Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal-tube campanulate to cupulate, the lobes triangular to linear, often unequal; cor irregular, the tube somewhat oblique, the 5 lobes shorter than the tube, all equally distinct, broadly rounded; stamens 4, paired, the lower pair the longer; filaments slender, villous; anthers pubescent, each sac tipped with a short awn at base; style slender; stigma 1, ovoid- capitate; fr ovoid to ellipsoid, loculicidal; seeds winged except in A. pedicularia; large, branching hemiparasitic herbs, the principal lvs opposite, entire to deeply lobed, the upper reduced and often scattered, subtending large, yellow, solitary, pediceled fls. 9, e. N. Amer., and 1 Mex.