Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal deeply divided into triangular or lanceolate lobes; cor campanulate, short-cylindric, or urceolate, with short lobes, bearing within a tooth or scale near the base of the tube opposite each lobe; filaments very short; anthers lance-triangular, connate, adherent to the stigma and prolonged into a cone beyond it; ovaries 2, subtended by 5 nectaries; ovules numerous; seeds numerous, comose; colonial perennial herbs with tough fibrous bark, mostly with erect or ascending stems, opposite mucronate lvs, and small white or pink fls in terminal cymes. 3, N. Amer. Hybridization is rampant in the group. Hybrids and hybrid segregates derived from A. androsaemifolium × either A. cannabinum or A. sibiricum are called A. ×floribundum Greene (A. medium).