(ˈ)prō+ noun
( plural prothallia )
Etymology: New Latin, from pro- (I) + thallus + -ium
1. : the gametophyte of a fern or other pteridophyte that is typically a small flat green thallus attached to the soil by rhizoids but is sometimes filamentous and branching, that occasionally forms a subterranean tuberous mass, or that rarely (as in the club mosses) develops within the megaspore by which it is produced
2. : any of various structures (as several cells of the pollen grain or in gymnosperms of the megaspore) that in seed plants correspond to the pteridophyte prothallium