kidney fern

Hymenophyllum nephrophyllum is a fern distinctive for its undivided, kidney-shaped fronds, which give the plant its English common name, the kidney fern. The fronds, or laminae, are 3–10 cm by 4–13 cm and are a shiny, translucent green. 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. The fronds are supported on brittle stipes 5–25 cm in length. 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. 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). After rain or when conditions improve, it unfurls and recovers.

Hymenophyllum nephrophyllum is endemic to New Zealand, meaning it is native to and found only in New Zealand. Other members of the Hymenophyllum genus can be found throughout the world as well as New Zealand.

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. Little else grows in mats of Hymenophyllum nephrophyllum as the rhizomes produce a compound inhibiting the root growth of seedlings of other species.