§ 1. Styles thickened and glandular toward the base; achenes glabrous, numerous; inflorescence cymose.
[*] Style nearly basal; stamens 25–30; perennial glandular-villous herbs, with pinnate leaves, and rather large white or yellow flowers.
1. P. argùta, Pursh. Stems erect, usually stout (1–4° high), brownish-hairy, clammy above; leaflets 7–11, oval or ovate, cut-serrate, downy beneath; cyme strict and rather close; stamens mostly 30, on a thick glandular disk.—Rocky hills, N. Brunswick to N. J., Minn., Kan., and westward.
[*][*] Style terminal; flowers small, yellow; leaves pinnate or ternate.
[+] Annual or biennial; leaflets incisely serrate, not white-tomentose; stamens 5–20.
2. P. Norvégica, L. Stout, erect, hirsute (½–2° high); leaves ternate; leaflets obovate or oblong-lanceolate; cyme rather close, leafy; calyx large; stamens 15 (rarely 20).—Lab. to N. J., west to Minn. and Kan. (Eu.)
3. P. rivàlis, Nutt. More slender and branched, softly villous; leaves pinnate, with two pairs of closely approximate leaflets, or a single pair and the terminal leaflet 3-parted; leaflets cuneate-obovate or -oblong; cyme loose, often diffuse, less leafy; calyx small; petals minute; stamens 10–20 (rarely 5).—Neb. to Mo. and N. Mex., and westward.
Var. millegràna, Watson. Leaves all ternate; stems erect, or weak and ascending; achenes often small and light-colored.—Minn. to Mo., N. Mex., and westward.
Var. pentándra, Watson. Leaves ternate, the lateral leaflets of the lower leaves parted nearly to the base; stamens 5, opposite to the sepals.—Iowa, Mo., and Ark.
4. P. supìna, L. Stems decumbent at base or erect, often stout, leafy, subvillous; leaflets pinnately 5–11, obovate or oblong; cyme loose, leafy; stamens 20; achenes strongly gibbous on the ventral side. (P. paradoxa, Nutt.)—Minn. to Mo., and westward; also eastward along the Great Lakes.—Var. Nicollétii, Watson. Slender; leaflets mostly but 3; inflorescence much elongated, leafy, and falsely racemose.—Devil's Lake, Minn.