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..
Annual or perennial scapose marsh or aquatic herbs. Leaves erect or ascending, or floating, narrow and gradually narrowed into the petiole or broad and deeply cordate at the base, 3-several-ribbed. Scapes as long as the leaves or longer, terminating in a few-flowered whorl or a many-flowered panicle, the pedicels spreading or recurving in fruit. Flowers perfect. Sepals 3, broad, embracing the fruit-head or reflexed beneath it. Petals 3, mainly white or pink, about as long as the sepals. Stamens 6 or 9; filaments elongate; anthers very short, often broader than long. Carpels relatively few, borne in few series on an elevated receptacle. Style not apical, minute; stigma acute. Achenes forming a globular or depressed head, turgid, crested-ribbed, obscurely beaked or beakless. [Name from the Greek, meaning sunflower.]
Two known species, the following, and one in Cuba. Type species: Echinodorus parvulus Engelm.
? Alisma tenellum Mart.; R. & S. Syst. Veg. 7:
1600. 1830. Echinodorus parvulus Engelm. in A. Gray,
Man. Ed. 2, 438. 1856. ?Echinodorus tcnellus Buchenau, Abh. Nat.
Gesell. Bremen 2: 18. 1868. Helianthium tenellum Britton, Man. Ed. 2, 54.
1904. Helianthium parvulum Small, N. A. Fl. 171:
Plants 6' tall or less; leaves linear to elliptic or oblong, 4"-15" long, acute or acutish at the apex, 3-veined, gradually narrowed into the slender petioles which usually somewhat exceed the blade in length; scapes solitary or few together, mostly as long as the leaves or longer; pedicels mostly 2-8, recurved in fruit, 1 1/4"-2 1/2" long; sepals orbicular-ovate or deltoid-ovate, 3/4"- 2" long; petals suborbicular, about as long as the sepals, emarginate at the apex; fruit-heads globular, 1 1/2" -2" in diameter, embraced by the persistent calyx; achenes 1/2"-3/4" long, the ribs obscurely crested.
In mud and shallow water, Massachusetts to Western Ontario, Minnesota, Florida, Texas and Mexico. Also in Cuba. April-Aug. This species was referred in the first edition of this work to Alisma tenellum Mart, a plant similar in habit, which appears to be confined to South America; it has been regarded by other authors as an Echinodorus.