Any of the green, seedless land plants that make up the division Bryophyta, numbering at least 18,000 species and divided into three classes: mosses, liverworts, and hornworts. They are distinguished from vascular plants and seed plants by the production of only one spore-containing organ in their spore-producing stage. Most bryophytes are 0.8–2 in. (2–5 cm) tall or, if reclining, generally less than 12 in. (30 cm) long. Found throughout the world, from polar regions to the tropics, they are most abundant in humid environments, though none is marine. Bryophytes are extremely tolerant of dry and freezing conditions. Peat moss is economically important to humans in horticulture and as an energy source. Some bryophytes are used ornamentally, as in moss gardens. In nature, bryophytes initiate soil formation on barren terrain and maintain soil moisture, and they recycle nutrients in forest ecosystems. They are found on rocks, logs, and forest litter.