The development of the sporocarp shows that it corresponds to a pinna, although when mature it may appear to occupy a ventral position in relation to the vegetative portion of the leaf.
The sori are developed in depressions and are thus protected within the resistent outer wall of the sporocarp. There are usually four sori in Pilularia, while in Marsilia they form two longitudinal rows.
Enclosed within the sporocarp they can endure a period of drought, but on the return of moist conditions are extruded from the sporocarp by the swelling of a special mucilaginous tissue and the spores become free.
The resemblance of the sporocarp-like bodies - discovered by Nathorst in association with Rhaetic Sagenopteris leaves, and more recently figured by Halle under a new generic name (Hydropterangium) - to the sporocarps of Marsilia is an argument in favour of including Sagenopteris in the Hydropterideae.