Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal regular, deeply 5-cleft; cor strongly bilabiate, the tube scarcely enlarged upward, the lips entire or inconspicuously toothed at the top; stamens 2, inserted near the top of the cor-tube; pollen-sacs distinct, one apparently lateral, the other terminal; fr elliptic, flat, 2–4-seeded, the thin sides torn loose by the recurving of the indurate margins; herbs with long-petioled lvs and axillary, cymose, sessile or subsessile, few-fld clusters of small, pink to purple or red fls subtended by foliaceous bracts. 150, warm regions.