Amphidium Schimp.

Plants small to medium-sized, usually light green above and dark or olive-green below, caespitose on rock or soil. Stems erect, branching repeatedly by subperichaetial innovation, sparsely beset with smooth, brown rhizoids, in cross-section with small, firm-walled cortical cells and lacking a central strand. Leaves crowded, in moderately distinct spiral ranks, erect-spreading and flexuose when moist, strongly contorted and often reflexed when dry, linear-lanceolate or oblong-linear, acute or acuminate, keeled, entire or irregularly and weakly toothed, ± recurved or plane at margins; upper laminal cells rounded-subquadrate or rounded-oblate, firm-walled, unistratose, not bulging, densely and finely papillose (sometimes obscuring the cell outlines), with papillae not restricted to the cell lumens, appearing longitudinally striate; basal laminal cells pale or hyaline, becoming gradually more elongate-rectangular, thin- or thick-walled, and less papillose-striolate towards leaf base; alar and marginal cells not differentiated. Costa strong and subpercurrent, in N.Z. species rounded and projecting abaxially in cross-section, with a single layer of median guide cells, 1 layer of adaxial substereids, and (1–)2 layers of abaxial substereids.

Autoicous in N.Z. species. Perichaetial leaves differentiated or not, sheathing the costa or erect-spreading. Perigonia on short bud-like branches immediately below the perichaetium. Setae short, straight or ± curved; capsules emergent or weakly exserted, erect or slightly inclined, urceolate, 8‑ribbed when moist, the mouth transverse or nearly so, becoming very strongly ribbed and flared at mouth when dry, at maturity red-brown and often with a distinctly green neck; exothecial cells ± rectangular, in alternate bands of thin- and thick-walled cells; stomata superficial; annulus lacking; operculum apiculate or rostrate, oblique. Peristome absent. Calyptra cucullate, smooth, and naked. Spores spherical, unicellular, red-brown, nearly smooth.

Amphidium is a modestly sized genus of 12 or fewer species (Magill & van Rooy 1998) occurring mostly in temperate regions in the northern and southern hemispheres; two species are documented from N.Z.

The familial placement of Amphidium is problematic, largely because of its lack of a peristome. Brotherus (1924) included it in a subfamily Rhabdoweisioideae within the Dicranaceae. The family placement has been discussed by many more recent workers, including Lewinsky (1976), Crum & Anderson (1981, p. 685), Goffinet & Vitt (1998), and Stech (1999), who placed it variously in the Orthotrichaceae, Dicranaceae, and a segregated Rhabdoweisiaceae. It is also sometimes placed in its own family in the general relationship with the Orthotrichaceae (Smith 2004). Its placement here in the Rhabdoweisiaceae follows Goffinet et al. (2009) and is consistent with the phylogenetic reconstructions using nucleotide sequence data of the chloroplast rbcL gene by Goffinet & Vitt (1998) and Goffinet et al. (1998).

Amphidium can be recognised, even when sterile, by the faintly striate appearance (under a compound microscope) of its laminal cells and the adjacent cell walls. The relationship of the striations to the papillae seen in the upper laminal cells is obscure. The upper laminal cells in both N.Z. species have very numerous (to 12 or more per cell) small papillae over the lumina. The very small (≤3 µm diam.) papillae extend over both the cell lumina and the contiguous cell walls; they may be protrusions of the cuticle. The papillae appear to become elongate towards the leaf base and are most conspicuous in the juxtacostal cells and in the transition region between the upper laminal cells and the smooth basal cells. Here they are mostly c. 3–4 µm long, 3–4:1, and lack pigmentation. They are most easily viewed at medium (c. 300×) magnification under the compound microscope and give the leaves a subtly striate appearance that is characteristic of the genus as a whole, as well as the other family members occurring in N.Z. It is unclear whether the papillae of the upper laminal cells transition to the "striations" or whether these represent two distinct morphological structures. The striations, if they are present on the upper laminal cell walls, are obscured. This relationship would be best resolved by study with the scanning electron microscope.

The cyathiform (cup-shaped) immersed or weakly exserted capsules usually retain a green neck, even at maturity when the urn is red-brown. These features, together with the strongly ribbed urn, give this genus a recognisable appearance when fresh.

Both Dixon (1926) and Sainsbury (1955) accepted one species of Amphidium in N.Z. Both, however, noted considerable variability, even within single collections. Dixon noted the close similarity of N.Z. A. cyathicarpum to A. mougeotii (Bruch & Schimp.) Schimp. [Coroll. Bryol. Eur. 40 1856] of northern hemisphere temperate and cold regions. Most N.Z. material is characterised by having unsheathing perichaetial leaves that are scarcely differentiated from the vegetative leaves; these are also features of A. mougeotii. Both N.Z. material of A. cyathicarpum and northern hemisphere material of A. mougeotii have variably toothed vegetative leaves. However, N.Z. material appears to be predominantly or completely (as noted by Dixon) autoicous, while A. mougeotii is considered by both Crum & Anderson (1981) and Smith (2004) to be dioicous.

Sainsbury (1955, p. 198) seemed to suspect that some of his N.Z. collections might belong to A. lapponicum, but he was reluctant to commit to this interpretation.

Magill & van Rooy (1998) treated two species of Amphidium for southern Africa, and suggested that both these species occurred in Australasia. Their application of the African name A. tortuosum (Hornsch.) Cufod. is not accepted here, for reasons discussed below.

Both the species of Amphidium recognised here could easily be confused with epilithic species of Zygodon, particularly Z. intermedius. The vegetative leaves of our species of Amphidium are longer (2.5–4.0 mm vs. c. 0.6–1.8 mm) and more contorted when dry than those of Zygodon. The upper laminal cells of both species of Amphidium tend to be oblate and have lower, unbranched papillae that are not restricted to the cell lumen, while those of Z. intermedium are isodiametric, with relatively high and branched papillae restricted to the area over the cell lumen. When fruiting, the urceolate capsules borne on short setae in both species of Amphidium clearly contrast with the ellipsoid capsules on slender setae in Z. intermedius.