Plants heterosporous. Sori enclosed within a sporocarp, which represents a modified, folded pinna with connate margins, each sorus bearing both megasporangia and microsporangia. Sporangia thin-walled, with a vestigial annulus or none. Aquatic or semi-aquatic plants, rooted in the substrate.

MARSILEACEAE Mirbel

Perennial herbs with long thin hairy rhizomes. Fronds borne singly but often close together, spirally coiled when young, of two kinds, sterile and fertile, the sterile ones filiform, the fertile ones small, globose and shortly stalked. Sporangia borne in globose or ovoid-oblong, hard, hairy sporocarps at the base of the stipe, each sporocarp containing two or more sori, each of which is surrounded by an indusium; the sporangia of two kinds, both occurring in the same sorus, the microsporangia above the megasporangia, developing in regular sequence from the apex to the base of the sorus, the microsporangia with numerous microspores, the megasporangia with one megaspore. Male prothallus of two vegetative cells and two antheridia, each producing 16 multiciliate spermatozoids. Female prothallus with many vegetative cells and a single archegonium situated on a projection from the megaspore.

Contains 3 genera and about 73 species, cosmopolitan