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= Hymenophyllum applanatum =

Hymenophyllum applanatum, commonly known as the skeleton filmy fern, is a species of fern native to Tasmania, Australia. It is the only member of this genus that exists in Tasmania which belongs to the family Hymenophyllaceae. It is a rare epiphyte found in native rainforests and in subtropical biomes, which can be identified as clumps of small ferns with thin wiry stems covered in stellate hairs.

Description
The family of Hymenophyllum applanatum which is Hymenophyllaceae stands for “membranous leaf”, attributes to the fronds of the plant comprising of very thin semi-transparent tissues. This gave ferns with thin leaves the common name filmy fern. The leaf laminas are mostly only one cell thick and made up of flattened segments while being absent of stomata. Therefore, this fern is very susceptible to desiccation and exposed to natural elements.

Hymenophyllum applanatum can be identified from the other species of the same family through stellate hairs and papillae densely covering the leaves, contributing to the leaves appearing thicker and opaque. It generally has sporangia noticeably sticking out beyond the involucre rim by approximately 1.5mm eventually sheds and reveals a receptacle that resembles a bristle. It has branching and long-creeping rhizome that is shiny brown and finely ribbed in a longitudinal manner, while also covered with fulvous stellate hairs. The linear-oblong segments comprising of laminas are flattened, flexible, and winged. The measurements of the main pinnae are approximately 5-10mm wide and 5-15mm long in length. The smooth brown stipes of the fern are thread-like and unwinged with a length that ranges between 4-45mm long.

It is commonly mistaken interchangeably with Sphaerocionium malingii and can be differentiated through their laminal wings formed out of the sheathing basal parts of the fern papillae. It also can be distinguished through the convex or concave state of the upper and lower lamina. However, that would require the preparation of transverse sections of the leaves and the use of microscopes at high magnifications.

Habitat and distribution
Hymenophyllum applanatum is endemic to the western, central and southwestern regions of Tasmania. It only inhabits moist environments with high rainfall and humidity among the cracks of rocks or rocks sheltered from elements. It is generally present and abundant in high altitude areas by covering rocks in a dense pale grey layer despite being an epiphyte.

Hymenophyllum applanatum are host-specific and only grows on the trunks of the Athrotaxis species, including Athrotaxis selaginoides and Athrotaxis cupressoides which are more commonly known as King Billy Pine and Pencil Pine. There has been some rare occurrences where it has been found on Banksia marginata.

Taxonomy & Naming
Hymenophyllum applanatum has two homotypic synonyms which are Apteropteris applanata and Sphaerocionium applanatum.

Threats
The specific hosts that Hymenophyllum applanatum inhabits are Athrotaxis selaginoides (King Billy Pine or King William Pine) and Athrotaxis cupressoides (Pencil Pine). The two species have shown to be highly sensitive to fire and thrive in moist environments. Athrotaxis selaginoides has been categorised as an endangered species while Athrotaxis cupressoides was listed as a vulnerable species under the Australia’s Environment Protection and Biodiversity Conservation Act of 1999. Athrotaxis selaginoides are exclusively found in alpine habitats which are at risk of temperature changes and reduction of their altitudinal range due to climate change. Climate models were utilised in climate change studies to study the predictions of the Athrotaxis selaginoides species and the results showed population numbers diminishing significantly. Although 84% of the forests have been established as protected areas to curb logging activities, bushfires are considered a major hazard for the areas and are made more likely due to the warming climate. The risk on the hosts are a direct threat to Hymenophyllum applanatum as it only grows on the specific host species.

Cultivation
Hymenophyllum applanatum is hard to cultivate as a cold and moist environment with some airflow is essential to the survival of the species. It is most likely to thriven in a well-designed terrarium or a very humid and well-maintained fernery.