Rhizogonium Brid.

Plants small or medium-sized, tufted or forming turves. Stems prostrate or erect, unbranched, with upper leaves distichous and lower leaves scale-like and unranked, in cross-section with a central strand and thick-walled cortical cells. Leaves uniform in shape, distichous, spreading, ovate, oblong-ovate, or ovate-lanceolate, plane at margins, with or without a border of elongate cells, moderately or strongly asymmetric at base, not or rarely decurrent, serrate or dentate above by single or rarely multicellular teeth. Upper laminal cells ± oblong-hexagonal and rounded, <2:1, firm-walled, smooth or rarely with fine cuticular striations, becoming ± more elongate in lower leaf; marginal cells not differentiated or elongate and forming a unistratose or rarely bistratose border; alar cells not differentiated. Costa excurrent (often strongly) or ending below the leaf apex, lacking abaxial teeth.

Dioicous. Perichaetia clustered or single at base of stem. Perigonia bud-like, clustered at base or scattered on lower stem, often difficult to observe. Setae elongate and straight; capsules horizontal or pendent, oblong-cylindric with a poorly defined neck; operculum rostrate from a conic base. Peristome double and well-developed; exostome teeth yellow-brown in N.Z. species, bordered, finely papillose or striolate below; endostome with a high basal membrane, segments nearly as long as the teeth, keeled but lacking perforations, cilia 2–3, well-developed or rudimentary. Calyptra cucullate. Spores small, finely papillose.

A genus of c. nine species distributed in Australasia, East Asia, Pacific Islands, and South America. Gilmore (2006) reported four species from Australia.

A type has not been designated for this genus.

The genus is here treated in a narrower sense than by Brotherus (1924). Manuel (1980) emended the genus to include only those species placed in section Rhizogonium Mitt., and in so doing, elevated section Pyrrhobryum (Mitt.) Mitt. to generic rank. Manuel’s interpretation of the genus Rhizogonium is accepted here. Matteri & Piñero (1987) provided useful notes on the genus in temperate South America.

Koponen et al. (1986) noted that some species retained in Rhizogonium by Manuel (e.g., R. pennatum) have multistratose leaf borders, but that in such instances the marginal cells were elongate, rather than short as in species of Pyrrhobryum. Other discrepancies exist between Manuel’s classification summary and N.Z. species, such as the occurrence of decurrent leaves in R. distichum.