Epiphytes or lithophytes with thin creeping branched rhizomes appressed to the host, anchored by filamentous roots. Pseudobulbs small, single-noded, widely spaced. Leaf single, smallish, terminal, longer than wide, flat, thin-textured but firm. Inflorescence racemose, 1-few-flowered, wiry, arising from nodes along the rhizome. Flowers lasting a few days, small, firm-textured, porrect to semi-nodding, with no noticeable scent. Lateral sepal bases fused with the column foot. Petals smaller than the sepals. Labellum firmly hinged to the apex of the column foot. Labellum lamina 3-lobed, fleshy.
Similar Genera
Significant Generic Characters
Epiphytic/lithophytic orchids; plants appressed; roots filamentous; rhizomes creeping; pseudobulbs small, single-noded, widely spaced; leaf single, longer than wide, flat, firm-textured; inflorescence arising from rhizome nodes, 1-few-flowered; peduncle longer than the pedicel; flowers lasting a few days, small; perianth segments thin-textured; petals smaller than the sepals; labellum firmly hinged to the apex of the column foot; lamina 3-lobed, fleshy.
Size and Distribution
A genus of about 9 species, the majority occurring in eastern Australia with a single species in New Zealand and another in New Caledonia. The Australian species are distributed between the McIlwraith Range (13°46' S) on Cape York Peninsula, Queensland, and Mumbulla Mountain near Bega, (36°40' S), New South Wales. State occurrence: Queensland, New South Wales (including Norfolk and Lord Howe Islands).
Ecology
Species of Adelopetalum typically grow in humid forests, particularly rainforest, and are often prominent at moderate to high altitudes in the ranges and tablelands. They grow on trees or rocks in situations ranging from shade to moderately bright light where the humidity is generally high and with free air movement. In Australia the genus is best developed in the ranges and tablelands of the tropics and subtropics with a single species in temperate regions.
Biology
Pollination: The flowers of species of Adelopetalum last 3-5 days and are insect-pollinated, the vectors apparently being small flies that feed on nectar produced by the labellum. There is some indication that the flowers of A. argyropus may be self-pollinating.
Reproduction: Reproduction in Adelopetalum is solely from seed. Seed dispersal takes 3-4 months after pollination and the capsules develop in an erect position. There is no increase in the length or thickness of the peduncle or pedicel during this process. Apomixis is unknown in the genus.
Seasonal Growth: The plants grow strongly in the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: Flowering occurs mainly in spring and summer but some of the tropical species flower sporadically through the year.
Hybrids: Natural hybrids involving species of Adelopetalum are unknown.
Derivation
The generic name Adelopetalum is derived from the Greek adelos, unclear and petalon, petal. The flowers generally have very small petals.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs, sympodial. Roots filamentous, produced from the base of a pseudobulb. Rhizomes appressed to the host, thin, creeping, much branched, covered by closely sheathing, imbricate, scarious bracts. Pseudobulbs appressed to the rhizome or erect, in-line, single-noded, moderately to widely spaced, small, conical, smooth, fleshy, ribbed or wrinkled. Trichomes absent. Leaves sessile to subsessile or shortly petiolate, 1- per shoot, terminal on a pseudobulb, not sheathing at the base, flat, longer than wide, smooth, firm-textured, coriaceous; apex unequal, entire or notched, flat. Inflorescence arising from the stem, 1- few-flowered, erect. Peduncle longer than the pedicel, thin and wiry, with 1-few tubular or flat closely sheathing to spreading, membranous to scarious sterile bracts. Floral bracts tubular or flat, sheathing to spreading, membranous to scarious. Pedicel shorter than the peduncle, thin, straight or curved, smooth. Ovary short, straight, merging with the pedicel, smooth or wrinkled. Flowers resupinate, pedicellate, porrect to nodding, lasting a few days, dull coloured (white, cream or yellowish, sometimes spotted and blotched or striped with red or purple), unscented. Perianth segments thin-textured to firm-textured, spreading to incurved. Dorsal sepal free, similar or subsimilar to the lateral sepals; apex flat or cymbiform. Lateral sepals similar or subsimilar to the dorsal sepal, attached by their bases to the column foot; apex flat or cymbiform. Petals free, much smaller than the sepals, entire. Labellum hinged by a short claw to the apex of the column foot, firmly attached, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina more or less ligulate, three-lobed, thick and fleshy, curved; lateral lobes basal, suberect; margins entire or fringed with hairs; apex entire. Callus obscure, consisting of a shallow basal channel. Nectar present. Column lacking free filament and style, short, straight. Column wings present, ventral and with tooth-like, triangular or elongate apical stelidia. Column foot well developed, curved upwards at the apex. Anther terminal, incumbent, 2-celled, persistent, hinged, smooth or papillate, erostrate. Pollinarium absent. Pollinia 4 in 2 pairs, crescentic, yellow or orange, hard, waxy. Viscidium absent. Rostellum ventral, transverse. Stigma entire, transverse, concave. Capsules small, dehiscent, glabrous, erect; peduncle not elongated in fruit; pedicels not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Adelopetalum has generally been included in Bulbophyllum but was recently reinstated at generic rank (Jones and Clements 2002).
Nomenclature
Adelopetalum Fitzg., J. Bot. 29: 152 (1891). Type species: Adelopetalum bracteatum Fitzg.
Infrageneric Taxa: No infrageneric taxa are currently recognised.
References
Clements, M.A. and Jones, D.L. (2002). Nomenclatural changes in the Australian and New Zealand Bulbophyllinae and Eriinae (Orchidaceae). Orchadian 13(11): 498-501.
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.
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.