Robert R. Ireland
Hypnum undulatum Hedwig
Plants large, in thin to loose, prostrate, complanate-foliate or sometimes erect and julaceous, light- or whitish-green, sometimes yellowish green, dull or somewhat glossy mats. Stems to 15 cm or longer, sparingly and irregularly branched, occasionally simple, branches sometimes attenuate at apices, epidermal layer of stem cells large and thin-walled, central strand sometimes present, rhizoids brown to reddish brown, smooth or minutely papillose, on leaves on undersurface of stems from base of stems and branches about halfway up and borne on base of costa and adjacent cells on abaxial surface. Leaves of stem and branches similar, crowded, imbricate, somewhat contorted, strongly undulate, especially near apex, concave, usually symmetric, weakly decurrent, ovate to ovate-lanceolate, rarely oblong-lanceolate, acute to acuminate, rarely somewhat obtuse; margins plane, entire or usually serrulate to serrate at apices; costa short and double, ending a short distance distal to base, rarely one branch reaching 1/3 length of leaf; leaf cells covered with minute, granular, cuticular papillae, the papillae much more abundant on the abaxial surface, walls of basal cells pitted; decurrent alar region triangular in outline, consisting of 1--3 longitudinal rows of rectangular cells, terminating at the base in a single cell. Specialized asexual reproduction lacking. Sexual condition dioicous, often fruiting; perigonia and perichaetia at bases of stems, perigonial bracts broadly ovate-lanceolate, acute, perichaetial bracts narrowly ovate-lanceolate to oblong-lanceolate, acute to acuminate, bract margins plane. Seta solitary, dark red, reddish- to light-brown, often curved, rarely circinate, smooth. Capsule inclined to pendulous, arcuate, sometimes straight, light- to orange-brown, cylindrical, smooth, wrinkled and contracted below mouth when dry; annulus of 1--2 rows of large cells, deciduous; operculum conic to rostrate, shorter than urn; stomata sparse, present in the neck, superficial; peristome double, exostome teeth papillose in distal half, with fine striations between the lamellae in proximal half; endostome with 2--3 cilia, as long or nearly as long as segments. Calyptra cucullate, smooth, naked. Spores globose to ovoid, smooth or minutely papillose.
Species 2 (1 in the flora): North America, Europe, Asia, Pacific Islands (Hawaii, New Guinea).
This genus occurs in terrestrial habitats in temperate, boreal, and rarely tropical woods.
SELECTED REFERENCES
Ireland, R.R. 1969. A taxonomic revision of the genus Plagiothecium for North America, north of Mexico. National Museums of Canada, Nat. Mus. Nat. Sciences, Publs. in Botany 1: 1--118. Ireland, R.R. 2001. Buckiella, a new genus in the Hypnaceae (Musci). Novon 11: 55--62. Pederson, N. and L. Hedenäs. 2002. Phylogeny of the Plagiotheciaceae based on molecular and morphological evidence. Bryologist 105: 310--324.
