Deciduous terrestrials with few leaves, the stems developed into irregular-shaped subterranean rhizomes. Leaves usually long, narrow, pleated, stalked at the base. Inflorescences usually long, terminating a rhizome, unbranched, with smallish pink semi-pendant flowers that are hairy on the exterior. The labellum, which is stiffly attached to the base of the column or apex of the column foot, has large lateral lobes, a projecting midlobe and a ridged or keeled callus with irregular thickened areas and short hairs. The column is long and narrow, hairy, usually with a column foot but sometimes without.
Similar Genera
Significant Generic Characters
Terrestrial orchids; stems rhizomatous, subterranean, multinoded, irregular; leaves 1-2 per shoot, stalked, plicate; inflorescence terminal on a rhizome, racemose; flowers small, lasting a few days, semi-pendant, externally glabrous or hairy; sepals and petals subsimilar; lateral sepals fused with the base of the dorsal sepal and column to form a mentum; labellum stiffly hinged to the base of the column or apex of the column foot; lamina 3-lobed, with or without a short saccate basal spur; lateral lobes large, erect; midlobe narrow or broad; callus ridged or keeled, with thickened areas, hairy; column elongate, hairy, with a short foot; pollinia 8, shortly stalked; viscidium present or absent.
Size and Distribution
A genus of about 8 species distributed in Africa, India, South-east Asia, Malaysia, Indonesia, New Caledonia, New Guinea and Australia where there is a single non-endemic species, Pachystoma pubescens, rarely found and sporadically distributed in northern areas. State occurrence: Northern Territory, Queensland.
Ecology
Pachystoma pubescens grows in grassy forests, grassy woodland and grassland in heavy soils that become seasonally wet. The plants can occur among colonies of Blady Grass or Kunai Grass (Imperata sp.) with the orchid leaves difficult to differentiate from the grass.
Biology
Pollination: The flowers of Pachystoma pubescens are insect-pollinated but the vector(s) is unknown.
Reproduction: Reproduction in Pachystoma pubescens is solely from seed. Seed dispersal takes 2-4 months from pollination and the capsules develop in a pendant position.
Seasonal Growth: Plants of Pachystoma pubescens are deciduous for short periods during the dry season.
Flowering: Pachystoma pubescens flowers in November and December, coinciding with the onset of the wet season.
Hybrids: Natural hybrids involving Pachystoma pubescens are unknown.
Fire: Pachystoma pubescens grows in fire-prone habitats and the plants reshoot after fire but there is no evidence of fire stimulation.
Derivation
The name Pachystoma is derived from the Greek pachys, thick and stoma, mouth, apparently in reference to the thickened labellum callus.
Botanical Description
Perennial, autotrophic, geophytic herbs, deciduous, sympodial. Plants hairy on some parts. Roots filamentous. Rhizomes subterranean, multinoded, irregularly shaped, sparsely branched, generally short. Trichomes absent or present on the flowers. Leaves 1-2 per shoot, 1 larger than the other, lasting 1 season, flat, plicate, petiolate. Venation unknown. Flowering and non-flowering plants monomorphic. Inflorescence racemose, terminal on a subterranean rhizome, multiflowered. Peduncle longer than the rhachis, with several tubular sterile bracts. Rhachis straight, shorter than the peduncle. Floral bracts narrow, scarious, partially sheathing the base of the pedicel. Pedicel moderately long, glabrous or pubescent, thin, distinct from the ovary. Ovary short, straight, glabrous or pubescent. Flowers resupinate, few-several, moderately small, often semi-pendant, lasting a few days, opening sequentially, pinkish, pedicellate. Perianth segments thin, narrow, porrect, glabrous or externally pubescent. Dorsal sepal free or fused at the base to the lateral sepals, subsimilar to the lateral sepals, externally pubescent. Lateral sepals attached by their bases to the dorsal sepal and the column foot, gibbous at the base and forming a short mentum, subsimilar to the dorsal sepal, externally pubescent. Petals subsimilar to the sepals but narrower, asymmetric, glabrous. Labellum stiffly attached to the column base or the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate or shallowly saccate. Labellum lamina 3-lobed; spur, if present, short, shallow; lateral lobes large, erect; midlobe porrect, narrow or dilated. Spur (see labellum lamina). Callus variable, consisting of thickened areas, papillae, keels, or ridges, glabrous or pubescent. Nectar unknown. Column at an angle from the end of the ovary, narrow, elongate, curved, glabrous or pubescent, lacking free filament and style, fleshy. Column wings small, terminal. Column foot either absent or very short. Pseudospur absent. Anther terminal, incumbent, 2-celled, persistent, smooth, with a short rostrum. Pollinarium present. Pollinia 8, in 2 groups of 4, clavate, hard, waxy, tapered to a short caudicle. Viscidium present or absent. Rostellum small, entire. Stigma entire, small, concave. Capsules dehiscent, glabrous or pubescent, pendant; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Species of Pachystoma have a general similarity in plant habit and aspects of floral morphology to some species of Eulophia but are well separated from that genus by the development of 8 pollinia.
Notes
The Australian species was well known as Pachystoma holtzei (F.Muell.) F.Muell. until it was sunk under Pachystoma pubescens which is generally regarded as a widespread variable taxon. The native taxon does not fit comfortably within that species and a detailed molecular study of the group is needed, combined with morphology. Unfortunately the native species is rarely encountered which precludes studying collections from a wide number of localities.
Nomenclature
Pachystoma Blume, Bijdr. 6: t.3, fig. 29; 8: 376 (1825). Type species: Pachystoma pubescens Blume.
References
Clements, M.A. (1989). Catalogue of Australian Orchidaceae. Austral. Orch. Res. 1: 1-160.
Dockrill, A.W. (1967). Australasian Sarcanthinae. The Australasian Native Orchid Society, Sydney.
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.