Gard. Dict. Abr. ed. 4, vol. 1. 1754.
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.
North America, Mexico, West Indies, Central America, South America, Europe, e Asia, n Africa.
Species ca. 50 (7 in the flora).
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.
|1||Leaves persistent, semideciduous, or deciduous, blades distinctly coriaceous.||> 2|
|2||Leaves persistent; drupes 10–15 mm.||Frangula californica|
|2||Leaves semideciduous or deciduous; drupes 5–10 mm.||> 3|
|3||Leaf blades glaucous adaxially when fresh; Plumas County, California.||Frangula purshiana|
|3||Leaf blades not glaucous adaxially; Arizona, Colorado, Nevada, Utah.||Frangula obovata|
|1||Leaves deciduous, blades herbaceous to subcoriaceous.||> 4|
|4||Eastern North America, as far west as Manitoba, Nebraska, and central Texas.||> 5|
|5||Inflorescences umbels, pedunculate; drupes 8–10 mm; leaf blades oblong to elliptic or obovate-elliptic, margins serrulate to crenulate to nearly entire; drupe stones 3.||Frangula caroliniana|
|5||Inflorescences fascicles, sessile; drupes 6–8 mm; leaf blades broadly elliptic-obovate to broadly elliptic or broadly oblong, margins entire; drupe stones 2(–3).||Frangula alnus|
|4||Western North America, as far east as Colorado, New Mexico, trans-Pecos Texas, and Wyoming.||> 6|
|6||Inflorescences fascicles, sessile.||Frangula alnus|
|6||Inflorescences umbels, pedunculate.||> 7|
|7||British Columbia, California, Idaho, Montana, w Nevada, Oregon, Washington.||> 8|
|8||Leaf blades 1.5–8.5 cm; drupe stones 2(–3); inflorescences (2–)4–15-flowered; California, Nevada, Oregon.||Frangula rubra|
|8||Leaf blades 6–15 cm; drupes stones 3; inflorescences 10–25-flowered; British Columbia; California, Idaho, Montana, Nevada, Oregon, Washington.||Frangula purshiana|
|7||Arizona, Colorado, s Nevada, New Mexico, Texas, Utah.||> 9|
|9||Leaf blades elliptic to oblong, elliptic-ovate, or narrowly ovate, 1.6–2.6(–2.9) times longer than wide, ± herbaceous, secondary veins (8–)9–13 pairs; se Arizona, New Mexico, Texas.||Frangula betulifolia|
|9||Leaf blades obovate to oblong-obovate or oblong, 1.2–1.8(–2.5) times longer than wide, subcoriaceous, secondary veins (5–)6–8(–9) pairs; n Arizona, Colorado, Nevada, Utah.||Frangula obovata|