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..
Marsh or aquatic plants, the leaves commonly floating on the surface of shallow water, slender-petioled, 4-foliolate. Peduncles shorter than the petioles, arising from their bases or more or less adnate to them. Sporocarps ovoid or bean-shaped, composed of two vertical valves with several transverse compartments (sori) in each valve. [Name in honor of Giovanni Marsigli, an Italian botanist, who died about 1804.]
About 53 species, widely distributed. Besides the following 2 or 3 others occur in Texas.
Sporocarps glabrous and purple when mature.
Sporocarps densely covered with hair-like scales.
Marsilea quadrifolia L. Sp. PI. 1099. 1753-
Rootstock slender, buried in the muddy bottoms of shallow lakes or streams. Petioles usually slender, 2-5' high, or when submerged sometimes elongated to 1° or 2°. Leaflets mostly triangular-obovate, variable in outline, 3"-8" long, 2"-6" wide, glabrous or rarely with scattered hairs when young, the margins entire; sporocarps 2 or rarely 3 on a branching peduncle which is attached to the petiole at its base, covered with short yellowish-brown hairs when young, becoming glabrous and dark purple when mature; sori 8 or 9 in each valve.
Bantam Lake, Litchfield Co., Conn.; thence introduced into other parts of the country, from Massachusetts to Maryland. Native of Europe and Asia.
M. vestita Hook. & Grev. Ic. Fil. pl. 159. 1831.
Marsilea mucronata A. Br. Amer. Journ. Sci. (II.) 3: 55. 1847.
Rootstocks slender, creeping. Petioles slender,. 2'-5' high; leaflets similar to those of the preceding species, entire or toothed; sporocarps 2"-4" long, 2"-3" wide, with a short raphe, a short and blunt lower tooth and an acute and sometimes curved upper one, densely covered with soft spreading narrow hair-like scales or (in the forms known as M. mucronata) these short and appressed or almost wanting; sori 6-11 in each valve.
In wet sand or in shallow ditches, Florida to Kansas, Arizona and Mexico, California and British Columbia.