moss, any species of the class Bryopsida, in which the liverworts are sometimes included. Mosses and liverworts together comprise the division Bryophyta, the first green land plants to develop in the process of evolution. It is believed that they evolved from certain very primitive vascular plants and have not given rise to any other type of plant. Their rootlike rhizomes and leaflike processes lack the vascular structure (xylem and phloem) of the true roots, stems, and leaves found in higher plants. Although limited to moist habitats because they require water for fertilization, bryophytes are usually extremely hardy and grow everywhere except in the sea. Mosses, the more complex class structurally, usually grow vertically rather than horizontally, like the liverworts. The green moss plant visible to the naked eye, seldom over 6 in. (15.2 cm) in height, is the gametophyte generation (see reproduction). Except for the commercially valuable sphagnum or peat moss, mosses are of little direct importance to humans. They are of some value in soil formation and filling in of barren habitats (e.g., dried lakes) prior to the growth of higher plants and also provide food for certain animals. Unrelated plants sharing the name moss include the club moss, flowering moss, or pyxie (of the diapensia family), Irish moss, or carrageen (see algae), reindeer moss (a lichen), and Spanish moss. Mosses are classified in the division Bryophyta, class Bryopsida.
See A. J. Grout, Moss Flora of North America (3 vol., 1928–39, repr. 1972).