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.

Helianthium Parvulum (Engelm.) Small. Dwarf Water-Plantain

Fig. 224

? 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:

45. 1909.

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.