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..
Slender aquatic herbs, with linear entire verticillate leaves, and solitary (rarely 2-4) discoid peduncled terminal heads of small purplish flowers. Involucre broadly campanulate or hemispheric. Bracts imbricated in about 2 series. Receptacle conic, naked. Corolla regular, its tube short, its limb campanulate, 5-lobed. Anthers obtuse at the base. Style-branches slender, obtuse. Pappus of 5 broad cartilaginous obtuse scales. Achenes 5-angled. [Greek, hard scale, referring to the pappus.]
A monotypic genus of eastern North America.
Perennial; stem simple, decumbent at the base, erect or ascending, glabrous or slightly pubescent, 1°-2° long, leafy. Leaves sessile, verticillate in 4's-6's, linear, 1-nerved, 4"-12" long, \"-\" wide, or the submerged ones filiform; head about 5" broad; bracts of the involucre linear-oblong, acutish, usually puberulent.
In shallow ponds and streams, New Hampshire to Florida. July-Sept.