Alternation of generations heteromorphic; the sexual generation, the leafy gametophyte dominant (‘the moss plant’), the asexual generation, the sporophyte dependent on the gametophyte for survival. Gametophyte stems without xylem or phloem but sometimes with tubular conducting cells; multicellular rhizoids often present; leaves typically unistratose but sometimes thicker; costa present or not; cells isodiametric to linear. Female gametangia, archegonia, flask-shaped and containing a single female gamete; male gametangia, antheridia, producing numerous biflagellate male gametes. Fusion of gametes resulting in the development of an embryo in the venter of the archegonium. Sporophyte consisting of foot embedded in gametophyte tissue, seta, frequently with tubular conducting cells and capsule; capsules usually with stomata and photosynthetic tissue, usually dehiscing by lid, occasionally cleistocarpous, rarely by longitudinal slits; later development of capsule controlled by the calyptra derived from neck of archegonium; spores produced by meiosis in spore sac of the capsule. About 12 000 species (almost certainly an overestimate). A highly successful group of plants occurring in almost every available natural habitat except the sea.