Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal subtended by 1 or 2 minute bractlets, tubular, split down the lower side, irregularly toothed; cor tubular, curved downward, very irregular, the upper lip straight, concave, entire or nearly so, the lower lip decurved, 3-lobed; stamens and style about equaling the cor; fr ovoid, tipped by the persistent style and capitate stigma; unbranched herbs, the stout stem mostly or wholly concealed by the numerous, fleshy, overlapping lf -scales; fls numerous, subsessile, crowded in a dense spike, each subtended by a bract smaller than the lf-scales. 2, N. Amer.