Buckthorn [Probably from Latin frango, to break, and -ula, diminutive, alluding to brittleness of twigs] Buckthorn [Probably from Latin frango, to break, and -ula, diminutive, alluding to brittleness of twigs]

John O. Sawyer†

Guy L. Nesom

Rhamnus Linnaeus sect. Frangula (Miller) de Candolle; Rhamnus subg. Frangula (Miller) Gray

Shrubs or trees, unarmed; bud scales absent, buds hairy. Leaves usually deciduous, sometimes persistent, alternate [rarely opposite], rarely fascicled on short shoots; blade not gland-dotted; pinnately veined, secondary veins nearly straight, parallel. Inflorescences axillary, within foliage, umbels or fascicles, or flowers solitary; peduncles and pedicels not fleshy in fruit. Pedicels present. Flowers bisexual; hypanthium cup-shaped, 1–3 mm wide, usually circumscissile far below sepal bases, rarely not circumscissile; sepals (rarely 4–)5, usually ± erect, sometimes spreading, yellowish to green or white, ovate-triangular, fleshy, keeled adaxially; petals (rarely 4–)5, yellowish, hooded, broadly obovate to obcordate, clawed; nectary thin, lining hypanthium; stamens (rarely 4–)5; ovary superior, (2–)3-locular; style 1. Fruits drupes; stones 2–3(–4), indehiscent but open at base. Seeds obovoid to lenticular, with cartilaginous beak protruding through opening in stone, not furrowed. x = 20–26.

Species ca. 50 (7 in the flora): North America, Mexico, West Indies, Central America, South America, Europe, e Asia, n Africa.

The difference between Frangula and Rhamnus has long been recognized, but treatments in taxonomic rank have been inconsistent. Frangula was treated within Rhamnus by M. C. Johnston and L. A. Johnston (1978), who noted that Frangula and Rhamnus are more closely related to each other than to other taxa. This observation has been corroborated by molecular studies that show them as sister taxa (J. E. Richardson et al. 2000; K. Bolmgren and B. Oxelman 2004). Frangula was also included within Rhamnus by D. Medan and C. Schirarend (2004) and by Chen Y. L. and C. Schirarend (2007), but various other recent treatments, as here, have maintained them as separate genera, emphasizing the differences outlined above in couplet 13 of the key to genera.

The key to species emphasizes geography as a primary character, reflecting the close similarities among the species and allowing morphological contrasts between sympatric taxa.