Typically, the prothallus is the gametophyte phase in the life of a fern or related pteridophytes. Rarely, this term may also be used to describe young gametophytes of bog moss or liverworts.
Prothallus usually emerges from germinating spores. These are short-lived and obscure heart-shaped structures. Their appearance however, may vary between species. Some are colourless, nourishing themselves while some others are green conducting photosynthesis.