This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Floating annual plants with slender stems bearing rather broad 2-ranked leaves, these finely papillose on the upper surface. Sporocarps globose, depressed, 9-14-sulcate, membranous, arranged in clusters, 1 or 2 of each cluster containing 10 or more sessile megasporanges, each containing a single megaspore, the others containing numerous smaller globose pedicelled microsporanges with very numerous microspores. [Name in honor of Antonio Maria Salvini, 1633-1729, Italian scientist.]
Marsilea natans L. Sp. PI. 1099. 1753. Salvinia natans Hoffm. Deutschl. Fl. 2: 1. 1795.
Leaves oblong, rather thick, obtuse or emarginate at the apex, rounded or cordate at .the base, entire, spreading, 6'-12' long, pinnately veined, bright green and papillose above, the lower surface densely matted with pellucid brown hairs; sporocarps 4-8 in a cluster, the upper ones containing about 10 megaspo-ranges, each containing a single megaspore, the remainder containing numerous microsporanges each with numerous microspores; megaspores marked with 3 obtuse lobes, these meeting at the apex.
Bois Brule Bottoms, Perry Co., Missouri, and near Minneapolis, Minn. Introduced into ponds on Staten Island, N. Y. Reported by Pursh in 1814 from central New York, but his exact station is unknown. Widely distributed in Europe and Asia.