Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - 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.
water horehound, bugle-weed