The Marsileaceæ, a natural tribe of small perennial aquatic herbs, have a filiform creeping rhizome with alternate erect leaves, curled in vernation like those of the ferns. The sporangia are enclosed in oval or spherical leathery capsules, or receptacles, which contain two dissimilar forms of reproductive organs—sporangia and antheridia, and are sessile, or nearly so, on the rhizome at the base of the leaves, whence the general name of Rhizospermæ. The Pilularia globulifera, or Pillwort, the only British species of its genus, may be taken as a type. Its rhizome creeps over sand or mud, at the margins of lakes and pools, where it is always submerged, or in sandy or gravelly places, which are only occasionally overflowed. At regular intervals the rhizome sends off a tuft of roots and a tuft of leaves opposite to them; the leaves are smooth, erect, and very slender, in deep water almost hair like, varying from four to five inches in height. The solitary globular receptacles, brown, hairy, and about the size of a small peppercorn, spring from the axils of the leaves, supported on so short a stem that they appear to be sessile. Fig. 71 shows an Algerian species of its natural size. The receptacles are divided by cross partitions into two or more cells, and separate at maturity into four equal valves. Each cell has a sort of placenta to which the sporangium and antheridia are attached; and its upper half is lined with three minute, sessile, obovate, yellowish bodies, which are the antheridia; the other half is occupied by a larger, roundish, or oblong sessile sporangium, containing one spore, which has a firm outer coat, tapering to a point, and leaving a cavity at the top of the nucleus. According to Hofmeister, this cavity becomes filled with cellular tissue, constituting a conical prothallus confluent with the nucleus. A single archegonium is formed in the centre, the orifice of which corresponds with the apex of the prothallus. The antheridia contain numerous granules, from which long spiral delicate spermatozoids are ultimately developed.^[76] The embryos of the young plants push forth their radicle in one direction to fix them to the soil, and a frond or leaf in the opposite direction. Thus these minute plants, approaching so nearly to a monocotyledon, exhibit a high organization, only inferior to that of the Lycopods.

The group contains the genera Marsilea, which has leaves made up of cuneiform lobes, and resembling those of some leguminous plant; and Salvinia and Azolla, both of which consist of small floating plants, which mostly occur in tropical or subtropical countries.