Joseph R. Rohrer
Plants medium-sized to robust, forming wefts, often in extensive sheets, growth sympodial or monopodial. Stems prostrate to ascending or erect, often with ascending-arching innovations, irregularly branched to regularly pinnate; paraphyllia sometimes present, branched, cells elongate; pseudoparaphyllia foliose; rhizoids sparse, at base of stems and tips of attenuate branches. Stem leaves often differentiated from branch leaves, very broadly ovate to lanceolate; margins serrate to serrulate, less commonly nearly entire; apex generally acuminate, sometimes acute to rounded; costa typically double, extending 1/4--2/3 leaf length, sometimes single or nearly absent; median cells prosenchymatous, narrowly elliptic to linear-flexuose; alar cells sometimes differentiated. Branch leaves usually smaller, narrower, more sharply toothed, with stronger costa. Sexual condition dioicous. Seta elongate, smooth. Capsule mostly inclined to horizontal; peristome double, exostome commonly reticulate on abaxial surface, but variable, papillose distally; endostome generally with high basal membrane, segments broad, perforate along the keel by narrow slits or broadly elliptic gaps, cilia 1--4. Calyptra cucullate, smooth, naked. Spores spherical, 10--25[--50] µm, finely papillose to nearly smooth.
Genera 11, species 26 (7 genera, 12 species in the flora): primarily cool-temperate to subarctic regions worldwide, extending to the tropics in montane habitats.
The Hylocomiaceae include some of the largest pleurocarpous mosses of North America. Hylocomium splendens, Pleurozium schreberi, and Rhytidiadelphus triquetrus, which carpet vast areas of forest floor across the northern half of the continent, are among our most common and widespread mosses. The genera of Hylocomiaceae are poor in species but rich in individuals. Many of the North American species are also common across northern Europe and Asia. Because of their robust size, handsome appearance, and abundance, several species in this family are commercially harvested for decorative use in planters. Since its original description, the family has been studied and redefined several times (A. L. Andrews 1954; A. Noguchi 1972; J. R. Rohrer 1985a). Many of the genera are isolated phenetically, and their cladistic relationships are debatable. The genera of Hylocomiaceae, as defined here, share various combinations of the following derived characters: sympodial growth, branched paraphyllia with elongate cells, differentiated stem and branch leaves, sharply serrate leaf margins, highly variable costa that are often long and double, or single and branched, prosenchymatous prorate leaf cells, reticulate exostome ornamentation, and endostome segments with broadly elliptic perforations.
Recently W. R. Buck (1997) described the new genus Schofieldiella, which he placed in the Hylocomiaceae, to accommodate a species usually called either Hygrohypnum micans (Mitten) Brotherus or Sematophyllum micans (Mitten) Braithwaite. Although this species is similar in various characteristics to some genera of Hylocomiaceae, such as Leptocladiella to which Buck compared Schofieldiella, it lacks nearly all of the critical apomorphies listed above that define the Hylocomiaceae.
Andrews, A. L. 1954. Taxonomic notes. XII. The families Rhytidiaceae and Hylocomiaceae. Bryologist 57: 251--261. Noguchi, A. 1972. On the delimitation of the genera of Hylocomiaceae and Rhytidiaceae. J. Hattori Bot. Lab. 35: 155--168. Rohrer, J. R. 1985a. A phenetic and phylogenetic analysis of the Hylocomiaceae and Rhytidiaceae. J. Hattori Bot. Lab. 59: 185--240. Rohrer, J. R. 1985b. A generic revision of the Hylocomiaceae. J. Hattori Bot. Lab. 59: 241--278.