Epiphyes or lithophytes with well developed creeping rhizomes anchored by thin branched roots that arise from nodes on the rhizome. Pseudobulbs shortly stalked, 1-noded, short to long, either of uniform shape or thickened at the base then tapered, with an elongated apical section (non-Australian), smooth or furrowed. Leaf solitary, from the apex of a pseudobulb, sessile, thin- to firm-textured, longer than wide, flat or channelled, with an unequally notched apex. Flowers produced in spasmodic bursts, arising singly or in pairs from a perennial meristem in the leaf axil covered by a large scarious bract. Each flower lasts less than a day and has relatively thin-textured narrow segments. Dorsal sepal subsimilar to the lateral sepals. Lateral sepals subsimilar to the dorsal sepal, their bases partly fused with each other and with the column foot. Petals much narrower than the sepals. Labellum hinged to the apex of the column foot. Labellum lamina distinctly three-lobed; lateral lobes small; mid-lobe well developed. Callus consisting of low ridges or keels.
Epiphytic/lithophytic orchids; rhizomes creeping; pseudobulbs shortly stalked, 1-noded, either of uniform shape or thickened at the base then tapered, with an elongated apical section (non-Australian); leaves lasting several seasons, solitary on a pseudobulb, sessile, longer than wide, flat or channelled; leaf axil with a solitary perennial floral meristem covered by a large scarious bract; flowers produced singly or in pairs in spasmodic bursts, lasting less than a day; perianth segments relatively thin-textured, narrow; dorsal sepal subsimilar to the lateral sepals; lateral sepal bases partly fused with each other and with the column foot; petals much narrower than the sepals; labellum hinged to the apex of the column foot; lamina three-lobed; lateral lobes small; mid-lobe large; callus with low ridges.
Significant Generic Characters
A genus of about 105 species, highly developed in New Guinea and the adjacent islands, but also in Indonesia, Moluccas, Vanuatu, Solomon Islands, Philippines, Fiji, Samoa and northern Australia. In Australia there is a single non-endemic species (Diplocaulobium glabrum) and another that is doubtfully recorded. The native species occurs in northeastern Queensland, mainly between Cape York (10°41' S) and the Whitfield Range near Cairns (16°55' S). State occurrence: Queensland.
Size and Distribution
The native species, Diplocaulobium glabrum, grows on trees and rocks in sparse rainforest and humid, sometimes exposed sites in open forest and woodland. The species is commonest in the coastal lowlands but also extends to moderate altitudes in the ranges. The prevailing climate is tropical with the majority of rain falling during the summer wet season (December to March) and the remaining months being much drier.
Ecology
Pollination: The flowers of Diplocaulobium species last for 8-10 hours and are insect-pollinated, but the vectors are unknown. It is usual for a massed flowering to occur on a plant and all the plants in an area flower synchronously.
Reproduction: Reproduction in Diplocaulobium is mainly from seed, although some species also produce aerial growths. Seed dispersal occurs 6-10 months after pollination and the capsules develop in a nodding position. Apomixis is unknown in the genus.
Seasonal Growth: The plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: Flowering occurs in spasmodic bursts, mainly in the warmer months of the year.
Hybrids: Natural hybrids involving Diplocaulobium glabrum are unknown.
Biology
Diplocaulobium is derived from the Greek diplous, double, caulos, stem and bios, life; the pseudobulbs of some species are borne in pairs, one capable of flowering, the other not.
Derivation
Perennial, evergreen, epilithic herbs, sympodial. Plants glabrous. Roots elongate, branched, filamentous, produced from nodes on the rhizome. Rhizome short to long, covered with scarious bracts. Pseudobulbs shortly stalked, 1-noded, short to long, either of uniform shape or thickened at the base then tapered, with an elongated apical section (non-Australian), smooth or furrowed. Trichomes absent. Aerial growths occasional. Leaves solitary on a pseudobulb, sessile, longer than wide, firm-textured, coriaceous, smooth, flat, not grooved or channelled, base not sheathing; margins entire; apex unequally emarginate. Inflorescence single-flowered, arising spasmodically from a perennial meristem situated in the leaf axil and covered by a large scarious bract; each meristem can produce a number of flowers over time, with one or two flowers open at any time from each meristem, the flowers emerging through the scarious bract on the side nearest the leaf. Peduncle absent. Floral bracts membranous, sheathing the base of the pedicel. Pedicel long, thin, merging with the ovary. Ovary short, straight or curved, glabrous. Flowers resupinate, stalked, lasting less than a day, white, cream, yellow or red. Perianth segments thin-textured, widely spreading, entire, flat. Dorsal sepal free, subsimilar to the lateral sepals; straight; apex entire, flat. Lateral sepals subsimilar to the dorsal sepal but broader at the base, widely spreading, straight, partly fused with each other and attached by their broad bases to the column foot; apex entire. Petals free, shorter and narrower than the lateral sepals; apex entire. Labellum hinged to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina thin- to firm-textured, three-lobed; lateral lobes relatively small, erect, entire; mid-lobe well-developed, porrect or recurved. Spur absent. Callus consisting of low keels or ridges. Nectar absent. Column lacking free filament and style, fleshy, shorter than the perianth segments, nearly straight. Column wings present, poorly developed, ventral, with tooth-like, apical stelidia. Column foot well developed, as long as or longer than the column, at right angles to it, straight. Pseudospur present. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, rostrate; apex smooth. Pollinarium absent. Pollinia 4 in 2 pairs, straight or slightly falcate, yellow or orange, hard, waxy. Viscidium absent. Rostellum ventral, small, more or less triangular. Stigma entire, concave. Capsules dehiscent, glabrous, cernuous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Botanical Description
Diplocaulobium (H.G.Reichb.) Kraenzl. Pflanzenreich Orch.-Mon.-Dendr. 45: 331 (1910); Dendrobium section Diplocaulobium H.G.Reichb.
Type species: Dendrobium nitidissimum H.G.Reichb. [Diplocaulobium nitidissimum (H.G.Reichb.) Kraenzl.].
Nomenclature
Clements, M.A. and Jones, D.L. (2002). Nomenclatural changes in the Dendrobieae (Orchidaceae). 1: The Australasian Region. Orchadian 13(11): 485-492.
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.