Greenhoods, Greenhood Orchids
Deciduous terrestrial orchids that reproduce from seed and also form clonal colonies. Plants are monomorphic. Sterile plants have either a compact rosette of petiolate leaves or a loose cluster of sessile leaves. Fertile plants have either a basal rosette of scape-encircling petiolate leaves held on the soil surface or sessile leaves scattered in a loose spiral on the basal part of the scape. Flowers erect to nodding, right-way-up, solitary, distinctly hooded, mainly green with white or brown suffusions. Dorsal sepal and petal margins united to form a hood that encloses the column and the basal part of the labellum. Lateral sepals are fused in the basal half to form a structure that is held erect in front of the flower, effectively closing off the flower to the outside, with the exception of a gap between the upper margins of the fused sepals and the galea through which the labellum can protrude. In many species there is also a lateral gap between the margins of the fused sepals and the petals. Labellum actively motile, attached by an elastic strap; in the set position it is held away from the column and is triggered by the movement of an insect contacting the basal appendage; when triggered it closes against the column wings. Labellum long, slender, with a curved brush-like basal appendage. Column curved, with a narrow stigma.
Significant Generic Characters
Terrestrial orchids; plants monomorphic; leaves in a basal rosette or spiral on the scape; scape fleshy; flowers hooded, resupinate; dorsal sepal acute to shortly acuminate; lateral sepals fused, held erect in front of the flower; free points long, erect, filamentous. It is distinguished from Diplodium by its monomorphic sterile and fertile plants, fleshy scape and acute to shortly acuminate dorsal sepal. Species of Pterostylis mainly flower in spring and early summer, whereas species of Diplodium mainly flower in autumn and winter.
Size and Distribution
A genus of about 40 species occurring mainly in eastern Australia, but also distributed in New Zealand (including Stewart Island and Chatham Island), Lord Howe Island, New Caledonia (including Isle des Pines), New Guinea, New Britain, New Ireland and Indonesia (Ceram). Pterostylis has a wide latitudinal distribution, extending from Ceram (about 3° S) to Stewart Island (about 47° S). In Australia, the genus is distributed from about 16°12’ S on the Windsor Tableland in northern Queensland to about 43° S in southern Tasmania. Over its range the genus is distributed from near sea level to about 3660 m. alt., although the Australian species occur from near sea level and coastal lowlands to subalpine zones up to about 1650 m alt. State occurrence: Queensland, New South Wales (includinf Lord Howe Island), Australian Capital Territory, Victoria, Tasmania and South Australia.
Ecology
The majority of Pterostylis species grow as terrestrials and various species can be found in an extensive range of diverse habitats. A couple of species, notably Pterostylis pedunculata, also grow on the fibrous trunks of treeferns; some tropical species, particularly those from New Guinea, favour the mossy trunks and larger branches of trees.
The greatest concentration and diversity within Pterostylis occurs in Australia. Common habitats in lowland areas include wet sclerophyll forest, open forest, grassland, woodland, heathy forests, grassy forests, shrubland, mallee communities (often developed on limestone), coastal scrubs (on both acid and calcareous sands) and heathland. In montane and subalpine regions suitable habitats include snowgum woodland, tussock grassland, herbfield, bogs and riparian patches near small streams. Within these habitats some species favour moist to wet sites such as swamps, soaks and streambanks, whereas others occur on slopes and ridges in well-drained soils. Rainforest is not a prominent habitat for the genus but at least 3 species, Pterostylis hildae, P. scabrida and P. stricta, are known from this vegetation type. Common and widespread species, including P. curta, P. nutans and P. pedunculata, often colonise pine plantations.
Biology
Pollination: Most species of Pterostylis are insect-pollinated, the vectors being species of microdipterans in the families Mycetophilidae and Culicidae. The pollinating insects fly into the wind as if following a perfume trail, usually landing on the top of the galea. Entry to the inside of the flower is either via the sinus opening or the lateral gap between the synsepalum and the petal margins. The insect enters the flower and moves down the labellum that triggers when the insect contacts the basal appendage, closing against the column wings and trapping the insect inside the galea. While the labellum remains triggered the only exit from the flower is via the tunnel formed by the column wings.
Autogamy is known to occur in a few species, such as Pterostylis foliata. Apomixis is unknown in the genus.
Reproduction: All species of Pterostylis reproduce vegetatively by the production of daughter tubers on the end of stolonoid roots, as well as forming replacement tubers. In some species this reproduction is minimal, but others multiply by 3-5 times annually and as a result occur in congested and often extensive colonies.
Species of Pterostylis take 6-10 weeks to disperse seed after pollination.
Seasonal Growth: Species of Pterostylis occur in areas with a seasonal climate and the plants have distinct periods of active growth and dormancy. The dormant period coincides with extremes of heat or long dry spells and the plants survive as individual small, fleshy, naked tubers that sprout when conditions become favourable. Dormant periods are not so pronounced in species from the tropics and in this region growth and flowering can be stimulated sporadically by prevailing conditions. In the tropics it is not uncommon for periods of dormancy to be brief, resulting in the new tubers sprouting while still attached, via the stolonoid root, to the parent tuber.
Flowering: Flowering is distributed over most months with a concentration in spring.
