The New Student's Reference Work/Protonema

Protonema (prō′tō̇-nē′ma) (in plants). When the asexual spores of mosses germinate, they at first produce green, branching filaments, which are prostrate and resemble ordinary green algæ. Upon this filamentous body the ordinary leafy branches arise. This algæ-like body is known as the protonema. See Musci.