1. The juvenile form of a moss or liverwort that develops on germination of a spore. In mosses and foliose liverworts it is usually a branched green filament, resembling a filamentous green alga, but in Sphagnum it becomes thallose and in thallose liverworts it is not clearly differentiated The familiar adult form develops from buds on the protonema.
2. The erect green filament that develops on germination of the zygote in algae of the Charophyta.