Plants (3-)10-40(-180) mm, in dense cushions to loose mats, olivaceous, green, brown, or black, often with yellow, orange, or red tones. Leaves ovate-lanceolate, occasionally ovate-triangular, less commonly lanceolate to linear-lanceolate or elliptical to ligulate, keeled or concave proximally, sharply keeled or nearly flat distally, margins recurved, rarely plane or incurved, distal lamina usually 1-stratose or 2-stratose in striae or patches, rarely 2-stratose, specialized laminal and marginal chlorophyllose structures absent, muticous to long-awned, sometimes ending in a fleshy, multistratose apiculus; basal cells rectangular, with straight or sinuose and thin to thick cell walls; mid leaf and distal cells quadrate, rectangular, or ovate, rarely sub-triangular, smooth or papillose, usually sinuose and thick-walled. Gemmae absent. Sexual condition autoicous, rarely dioicous; perichaetial leaves usually enlarged. Seta short, straight. Capsule erect, immersed, symmetric, cylindric or campanulate; annulus rudimentary or absent; operculum rostrate or rarely mamillate, usually falling attached to columella (except S. trichodon). Calyptra cucullate or mitrate, not erose, not fully covering operculum, smooth.
Species ca. 120 (30 in the flora): North America, Mexico, Central America, South America, Eurasia, Africa, Atlantic Islands, Pacific Islands, Australia, Antarctica.
The genus Schistidium has consistently fascinated yet confounded bryologists across North America. Treatments vary from region to region, and names applied to specimens at both the species and varietal levels have been as inconsistent as the characters used to differentiate the taxa. The treatment of the S. apocarpum complex by H. H. Blom (1996) and a survey of Nordic species of Schistidium (Blom 1998) assist in a better understanding of the taxonomy of this complex genus in North America, but many problems of taxonomic interpretation remain. Although Schistidium offers a great number of both gametophytic and sporophytic characters for study, some traits are not well understood and further detailed field and laboratory research is needed.
This treatment follows H. Deguchi (1979) and H. H. Blom (1996, 1998), with reservations. For example, North American specimens identified as Schistidium recurvum H. H. Blom seem to differ from the type in significant character states (Blom, pers. comm.), so this species has been omitted. Schistidium lancifolium H. H. Blom and S. umbrosum (J. E. Zetterstedt) H. H. Blom have been included within S. apocarpum. Although S. ambiguum Sullivant has been reported from North America and may be present, all of the North American collections named S. ambiguum that were examined have proved to be another species.
When examining a specimen of Schistidium certain steps are helpful. It is important to examine the leaves proximal to the perichaetial region. Transverse-sections from the distal region to mid leaf of multiple leaves are also critical in most cases. The necessity of numerous transverse-sections is apparent when studying S. papillosum or S. boreale, for example, as some leaves can be slightly papillose and unless numerous sections are made the papillae may be missed. Mature, empty capsules that are not overly degraded, although not always available, should be used. Transverse-sections of the capsules assist in the easier examination of the exothecial cells and peristome teeth. It is also useful to examine more than one capsule if available, as there can be some variation in exothecial cell makeup. As H. H. Blom (1996) pointed out, mixed populations are present in some sites, especially in more humid areas, so care must be taken to ensure that all species in a collection are separated. Blom also provided a great amount of supplementary detail about many of the species treated here.