Hymenophyllum nephrophyllum

Hymenophyllum nephrophyllum is a fern distinctive for its undivided, kidney-shaped fronds, which give the plant its English common name, the kidney fern.[4] The fronds, or laminae, are 3–10 cm by 4–13 cm and are a shiny, translucent green.[5] The translucent nature of the kidney fern's fronds is due to them being very thin, only one cell thick as a sporeling, which increases to 3–4 cells when the plant reaches maturity.[4] The fronds are supported on brittle stipes 5–25 cm in length.[5] Mature plants have a row of sori (a collection of sporangia) crowning the upper margin of their frond, where they resemble a row of small brown pegs.[6] A feature Hymenophyllum nephrophyllum shares with some other filmy ferns is the ability to curl up tightly during dry conditions in order to reduce moisture loss; this gives kidney fern one of its Māori common names, kopakopa (to wrap or clasp).[7] After rain or when conditions improve, it unfurls and recovers.[7]

The scientific names Trichomanes reniforme and Cardiomanes reniforme have previously been used for kidney fern. However, phylogenetic analyses of DNA sequences have shown kidney fern to be part of Hymenophyllum, and a name in that genus is used in the Flora of New Zealand and the Checklist of Ferns and Lycophytes of the World.[4][1]

The species epithet for Hymenophyllum nephrophyllum is a reference to its fronds: from the Greek nephro- (kidney-shaped) and -phyllus (leaved).[4] The reniforme epithet of the two synonyms Trichomanes reniforme and Cardiomanes reniforme is also in reference to the frond shape. The genus name Cardiomanes also arises from reference to the distinctive frond shape: Greek kardia (heart) and manos (thin).

Like all ferns, Hymenophyllum neprhophyllum reproduces and disperses offspring through spores. It has tubular indusia (spore protecting structures) that stick out from the edge of the fronds. Stalks carrying sori, with sporangia that develop sequentially from base to apex, grow out of the indusia until at plant maturity they emerge and release their spores. Hymenophyllum nephrophyllum also spreads by vegetative reproduction, putting out far-creeping rhizomes (an underground stem that puts out adventitious shoots and roots) that form the distinctive mats of fronds on the forest floor.[5] Little else grows in mats of Hymenophyllum nephrophyllum as the rhizomes produce a compound inhibiting the root growth of seedlings of other species.[7]

The lifecycle of the fern could not occur without damp soil as its spores prefer a moist environment.[9] As Hymenophyllum neprhophyllum mainly inhabits moist forests, it requires a relatively damp soil; however, the kidney fern is poikilohydric[10] and has adapted to a variety of habitats. This has given it the ability to store water in drier weather, which it does by shrivelling up and expanding again once rain returns.[11]

The caterpillars from the moth family Tortricidae are known to feed on Hymenophyllum nephrophyllum.[7] The caterpillars of these filmy-fern leaf-tyer moths can be found in silk tubes attached to the plants fronds, which may have been woven together or bent inwards to protect the caterpillar from predation.[7] Due to the specific climatic and soil conditions that ferns require human based threats to their survival include habitat loss through forest clearance and introduced invasive plants.[12]

Hymenophyllum nephrophyllum has several traditional uses to Māori including being a treatment for bowel disorders (however this is disputed), a perfume plant and also worn during mourning.[7] Other Māori common names for Hymenophyllum nephrophyllum are konehu and raurenga.[5]