Leafless terrestrials with fleshy subterranean rhizomes bearing short thin apical roots and short, erect, fleshy scapes which carry 1-few porrect, bell-shaped, white, stalked flowers. The sepals and petals are fused together at the base with the upper parts free. The labellum, which can be unlobed or lobed, is narrowest at the base and widest towards the apex. Its margins curve up close to the column and there are 2 rows of irregular-shaped interlocking yellow papillae along the centre line. The column is relatively long with a short curved column foot.
Significant Generic Characters
Leafless mycotrophic terrestrial orchids; rhizomes fleshy, subterranean, horizontal, constricted between growths, with short filamentous roots from the apex; inflorescence terminal, 1-few-flowered; peduncle pale brown, fleshy; flowers resupinate, campanulate, white; sepals and petals fused with the distal parts free; labellum attached to the apex of the column foot, unlobed or 3-lobed, widest towards the apex; callus of 2 central rows of irregular yellow interlocking appressed calli extending to the labellum apex; column long, with a short curved foot; stigma apical; pollinia 4, attached to a transverse viscidium.
Size and Distribution
A genus of about 24 species distributed in Africa, Madagascar, India, South-east Asia, Malaysia, Philippines, Indonesia, New Caledonia, New Guinea, Fiji and Australia where there is a single non-endemic species, Didymoplexis pallens, in tropical parts of the north. State occurrence: Queensland (including Saibai Island), Northern Territory, Western Australia (including Christmas Island).
Ecology
Didymoplexis pallens grows in grassy forest, bamboo thickets and rainforest, mainly in the lowlands but extending to about 450 m alt. Plants usually occur as scattered individuals but can sometimes be locally frequent. Fruiting plants are more conspicuous than flowering plants. The whole flowering/fruiting cycle is rapid with plants only being above-ground for 3-5 weeks.
Biology
Pollination: The flowers of Didymoplexis pallens are insect-pollinated but the vector is unknown.
Reproduction: Reproduction in Didymoplexis pallens is mainly from seed although the rhizomes can separate by decay and become individual plants. Didymoplexis pallens may be able to form small vegetative structures from the roots but this requires further investigation. Seed dispersal takes 1-2 weeks from pollination and the capsules develop in an erect position. Apomixis is unknown in the genus.
Seasonal Growth: Plants of Didymoplexis pallens survive as subterranean rhizomes with the inflorescence the only part to emerge above ground.
Flowering: Didymoplexis pallens flowers November to March.
Hybrids: Natural hybrids involving Didymoplexis pallens are unknown.
Fire: Didymoplexis pallens sometimes occurs in fire-prone habitats but the plants are dormant in the fire season.
Derivation
The name Didymoplexis is derived from the Greek didymos, twin, and plexis, plaiting, in reference to the 2 rows of interlocking papillae on the labellum.
Botanical Description
Perennial, leafless, geophytic herbs, mycotrophic, sympodial. Plants glabrous. Rhizomes fleshy, subterranean, thin, elongate, smooth, single, mostly horizontal, lasting many years, often with a constriction between successive years growth. Roots filamentous, short, arising from the apex of the rhizome. Stem absent. Trichomes absent. Leaves absent. Inflorescence racemose, terminal, 1-few-flowered. Peduncle longer than the rhachis, thin, pale brown, fleshy. Sterile bracts tubular, sheathing. Floral bracts very small, partly sheathing. Pedicel short, thin, distinct from the ovary. Ovary straight, smooth. Flowers resupinate, campanulate, white, porrect to suberect, lasting 1 day. Perianth segments thin, connate basally with free tips, the dorsal sepal fused for a greater length with the petals than the lateral sepals. Dorsal sepal incurved, cucullate, similar to the lateral sepals. Lateral sepals fused to form a synsepalum, base porrect and saccate, tips recurved, free. Petals partially fused with the sepals, the tips free, porrect to incurved. Labellum flexibly attached to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina unlobed or 3-lobed (non-Australian), narrow at the base and widening distally, the margins erect and incurved close to the column; ventral surface smooth; apex broadly truncate to emarginate. Spur absent. Callus consisting of 1-2 irregular rows of appressed yellow interlocking papillae extending along the midline to the labellum apex, the basal papillae sometimes fused. Nectar unknown. Column porrect or at an angle from the end of the ovary, relatively long, fleshy, dilated distally, lacking free filament and style. Column wings absent or small and dentiform. Column foot short, curved. Pseudospur absent. Anther terminal, incumbent, erostrate. Pollinarium present. Pollinia 4, narrow, sectile, yellow, sessile. Viscidium broad, transverse, sometimes absent. Rostellum flat, entire, plate -like. Stigma apical, concave. Capsules dehiscent, smooth or verrucose (non-Australian), erect; peduncle elongated in fruit; pedicel elongated in fruit. Seeds numerous, light coloured, winged.
Notes
A detailed investigation is needed of Didymoplexis pallens throughout its considerable range. Specimens from different countries exhibit variation in labellum shape, lobing and the callus papillae, suggesting that more than one taxon is involved.
Nomenclature
Didymoplexis Griffith, Calcutta J. Nat. Hist. 4: 383, t.17 (1843). Type species: Didymoplexis pallens Griffith.
Infrageneric Taxa: There has been no formal division of Didymoplexis into infrageneric taxa.
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.