protonema An early stage in the development of a moss or liverwort. A protonema is produced when a moss or liverwort spore germinates. In mosses it usually consists of green, branching filaments; but it is thalloid (a flat sheet or disc of cells) in Sphagnum and Andreaea, for example, and in many liverworts. The familiar moss or liverwort plant (the gametophyte) arises from buds which develop on the protonema.

protonema The first, usually filamentous, structure produced by a germinating moss or fern spore under dark conditions. The protonema of mosses bears buds that develop into the gametophyte plant; in ferns the protonema becomes the prothallus.