Gleason, H.A. & Cronquist, A. (1963) Manual of Vascular Plants of Northeastern United States and Adjacent Canada D. van Nostrand Co, Inc. Princetown New Jersey.
Description of the genus
Shrubs or woody herbs and climbers. Leaves opposite, simple or compound. Flowers hypogynous, irregular, 5-merous; corolla large, campanulate, tubular; lobes shorter than the tube; stamens 2 or 4, inserted on the corolla tube and alternate with its lobes; disc often conspicuous. Fruit a capsule.