1. Nuphar microphylla (Persoon) Fernald, Rhodora. 19: 111. 1917.
Petit nénuphar jaune
Nymphaea microphylla Persoon, Syn. Pl. 2: 63. 1807; Nuphar kalmiana (Michaux) W. T. Aiton; N. minima (Willdenow) Smith
Rhizomes 1-2 cm diam. Leaves mostly floating, occasionally submersed; petiole flattened to filiform. Leaf blade abaxially often purple, adaxially green to greenish purple, broadly elliptic to ovate, 3.5-10(-13) × 3.5-7.5(-8.5) cm, 1-1.5 times as long as wide, sinus 2/3 or more length of midrib, lobes divergent and forming V-shaped angle; surfaces abaxially glabrous to densely pubescent. Flowers 1-2 cm diam.; sepals 5(-10), abaxially green to adaxially yellow toward base; petals broadly spatulate and thin, or notched and thickened; anthers 1-3 mm, shorter than filaments. Fruit yellow, green, brown, or rarely purple, mostly globose-ovoid, occasionally flask-shaped, 1-2 cm, smooth basally, faintly ribbed toward apex, deeply constricted below stigmatic disk, constriction 1.5-5 mm diam.; stigmatic disk red, 2.5-7 mm diam., with 6-10 deep crenations; stigmatic rays 6-11, linear, terminating 0-0.2 mm from margin of disk. Seeds ca. 3 mm. 2 n = 34.
Flowering summer-early fall. Ponds, lakes, sluggish streams, sloughs, ditches, and occasionally tidal waters; 0-400 m; Man., N.B., N.S., Ont., Que.; Conn., Maine, Mass., Mich., Minn., N.H., N.J., N.Y., Pa., Vt., Wis.; Europe; n Asia.
Intermediates between Nuphar microphylla and N . variegata , probably of hybrid origin, are treated as N . rubrodisca . A form with ten sepals ( Nuphar microphyllum forma multisepalum Lakela) occurs in northeastern Minnesota. Recent observations of the Eurasian N . pumila (Timm) de Candolle by C. B. Hellquist in Siberia suggest that Beal's lumping of N microphylla under N . lutea subsp. pumila (Timm) E. O. Beal should be further studied.