Green Midge Orchids
Deciduous terrestrial orchids that mainly reproduce from seed, although there may be limited vegetative increase to form small clonal groups. Tubers very small, globose, subterranean. Leaf single, heart-shaped, entire or deeply lobed, usually green or pellucid beneath. Leaf blade held horizontally above the soil surface on a slender stalk. In some species there is dimorphism between the leaves of sterile and fertile plants, with sterile plants having entire leaves held close to the ground and fertile plants with lobed leaves held well above ground level. Inflorescence a multiflowered raceme, arising in the leaf base. Flowers tiny, right-way-up, green, borne on long thin ovaries. Sepals very thin, without needle-like tails. Petals thin. Labellum proportionately large, fixed to the base of the column. Labellum lamina projecting forwards, thin-textured, often apically lobed or toothed. Callus consisting of a thin central plate, some non-Australian species also with small basal glands. Column relatively long, thin, arching over the labellum base, not winged.
Significant Generic Characters
Deciduous terrestrial; leaf single, about as long as wide, entire or lobed; raceme multiflowered; flowers tiny, resupinate, green, on long thin ovaries; sepals lacking osmophores; petals of similar size to the sepals; labellum proportionately large, often apically lobed; callus a thin plate, with or without small basal glands; column not winged.
Size and Distribution
A genus of about 7 species of terrestrial orchid with the majority occurring in New Caledonia (Halle 1978) and 2 species endemic in eastern Australia, one extending into the tropics. In Australia the genus is distributed from the Atherton Tableland (17°20' S) in north-eastern Queensland to near Taree, New South Wales (32° S). State occurrence: Queensland, New South Wales.
The Australian species of Acianthella are restricted to coastal lowlands and near-coastal ranges and tablelands. The plants grow in moist shady conditions amongst shrubs in closed forests, particularly rainforest. They also colonise disturbed sites such as track verges and embankments. Soils are well-drained sandy loam and loam and accumulations of litter are also colonised.
Pollination: Both of the native species of Acianthella are self-pollinating and the ovaries swell rapidly as flowering progresses. Apomixis is possible in this genus but its occurrence has not been proved.
Reproduction: Both native species of Acianthella grow in small, loose groups and reproduce solely from seed. The plants grow actively between February and May, with the inflorescences emerging with the leaves. The flowers open in succession up the raceme with each flower short-lived, lasting only 2-3 days. After pollination the capsules develop in an erect position and seed dispersal occurs 10-14 days later. Seed shed occurs rapidly and it is not uncommon to have open flowers and dehisced capsules present concurrently on an inflorescence.
Flowering: Flowering occurs between February and June.
Seasonal Growth: The prevailing climate where these orchids grow is strongly seasonal and growth in the tropical regions is markedly influenced by the wet season. Acianthella plants emerge and grow in summer-autumn, dying back to the tiny tubers in late winter-spring to avoid the extremes of summer heat and dryness.
Fire: Fires do not generally occur in habitats where this species grows.
Perennial geophytic herbs, sympodial. Plants glabrous except for multiseriate hairs on the stem and collar. Roots absent. Tubers globose, small, paired, fleshy; replacement tubers formed at the end of short droppers; daughter tubers absent. Subterranean axes with minute papillae each with apical unicellular hairs. Stem erect, short, unbranched, terete, of 3-6 internodes, with membranous cataphylls at each node. Leaf single, basal, in some species dimorphic between sterile and fertile plants; fertile plants with petiolate leaves, sterile plants sessile or petiolate. Leaf lamina membranaceous, held horizontally above the soil surface, flat, hypostomatic, convolute in bud, sometimes deeply lobed, abaxially without anthocyanin pigments. Venation anastomosing, the main veins uniting apically; petiole fused with base of scape. Inflorescence racemose, one-several-flowered, erect, terminal; scape lacking sterile bracts. Floral bracts foliaceous, small. Ovary elongate, ribbed, glabrous. Flowers resupinate, small to tiny, insectiform, dull coloured, pedicellate. Nectar absent. Dorsal sepal free, subsimilar to lateral sepals, without a terete apical extension. Lateral sepals free, subsimilar to the dorsal sepal, without a terete apical extension. Petals free, subsimilar to the sepals. Labellum free, attached by its base to the anterior column base, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina membranaceous, straight or arcuate, lobed distally, flat; margins flat or weakly revolute, entire or coarsely dentate; apex acuminate. Callus fleshy, often reduced to a central plate, with (non-Australian) or without basal glands. Nectar absent. Spur absent. Column lacking free filament and style, short, slender; base not enlarged on anterior side; apex dilated or not, weakly incurved. Column foot absent. Pseudospur absent. Column wings absent. Anther terminal, 2-celled, persistent, basifixed, porrect, not rostrate. Pollinarium present, consisting of pollinia attached directly to a common viscidium. Pollinia 4 in 2 pairs, suborbicular, nearly entire, mealy, yellow; pollen grains in tetrads. Viscidium terminal. Rostellum ventral, transverse. Stigma small, concave, the posterior margin with well-developed appendages. Capsules dehiscent, thin-walled, erect; pedicels not elongating in fruit; peduncle not elongating in fruit. Seeds numerous, light coloured, winged; cells aligned in rows.
All species in Acianthella were previously included in Acianthus. Kores (1995) accommodated them in their own subgenus, viz Acianthus subgenus Univiscidiatus Kores and this was raised to generic rank by Szlachetko (2001). This latter name was ruled to be illegitimate at generic rank because it coincided with a technical term (Art. 20.2 of ICBN). In the meantime Jones and Clements (Jan. 2002) named the genus Acianthopsis and later Szlachetko (June 2002) also renamed the genus Acianthopsis. The interpretation by Jones and Clements (2002) was different to that of Szlachetko and after sorting out priorities of publication, the genus Acianthella was erected (Jones and Clements 2004) to accomodate species with small green flowers.
The leaves of these orchids are variable in the extent, depth and shape of the divisions or lobes.
The capsules mature rapidly and it is not unusual to have open flowers and dehiscent capsules present simultaneously on a plant.
Acianthella D.L.Jones and M.A.Clem., Orchadian 14(7): 331 (2004). Type species: Microstylis amplexicaulis F.M.Bailey (Acianthus amplexicaulis (F.M.Bailey) Kores; Acianthella amplexicaulis (F.M.Bailey) D.L.Jones and M.A.Clem.).
Univisciadatus (Kores) Szlachetko Polish Bot. J. 46(1): 20 (2001), nom. illeg.
Acianthopsis D.L.Szlachetko, Polish Botanical Journal 46(2): 143 (June 2002), non Jones et al. (Jan. 2002).
Halle, N. (1978). Orchidacees. Flore de la Nouvelle-Caledonie et Dependences 8: 1-565.
Jones, D.L., Clements, M.A., Sharma, I.K., Mackenzie, A.M. and Molloy, B.P.J. (2002). Nomenclatural notes arising from studies into the tribe Diurideae (Orchidaceae). Orchadian 13(10): 437-468.