sphagnum or peat moss, any species of the large and widely distributed genus Sphagnum, economically the most valuable moss. Sphagnums, the principal constituent of peat, typically grow as a floating mat on freshwater bogs. Their leaflike appendages have many large cells with circular openings that enable them to absorb liquids readily; hence they are commercially important as a soil structure enhancer (or component of potting soils), packing material, and absorbent dressings and for other uses. Sphagnum is classified in the division Bryophyta, class Bryopsida.