Mosses are small flowerless plants that usually grow in dense green clumps or mats, in damp or shady locations. The individual plants are usually composed of simple, one-cell thick leaves, covering a thin stem that supports them but does not conduct water and nutrients (nonvascular).osses are in the phylum (division) Bryophyta, which formerly also included hornworts and liverworts. These other two groups of bryophytes are now placed in their own divisions. There are approximately 12,000 species of moss classified in the Bryophyta.