Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep usually with posterior auricles; pet somewhat unequal, the 2 lateral ones often bearded internally at the base, the lower one usually with a basal spur or sac and sometimes bearded at the throat; stamens 5, the 2 lowermost bearing appendages that extend into the spur of the lower pet; ours herbs with axillary or basal 1-fld peduncles bearing 2 small bracts near the middle. Most spp. produce normal petaliferous fls in spring, and very fertile cleistogamous fls in summer. All spp. with cyanic (rather than yellow) fls produce occasional white-fld forms. Hybrids between closely related spp. are common, and spp. 3–8 form an intergrading polyploid complex. 400, mainly N. Temp.