Herbs, to 50 cm. Roots branched; stolon buds with 10 or more roots. Leaves floating or emersed in dense vegetation and when stranded; blade 1--10 ´ 0.9--7.8 cm; primary veins forming 30--80° angle with midvein, ascending, aerenchyma extensive, nearly margin to margin, individual aerenchyma space (located ca. 1 mm from either side of midvein) , 0.4--1.6 mm wide, 1 mm from midveinacross its longest axis. Flowers: staminate flowers with 9--12(--18) stamens; pistillate flowers with 3--4 petals; ovary 6--9-carpellate, locules 6--9; styles 2-fid nearly to base; ovules 200. Fruits 4--12 mm diam. Flowering summer--fall. Floating on slow-moving water of streams, bayous, and lakes or stranded along shore; 0--100 m; Ala., Ark., Del., Fla., Ga., Ill., Ky., La., Md., Miss., Mo., N.Y., N.C., Okla., S.C., Tenn., Tex., Va. No specimens have been seen from New Jersey, but although the species is to be expected there. Limnobium spongia has two leaf forms, often on the same plant. The floating leaves have a thick layer of aerenchyma on the abaxial surface; the emersed leaves lack such tissue. Flowering and fruiting are predominantly on individuals with emersed leaves. Following pollination, the peduncle becomes recurved, forcing the developing fruit below the water surface.
Lf-blades broadly ovate (especially when emergent) to deeply cordate-orbicular but usually acute, 3-7 cm long and wide, 5-7-veined, the lateral veins arcuate- ascending; floating lvs aerenchymatous and spongy toward the base beneath; pedicels 3-10 cm; pet white, linear or linear-oblong, ca 1 cm, not much longer than the slender sep; anthers elongate, ca 3.5 mm; stigmas conspicuous, 10-15 mm; fr 4-12 mm thick. In ponds and bayous; trop. Amer., n. along the Atlantic coastal plain to N.J. and up the Mississippi embayment to s. Ill.; also in n. Ind. and w. N.Y. June-Sept.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.