Hybrids: Rare and sporadic hybrids occur between some species of Pterostylis that grow sympatrically. Typically these crosses are sustained by the formation of clonal colonies.
Fire: No species is fire dependent, indeed flowering is generally suppressed by hot summer fires until suitable microhabitats return.
Derivation
Pterostylis is derived from the Greek pteron, wing and stylos, pillar, pole – but in Orchidaceae applied generally to the column. The name refers to the prominent apical wings on the column.
Botanical Description
Perennial geophytic or facultatively epiphytic herbs, sympodial. Plants vegetatively glabrous. Roots both filamentous and stolonoid. Tubers fleshy, globose; replacement tubers formed at the end of short droppers; daughter tubers formed at the end of lateral stolon-like roots. Stem erect, short, unbranched, with membranous cataphylls at each node. Trichomes present on the column wiongs. Flowering and non-flowering plants monomorphic. Leaves either petiolate in a basal stem-encircling rosette or subsessile to sessile and cauline; in the former group the leaves on the scape are usually reduced to sheathing, bract -like structures. Venation acrodromus, without included veinlets. Inflorescence terminal, one-flowered (rarely 2). Peduncle fleshy, with a few sheathing bracts. Floral bracts foliaceous. Pedicel fleshy. Ovary erect, smooth or rough. Flowers resupinate, erect to nodding, pedicellate, green or green and white, sometimes with brown, without any noticeable scent. Nectar absent. Dorsal sepal about as long as the petals, overlapping the petal margins and adherent with them to form a galea; apex usually acute to acuminate. Galea with a single opening, facing outwards or downwards. Lateral sepals fused in the proximal half to form an erect synsepalum; synsepalum usually with a lateral gap between the margins of the synsepalum and the petals, rarely closely embracing the anterior part of the galea, about as long as wide; distal parts free, each sepal ending in a free point; free points narrowly tapered or suddenly contracted and filiform, erect above the galea or reflexed. Petals sessile, asymmetrical, about as long as the dorsal sepal, usually falcate; dorsal surface less developed than the anterior surface; dorsal flange present, deltate. Labellum free, actively motile, attached to the column foot by a short irritable ligulate claw; when set either fully enclosed within the flower or partially protruding; when triggered travelling in an arc of c. 30°, either becoming completely enclosed within the flower or remaining partly exposed, blocking off exit from the flower except via the column wings. Labellum lamina entire, unlobed; basal appendage extending porrectly from the labellum base, incurved; apex penicillate. Spur absent. Callus consisting of a rounded longitudinal ridge, hollow beneath. Column completely enclosed within the galea, curved, lacking free filament and style. Column wings hatchet-shaped, fused to the column basally, distally free, 2-lobed; upper margin lacking ornamentation, with a short entire apical lobe; medial area lacking barrier trichomes; basal lobe free, the anterior margin ciliate. Column foot prominent, much shorter than the column. Pseudospur absent. Anther erect to incumbent. Pollinarium absent. Pollinia 4, free, linear, soft and mealy, yellow. Stigma bilobed, medial, longer than wide, about as wide as the column. Rostellum terminal. Capsules dehiscent, erect, smooth; pedicel not elongating noticeably in fruit; peduncle not elongating in fruit. Seeds numerous, light-coloured. winged.
Notes
Cytology: Pterostylis nutans, 2n=42 (pers. comm. B. Molloy).
Nomenclature
Pterostylis R.Br., Prod. 326 (1810) (nom. cons.).
Type species: Pterostylis curta R.Br. (type cons.).
Infrageneric taxa: A genus of 3 subgenera:-
1. Pterostylis subgenus Pterostylis. Leaves distinctly petiolate, arranged in a compact, basal, scape-encircling rosette; scape fleshy, with reduced, closely sheathing, bract-like leaves. Type species: Pterostylis curta R.Br.
Pterostylis sect. Nudicaules G.Don in Loudon's Hortus Brittanicus 369 (1830). Lectotype species: Pterostylis curta R.Br., fide Jones and Clements 2002.
Pterostylis sect. Acuminatae Rchb.f., Beitr. Syst. Pflanz. 68 (1871). Lectotype species: Pterostylis acuminata R.Br., fide Jones and Clements 2002.
Pterostylis sect. Laminatae Rupp, Proc. Linn. Soc. New South Wales 58: 423 (1933). Lectotype species: Pterostylis curta R.Br., fide Jones and Clements (2002).
2. Pterostylis subgenus Cucullatae (Rchb.f.) D.L.Jones and M.A.Clem., Austral. Orch. Res. 4: 65 (2002). Leaves subsessile, arranged in a loose spiral; scape fleshy, lacking any reduced, closely sheathing, bract-like leaves. Lectotype species: Pterostylis cucullata R.Br., fide Jones and Clements (2002)
3. Pterostylis subgenus Graminifoliae D.L.Jones and M.A.Clem., Austral. Orch. Res. 4: 66 (2002). Leaves subsessile to sessile, narrow, grass-like, arranged more or less distichously; scape wiry, lacking any reduced, closely sheathing, bract-like leaves. Type species: Pterostylis banksii A.Cunn.
References
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Jones, D.L., Molloy, B.P.J. and Clements, M.A. (1997). Six new species of Pterostylis R.Br. (Orchidaceae) from New Zealand, The Orchadian 12(6): 266-281.
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