Flora of New Zealand Series

Type locality: Mt Egmont. Type: AK 24604 Herb. E. D. Hatch No. 568, North Egmont Hostel, 3,800 ft. 12/1948, O. E. Gibson.

Plant to c. 35 cm. tall at fl., us. less. Stem us. erect, smooth, internodes < lvs. Lvs mostly 3–5, the size changing from below upwards, the largest lf on a stem c. 4–15–(18) cm. × 5–12 mm.; all linear-lanceolate, ± keeled, acuminate, widest near sheathing base, us. overtopping fl. Fl. solitary, ovary us. erect. Dorsal sepal (1.5)–2.5–3 cm. tall, the erect part us. long and the ± horizontal part proportionately rather short, acuminate to shortly caudate; lateral sepals diverging at a narrow angle, tips acuminate to shortly caudate, not greatly exceeding galea. Petals little shorter than dorsal sepal, tips narrow-acuminate. Labellum narrow-triangular and protruding, gently arched but us. curled sharply downwards at the narrow, cucullate, obtuse tip; midrib evanescent distally, proximally prominent and ± papillose, terminating basally in a projecting dark callus. Column almost as tall as labellum; stigma long, narrow.

DIST.: N. Mt. Egmont; near Kaitoke, Wellington. S. Nelson and northern Westland, outlying records from Franz Josef, Otira and from Marlborough.

Endemic.

Forest margins, scrub, banks, locally abundant.

FL. (11)–12–1.

The labellum, with its median basal callus and very characteristic apex, distinguishes this sp. from all others. Some plants are almost wholly green, but in many the stems and the midribs of the lvs are red while the rather narrow fls are often elegantly striped with green, red and white.

Named "to acknowledge the labours and enthusiasm of Messrs J. B. Irwin and O. E. Gibson, who between them have done much to elucidate the orchid flora of Mount Egmont".

P. irsoniana is represented in Laing's Arthur's Pass herbarium at CANTY in two collections from Otira Gorge, that of 13/1/31 mounted with an approximately equal number of plants of P. montana. Both spp. are clearly recognizable in sketches made by Jessie Brownlee, 24/1/30, of "Three forms of Pterostylis from creek near Gaya Cottage, Arthur's Pass" in T.R.S.N.Z. 64, 1934, Pl. 1.