Source : Webster's Revised Unabridged Dictionary (1913)

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
   o["o]phoric generation of ferns, etc.

Source : WordNet®

prothalli
     See {prothallus}