Durabaculum undulatum

Epiphytes or lithophytes with very short indiscernible rhizomes anchored by roots that arise from nodes at the base of the pseudobulb. Pseudobulbs elongated, hard and cane-like, more or less cylindrical, leafy over much of their length. Leaves thick and fleshy, flat, without any channel or groove, not much longer than wide, basally sheathing, with an unequally notched apex. Inflorescence racemose, arising from the upper nodes of a pseudobulb. Flowers long-lasting, relatively large, thick-textured, sometimes scented. Perianth segments often irregularly wavy or twisted. Sepals subsimilar. Bases of the lateral sepals fused with the column foot. Petals longer than the sepals. Labellum stiffly attached to the apex of the column foot. Labellum lamina distinctly three-lobed, relatively thick and fleshy, with prominent central ridges and a short basal spur.

Significant Generic Characters

Epiphytic/lithophytic orchids; rhizomes indiscernible; pseudobulbs elongated, cane-like, more or less cylindrical, leafy over much of their length; leaves lasting several seasons, thick, fleshy, flat, not much longer than wide, basally sheathing; inflorescence racemose, from the upper nodes; flowers long-lasting, relatively large, thick-textured; perianth segments often irregularly wavy or twisted; sepals subsimilar; lateral sepal bases fused with the column foot; petals longer than the sepals; labellum stiffly attached to the apex of the column foot; lamina distinctly three-lobed, relatively thick and fleshy, with a short basal spur; callus with prominent central ridges.

Size and Distribution

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A genus of about 52 species distributed in the Philippines, Indonesia, New Guinea, Solomon Islands, Vanuatu, Fiji, Samoa, New Caledonia and north-eastern Queensland where there are 4 species, one endemic. The Australian species are variously scattered between the Torres Strait islands (925' S on Dauan Island) and near Rockhampton (2322' S). State occurrence: Queensland (including Moa and Dauan Islands).

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Ecology

The Australian species of Durabaculum are most abundant in coastal and lowland areas, but at least one species extends into the ranges and tablelands at low to moderate altitudes. The plants are found growing on trees and rocks in moist or humid but airy habitats, including rainforest, open forest, woodland, coastal scrubs 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 periods of sporadic or intermittent rain.

Biology

Pollination: The flowers of species of Durabaculum last many days and are pollinated by large wasps and hornets. Durabaculum mirbelianum is self-pollinating.

Reproduction: Reproduction in Durabaculum 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: The plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year.

Flowering: Flowering occurs mainly in winter-spring and early summer.

Hybrids: Natural hybrids involving species of Durabaculum are rare (example Durabaculum nindii x Durabaculum undulatum). Intergeneric natural hybrids also occur rarely (examples Durabaculum undulatum x Vappodes bigibba; Durabaculum undulatum x Cepobaculum johannis; Durabaculum undulatum x Ceratobium antennatum).

Derivation

Durabaculum, which is derived from the Latin durus, hard or tough and baculus, cane, refers to the hard, cane-like pseudobulbs.

Botanical Description

Perennial, evergreen, epiphytic or lithophytic herbs, sympodial. Plants glabrous. Roots elongate, produced from nodes on the base of the pseudobulb. Rhizome superficial, branched. Pseudobulbs well-developed, crowded, elongate, basically cylindrical, hard, when young covered by scarious bracts. Trichomes absent. Aerial growths often present, arising from the apical nodes on the pseudobulbs. Leaves distichously arranged on the distal half to three-quarters of the pseudobulb, sessile, slightly longer than wide, flat, not grooved or channelled, thick and fleshy, coriaceous, smooth; base sheathing the pseudobulb; margins entire; apex unequally emarginate. Inflorescence racemose, erect to arcuate, arising from an apical node on a mature pseudobulb, multiflowered. Peduncle shorter than the rhachis, the base covered with imbricate scarious bracts.  Floral bracts scarious, sheathing the base of the pedicel. Pedicel short, thin, merging with the ovary. Ovary short, straight, sometimes at right angles to the pedicel. Flowers resupinate, stalked, lasting many days, brown, bluish or yellowish, often with cream or mauve suffusions; labellum with distinct striae or reticulations. Perianth segments thick-textured, widely spreading, entire or with undulate margins, straight or twisted. Dorsal sepal free, subsimilar to the lateral sepals; apex entire, flat.  Lateral sepals subsimilar to the dorsal sepal, attached by their bases to the column foot; apex entire. Petals free, longer than the sepals; apex entire. Labellum stiffly attached to the apex of the column foot (see also spur below), markedly dissimilar in size and shape to the sepals and petals, calcarate. Labellum lamina more or less oblong, fleshy, strongly three-lobed; lateral lobes large, erect, flanking the column or incurved, entire or the distal margins minutely lobed/fimbriate; mid-lobe short or narrow, porrect to recurved; margins entire or undulate; apex entire. Spur prominent, elongate. Callus consisting of narrow parallel ridges, often becoming irregular or wavy on the mid-lobe. Nectar absent. Column lacking free filament and style, fleshy, shorter than the perianth segments, nearly straight. Column wings present, reduced, ventral and with short tooth-like apical stelidia. Column foot well developed, as long as or longer than the column, straight or curved. Pseudospur absent. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, erostrate or with a short rostrum; apex papillate. Pollinarium absent. Pollinia 4 in 2 pairs, straight or falcate, yellow or orange, hard, waxy. Viscidium absent. Rostellum ventral, swollen, transverse. Stigma entire, vertical, concave. Capsules dehiscent, glabrous, pendulous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.

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Nomenclature

Durabaculum M.A.Clem. & D.L.Jones, Orchadian 13(11): 487 (2002); Dendrobium section Strebloceras Schltr. in K. Schum. et Laut., Fl. Schutzg. Sudsee, Nachtr. 165 (1905). Type species: Dendrobium undulatum R.Br. [Durabaculum undulatum (R.Br.) M.A.Clem. & D.L.Jones].

References

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.