Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal divided nearly to the base; cor tubular to rotate; filaments usually equal, attached to the cor-tube near its base; style shortly to deeply bifid; ovules 2–many on each of the 2 intruded, partition-like placentas; capsule 2-valved; taprooted herbs (ours annual or biennial), usually hairy, with entire (in extralimital spp.) to pinnately dissected lvs and mostly blue or white fls in 1 or more helicoid cymes that elongate and straighten with maturity. 150+, New World, esp. w. U.S.