prothallus

from The Collaborative International Dictionary of English v.0.48

Prothallus \Pro*thal"lus\, n.; pl. {Prothalli}. [NL., fr. Gr. ?
   before + ? a young shoot.] (Bot.)
   The minute primary growth from the spore of ferns and other
   Pteridophyta, which bears the true sexual organs; the
   oophoric generation of ferns, etc.
   [1913 Webster]