Herbs, to 20 cm. Rhizomes absent; stStolon buds with 1 root. Leaves floating or, in dense vegetation, emergent; blade 1.2--6 ´ 1.3--6.3 cm; primary veins forming 75--90° angle with midvein, broadly curving, aerenchyma confined to midvein region (not margin to margin as in Limnobium), individual aerenchyma space (located ca. 1 mm from either side of midvein) 0.1--0.5 mm across its longest axis, 0.1--0.5 mm wide, 1 mm from midvein. Flowers: staminate flowers 1--5 in each spathe; pedicel to 4 cm; stamens 9--12 in 4 whorls; filaments basally not obviously connate; pistillate flowers solitary; pedicels to 9 cm; styles 2-fid for less than ½ length. Seeds 1--1.3 mm. 2n = 28 (Netherlands). Flowering spring--fall. Ponds, bays of rivers; 10--50 m; introduced; Ont., Que.; N.Y.; Eurasia. Hydrocharis morsus-ranae was planted in ponds beside Dow´s Lake in the Central Experimental Farm Arboretum at Ottawa in 1932 (P. M. Catling and W. G. Dore 1982). It apparently escaped from these ponds; by 1939 it was found in the Rideau Canal and by 1967 in the St. Lawrence River from Montreal as far as Lake St. Peter. It had spread into Lake Ontario, Lake Erie, and a couple of localities in New York (Catling and Dore 1982).
Much like Limnobium spongia in aspect, but the lvs more consistently cordate-orbicular, less obviously spongy beneath, and with more broadly arching (even initially descending) lateral veins; dioecious; stolons with seasonally dimorphic terminal buds; pet ca 1 cm, 2-3 times as long as the sep, white with a yellow basal spot; outer (antesepalous) stamens basally connate with those of the third cycle; anthers ca 1 mm long and wide; styles 6, ca 4 mm overall; 2n=28. Slow-moving water, ditches, and pools; native of Europe, intr. and spreading, from Que. to s. Ont. and n. N.Y. along the St. Lawrence and Ottawa rivers and along lakes Erie and Ontario.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.