July in the hills with Chris Horne
Hymenophyllum demissum, irirangi, piripiri, drooping filmy fern
Origin of the names
Hymenophyllum means ‘membranous leaf’. The Greek word for ‘membrane’ is ‘hymen’; the Greek word for ‘leaf’ is ‘phyllon’; ‘demissum’ comes from the Latin word ‘demittere’, and its adjectival form ‘demissus’ means ‘drooping, arching and hanging down’. ‘Piripiri’ refers to the fern’s growth habit of ‘keeping close together’, densely populating part of a forest floor. ‘Drooping filmy fern’ refers to a distinctive feature of this fern.
Distribution and habitat
Irirangi grows on Te Ika a Māui/North Island, Te Wai Pounamu/South Island, and on the Kermadec, Rakiura/Stewart, Rekohu/Chatham and Antipodes islands. It is our most common filmy fern in lowland to montane forests except for the drier parts of the eastern South Island. Often forming carpets over extensive areas of the forest floor, it also grows on banks, logs, tree roots and rocks, and sometimes perches on trees as an epiphyte. You will sometimes see areas of the forest floor with a dense cover solely of irirangi. This fern may have a chemical defence against competition which prevents seeds from other plants germinating among it, or slowing their growth if they do germinate.
Irirangi has a slender, long-creeping rhizome.
The stalks/stipes are 4-17 cm long, stout, smooth and usually lack wings. There are narrow wings along the length of the stems/rachises. The fronds, which arch stiffly from the stalks, are 7-25 cm long x 3-15 cm wide, elliptic or ovate, pale green or bright green, and smooth. Because irirangi’s fronds are mostly one cell thick and translucent, like those of Leptopteris hymenophylloides and L. superba (The Tramper May and June 2017), it can easily be confused with those two species when they are immature. Despite irirangi’s fronds looking delicate, they rarely curl up in dry weather.
The spores are held in sori. These usually appear in pairs on the ends of the pinnae/segments which are oblong with smooth margins – see image. The fronds are often sterile.
Māori used the fronds as ‘a bitter tonic’, according to Charles Jeffs, 1888, as noted on a pressed specimen in the King Tawhiao Collection at Te Papa (Specimen 1499). The fronds were also used as a scent (H W Williams 1975).
Where to look for drooping filmy fern
This widespread fern is common in Wellington city’s reserves, where it is often the only member of the Hymenophyllum genus. You can see it in Otari-Wilton’s Bush, four of the Botanic Garden’s five native forest areas, Centennial Reserve, Miramar and Burrows Avenue Reserve, Karori, etc. Look for it when you tramp in lowland to montane fore