1) Prothallium 
a gametophyte body, usually flattened and delicate; e.g. in ferns and fern allies.
• (n.) The minute primary growth from the spore of ferns and other Pteridophyta, which bears the true sexual organs; the oophoric generation of ferns, etc.
A gametophyte body, especially in ferns and related plants. ... (09 Oct 1997) ... 
Independent gametophyte phase of horsetail or fern.
Pro·thal'lus noun ; plural Prothalli . [ New Latin , from Greek ... before + ... a young shoot.] (Botany) The minute primary growth from the spore of ferns and other Pteridophyta , which bears the true sexual organs; the oöphoric generation of ferns, etc. 
prothallium.