Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal campanulate, variously lobed; cor tubular, the tube usually downcurved, much longer than the lobes; stamens about equaling the cor-tube; fls in racemes or spikes or solitary. 100 +, cosmop.