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Polytrichaceae Schwägrichen in C. L. von Willdenow, Sp. Pl. ed. 4,. 5(2): 1. 1830.

Gary L. Smith Merrill

Plants small, medium to large, densely to loosely caespitose or scattered among other bryophytes, rarely with individual plants scattered on a persistent protonema. Stems erect, acrocarpous, from a ± developed underground rhizome, simple or rarely branched, bracteate below, grading gradually or abruptly to mature leaves. Leaves various, with a chartaceous, sheathing base and a divergent, firm-textured blade (polytrichoid), or the whole leaf membranous and sheath not or weakly differentiated, the blade rarely transversely undulate, crisped and contorted when dry; adaxial surface of blade with numerous closely packed longitudinal photosynthetic lamellae across most of the blade, the marginal lamina narrow, or the lamellae restricted to the costa, flanked by a broad, 1 (rarely 2)-stratose lamina, rarely with abaxial lamellae; margins 1(--3)-stratose, entire, denticulate, serrate, or toothed (in Atrichum bordered by linear, thick-walled cells); costa narrow in basal portion, in the blade abruptly broadened and diffuse, smooth or toothed adaxially, rarely with abaxial lamellae, in cross-section with a prominent arc of large diameter guide cells and an abaxial stereid band; lamellae entire, finely serrulate, crenulate, or coarsely serrate, the free margin smooth, finely striolate, pitted, or coarsely cuticular-papillose, the marginal cells in cross-section undifferentiated or sharply distinct in size and/or shape from those beneath; transition in areolation from sheath to blade gradual or abrupt, with “hinge-tissue” at the shoulders (except in Atrichum and Psilopilum); cells of back of costa (or cells of the membranous lamina) typically in longitudinal rows, ± isodiametric to transversely elongate-hexagonal. Vegetative reproduction none, or by proliferation of an underground rhizome. Sexual condition dioicous or rarely monoicous; male inflorescence indeterminate, innovating from the center and continuing the growth of the stem, often several successive perigonia per shoot; female inflorescence terminal, perichaetial leaves long-sheathing or not much differentiated. Seta solitary or rarely several from the same perichaetium. Capsules obtusely to sharply (2--)4(--6)-angled, with indistinct longitudinal angles or ridges, or terete; hypophysis tapering and indistinct or delimited by a constriction at base of capsule; exothecium smooth, mamillose, or scabrous; stomata present (absent in Atrichum and Pogonatum); peristome pale or strongly pigmented, nematodontous, with a single series of [16--]32--64 rigid, unjointed teeth composed of elongate, fiber-like, sinuate cells, the teeth simple or compound, attached by their tips to the epiphragm (tympanum) covering the capsule mouth. Calyptra cucullate, with a matted felt of hairs arising from its tip and covering all or part of the capsule, or the calyptra sparsely ciliate to smooth. Spores minute and echinulate, or larger and finely papillose.

Genera 20, about 400 species (9 genera, XXX species in the flora). Terrestrial mosses, chiefly on soil and soil-covered rocks, roadside banks, deciduous and coniferous forests, moorlands, and tundra, less frequently in wet meadows and boggy sites, at low, medium and upper elevations. Almost cosmopolitan and widely distributed in all climatic zones except the lowland tropics.

The Polytrichaceae include many large, common, and familiar N. Am. mosses. The “Hair-Cap Mosses” have no close living relatives, and have a long (but disjunctive) fossil record (Eopolytrichum) from the Late Cretaceous of Georgia, U.S.A. (A. S. Konopka et al. 1997). The family is remarkable for the structural complexity of both gametophyte and sporophyte found in many of its members. The stems of robust taxa (Polytrichum) have a conspicuous central strand composed of hydrome and leptome, and traces extending into the leaves; in other genera the central strand is weak and indistinct.

The following descriptions use the shorthand term “polytrichoid” for taxa with a growth form and leaves like those of Polytrichum. The best analogy to the polytrichoid leaf is the grass leaf, with a clear distinction between a sheath and divergent blade. Typically a wedge-shaped group of transversely elongate, incrassate cells (“hinge tissue”) is present at the shoulders or just above. Most polytrichoid taxa have a hyaline-margined sheath, with a sharply defined border of thin-walled, decolorate cells. The leaves of Atrichum have a slender costa and a broad, membranous lamina. In Bartramiopsis and Lyellia, however, the lamellae are restricted to the “costa” and the broad “lamina” is 2-stratose. In the polytrichoid leaf the distinction between costa and lamina becomes blurred. The conducting strands are confined to a median band, flanked by an ambiguous zone bearing closely packed lamellae but only a few cells in thickness, and a narrow, usually 1-stratose marginal lamina. G. L. Smith (1971) suggested that lamellae originally covered the adaxial surface of the leaf, and the broad membranous lamina developed as conducting strands were restricted to the median portion.

