Pottiaceae Hampe

The following family description is modified from Zander et al. (2007).

Plants minute to robust, turf-forming or gregarious, commonly found in harsh environments, usually on soil, sometimes on rock, concrete or bark. Stems irregularly branching, in cross-section with or without a central strand, sclerodermis sometimes present, hyalodermis usually absent. Leaves usually appressed and often contorted when dry, spreading when wet, ovate to lanceolate or lingulate, occasionally bordered, rounded to acuminate at the apex; margins incurved, plane, recurved or revolute, sometimes dentate distally; upper laminal cells usually firm-walled, subquadrate, and papillose; basal cells usually strongly differentiated, rectangular; alar cells not differentiated. Costa single, usually strong and protruding abaxially, failing shortly before the apex to long excurrent, occasionally bearing filaments or lamellae on the adaxial face, in cross-section with 1 or 2 stereid bands. Brood bodies occasional, as globose gemmae on leaves, modified leaves, modified leaf apices or rhizoid-borne tubers. Axillary hairs hyaline throughout, or with a brown basal cell. Laminal KOH colour reaction yellow to orange, or red, rarely colourless.

Dioicous or monoicous. Perichaetia terminal, the leaves usually longer than vegetative leaves, often sheathing in their basal portion where the cells are elongate-rhomboid. Setae usually elongate, but occasionally very short. Capsules usually exserted, sometimes immersed or emergent, usually stegocarpous; theca ovoid to cylindric, erect or rarely weakly arcuate; annulus sometimes present; operculum conic or rostrate, with cells in straight or oblique rows. Peristome single, rudimentary or well developed, occasionally absent (variable even within a genus), when well developed comprising 16 lanceolate teeth or 32 straight or spiralled linear rami. Calyptra cucullate, smooth, rarely papillose. Spores spherical, usually c. 10–18 µm, smooth to verruculose.

The Pottiaceae are a very large family, with a predominantly temperate distribution. Globally they are the largest family of mosses in terms of number of genera (Zander et al. 2007), and in the N.Z. moss flora they are the family with the largest number of species (Allan Herbarium 2023​​​​​​​). Brotherus (1901–1909) recognised 46 genera in the family and divided them into four subfamilies, and later recognised 71 genera in five subfamilies (Brotherus 1924). His circumscription formed the basis of modern taxonomies on a regional level; for example, for Japan by Saito (1975), and for N.Z. by Dixon (1923), whose treatment was closely followed by that of Sainsbury (1955). A monumental monograph for Pottiaceae at the generic level (Zander 1993), 'Genera of the Pottiaceae: Mosses of harsh environments', was based on cladistic analyses utilising 74 characters, the majority of them gametophytic. Zander’s conclusions were adopted for N.Z. by Fife (1995) in the first overall classification of the N.Z. moss flora since Sainsbury’s Handbook (Sainsbury 1955). A radical change was the transfer of species of Pottia to Hennediella, Microbryum, or Tortula. In a review of Zander’s monograph, Blockeel (1994) noted that "because of the size of the family ... and its intrinsic taxonomic difficulties, it is often the dismay of the inexperienced bryologist, and it continues to perplex the specialist". This has been no less true in a N.Z. context. Members of the genus Didymodon have recently been split into four genera (Jiménez et al. 2022; Beever et al. 2023), one of which, Tridontium, had previously been regarded as belonging to a different family, Scouleriaceae (Goffinet et al. 2009). The genera Trichostomum and Weissia similarly require critical reassessment. In some genera, a paucity of collections of certain species has exacerbated difficulties, as in Willia and Syntrichia, and more fieldwork will be beneficial. Molecular analysis, continually being applied to the family, will no doubt further elucidate some of the complexities.

A common feature in the family is the presence of well-developed peristome teeth, often comprising long, filamentous, spiral rami distal to basal cylinders of various heights. However, although useful in identification at the species level, little weight has been placed on peristome morphology in the higher taxonomy of the family. A justification for this is presented by Zander (2006), who suggested that the evolution of spiralled peristomes perhaps represents a one-time developmental response to selection involving many genes or gene clusters and one or more regulators. These may remain masked in subsequent evolution, and the reappearance of the phenotype in modern taxa thus appears sporadic, being widespread in the three main subfamilies. Its reappearance is estimated to have occurred some four to seven times within Pottiaceae.

