1. Plant truly aquatic, the submersed leaves dissected in a bipinnate pattern into filiform segments (midvein present, the lateral segments again dissected), frequently detaching readily from the stem; petals white.

R. aquatica

1. Plant terrestrial or aquatic but even if in water the leaves with definite flat lobes (not bipinnately dissected) and not falling from the stem; petals yellow.

2. Uppermost leaves (including those at base of branches of the inflorescence) deeply pinnatifid (lobed nearly or quite to the midrib); fruit linear, the ovary soon becoming more than 5 times as long as wide, but not (in Michigan) forming ripe seed; petals 2.5-3.7 mm long, distinctly exceeding the sepals.

R. sylvestris

2. Uppermost leaves often merely toothed or shallowly lobed; fruit nearly spherical to short-oblong or cylindrical, no more than 5 times as long as wide, normally setting copious seed; petals 0.5-2.3 mm long, barely if at all longer than the sepals (often shorter).

3. Pedicels of mature fruit somewhat curved or arching downwards; petals ca. 1 mm long or shorter; leaves all glabrous beneath; mature fruit ca. 2-5.2 mm long, up to 1.7 mm thick, mostly broadest well below the middle, strongly tapering to ± acute apex.

R. curvipes

3. Pedicels of mature fruit usually ± straight, ascending or spreading; petals ca. (1-) 1.5-2.3 mm long; leaves glabrous or hispid beneath; mature fruit thicker, the largest on a plant (1.6-) 2-2.7 mm thick at or near the middle, not strongly tapering, the apex blunter.

R. palustris