prothallium

thumb|right|Prothallus (prothallium) of the fern Polypodium vulgare seen under a light microscope.

A prothallus, or prothallium, (from Latin pro = forwards and Greek θαλλος (thallos) = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally, the term is also used to describe the young gametophyte of a liverwort or peat moss as well. In lichens, it refers to the region of the thallus that is free of algae.