Moss (division Bryophyta), any of at least 12,000 species of small non vascular spore-bearing land plants. They are small flowerless plants that typically grow in dense green clumps or mats, often in damp or shady locations. The individual plants are usually composed of simple leaves that are generally only one cell thick, attached to a stem that may be branched or unbranched and has only a limited role in conducting water and nutrients.