Marsileaceae

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 "pepperwort family" or as the "water-clover family" because the leaves of the genus ''Marsilea'' superficially resemble the leaves of a four-leaf clover. In all, the family contains 3 genera and 50 to 80 species with most of those belonging to ''Marsilea''.

Natural history

Members of the Marsileaceae are aquatic or semi-aquatic. Plants often grow in dense clumps in mud along the shores of ponds or streams, or they may grow submerged in shallow water with some of the leaves extending to float on the water surface. They grow in seasonally wet habitats, but survive the winter or dry season by losing their leaves and producing hard, desiccation-resistant reproductive structures.

There are only three living genera in the family Marsileaceae. The majority of species (about 45 to 70) 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 and neighboring parts of Argentina; it has only two leaflets per leaf. The third genus ''Pilularia'' grows widely in temperate regions of both the northern and southern hemispheres. Its leaves do not subdivide into leaflets but are slender and tapered to a point, so that it is often overlooked and mistaken for a grass. There are only about five species known.

The closest relatives of the Marsileaceae are the Salviniaceae, which are also aquatic and heterosporous. However, both of these other fern families float freely on the surface of ponds or lakes instead of rooting in soil or mud. The close relationship of these groups to the Marsileaceae is supported by both morphologic and molecular analysis, as well as by the discovery of an intermediate fossil named '' Hydropteris''. In general, the Salviniaceae and Azollaceae have a much better fossil record than the Marsileaceae. Until recently, '' Rodeites dakshinii'' was the oldest fossil member known; it is a preserved sporocarp containing spores, found in Tertiary chert of India. In 2000, the discovery of fossilized sporocarps from the Cretaceous of eastern North America was announced. 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 in Liaoning, China.

Morphology

The Marsileaceae share many of the basic structural characteristics common to most ferns, but the differences are more noticeable than the similarities. Species of this family have long, slender rhizomes that creep along or beneath the ground. Their fronds (leaves) grow in distinct clusters at nodes along the rhizome, with wide spacing between leaf clusters. As a result, the plants appear to be more stem than leaf, unlike other ferns.

Roots grow primarily from the same nodes as the leaves, but may also grow from other locations along the rhizome. The roots of ''Marsilea'' and ''Regnellidium'' are noteworthy for containing vessel elements. Vessels have also been found in the rhizome of two species of ''Marsilea''. These vessels have evolved independently of vessels in other groups of plants.

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. In ''Pilularia'', the leaves are narrowly cylindrical and taper to a point. Leaves of ''Regnellidium'' bear two broad leaflets, while leaves of ''Marsilea'' bear four leaflets at the tip. The four leaflets on the leaf of ''Marsilea'' are not borne equally. Instead, they are borne in pairs with one pair of leaflets attached slightly higher than the other. Thus in the developing leaf, the leaflets are folded more like the wings of a butterfly than like the leaflets of a clover.

As with other ferns, the leaves develop in a circinate pattern. They begin as small, tight spiral

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