Pterostylis is a genus of terrestrial orchids, commonly known as greenhoods, in the family Orchidaceae. The genus was formally described by Robert Brown in 1810 in his Prodromus Florae Novae Hollandiae, based on 19 species from Australia and Tasmania, and today contains roughly 300 species, of which about 200 have been formally named. The Global Biodiversity Information Facility records 367 descendant taxa under the accepted name Pterostylis R.Br., and the Atlas of Living Australia lists well over 400 documented taxa once subspecies, hybrids and undescribed forms are included.
Plants are sympodial herbs with filamentous roots, paired underground tuberoids, and additional tuberoids that often form at the tips of stolons. Each plant produces a rosette or stem leaves and, when flowering, a single bloom or short raceme. The most distinctive feature of the genus is the flower itself: the dorsal sepal and the two lateral petals fuse to form a hooded structure called the galea, which arches over and conceals the reproductive parts of the flower. Most species are predominantly green, sometimes striped with brown, red or white, giving the group its common name. The labellum, the third and highly modified petal, is hinged on an "irritable claw" and snaps inward when triggered. The column itself lacks a free filament and style, and its wings are fused to the column and expanded in the upper half.
Greenhoods are pollinated almost exclusively by tiny insects, most often fungus gnats of the family Mycetophilidae and, in some species, mosquitoes (Culicidae). When an insect lands on the trigger of the labellum it is flicked against the column; trapped briefly inside the galea, the insect can only escape past the anther and stigma, picking up or depositing pollen on the way out. This sensitive-labellum trap mechanism is shared across the genus and is one of the most specialised pollination systems in the Australasian orchid flora.
Pterostylis is centred on Australia, where it occurs in every state and territory except the Northern Territory, with the highest diversity in the south and east. Its range extends to New Zealand, New Caledonia, New Guinea, New Britain, New Ireland and the Indonesian island of Seram. New South Wales alone hosts approximately 100 species. A 2010 molecular study reinstated Pterostylis as a monophyletic genus and recognised two subgenera distinguished by the orientation of the lateral sepals, settling several decades of taxonomic instability in which the genus had been variously split into smaller segregate groups. Vernacular names reflect the variety of flower shapes within the group and include greenhoods, helmet orchids, maroonhoods, nun-flower, parson-in-the-pulpit, rustyhoods, shell orchids and snail orchids.
Etymology
The genus name Pterostylis is formed from the Greek roots pteron, meaning "wing", and stylos, meaning "pillar" or "post", a reference to the prominent winged column at the centre of the flower.
Distribution
Pterostylis is overwhelmingly Australasian. In Australia it grows in every state and territory except the Northern Territory, with particular concentrations in the cooler and wetter south and east; New South Wales alone hosts approximately 100 species. Beyond Australia, the genus extends to New Zealand, New Caledonia, New Guinea, New Britain, New Ireland and the Indonesian island of Seram. The Atlas of Living Australia recognises well over 400 named and unnamed taxa across this range once subspecies, hybrids and undescribed entities are included.
Ecology
Greenhoods are pollinated almost exclusively by very small flying insects, principally fungus gnats of the family Mycetophilidae and, in some species, mosquitoes (Culicidae). The labellum is attached by an "irritable claw" and snaps inward when an insect lands on it, briefly trapping the visitor inside the galea formed by the fused dorsal sepal and lateral petals. To escape, the insect must crawl past the anther and stigma, which transfers pollen and effects cross-pollination. This sensitive-trap system makes the genus a textbook example of mechanical pollinator manipulation in terrestrial orchids.
Taxonomy
Pterostylis sits in the family Orchidaceae and was described by Robert Brown in 1810 from 19 Australian and Tasmanian species. The flowers are diagnostic: the dorsal sepal adheres to the two lateral petals to form a hood-like galea, while the highly modified labellum is hinged on a movable, "irritable" claw. The column lacks a free filament and style, and its wings are fused to the column and broadly expanded in the upper half. A 2010 molecular phylogenetic study reinstated Pterostylis as a monophyletic genus and divided it into two subgenera based on the orientation of the lateral sepals, after a period in which several segregate genera had been proposed. The Global Biodiversity Information Facility currently lists Pterostylis R.Br. as accepted with no nomenclatural issues and 367 descendant taxa.
Propagation
Pterostylis plants overwinter and reproduce vegetatively through paired underground tuberoids, with many species producing additional daughter tuberoids at the tips of stolons. This stoloniferous tuberoid system is the natural means by which colonies expand and is the basis of vegetative propagation for the genus in cultivation.
Conservation
Many Pterostylis species have small, scattered populations tied to specific habitats, and several are listed under Australian state legislation. The Atlas of Living Australia notes, for example, that Pterostylis sp. Rock ledges is listed as Endangered in South Australia.
History
The genus was erected by the Scottish botanist Robert Brown in 1810 in his Prodromus Florae Novae Hollandiae et Insulae Van Diemen (page 326), based on material collected during the Flinders expedition to Australia and Tasmania. Brown's original treatment included 19 species and established the morphological characters — the hooded galea and movable labellum — that still define the genus today.
