Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls perfect; cal small, campanulate, truncate or 4-toothed; cor salverform, 4-lobed; stamens 2, included in our spp.; fr a 2-locular capsule with 2 seeds per locule; much branched deciduous shrubs with simple, entire lvs and fragrant fls in dense mixed panicles. 30, Eurasia.