The Moss Families of the British Isles

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L. Watson and M. J. Dallwitz

Polytrichaceae

Haircap- and Smoothcap-mosses.

Gametophyte. Plants “normal”, with leaves containing chlorophyll. Acrocarpous; forming tufts, or mat or turf forming, or forming patches, or the plants scattered. Primary stems the aerial shoots arising from a rhizome-like underground stem. Mature plants 3–100 mm high. Stems with a differentiated central strand. The leaves lanceolate to narrowly lanceolate (in Polytrichum), or ovate, or lingulate; spiral; crisped when dry, or not crisped when dry. Leaf bases sheathing (mostly), or not sheathing (though broad, in Atrichum). The leaves heavily single-nerved (the distal part of the lamina consisting in Polytrichum and Pogonatum, but not in Atrichum, mainly of adaxially expanded nerve). The leaf nerves extending beyond the middle of the leaf, but not to the tip, or extending to the leaf tip; excurrent to not excurrent. Leaf blades adaxially longitudinally lamellate (with two to many chloroplast-containing lamellae along the nerves). Leaf blade apices pointed; apically acute, or acuminate. Leaf blade margins unistratose to several-stratose. Leaf blades bordered, or not conspicuously bordered.

Plants dioecious.

Sporophyte. Capsules exserted; erect to horizontal; symmetrical, or asymmetrical; straight, or curved (Atrichum); when not cuboid or trapezoid, etc., sub-cylindric to ovoid, or pyriform; obscurely to sharply angular (in Polytrichum, being then four to six angled, and rectangular or trapezoid to hexagonal in section), or neither flattened nor angular; with an externally conspicuous apophysis (usually, in Polytrichum), or without an externally conspicuous apophysis. Calyptra large and covering the well-developed capsule; usually densely or sparsely hairy, or glabrous (in Atrichum). Capsules with a peristome (the teeth retaining a cellular structure, and in this respect contrasting with conventional moss peristomes, where these comprise only the thickened parts of cell walls remaining after the rest of the cells have degenerated; cf. Tetraphidaceae). The peristome single (of short teeth, derived from several concentric layers of cells, and not resolvable into one or two rings). The peristome teeth in European species, 32, or 64 (64 in Polytrichum, 32 in the other genera, the teeth usually joined at their tips by an epiphragm representing the expanded apex of the columella); joined basally to form a membranous ring, or not basally joined; not deeply cleft; solid, without transverse bars (derived from several concentric series of sporogonium cells). The processes of the inner peristome opposite the teeth of the outer peristome. The operculum rostellate to subulate. Setae long; straight, or flexuose.

Ecology. Occurring in mainly acid conditions. In diverse habitats.

Cytology. Haploid chromosome number, n = 7 and 14, or 21 (sometimes, in Atrichum undulatum only).

British representation. 17 species. Atrichum (Smoothcaps), Oligotrichum (Hercynian Haircap), Pogonatum (Haircaps), Polytrichastrum (Haircaps), Polytrichum (Haircaps). Northern Scotland, southern Scotland, northern England, English Midlands, East Anglia, Wales, southeast England, central southern England, southwest England, Isle of Wight, and Ireland.

Classification. Class Polytrichopsida; Order Polytrichales.

Illustrations. • Pogonatum and Polytrichastrum alpinum: Dixon. • 6 Polytrichum and Polytrichastrum. • Atrichum angustatum, A. crispum, A. tenellum and A. undulatum: Dixon. • Oligotrichum hercynicum: Dixon. Oligotrichum hercynicum. • Atrichum, Pogonatum: Berkeley. • Polytrichum alpinum and P. juniperinum: Berkeley.


To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The moss families of the British Isles. Version: 21st June 2009. http://delta-intkey.com’.

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