About Hymenophyllum rarum R.Br.
This species, commonly called the narrow filmy-fern, can be distinguished by its long, thin, creeping rhizome and membranous fronds that grow in moist areas. All parts of the plant are completely glabrous, meaning they lack any hairs, and fronds are a pale grey-green colour. Fronds are pendant and reach up to 15 cm in total length. The stipe measures 20 to 70 mm long, is very thin, and black in colour. The rachis is winged only along the uppermost section of the frond. The lamina is 1-pinnate to 1–2-pinnatifid, pale green, and can grow up to 100 mm long and 10 to 25 mm wide, with untoothed margins. Solitary sori grow at the apex of frond segments, are sunken at the base but are not tubular. The indusium, which is shaped like a rhomboid with a rounded or obtusely angled apex, completely covers the sporangia, and splits open to release spores for wind dispersal. The receptacle is slender and enclosed within the indusium. The narrow filmy-fern grows in rainforests, where it forms patches on rocks; it is also often epiphytic, growing on trees and tree ferns in moist gullies. It is widely distributed across tropical and south temperate zones. Within Australia, it occurs in New South Wales, Victoria, Tasmania, and the Bass Strait Islands. It is also widely distributed throughout New Zealand, where it grows on the North Island, South Island, Stewart Island, Chatham Island, and Auckland Island. It occupies habitats ranging from coastal to montane areas, and is common in rainforests, scrub, shaded cliff faces, among boulders, and in damp gullies. It often grows alongside other filmy-fern species. This species is closely related to and resembles Hymenophyllum cupressiforme and Hymenophyllum peltatum. Like H. peltatum, its pinnae are often divided on only one side. Unlike both H. cupressiforme and H. peltatum, the pinnae of H. rarum do not have serrated margins.
