A plant, usually a pteridophyte, in gametophyte stage in the alternation of generations cycle
A prothallium, or prothallus; from Latin pro = forwards and Greek θαλλος (thallos) = twig; is usually a pteridophyte, i.e. spore-bearing plant, (almost exclusively ferns) in gametophyte stage in the alternation of generations cycle. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well