Rhomboda polygonoides

Velvet Orchids

Terrestrials with succulent, above-ground, creeping rhizomes with an erect apex, anchored by wiry roots that appear woolly.  The relatively narrow leaves, which arise in a basal rosette, are thin-textured, petiolate, dark green to brownish purple with a pale band along the midrib, appearing velvety. Spikes short, terminal on a shoot, hairy. Flowers small, wider than long, right-way-up, dull-coloured, sparsely hairy. Dorsal sepal and petals overlapping to form a hood. Lateral sepals widely divergent. Labellum attached to the anterior base of the column, with a broad pouched base  and either a narrow entire apex or broad and deeply cleft. Basal pouch with two large stalkless glands. Column elongate, winged.

Similar Genera

Anoectochilus

Significant Generic Characters

Terrestrial orchids; stems succulent, above-ground, creeping with an erect apex, anchored by wiry nodal roots that appear woolly; leaves petiolate, in a central rosette, glabrous, relatively narrow, thin-textured, dark green to brownish purple with a pale median band; inflorescence spicate, short, terminal on a shoot; flowers resupinate or nearly so, relatively small, often short-lived, dull-coloured (green, reddish, white), glabrous or externally hairy; dorsal sepal and petals overlapping to form a galea; lateral sepals divergent. Labellum attached to the anterior base of the column, with a large basal spur; hypochile larger than the epichile, the base saccate, with 2 large stalkless glands, distally with a bicarinate dome and unusual external fleshy flanges; epichile reduced or well-developed, entire or bilobed. Column short, medially winged, with 2 stigmas.

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Size and Distribution

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A genus of about 23 species distributed in tropical regions from northern India, China, Myanmar and Japan south to Malaysia and east to New Guinea, Solomon Islands and Australia. A single non-endemic species, Rhomboda polygonoides, occurs in northeastern Queensland, more or less between the Daintree River (1620' S) and the Paluma Range (19 S).  State occurrence: Queensland.

Ecology

Rhomboda polygonoides grows in sheltered positions in rainforest, often in litter accumulated on large rocks and boulders in sheltered gullies close to streams. It commonly grows in dark areas where there is constant high humidity. Elevation ranges from about 200 m to 800 m alt. The climate has a summer wet season (December to March) with the remaining months being much drier with sporadic or intermittent rain.

Biology

Pollination: The flowers of Rhomboda polygonoides are probably self-pollinating, although the pollinarium has a well-developed viscidium and the stigmas are deeply concave as if developed for cross-pollination.

Reproduction: Rhomboda polygonoides reproduces solely from seed. Seed dispersal takes 3-4 months from pollination and the capsules develop in an erect position. There is no elongation of the peduncle or pedicels. Apomixis is unknown in the genus.

Seasonal Growth: Rhomboda polygonoides survives the dry parts of the year as leafy quiescent plants.

Flowering: Rhomboda polygonoides flowers between June and August.

Hybrids: Rhomboda polygonoides does not participate in natural hybridisation.

Fire: Rhomboda polygonoides does not grow in fire-prone habitats.

Derivation

Rhomboda is derived  from the Greek, rhombos, rhomboid, an apparent reference to the shape of the glands in the saccate labellum base.

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Botanical Description

Perennial, evergreen, terrestrial herbs (rarely epiphytic), sympodial. Roots wiry, produced from rhizome nodes, appearing woolly from a covering of root hairs. Rhizome cylindrical, fleshy, unbranched or sparsely branched, prostrate. Stem erect, apical, usually 1 shoot per plant, similar to the rhizome. Pseudobulbs absent. Trichomes present on peduncle, rhachis, bracts, ovaries and sometimes the exterior of the sepals, unbranched, multiseriate, eglandular. Leaves lasting 1 season, few per shoot, petiolate, glabrous, sheathing at the base, forming a loose rosette. Leaf lamina longer than wide, thin, membranaceous, flat, smooth, entire, dark-coloured, often with a pale or reddish median band. Venation acrodromus to campylodromus, with cross-veinlets and few anastomoses. Inflorescence spicate, terminal on a growth, few-flowered, erect. Peduncle subequal to or longer than the rhachis, pubescent, with semi-sheathing to projecting, pubescent sterile bracts.  Floral bracts sheathing, pubescent. Ovary elongate, projecting at an angle from the rhachis, asymmetric, straight or curved, twisted, pubescent. Flowers resupinate or nearly so (sometimes partially twisted), not crowded, sometimes opening tardily, dull coloured (sepals green to reddish, petals and labellum white), sessile to subsessile. Dorsal sepal closely overlapping with the petals to form a projecting galea, externally glabrous or pubescentLateral sepals free, porrect to widely divergent, externally glabrous or pubescent. Petals of similar length to the sepals, forming a galea with the dorsal sepal, strongly asymmetric, membranousLabellum fixed by its basal margins to the anterior margins of the column base and the margin of the column wings, immoveable (rarely flexible), markedly dissimilar in size and shape to the sepals and petals, calcarate (saccate). Labellum lamina divided into a hypochile and an epichile; hypochile equal to or longer than the epichile, entire, with a pouched (saccate) base containing 2 large stalkless smooth or denticulate glands; external lateral surface with a smooth or rugose, membranous to fleshy longitudinal flange (rarely projecting at right angles to the surface); epichile sessile or with a narrow basal claw with involute margins; lamina small to large; apex entire or bilobed. Callus consisting of two ridges that thicken distally to a bicarinate dome-shaped structure.  Nectar absent. Column lacking free filament and style, elongated. Column wings present, ventral, large, running the length of the column on the anterior side, the basal margin sometimes enlarged and attached to the labellum base. Column foot absent. Pseudospur absent. Anther dorsal, 2-celled, persistent, basifixed, with an acuminate rostrum. Pollinarium present. Pollinia 2, clavate, deeply grooved, curved, yellow, sectile, sometimes with basal stalks. Viscidium present. Rostellum ventral, short, bifid. Stigmas 2, separate or united, situated on the distal corners of the column, deeply concave. Capsules dehiscent, pubescent, erect, often with persistent floral parts; peduncle not elongated in fruit; pedicels not elongated in fruit. Seeds numerous, light coloured, winged.

Taxonomy

Within the subtribe Goodyerinae Rhomboda is distinguished by the resupinate flowers, labellum base with a fleshy exterior flange, ridged callus and broad basal labellum spur containing 2 stalkless glands.

Nomenclature

Rhomboda Lindl., J. Linn. Soc. 1: 181, (1857).  

Type species: Rhomboda longifolia Lindl.

Infrageneric Taxa: No infrageneric taxa are recognised.

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.

Ormerod, P. (1995). A Reinstatement of Rhomboda Lindl. (Orchidaceae Subtribe Goodyerinae). Orchadian 11(7): 323-339.

Pridgeon, A.M., Cribb, P.J., Chase, M.W. and Rasmussen, F.N. (eds), (2003). Genera Orchidacearum, Vol. 3. Oxford University Press.

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