A bryophyte is any member of the phylum Bryophya – small, green, rootless, non-vascular plants, including mosses (Musci), liverworts (Hepacticae), and hornworts (Anthoceratae), i.e., the most primitive of the green plants. Bryophytes grow on damp surfaces exposed to light, including rocks and tree bark, almost everywhere from the Arctic to the Antarctic. They have a characteristic life cycle in which the gametophyte is dominant and the sporophyte is attached to an dependent on the gametophyte for nutrition. (See also alternation of generations.) About 24,000 species of bryophytes are known.