Leptotheca Schwägr.

The following generic description draws on that of Churchill & Buck (1982).

Plants rather small to medium-sized, slender, yellow- to dark-green, turf-forming. Stems erect, branching by both forking and innovation, in cross-section with a distinct central strand, beset below with red-brown rhizoids. Leaves spreading when moist, more erect when dry, arranged in several rows, ovate-lanceolate or linear-lanceolate, weakly concave, plane or narrowly recurved at margins, aristate, not bordered, symmetric, narrowly decurrent, ± coarsely and irregularly toothed above; reduced in size at stem base and ± reduced at stem apex; upper laminal cells ± isodiametric (in N.Z. species) or irregular and 1–3:1, incrassate, smooth; marginal and alar cells not differentiated. Costa stout, sinuose, variably excurrent, smooth abaxially, in cross-section strongly protruding and rounded abaxially, with median guide cells and abaxial and adaxial stereids, a single abaxial surface layer of cells ± enlarged. Brood bodies (axillary gemmae) usually present, linear, orange-brown to nearly black, uniseriate and transversely septate, often conspicuous and present in large numbers on sterile stems.

Dioicous. Perichaetia terminal, with the leaves narrower and longer than the vegetative. Perigonia terminal and often obscure. Setae elongate and straight, slender, pale, twisted to the right above; capsules erect, straight or weakly curved, cylindric, 8-ribbed, with a weak neck; exothecial cells oblong, firm-walled; stomata superficial, restricted to neck. Peristome double, inserted close to mouth; exostome teeth pale, lanceolate and obtuse at apex; endostome pale, with a very low membrane, the segments nearly equal the teeth, papillose and narrowly perforate throughout, alternating with single, variably developed, sometimes nodose cilia. Spores spherical, nearly smooth.

A genus of perhaps three species distributed in Australasia, temperate and tropical South America, and South Africa. Leptotheca boliviana occurs at high elevations in the tropical Andes and adjacent regions and is distinct from the N.Z. species in terms of leaf shape, rhizoid morphology, and geographic distribution (fide Churchill & Buck 1982). Crum (1987) subsequently described L. hamiltonii from Peru. Bell et al. (2007) have questioned the relationship of Leptotheca boliviana and L. gaudichaudii.

Leptotheca was placed by Brotherus (1924) and several subsequent workers (e.g., Sainsbury 1955) in the Aulacomniaceae, with the northern hemisphere genus Aulacomnium. Brotherus was probably influenced by the earlier inclusion of Leptotheca as a section of Aulacomnium by Mitten (1869, p. 317). Churchill & Buck (1982) argued for its removal from the Aulacomniaceae, based partly on chromosome number differences and geography. They argued instead for placement in the Rhizogoniaceae, but noted that Leptotheca would be anomalous in this family by having terminal rather than basal or lateral sex organs. Also, Leptotheca has erect, narrowly cylindric, and ribbed capsules, which differ strongly from those of other genera of Rhizogoniaceae.

More recently Goffinet et al. (2009) have proposed the transfer of Leptotheca from the Rhizogoniaceae to the Orthodontiaceae. This placement is tenable for Leptotheca on the basis of peristome and capsule morphology, and geography. However, Leptotheca differs from Orthodontium by its isodiametric and incrassate laminal cells, its dioicous sexuality, and the presence of abundant specialised linear axillary gemmae. Orthodontium, by contrast, has linear, firm-walled laminal cells, autoicous sexuality (gonioautoicous or paroicous in N.Z. species), and no specialised gemmae (except for reduced, caducous leaves). The placement of Leptotheca in the Orthodontiaceae is accepted here with reservation.