Pterostylis irsoniana

Pterostylis irsoniana is a species of greenhood orchid endemic to New Zealand. There are up to five erect leaves on the flowering stem with a single dark green flower with white stripes and a reddish tinge on the tips. It has a distinctive curved labellum with a curled tip with a dark red callus.

Description

Pterostylis irsoniana is a terrestrial, perennial, deciduous, herb with an underground tuber and three to five stem leaves increasing in size up to stem where the largest leaf is up to long and wide. The leaves are linear to lance-shaped, green with a reddish tinge and the uppermost leaf is taller than the flower. There is a single dark green flower with a reddish tinge near the tips borne on a flowering stem up to tall. The dorsal sepal and petals are fused, forming a hood or "galea" over the column. The dorsal sepal is long and curves forward with a short-pointed tip. The lateral sepals are erect, in close contact with the galea with their tips spreading apart slightly from each other. The labellum is gently curved then suddenly curls downward at its tip with a dark red or blackish callus. Flowering occurs from October to January.

Taxonomy and naming

Pterostylis irsoniana was first formally described in 1950 by Edwin Hatch from a specimen collected at in the Egmont National Park. The description was published in Transactions and Proceedings of the Royal Society of New Zealand. The specific epithet is a derived from the names of J. Bruce Irwin and O.E. Gibson "who between them have done much to elucidate the orchid flora of Mount Egmont".

Distribution and habitat

This greenhood grows in grass and montane forests of Nothofagus and subalpine Podocarpus scrub, often in deep leaf litter or on the edge of bogs. In the North Island it occurs from East Cape to Wellington and in the South Island on the western side as far south as Haast.