Cal evidently to obscurely 4-5-nerved, sometimes with additional lesser nerves, the 4-5 teeth equal or nearly so; cor small, the tube short, internally hairy at the throat, the limb nearly regularly 4-5-lobed, when 4-lobed the upper lobe tending to be broader than the others and emarginate; fertile stamens 2, slightly exserted, the upper pair obsolete or represented by small staminodes; pollen-sacs parallel, separately attached to the short connective; nutlets widened upward, bearing a corky ridge along the lateral angles and often across the top, the outer surface smooth and nearly plane, the inner convex and commonly glandular; rhizomatous and/or stoloniferous perennials, scarcely aromatic, with serrate to pinnatifid, wholly cauline lvs and numerous dense axillary clusters of small white fls in the summer. Our spp. are habitally much alike, 1-10 dm, growing in wet places. 14, widespread but mainly N. Amer.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.