''Hymenophyllum rarum'' (Hymen-O-FIL-lum rare-um), the narrow filmy-fern, is a species of

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...

from the family

Hymenophyllaceae The Hymenophyllaceae, the filmy ferns and bristle ferns, are a family of two to nine genera (depending on classification system) and about 650 known species of ferns, with a subcosmopolitan distribution, but generally restricted to very damp pla ...

. This thin-leaved fern is commonly found in New Zealand and Tasmania, growing in patches on rocks and is

epiphytic An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

on trees and tree ferns, growing in moist gullies or rainforests. A rather drought tolerant species often found at exposed sites ranging from coastal to montane areas. Forming extensive, interwoven and creeping patches with its thin long (creeping) rhizomes sparsely covered in red-brown hairs, easily recognised by its membranous grey-green fronds, the smooth margins of the pinnae, ultimate segments and indusia; and by the sunken sori in the uppermost segments of the uppermost pinnae. The species can be found throughout Tasmanian rainforests as well as occurring in New South Wales, Victoria and New Zealand on the North and South Islands as well as, Stewart, Chatham and Auckland Islands.

Etymology

''Hymenophyllum'': Membranous leaf, from the Greek and . ''rarum'': thin-leaved; from the Latin ''rarus''.

Description

The narrow filmy-fern is distinct by its long thin, creeping rhizome, membranous fronds, that grow in moist areas. Fronds are pale grey-green, and the entire plant is glabrous (devoid of hairs). Fronds are pendant and up to 15 cm long; stipe to 20–70 mm long, very thin, black; rachis winged in the uppermost section of the frond; lamina 1-pinnate 1–2-pinnatifid, pale green, and up to 100 mm in length and 10–25 mm wide, with no toothed margins. Solitary sori borne at apex of segments, and sunken at the base but not tubular; a whorl or rosette of bracts surrounding the inflorescence or at the base of an umbel, shaped like a rhomboid, apex rounded or obtusely angled; receptacle slender, included. Indusium completely covers the sporangia, and splits apart when spores are ready to be dispersed by wind.

Habitat and distribution

The narrow filmy-fern is found in rainforests, forming patches on rocks and is often epiphytic on trees and tree ferns in moist gullies. It is highly dispersed throughout the tropical and south temperate zones occurring across Australia in New South Wales and Victoria. This species can also be found in Tasmania and on the Bass Strait Islands, While being highly dispersed throughout New Zealand, occurring on North and South Islands as well as, Stewart, Chatham and Auckland Islands. Occurring in coastal to montane habitats, this species is common in rainforests, scrub, shaded cliff faces or amongst boulders and damp gullies. Often growing amongst other filmy-ferns, this species closely resembles is relatives ''H. cupressiforme'' and ''H. peltatum''. As with H. peltatum, the pinnae are often divided on only one side. Unlike both these species, however, the pinnae of ''H. rarum'' do not have serrated margins.

Taxonomy and evolution

The family

is a largely successful family under the division of

Pteridophytes A pteridophyte is a vascular plant (with xylem and phloem) that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden. Ferns, ...

(ferns and fern allies). This family is commonly referred to as the filmy fern family, with around 670–700 named species, only 131 are accepted. Divided into two genera, ''

Hymenophyllum ''Hymenophyllum'' is a genus of ferns in the family Hymenophyllaceae. Its name means "membranous leaf", referring to the very thin translucent tissue of the fronds, which gives rise to the common name filmy fern for this and other thin-leaved fern ...

'' and ''

Trichomanes ''Trichomanes'' is a genus of ferns in the family Hymenophyllaceae, termed bristle ferns. The circumscription of the genus is disputed. All ferns in the genus are filmy ferns, with leaf tissue typically 2 cells thick. This thinness generally nec ...

''. The Genera are separated based on the

sorus A sorus (pl. sori) is a cluster of sporangia (structures producing and containing spores) in ferns and fungi. A coenosorus (plural coenosori) is a compound sorus composed of multiple, fused sori. Etymology This New Latin word is from Ancient Gr ...

and

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

structures, particularly the indsium structure (membrane covering sorus) and the receptacle. The Australian plants of H. rarum differ from their New Zealand counterparts with widely spaced pinnae rather than imbricating pale green pinnae. The Sori of the Australian ''H. rarum'' are not wholly sunken within the pinnae nor are they bound to the uppermost segments of the uppermost pinnae. Members from the genus ''

'' are all small, thin and delicate. They are either

plants or rock plants, forming dense patches or mats along rainforest floors. Members of this family most commonly occur in moist habitats with few members being able to withstand dryer conditions, in which they are able to later 'revive' themselves when moisture becomes readily available.

References

{{Taxonbar, from=Q3144028 rarum Flora of the Auckland Islands