Hymenophyllum lyallii Hook.f.

Usually epiphytic ferns, rarely terrestrial or rupestral. Rhizomes long-creeping, 0.1–0.2 mm diameter, bearing unbranched pale brown hairs up to 1.5 mm long, clustered at bases of stipes, more evenly distributed elsewhere. Fronds 9–90 mm long. Stipes 4–70 mm long, dark brown, not winged, bearing long brown bifurcate hairs. Laminae 2–4-pinnatifid or branching pseudodichotomously 2–4 times, ovate or flabellate or almost orbicular, 4–40 mm long, 6–34 mm wide, pale green, membranous, with marginal teeth bearing long bifurcate hairs, and sparse bifurcate hairs on costae. Rachises winged, dark brown, bearing sparse bifurcate hairs. Primary pinnae in 2–5 pairs, not overlapping, winged throughout, obovate or flabellate, all adnate; distal portion of primary pinnae straight; the longest primary pinnae at the base, 6–21 mm long, 3–13 mm wide. Secondary pinnae arising basiscopically or both basiscopically and acroscopically, not overlapping, obovate, winged throughout, adnate; the longest secondary pinnae 2–18 mm long, 2–10 mm wide. Ultimate lamina segments oblong, up to 6 mm long, 1.1–2.4 mm wide; apices obtuse or truncate or emarginate; margins with minute teeth, lacking a distinct border; distal segments on primary pinnae divergent. Sori terminating ultimate segments throughout primary pinnae, solitary, many on each primary pinna, immersed in lamina; indusia bivalvate; indusial flaps obovate or obtriangular or broader than long, 1–2 mm long, apices obtuse or truncate, margins entire, hairy on margins; receptacles included within indusial flaps.

Hymenophyllum lyallii is distinguished by its flabellate fronds with minute marginal teeth bearing bifurcate hairs. No other New Zealand species of filmy fern has flabellate fronds or bifurcate hairs, although stellate hairs are present in H. frankliniae and H. malingii. However, sterile fronds of H. lyallii are often confused with a group of common liverworts, including Hymenophyton flabellatum and Symphyogyna species.

Occurs in kauri, podocarp, beech and broadleaved forest, usually as an epiphyte or at the base of trunks, but also on old stumps, mossy banks, rock faces and under overhangs. It has been recorded growing on Cyathea dealbata, C. smithii, Dicksonia squarrosa, Dacrydium cupressinum, Lepidothamnus intermedius, Libocedrus bidwillii, Lophozonia menziesii, Metrosideros sp., Quintinia serrata and Weinmannia racemosa.

n = 36 (Brownlie 1965).

Hymenophyllum lyallii has been recorded for New Caledonia by Ebihara et al. (2004). However, DNA sequencing suggests that New Caledonian plants are not closely related to Australian and New Zealand plants and may belong to a different species (Perrie et al. 2016).