protonema
noun
pro·to·ne·ma ˌprō-tə-ˈnē-mə 
plural protonemata ˌprō-tə-ˈnē-mə-tə
-ˈne-
: the primary usually filamentous thalloid stage of the gametophyte in mosses and in some liverworts comparable to the prothallus in ferns
protonemal
ˌprō-tə-ˈnē-məl
adjective protonematal
ˌprō-tə-ˈnē-mə-tᵊl -ˈne-
adjective -ˈne- 
Word History
Etymology
New Latin protonemat-, protonema, from prot- + Greek nēma thread — more at nemat-
First Known Use
1857, in the meaning defined above