A portion of chlorenchyma tissue that develops directly from a spore and grows by division of an apical cell to form filaments that are one cell wide and dichotomously branching. [source: POC:Ramona_Walls]
The protonema is the first structure that develops in the gametophytic phase of mosses, some other bryophytes, and leptosporangiate ferns. The term protonema is also used to refer to the phase of development in which protonemal tissue is produced (see PO:0030006 protonema phase). In some species of moss, the protonema is perennial. A protonema may develop from sporophytic tissue during apospory.