Hymenophyllum australe, commonly known as austral filmy fern, is a relatively large rupestral and epiphytic fern, indigenous to eastern Australia and New Zealand. It belongs to the unique Hymenophyllum genus (filmy ferns), which are characterised by their thin membranous fronds that are seldom more than one cell thick, with the exception of regions over and around veins. Hymenophyllum australe is distinctive in that the fronds are typically thicker than other Hymenophyllum species, often being up to 2-3 cells thick. Hymenophyllum australe is inconspicuous and will typically form matted patches together with mosses and liverworts. In Australia it takes on an epiphytic approach, commonly seen to clothe roots, trunks and rocks to form a dense mat of foliage; while in New Zealand it adopts a more rheophytic approach, preferring rocks or tree trunks in or close to water. It has dark green fronds, which are distinctly deltoid in shape, with entire ultimate segment margins, each containing a single conspicuous vein. It is recognised by its winged fronds, which are initially relatively broad (several millimetres wide) but gradually taper from the rachis along the stipe. Description Hymenophyllum australe are rupestral or epiphytic ferns. The rhizomes are long-creeping, fine (typically 0.2-0.5 mm in diameter), branched and possess bare scattered hairs typically clustered near the base of the stipe. Stomata are absent. Characteristic of the Hymenophyllum genus, the sori are terminal on segments (see sketch below) and encompass a two-lipped indusium (protective cover), which often has notched margins. The sori are numerous and borne singly or paired. The receptacle is short and enclosed within indusial flaps. The species is homosporous, with spores trilete, greenish and slightly roughened. The stipes are winged almost to the base, often crinkled and emanate at intervals along the rhizome. The fronds are dark green, dull and membranous, ranging in size from 38–210 mm long; they are triangular to ovate in shape and pinnate to tripinnate. The rachises are winged, glabrous, dark brown proximally and pale brown distally. The ultimate lamina segments are narrowly oblong and have a spreading finger-like appearance; the margins are entire, with each ultimate segment containing a single conspicuous vein. Taxonomy and naming Hymenophyllum australe was first formally described in 1810 by Carl Ludwig Willdenow in the published Species Plantarum Edition 4, under the basionym of Mecodium australe. The botanical genus name Hymenophyllum is derived from the Greek hymen, meaning membrane, and , a leaf, referring