Pterostylis is a genus of about 300 species of plants in the orchid family, Orchidaceae. Commonly called greenhood orchids, they are terrestrial, deciduous, perennial, tuberous, herbs found in Australia, New Zealand, New Guinea, New Caledonia and one Indonesian island. The flowers are mostly green, sometimes with brown, reddish or white stripes, and are distinguished from other orchids by their unusual flower structures and pollination mechanism.
Greenhood orchids are all terrestrial herbs with an underground tuber like many other genera of orchids but are distinguished by a hood-like "galea" formed by the fusing of the dorsal sepal and two lateral petals. The galea curves forward, covers the sexual parts of the flower, is important in the pollination process and is about as long as the two petals. The dorsal sepal is translucent white with green, reddish or brown stripes. The two lateral sepals are joined at their base, form the front of the flower and usually protrude to form "points" or "ears" which extend above or to the side of the galea. The third (medial) petal forms the highly modified labellum. As in other orchids, the sexual parts of the flower form part of the column and in all greenhoods, this structure has a pair of mostly translucent wings and is also important in pollination.[3][4][5]
The tuber of a greenhood is usually more or less spherical in shape and lasts for about a year then dies. A new one is produced early in the life of the parent tuber at the same time as a new shoot is produced and continues to grow, reaching maturity at the end of the growing season. In some species, a larger number of "daughter" tubers grow, and a colony of orchids is produced. The leaves are either attached in a rosette to the base of the peduncle or "stalk" of the plant by a short petiole, or further up it, in which case the leaves are sessile. There may be one or several flowers on the peduncle.[3][4][5]
The fruit of a greenhood is a dehiscent capsule containing up to 500 tiny seeds and is oblong to elliptic in shape with three ridges. Other parts of the flower wither soon after pollination and the dry remains are attached to the end of the capsule.[3][5]
The first observation of Pterostylis by European botanists was probably by Joseph Banks when HMS Endeavour visited Botany Bay in 1770 but any specimens collected were lost later in the voyage. The first formal description of a greenhood was by Jacques Labillardière who collected a specimen from Bruny Island and gave it the name Disperis alata.[6] After several name changes, Disperis alata is now known as Pterostylis alata. John White, the first surgeon-general of the colony of New South Wales collected several species and George Caley collected 208 greenhood specimens from 16 species and described them in great detail, including details of their habitat and gave them names like Druids Cap patersoni but did not publish his work.[3]
The genus Pterostylis was first raised by Robert Brown who formally described 19 species but did not nominate a type species. The species he described were collected from the east coast of the mainland and from Tasmania and the descriptions were published in Prodromus Florae Novae Hollandiae.[1][3][7]