Dichopus insignis

Epiphytes or lithophytes with very short indiscernible rhizomes anchored by thickish roots that arise from the swollen base of the stems. The cane-like erect to straggly stems, which grow over 3-4 seasons, are laterally flattened, moderately thick, narrowed in the upper part and leafy throughout, with a leaf present at most nodes. The leaves on the lower half of the stem are persistent whereas those in the upper parts shed early leaving a naked apical section. The leaves, which are arranged distichously, are longer than wide, fleshy and basally sheath a node. The short inflorescences emerge from a node opposite a leaf, breaking through its sheath. Each node produces a single inflorescence and after flowering a prominent protruding bract remains. Flowers are usually borne in pairs, not facing each other, lasting for two days. Sepals all of similar shape and size, firm-textured. Petals narrower. Bases of the lateral sepals fused with the column foot. Labellum hinged to the apex of the column foot, prominently lobed; midlobe adorned with hairs and large linear calli. Column with an unusual appendage projecting from the base of the stigma.

Similar Genera

Grastidium

Significant Generic Characters

Epiphytic/lithophytic orchids; stems cane-like, laterally flattened, growing over 3-4 seasons, thickened proximally, narrowed distally, leafy throughout; leaves distichous, planar, longer than wide, fleshy, basally sheathing a node; proximal leaves lasting several seasons; distal leaves shedding early and leaving a naked stem section; floral nodes flowering once; inflorescence leaf opposed, short; floral bracts persistent, protruding; flowers 1-3, usually in pairs, not facing each other, lasting for two days; sepals similar in shape and size; petals narrow; lateral sepal bases fused with the column foot; labellum hinged to the apex of the column foot; lamina three-lobed; midlobe adorned with hairs and large linear calli; stigma with a projecting basal appendage; capsule large.

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Size and Distribution

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A small genus of 3-4 species occurring in Indonesia, Solomons and New Guinea, with a single species, Dichopus insignis, extending to Saibai Island (924' S) on the northern side of Torres Strait. State occurrence: Queensland (Saibai Island).

Ecology

Dichopus insignis frequents coastal forests and mangroves, growing on trees in situations of strong light where the humidity is relatively high and there is ample air movement. 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 sporadic or intermittent rain, particularly localised coastal showers.

Biology

Pollination: The flowers of Dichopus insignis last 2 days and are fragrant during the day. They are insect-pollinated but the vector is unknown.

Reproduction: Reproduction in Dichopus insignis is solely from seed. Seed dispersal takes 6-10 months after pollination and the capsules develop in a pendulous position. Apomixis is unknown in the genus.

Seasonal Growth: The plants grow throughout the year, particularly during spring and summer.

Flowering: Flowering occurs in spasmodic bursts throughout the year.

Hybrids: Natural hybrids involving Dichopus insignis are unknown.

Derivation

The name Dichopus refers to fleshy appendage that projects from the base of the stigma (Greek, dicha, double and pous, foot).

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Botanical Description

Perennial, evergreen, epiphytic herbs, sympodial. Plants glabrous. Roots elongate, produced from the swollen bases of the canes. Rhizome superficial, branched. Pseudobulbs absent. Stems cane-like, crowded, moderately thick in the proximal part, distally thinner, laterally flattened, capable of growth over 3-4 seasons, erect or straggly, when young covered by scarious bracts. Trichomes absent from vegetative parts. Aerial growths absent. Lower leaves lasting several seasons, distal leaves lasting 1-2 seasons, distichously arranged along the stem, sessile, much longer than wide, thick, fleshy, smooth, not grooved or channelled; base sheathing, the sheath completely enclosing a node; margins entire; apex unequally emarginate. Inflorescence short, porrect, arising from a node opposite a leaf and breaking through its sheath, two-flowered, the flowers not facing each other. Floral nodes capable of producing a single inflorescence only.  Peduncle very short, with a group of imbricate scarious bracts.  Floral bracts scarious, small, subtending the base of the pedicel. Pedicel short, thin, merging with the ovary. Ovary short, straight, merging with the pedicel. Flowers resupinate, stalked, lasting 2 days, yellow with orange markings and a white labellum. Perianth segments firm-textured, mostly spreading widely, entire. Dorsal sepal free, subsimilar to the lateral sepals; apex entire or cymbiformLateral sepals subsimilar to the dorsal sepal, attached by their bases to the column foot; apex entire or cymbiform. Petals free, subsimilar to the 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 more or less oblong, thin, three-lobed; lateral lobes short, entire; midlobe elongated; apex entire or emarginate. Spur absent. Callus obscure, adorned with hairs and pairs of linear, fleshy calli. Nectar present. Column lacking free filament and style, fleshy, much shorter than the perianth segments, nearly straight; ventral surface smooth. Column wings present, flared, ventral, without apical stelidia. Column foot well developed, nearly at right angles to the column, longer than the column, straight or curved. Pseudospur absent. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, shortly rostrate; apex smooth. Pollinarium absent. Pollinia 4 in 2 pairs, straight, orange, hard, waxy. Viscidium absent. Rostellum ventral, transverse. Stigma transverse, concave, the base with a projecting linear appendage. Capsules dehiscent, large, glabrous, pendulous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, relatively large, light coloured, winged.

Taxonomy

Although the genus was described in 1856 it has generally been treated as a synonym of Dendrobium.

Nomenclature

Dichopus Blume, Mus. Bat. 2: 176 (1856). Type species: Dichopus insignis Blume.

References

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.  

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