As demonstrated by S. O. Lindberg (1868) the form of the marginal cells of the lamellae is quite sufficient to distinguish between species of Polytrichum (and Polytrichastrum). Free-hand sections with a razor blade are adequate in most cases. Sections should be made from ordinary vegetative leaves, in the middle 1/3 (or in polytrichoid leaves, midway between the tip and the base of the blade). Lengths of lamellae should also be scraped from the leaves to be viewed in profile. Median cells of the lamina are best measured individually. The leaf cells are typically aligned in ± regular longitudinal rows, but it is difficult to measure a group of cells and divide to obtain an average cell width since the cells are not aligned transversely.

The peristome teeth are unique among mosses in being composed of compact bundles of whole, fiber-like cells. The teeth are not composed of remnants of wall thickenings as in arthrodontous mosses, but form by intrusive growth and elongation of living cells. Accordingly, the peristomes are not homologous. The teeth may be simple (with a single median line), or compound (the outlines of two teeth visible on the outer face of each tooth), although the exact nature of this difference is not well understood. The sinus between these “teeth” may be broad (Oligotrichum), or narrow and obscured (Pogonatum). The height of the peristome is measured from the tips of the teeth to the capsule rim, seen from without. The term “basal membrane” is not used. The generic classification used in this treatment follows that of G. L. Smith (1971) with few modifications.


Frye, T. C. 1910. The Polytrichaceae of western North America. Proc. Wash. Acad. Sci. 12: 275--281. Lindberg, S. O. 1868. Observationes de formis praesertim europaeis Polytrichoidearum (Bryacearum nematodontearum). Not. Sällsk. Fauna Fl. Fenn. Förh. 9: 91--158. Long, D. G. 1985. Polytrichaceae. In: Mogensen, G. S. (ed.), Illustrated Moss Flora of Arctic North America and Greenland. Fasc. 1. Meddel. Grønland, Biosci. 17: 9--57. Smith, G. L. 1971. A conspectus of the genera of Polytrichaceae. Mem. New York Bot. Gard. 21(3): 1--83. fig. 1--128.


Konopka, A. S., P. S. Herendeen, G. L. Smith Merrill, and P. R. Crane. 1997. Sporophytes and gametophytes of Polytrichaceae from the Campanian (Late Cretaceous) of Georgia, U.S.A. Int. J. Plant Sci. 158: 489-499, fig. 1--45.

Alternative Key

1 Calyptra densely hairy.   (2)
+ Calyptra sparsely hairy or smooth.   (4)
2 (1) Capsules without stomata, terete or with 6--8 longitudinal ridges; peristome colored; exothecium scabrous.   Pogonatum
+ Capsules with stomata, angled or cylindric; peristome teeth pale; exothecium smooth or mammillose.   (3)
3 (2) Urn sharply alate, especially with age; exothecium mammillose, the exothecial cells bulging, with a sharply-defined pit; teeth with a thin vertical keel on the inner face.   Polytrichum
+ Urn rather bluntly 4(--6) angled or terete; exothecium smooth, the exothecial cells without pits or with a diffuse thin-spot; peristome teeth not keeled.   Polytrichastrum
4 (1) Lamellae numerous, occupying most of the leaf width; capsule bilaterally compressed, 4-angled, the two uppermost angles often more prominent, forming a crest.   Meiotrichum
+ Lamellae 10 or fewer, confined to the median portion of leaf; capsule 2-angled or terete.   (5)
5 (4) Leaf lamina 2-stratose; peristome none.   (6)
+ Leaf lamina 1-stratose; peristome present.   (7)
6 (5) Lamellae entire; leaf sheath entire; leaf tip with a caducous, uniseriate hair; plants polytrichoid; [capsule sharply 2-angled and lunate in section].   Lyellia
+ Lamellae coarsely serrate; leaf sheath ciliate; leaf tip not ending in a slender, uniseriate hair; plants slender and wire-like; capsule turbinate.   Bartramiopsis
7 (5) Peristome teeth compound (double), with a broad U-shaped sinus on the outer face of each tooth; lamina and costa often with abaxial lamellae.   Oligotrichum
+ Peristome teeth simple, without a U-shaped sinus; lamina without abaxial lamellae (except on back of costa near apex), the costa often toothed at back.   (8)
8 (7) Capsules without stomata; leaf margins toothed and strongly bordered by thick-walled, linear cells.   Atrichum
+ Capsules with stomata; leaf margins entire, bordered (at least in part) by thin-walled, hyaline, cells.   Psilopilum

List of Keys

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