Stems glabrous. Leaves laminate and filiform-dissected; laminate leaf blades reniform, 3-parted, 0.4-1.1 × 0.7-2.3 cm, segments obovate or fan-shaped, shallowly cleft, margins crenate; filiform-dissected leaves with stipules gradually tapering upward, connate for their whole length. Flowers: receptacle hispid, rarely glabrous; sepals spreading or reflexed, 2-4 × 1-2 mm, glabrous; petals 5, 4-7 × 1-5 mm; style 0.1-0.8 mm. Fruiting pedicels usually recurved. Heads of achenes hemispheric to ovoid, 2-4 × 2-5 mm; achenes 1-2 × 0.8-1.4 mm, glabrous or hispid; beak persistent, filiform, 0.1-1.2 mm.
Ranunculus aquatilis is very variable. In the past it has often been treated as three or four species and many varieties. These segregate taxa have been based on the size, petiolation and rigidity of the leaves (now known to be primarily under environmental control), petal size and curvature of the fruiting pedicel (both sometimes variable along single stem), and number of achenes per head and length of achene beak (which vary continuously and are not correlated with one another). Unless reliable characters can be found, only two of the taxa recognized in older floras can be maintained.
Welsh et al. 1993, Kearney and Peebles 1980
Duration: Perennial Nativity: Native Lifeform: Forb/Herb General: Aquatic perennial with glabrous stem 1-2.5 mm wide, 20-80 cm long, submersed or floating and rooting at the nodes, stems 1-2.5 mm wide. Leaves: Alternate and cauline, submersed ones dissected into filiform segments, blades 1-3 cm long, on petioles 3-18 mm long, floating ones simple but deeply 3-parted and 1-2.5 cm broad by 1-1.5 cm long on petioles 1-3 cm long. Flowers: Solitary on pedicel 1-6 cm long, recurved at tip, 5 sepals, green or purplish, 3-5 mm long, glabrous, deciduous, 5 white petals, 1.5-2 mm long, subglobose fruiting head 4-6.5 mm high, 4-6 mm wide. Fruits: Achene transversely ridged, glabrous to somewhat hispid, especially dorsally, beaks absent or minute. Ecology: Found in ponds or streams from 4,500-9,000 (1372-2743 m); flowers May-July. Notes: Distinctive for its minute buttercup white flowers poking above the surface of the water. Ethnobotany: The entire plant was boiled and eaten. Etymology: Ranunculus is from Latin rana, for little frog, while aquatilis means growing in or near water. Synonyms: Batrachium aquatile, Ranunculus aquatilis var. hispidulus, R. aquatilis var. typicus, R. trichophyllus var. hispidulus Editor: SBuckley, 2010