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 deeply 4-lobed; cor deeply 4- parted, the short tube campanulate, the lobes oblong, widely spreading; stamens 2, very short, borne on the short cor-tube; fr a bilocular, many-seeded capsule; much branched shrubs with simple, entire or toothed lvs. 7, Eurasia.
Several spp. are commonly cult. and sometimes persist, but apparently do not become naturalized. The most common, with many cultivars, are F. viridissima Lindl., suberect, with cross-chambered pith, merely toothed lvs, and rather light yellow or greenish-yellow fls; F. suspensa (Thunb.) Vahl, more arching, with hollow branches solid at the nodes, toothed or often 3-parted lvs, and golden yellow fls; and their hybrid, F. ×intermedia Zabel.