Stems erect or ascending, rooting at most proximal nodes, glabrous. Roots not thickened basally, glabrous. Proximal cauline leaf blades ovate or lanceolate, 1.2-4.2 × 0.5-1.2 cm, base acute to truncate, margins entire or denticulate, apex acuminate to rounded. Inflorescences: bracts linear to lanceolate or oblanceolate. Flowers: receptacle glabrous; sepals 4-5, spreading or reflexed from base, 1.5-3 × 1-1.5 mm, glabrous or sparsely hirsute; petals 1-3, 1.5-2 × 0.5-1 mm; nectary scales glabrous. Heads of achenes hemispheric to cylindric, 2-8 × 2-3 mm; achenes 1-1.2 × 0.6-0.8 mm, ± tuberculate, glabrous; beak absent or nearly so, to 0.1 mm. Flowering spring (Apr-Jun). Ditches, ponds, and swamps; 0-300 m; Ala., Ark., Calif., Del., D.C., Fla., Ga., Ill., Ind., Ky., La., Md., Miss., Mo., N.J., N.Y., N.C., Ohio, Okla., Pa., S.C., Tenn., Tex., Va., W.Va. In most specimens of Ranunculus pusillus , the heads of achenes are hemispheric to short-ovate and only 2-3 mm. Occasional plants with cylindric heads of achenes 4-6 mm from the Gulf Coast states have been called R . pusillus var. angustifolius .
Annual; stems weak, ascending or erect, 1-5 dm, branched from the base, many-fld; lower lvs long-petioled, ovate or lance-ovate, 1-4 cm, basally obtuse or rounded; cauline lvs progressively narrower, most of them linear, scarcely differentiated into blade and petiole; pet 1-5, 2 mm, equaling or a little shorter than the sep; stamens 10 or fewer; achenes 1 mm, very shortly beaked. Ditches, muddy ground, and shallow water, mostly on the coastal plain; s. N.Y. to Fla. and Tex., n. in the interior to Mo., Ind., and O. May, June.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.