protonema
/ˌprəʊtə(ʊ)ˈniːmə/Botany
noun
1The juvenile form of a bryophyte (usually consisting of a branched filamentous structure, but sometimes flat or platelike), developing on the germination of a spore and producing buds from which the adult plants arise.
2An analogous thread-like structure in other lower plants; specifically the filamentous form of a developing gametophyte of a fern or charophytic alga.
Origin
Mid 19th century; earliest use found in Miles Berkeley (1803–1889), Church of England clergyman and naturalist. From scientific Latin protonema, former genus name from proto- + -nema.