Tmesipteris, commonly known as hanging fork ferns, is a small genus of primitive vascular plants in the family Psilotaceae, order Psilotales, within the class Polypodiopsida (ferns and fern allies). It is one of only two living genera in the family, the other being Psilotum (whisk ferns). The genus belongs to an ancient, morphologically simplified lineage whose members lack true roots and true leaves — what appear to be leaves are actually flattened, photosynthetic stems, and the plants anchor themselves via rhizoids rather than a conventional root system.
Plants grow as pendant or semi-erect shoots emerging from a brittle, dichotomously branched rhizome densely covered in dark brown rhizoids. The aerial shoots can reach from a few centimetres to over a metre in length and bear spirally or distichously arranged leaf-like appendages that are sub-leathery, oblong to lanceolate, and typically 10–42 mm long. Reproduction is via spores produced in fused sporangia called synangia, which are paired and positioned along the fertile shoot; in T. elongata the synangia are testiculate (rounded) and greenish yellow to light brown at maturity.
Tmesipteris is restricted to the Southern Pacific, with species recorded from Australia, New Zealand, New Caledonia, Norfolk Island, Vanuatu, the Solomon Islands, and the Philippines. In New Zealand it is a familiar component of warm temperate rain forest, growing epiphytically on the fibrous root-mats that clothe the trunks of tree ferns such as Dicksonia and Cyathea, as well as on living trees and at the bases of tank lilies (Astelia spp.). It occasionally grows terrestrially among mosses or in rock crevices. The genus comprises around 15–18 species; the best-known include T. elongata, T. tannensis, T. lanceolata, and T. sigmatifolia.
Etymology
The name Tmesipteris derives from the Greek tmesis (cutting) and pteris (fern), alluding to the truncated or forked appendages on the fertile fronds.
Distribution
Tmesipteris is confined to the Southern Pacific region, with species occurring in Australia (including Victoria, Tasmania, New South Wales, and Queensland), New Zealand (North and South Islands, Stewart Island/Rakiura, Chatham Islands, Banks Peninsula), New Caledonia, Norfolk Island, Vanuatu, the Solomon Islands, and the Philippines. In New Zealand it is widespread in warm temperate rain forests on both main islands.
Ecology
Most species grow as epiphytes on the fibrous root-mats of mature tree ferns, especially Dicksonia, and on the trunks of other forest trees; they also occur at the bases of tank lilies (Astelia spp.). Less commonly, plants grow terrestrially among mosses, on cliff faces, and in rock crevices. The genus is associated with humid, shaded forest environments from coastal to montane elevations, though it is most common in lowland areas.
Taxonomy Notes
The Smith et al. (2006) classification, based on molecular phylogeny, placed Tmesipteris in Psilotaceae alongside Psilotum; subsequent classifications have maintained this placement. GBIF places the genus in order Psilotales, class Polypodiopsida. The family Psilotaceae is considered among the most morphologically reduced of all vascular plants, retaining a body plan that lacks conventional roots and true leaves.
