In typical ferns, the gametophyte is a prothallus that is most commonly a small green heart-shaped structure up to about 1 cm across that lives on the surface of damp soil (Fig. 7), sometimes in dense patches. The body of the prothallus consists of undifferentiated parenchyma cells with a thickened central part bearing rhizoids that absorb water. Antheridia and archegonia are produced on the underside. The antheridium is a rounded jacket containing 16–32 motile sperm. The archegonia are produced mainly near the notch, slightly later than the antheridia. Each archegonium has a neck of several cells surrounding two canal cells that degenerate at maturity, and an egg cell at their base, similar to those of bryophytes .
The eusporangiate ferns and royal ferns have larger and longer-lived prothalli than typical ferns and all have fungi associated with them acting as mycorrhizae ; some are colorless and subterranean. The heterosporous water ferns have much reduced gametophytes that develop within the spore wall, as in heterosporous lycopods . They develop in similar ways to Selaginella, although they are unrelated. In the floating group, the microspores aggregate together and hooked hairs develop that may aid attachment to the megasporangia.
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