The subfamily and generic classifications used here are based on those of Zander (2006), with the addition of Tridontium to the family, and with Hyophila retained in subfamily Pottioideae, as advocated by Zander (1993). The subfamily name Barbuloideae (Zander 2006) is used in preference to Merceyoideae (Zander 1993).

The Pottiaceae in N.Z. thus includes 31 genera, assigned to three subfamilies: subfamily Barbuloideae (Anoectangium, Ardeuma, Barbula, Bryoerythrophyllum, Geheebia, Gertrudiella, Gymnostomum, Leptodontium, Pseudocrossidium, Streblotrichum, Trichostomopsis, Tridontium, and Triquetrella); subfamily Pottioideae (Acaulon, Aloina, Chenia, Crossidium, Hennediella, Hyophila, Microbryum, Phascopsis, Pterygoneurum, Syntrichia, Tortula, and Willia); and subfamily Trichostomoideae (Calyptopogon, Chionoloma, Tetracoscinodon, Tortella, Trichostomum, and Weissia).

The following key to subfamilies is based on that of Zander 1993.

A workable generic key, even for N.Z. members only, has not yet been achieved due to the complexity of the family. As an alternative, generic keys for each subfamily are presented separately.

Members of the family are generally characterised by erect shoots, with leaves of various shapes and forms, but never falcate. Leaves are usually with a long costa, which is often excurrent, sometimes extending as a hair-point. Upper laminal cells are often small, quadrate to rounded or hexagonal, frequently papillose, occasionally smooth and generally firm- to thick-walled. Lower laminal cells are usually differentiated, thinner-walled, smooth and elongate. Brood bodies of various forms are characteristic of individual species. Confusion can occur with members of the Ditrichaceae, especially Ceratodon purpureus, which can be distinguished from any member of the Pottiaceae by the combination of unbordered leaves, usually with a slightly toothed distal margin and smooth laminal cells.

Ornamentation of periclinal walls of upper laminal cells is often a useful diagnostic character for identifying species. The term 'papillae' is used here for both solid and hollow protuberances, which are described as being simple, complex (i.e., branched or apparently C-shaped) or absent, based on light microscopy with fine-focusing on high-power (×600). In the illustrations provided the extent of papillae is, for clarity, often only partially indicated.

Detail is enhanced by mounting moist leaves in lactic acid. Cells may be observed in face view, or papillae may be seen in profile along folds in leaves, where they are constrained under a coverslip. In some species papillae may be either present or apparently absent (e.g., Tortula truncata), or reliably absent (e.g., Aloina spp. and Tridontium tasmanicum). A mixture of papilla forms may be present on a single leaf (e.g., in Calyptopogon mnioides). Detailed SEM studies of moss leaf surfaces are widely reported in the literature, and include some pottiaceous taxa found in N.Z. (e.g., Cano 2008; Kou et al. 2014).

In the Pottiaceae the colour reaction of the walls of upper laminal cells with KOH is a useful character for discriminating certain taxa (Zander 1993). The KOH may be applied as a 2% aqueous solution by means of a dropper onto moist leaves on a slide, before adding a coverslip. Reaction of the walls may be a clear yellow to yellow-orange or red. It needs to be assessed under high power (×600) of the compound microscope, with focusing on the anticlinal walls.

Stem lengths in this account are based on herbarium material, and because mosses are often collected with their bases broken off, especially when shoots are deeply set in a soft substrate, measurements given are conservative.

Doubtful and excluded taxa: a "very small, annual, pale-green" moss was described as Pottia obliqua R.Br.bis. by Robert Brown (1894​​​​​​​). From the protologue it has a resemblance to Tortula truncata (Hedw.) Mitt., but as no types nor any other specimens have been located it is not considered further.

The holotype of Trichostomum rostratum R.Br.bis (CHR 335722!) is Orthotrichum cupulatum Brid., and a duplicate (BM 001006852!) is a mixture, predominantly Syntrichia phaea.

​​​​​​​Anacalypta stevensii R.Br.bis, holotype CHR 335653!, is Fissidens taylorii Müll.Hal.