Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal 10–13-nerved, regular or weakly 2-lipped, the broadly triangular to subulate teeth equal or unequal; cor with short tube and nearly regularly 4-lobed limb, the upper lobe formed by fusion of 2, tending to be broader than the others and often apically emarginate, rarely and casually the cor equally 5-lobed; stamens 4, straight, somewhat divergent, exsert, about alike, the pollen-sacs parallel; rhizomatous perennial herbs with toothed, wholly cauline lvs and small, blue to lavender or white fls in the axils of lvs or in terminal "spikes" or heads. Many spp. produce small-fld pistillate plants as well as normal perfect-fld ones. Ca 25 spp., mainly of Eurasia and Australia, 1 circumboreal. The European spp., long in cultivation, have given rise through hybridization, polyploidy, and other kinds of chromosomal aberration to numerous ± stabilized additional populations that have become established in the wild as well as being retained in cult.