, larger than the lateral sepals, recurved. Lateral sepals mostly free, anteriorly decurrent on the column foot, smaller than the dorsal sepal, reflexed against the ovary. Petals free, narrower than the sepals. Labellum free, hinged to the underside of the column foot by a claw, projecting as an insectiform lure, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina simple, unlobed, almost vestigial, lorate; base swollen; margins entire. Callus dominating the labellum, proximally fused to the lamina, distally free and ornamented with shiny clavate calli. Nectar absent. Spur absent. Column lacking free filament and style, elongate, incurved. Column foot short, swollen into an apical knob. Pseudospur absent. Column wings fused to the column, projected forwards, with 2 pairs of elongate, incurved, unequal, wing-like projections, the larger pair situated medially, the smaller pair projecting over the anther. Anther terminal, 4-celled, persistent, basifixed, porrect. Pollinarium absent. Pollinia 4, bilobed, flat, mealy, yellow. Viscidium absent. Rostellum ventral. Stigma entire, circular, concave. Capsules dehiscent, glabrous, erect; sepals and petals persistent as the fruit develops; peduncle not elongating in fruit; pedicels not elongating in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Two genera have recently been segregated from Arthrochilus - see Phoringopsis and Thynninorchis (Jones et al. 2002).
Notes
Flowering plants of most species of Arthrochilus produce the inflorescence long before the leaves, these appearing later in a rosette arising on a lateral growth from the base of the peduncle. By contrast non-flowering plants develop a rosette only, these rosettes usually emerging later than the inflorescences, which frequently develop rapidly after the first rains of the wet season. An exception is Arthrochilus rosulatus in which the rosette leaves encircle the base of the inflorescence, both parts emerging more or less together. Flower production continues as long as favourable growing conditions persist and the inflorescences can become very tall with all stages present, from immature buds to dehisced capsules.
Nomenclature
Arthrochilus F.Muell., Fragm. 1: 42 (1858). Type species: Arthrochilus irritabilis F.Muell.
Drakaea Lindl. sect. Akaedra Schltr., Bot. Jahrb. Syst. 45: 383 (1911), pro parte. Types: Drakaea irritabilis Rchb.f. and D. huntiana F.Muell.
Infrageneric Taxa: No infrageneric taxa are recognised.
References
Bishop, T. (1996). Field Guide to the Orchids of New South Wales and Victoria. University of New South Wales Press, Sydney.
Blaxell, D.F. (1972). Arthrochilus F.Muell. and related genera (Orchidaceae) in Australasia. Contr. New South Wales Nat. Herb. 4(5): 275-283.
Bower, C.C. (2001). Pollination of Arthrochilus, in Pridgeon, A.M., Cribb, P.J. and Chase, M.W. (2001). Genera Orchidacearum, vol. 2: Orchidoideae (Part 1), 138. Oxford University Press.
Brown, G.R. (1996). Arthrothynnus, a new genus of orchid-pollinating Thynninae (Hymenoptera: Tiphidae). The Beagle, Records of the Museums and Art Galleries of the Northern Territory.
Clements, M.A. (1990). Catalogue of Australian Orchidaceae. Austral. Orch. Res. 1: 1-160.
Harden, G. (ed.) (1993). Flora of New South Wales, vol. 4. New South Wales University Press, Sydney.
Jones, D.L. (1991). New taxa of Australian Orchidaceae. Austral. Orch. Res. 2: 8-13.
Jones, D.L., Clements, M.A., Sharma, I.K., Mackenzie, A.M. and Molloy, B.P.J. (2002). Nomenclatural notes arising from studies into the tribe Diurideae (Orchidaceae). Orchadian 13(10): 437-468.
Nicholls, W.H. (1969). Orchids of Australia. Thomas Nelson, Melbourne.
Ross, E.M. and Jones, D. (1989), in Stanley, T.D. and Ross, E.M., Flora of South-eastern Queensland, vol. 3. Queensland Department of Primary Industries.
Rupp, H.M.R. (1943). The Orchids of New South Wales. Australian Medical Publishing Coy, Sydney.