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, the narrow lobes much longer than the tube; cor large, funnelform, the tube equaling or much longer than the campanulate or obconic throat, the limb slightly oblique, spreading, subequally 5-lobed; stamens 4, weakly united in pairs toward the base, the lower pair somewhat the longer; a staminodium commonly present; fr clavate or obovate; seeds 3–8 per locule, compressed, suborbicular; ours perennial herbs with large blue- lavender fls in axillary few-fld cymes or terminating short axillary branches. 300+, mostly trop. and warm-temp.