Marsileaceae is a small family of heterosporous aquatic and semi-aquatic ferns, though at first sight they do not physically resemble other ferns. The group is commonly known as the "pillwort family" after the genus Pilularia (pillwort), "pepperwort family" or as the "water-clover family" because the leaves of the genus Marsilea superficially resemble the leaves of a four-leaf clover. The family contains three genera; most of the species are in Marsilea. It is sister to the Salviniaceae, which is also aquatic and heterosporous. Fossils of the family are known as far back as the Triassic. The family is distinctive in producing its spores inside sporocarps, which are bean-shaped structures borne near the bases of the leaves. Some species of Marsilea are grown in garden pools or aquaria.
Taxonomy
There are only three extant genera in the Marsileaceae. The majority of the species belong to the genus Marsilea, which grows worldwide in warm-temperate and tropical regions. Marsilea can be distinguished from the other two genera by the presence of four leaflets on each leaf, although some species occasionally produce six leaflets per leaf. A second genus Regnellidium includes a single living species that grows only in southern Brazil The close relationship of the families is supported by both morphologic and molecular analysis, In general, the Salviniaceae and Azollaceae have a much better fossil record than the Marsileaceae. Rodeites dakshinii is a preserved sporocarp containing spores, found in Tertiary chert of India. These fossils were assigned to the species Regnellidium upatoiensis, and pushed the known history of the Marsileaceae back into the Mesozoic. Other remains include Regnellites nagashimae from the Upper Jurassic or Lower Cretaceous of Japan. The fossils include leaves with visible veins, as well as sporocarps. The currently oldest known member of the family is Flabellariopteris, described in 2014 from isolated leaves dating to the Late Triassic (237–201 mya) in Liaoning, China.
Morphology
The Marsileaceae differ from most ferns in having long, slender rhizomes that creep along or beneath the ground, and in not having finely-divided pinnate leaves. Their fronds (leaves) grow in distinct clusters at nodes along the rhizome, with wide spacing between leaf clusters.
Roots grow primarily from the same nodes as the leaves, but may also grow from other places along the rhizome. The roots of Marsilea and Regnellidium contain vessel elements. These have evolved independently of vessels in other groups of plants.
thumb|left|Leaves of the Hawaiian species Marsilea villosa
The leaves are the most easily observed characteristic for the Marsileaceae; they have a long slender leaf stalk ending in zero, two, or four (occasionally six) leaflets. The number of leaflets differs among the three genera and can therefore be used for identification.
As with other ferns, the leaves develop in a circinate pattern, meaning that they unroll as they mature. Each growing season, only one sporocarp typically develops per node along the rhizome near the base of the other leaf-stalks, though in some species of Marsilea there may be two or occasionally as many as twenty. although they have much shorter stalks than the vegetative leaflets. Inside the sporocarp, the modified leaflets bear several sori, each of which consists of several sporangia covered by a thin hood of tissue (the indusium). Each sorus includes a mix of two types of sporangium, each type producing only one of two kinds of spores. Toward the center of each sorus and developing first are the megasporangia, each of which will produce a single large female megaspore. Surrounding them at the edge of the sorus and developing later are the microsporangia, each of which will produce many small male microspores. The spores remain dormant inside the sporocarp through unfavorable conditions, but when conditions are suitable and wet, the sporocarp will germinate. It splits into halves, allowing the tissue coiled inside to become hydrated. As this internal tissue swells with water, it pushes the halves of the hard outer covering apart, and emerges as a long gelatinous worm-like sorophore. The sorophore is a sorus-bearing structure unique to the Marsileaceae; it may extend to more than ten times the length of the sporocarp inside which it was coiled. This extension carries the numerous spore-producing sori attached along each side of the sorophore out into the water.
Human uses
Some species of Marsilea are cultivated in garden pools or aquaria.