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; cor broadly campanulate to salverform; stamens included; ovary superior; style short; capsule many-seeded; ornamental shrubs, usually with serrate lvs and numerous fls in heads or panicles. 100, mainly of warm reg.