This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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..
Perennial aquatic herbs, with opposite or whorled leaves, those of the submersed ones filiformly dissected, those of the emersed ones serrate to laciniate, and solitary peduncled showy heads with both tubular and radiate flowers. Involucre hemispheric, its bracts in 2 series, rather broad, distinct or nearly so, the outer ones smaller than the inner, often lax. Receptacle chaffy. Rays neutral, yellow. Disk-flowers perfect, fertile, their corollas tubular, 5-toothed. Anthers minutely sagittate at the base. Style-branches with subulate tips. Achenes nearly terete, truncate at both ends. Pappus of 3-6 long-subulate awns which are retrorsely barbed, except near the base. [Greek, large-toothed, referring to the pappus awns.]
Two known species, the following typical, the other occurring in Washington State.
Bidens Beckii Torr.; Spreng. Neue Entdeck. 2: 135. 1821.
M. Beckii Greene, Pittonia 4: 271. 1901.
M. nudata Greene, loc. cit. 1901.
Stems simple, or little branched, 2°-8° long. Submersed leaves sessile, 1'-2' long, repeatedly divided into numerous capillary segments; emersed leaves few, sessile, opposite, or sometimes in 3's, lanceolate or oblong, acute, serrate or laciniate, 1 - 1 1/2' long; heads solitary or few, short-peduncled, l'- 1 1/2' broad; involucre hemispheric, its bracts oval or oblong, obtusish, glabrous, the outer somewhat shorter than the inner; rays 6-10, obovate, or oblong, notched, golden yellow; achenes nearly terete, 5" - 7" long; pappus of 3-6 slender awns, downwardly barbed above, smooth below, divergent, 6"-12" long.
In ponds and streams, Quebec to New Jersey, west to Manitoba and Missouri. Aug.-Sept.