5. Ditrichum Hampe, Flora. 50: 181. 1867.
Plants often minute to small, rarely medium-sized or large, yellowish green, in loose or dense tufts. Stems erect, simple or sparsely branched, sparsely to densely radiculose; central strand present. Leaves often homomallous, lanceolate, gradually or abruptly narrowed from a lanceolate or oblong-ovate, sometimes clasping base to a long, slender or setaceous acumen; margins plane or reflexed, entire or serrulate above; costa single, stout, percurrent to excurrent, usually filling most or nearly the entire subula; upper cells subquadrate to oval or elongate, smooth; basal cells rectangular to linear, thin-walled; alar cells not differentiated. Autoicous or dioicous. Perichaetial leaves similar to the upper stem leaves or slightly larger and clasping at base. Setae elongate, slender, straight or flexuous; capsules oblong-ovoid to cylindric, erect to suberect or inclined, symmetric or curved, sometimes striate or furrowed when dry and empty; opercula conic, or shortly rostrate; annuli differentiated, consisting of large cells, revoluble, deciduous or persistent; stomata present; peristome single, 16 teeth, linear or filiform, divided nearly to the base, basal membrane low, papillose. Calyptrae cucullate, smooth, entire. Spores spherical, small, smooth or faintly papillose.
The genus consists of about 50 species in the world. Redfearn et al. (1996) listed 13 species of Ditrichum from China. Of these 13 species, Ditrichum microcarpum Broth. and D. setschwanicum Broth. are synonyms of D. pusillum (Hedw.) Hampe (T. Cao 1994). Ditrichum darjeelingense Ren. & Card. and D. tortuloides Grout reported from Yunnan by S.-Y. Zeng (1990) and D. divaricatum Mitt. listed by Redfearn et al. (1996) cannot be confirmed. In this study, eight species are treated for China.
The gametophytes of several Ditrichum species are similar to those of Dicranella and Pleuridium, but the threadlike division and papillose markings of the peristome teeth are significantly different from Dicranella, and its stegocarpous sporophyte condition affirms the difference from Pleuridium.