Polytrichum Hedw., Sp. Musc. Frond. 88. 1801.
[Greek polys, many, and thrix, hair, applied in antiquity to plants with fine, hairlike parts, including mosses; now alluding to the hairy calyptra]
Gary L. Smith Merrill
Plants medium sized to tall and robust, in loose to compact tufts, arising from a horizontal underground rhizome. Stems loosely to densely leafy distally, bracteate proximally, rhizoidous at base or rarely wooly-tomentose throughout. Leaves with differentiated sheath and blade; sheath entire, hyaline-margined, often highly nitid (glossy), with a well-developed hinge-tissue at the junction of sheath and blade; marginal lamina narrow, plane or erect, sharply toothed with stout, unicellular teeth, or entire, broadened and ± sharply inflexed, enclosing the lamellae; costa short-excurrent or prolonged as a long, spinulose awn; lamellae numerous, closely-spaced, the marginal cells in section distinctly differentiated, pyriform, flat-topped, or retuse. Sexual condition dioicous; male plants with conspicuous rosettes formed by the broadly overlapping, apiculate perigonial bracts, commonly innovating and producing several successive male inflorescences per shoot; perichaetial leaves typically long-sheathing, the sheath broadly hyaline-margined, with a weakly-developed and greatly abbreviated blade. Seta solitary. Capsule 4(--5)-angled, often somewhat broader toward the base, alate and prismatic with knife-edge angles after the operculum is shed, reddish to purplish brown, glaucous in fresh capsules, suberect when young but becoming sharply bent at the attachment to the seta and almost horizontal; hypophysis discoid, sharply delimited from the urn by a deep basal constriction; stomata rather few and confined to the constriction; exothecial cells bulging-mammillose, often transversely elongate, with a sharply defined, circular or slit-like pit in the outer wall; operculum umbonate, with a short beak; peristome teeth 64, pale, single, with a thin vertical keel and delicate spur-like projections in the inner face; epiphragm thin and delicate, remaining attached to the peristome teeth, the margin thicker and dissected into pendent lobes alternating with the peristome teeth. Calyptra with a densely interwoven, matted felt of hairs, enveloping the whole capsule and entwined beneath. Spores small, smooth (minutely echinulate with SEM).
Species about 70 (7 in the flora): cosmopolitan.
Polytrichum is characterized by a “unique set of tightly correlated sporophytic characters” (G. L. Merrill 1992). The capsules are sharply angled with a discoid hypophysis separated from the body of the capsule by a deep basal constriction, and a glaucous appearance when fresh (see E. Lawton 1971, frontispiece). The exothecial cells are bulging-mammillose, with a sharply-defined pit, and the peristome and epiphragm are of the pterygodont type (S. O. Lindberg 1868; G. L. Smith 1971, 1974). Polytrichum species are distinct genetically from other members of the family, suggesting an early origin for this lineage (G. Derda et al. 2000). Capsules with bulging-mammillose, pitted exothecial cells, discoid hypophysis, and spores echinulate with “Christmas-tree” projections are already present in the Late Cretaceous fossil genus Eopolytrichum (A. S. Konopka et al. 1997). Two groups of Polytrichum species are represented in our area, one with narrow, toothed, ± erect leaf margins (sect. Polytrichum), and the other with broad, entire, sharply inflexed leaf margins, enclosing and sheltering the adaxial lamellae (sect. Juniperifolia).
Lindberg, S. O. 1868. Observationes de formis praesertim europaeis Polytrichoidearum (Bryacearum nematodontearum). Not. Sällsk. Fauna Fl. Fenn. Förh. 9: 91-158. Smith, G. L. 1971. Conspectus of the genera of Polytrichaceae. Mem. N.Y. Bot. Gard. 21(3): 1--83. Merrill, G. L. 1992. Notes on North American Polytrichaceae: Polytrichastrum G. Sm. Bryologist 95: 270--273.
Bijlsma, R., M. van der Velde, L. van de Zande, A. C. Boerema, and B. O. van Zanten. 2000. Molecular markers reveal cryptic species within Polytrichum commune (Common Hair-Cap Moss). Plant Biol. (Stuttgart) 2: 408--441.
Crum, H. A. and L. E. Anderson. 1981. Polytrichaceae. Moss Flora of Eastern North America. 2: 1247--1284. New York.
Derda, G. and R. Wyatt. 1999. Levels of genetic variation and its partitioning in the wide-ranging moss Polytrichum commune. Syst. Bot. 24: 512--528.
Derda, G., R. Wyatt, and J. Hyvonen. 1999. Genetic similarities among the Hair-Cap Mosses (Polytrichaceae) as revealed by enzyme electrophoresis. Bryologist 102: 352--365.
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.
Lawton, E. 1971. Moss Flora of the Pacific Northwest. Nichinan, Japan.
Long, D. G. 1985. Illustrated Moss Flora of Arctic North America and Greenland. I. Polytrichaceae. Meddel. Grönland, Biosci. 17: 1--57.
Smith, G. L. 1974. New developments in the taxonomy of Polytrichaceae: epiphragm structure and spore morphology as generic characters. J. Hattori Bot. Lab. 38: 143--150