Rhamnus

Shrubs or trees, erect or spreading, unarmed or armed with thorns; bud scales present, buds glabrate. Leaves deciduous or persistent, present at flowering, alternate or opposite to subopposite, sometimes fascicled on short shoots; blade not gland-dotted; pinnately veined, secondary veins arching, not parallel. Inflorescences axillary, within foliage, fascicles or cymes, or flowers solitary; peduncles and pedicels not fleshy in fruit. Pedicels present. Flowers usually unisexual, rarely some bisexual (plants polygamous); hypanthium campanulate to cupulate, 2–3 mm wide, usually not circumscissile, rarely so at or just proximal to sepal bases; sepals 4–5, spreading, yellowish, yellowish green, green, or greenish white to white, ovate-triangular, thin, not keeled or crested adaxially; petals 0 or 4[–5], yellowish to cream, hooded, spatulate, not clawed; nectary thin, lining hypanthium; stamens 4–5, (rudimentary in pistillate flowers); ovary superior, 2–4-locular; styles 2–4, connate proximally. Fruits drupes; stones 2–4, longitudinally dehiscent. Seeds obovoid or oblong-obovoid, not beaked, longitudinally furrowed. x = 12.

Distribution

North America, Mexico, Central America, South America, Europe, e Asia, Africa.

Discussion

Species ca. 150 (14 in the flora).

Rhamnus in the flora area includes six species (species 9–14) introduced primarily from Europe and Asia. The native species for the most part are clearly delimited, but the R. crocea complex needs taxonomic study using modern techniques.

Selected References

Johnston, L. A. 1975. Revision of the Rhamnus serrata complex. Sida 6: 67–79.

Wolf, C. B. 1938. The North American species of Rhamnus. Rancho Santa Ana Bot. Gard. Monogr., Bot. Ser. 1.

Lower Taxa

Rhamnus alaternus, Rhamnus alnifolia, Rhamnus arguta, Rhamnus cathartica, Rhamnus crocea, Rhamnus davurica, Rhamnus ilicifolia, Rhamnus japonica, Rhamnus lanceolata, Rhamnus pilosa, Rhamnus pirifolia, Rhamnus serrata, Rhamnus smithii, Rhamnus utilis

Key

1	Leaves persistent, alternate, blades distinctly coriaceous; petals 0 (rarely 4 in R. pilosa).	> 2
2	Leaf blade secondary veins with basal pair diverging much more acutely than distal pairs; inflorescences cymes or flowers solitary; sepals 5; drupes dark red, becoming black, stones 3.	Rhamnus alaternus
2	Leaf blade secondary veins all diverging at nearly same angle; inflorescences fascicles or flowers solitary; sepals 4(–5); drupes red, stones 2.	> 3
3	Leaf blade margins entire or coarsely serrate, blunt-serrate, or spinulose-serrate.	> 4
4	Shrubs, 1.2–2(–4) m, usually armed with thorns; leaf blades 0.7–2 cm, elliptic to obovate or ovate; California mainland.	Rhamnus crocea
4	Shrubs or trees, 2.5–6(–10) m, unarmed; leaf blades (1.5–)2–5(–6) cm, broadly elliptic to oblong or ovate-oblong; California Channel Islands.	Rhamnus pirifolia
3	Leaf blade margins spinulose to spinose-dentate.	> 5
5	Branchlets stiff, glabrous or densely and softly hirtellous; both leaf blade surfaces glabrous.	Rhamnus ilicifolia
5	Branchlets flexible, sparsely to densely softly hirtellous; both leaf blade surfaces sparsely to densely softly hirtellous.	Rhamnus pilosa
1	Leaves deciduous, alternate, subopposite, or opposite, sometimes fascicled on short shoots, blades herbaceous to subcoriaceous; petals 0 or 4.	> 6
6	Leaves usually alternate, sometimes subopposite, blade secondary veins 4–11 pairs; shrubs or small trees 0.5–3(–4) m, unarmed.	> 7
7	Sepals 5; petals 0; drupe stones 3; shrubs 0.5–1(–1.5) m; wetlands and seeps.	Rhamnus alnifolia
7	Sepals 4; petals 4; drupe stones 2 (3 in R. arguta); shrubs or small trees 0.5–3(–4) m; drier habitats.	> 8
8	Petioles 10–28 mm; pedicels 10–24(–30) mm; drupe stones 3; leaf blades usually broadly ovate, ovate, oblong-ovate, or ovate-cordate, sometimes ovate-orbiculate, margins spinulose-serrate.	Rhamnus arguta
8	Petioles 3–8 mm; pedicels (0–)0.5–4 mm; drupe stones 2; leaf blades lanceolate, elliptic-lanceolate, oblong-lanceolate, elliptic, or elliptic-oblong, margins crenulate, crenulate-serrate, or serrulate.	> 9
9	Branchlets 6+ cm; leaf blade apices acute to acuminate; c, e United States.	Rhamnus lanceolata
9	Branchlets 2–5 cm; leaf blade apices acute to obtuse or rounded; w United States.	> 10
10	Bud scales thin, glossy, pale golden to yellowish; leaf blades usually lanceolate, sometimes elliptic-lanceolate or oblong-lanceolate, 3–8 cm, both surfaces glabrous.	Rhamnus smithii
10	Bud scales coriaceous, dull, dark red to reddish brown; leaf blades usually elliptic to elliptic-lanceolate or elliptic-oblong, 1.5–5(–5.5) cm, both surfaces minutely hirtellous.	Rhamnus serrata
6	Leaves usually opposite to subopposite, sometimes alternate or fascicled on short shoots, blade secondary veins 2–7(–8) pairs; shrubs or trees (1–)2–10 m, armed with thorns.	> 11
11	Leaf blade bases rounded to truncate or cordate; drupe stones 3–4.	> 12
12	Leaf blade margins crenate-serrate, secondary veins 2–4 pairs; pedicels 2–4 mm; drupes 5–6(–8) mm.	Rhamnus cathartica
12	Leaf blade margins spinulose-serrate, secondary veins (3–)4–5 pairs; pedicels 10–24(–30) mm; drupes 7–10 mm.	Rhamnus arguta
11	Leaf blade bases cuneate or acute; drupe stones 2.	> 13
13	Leaf blades obovate to elliptic-oblanceolate or broadly oblanceolate, adaxial surfaces usually minutely hirtellous at least along main veins on distal 1/3, rarely glabrous.	Rhamnus japonica
13	Leaf blades usually elliptic to oblong, oblong-obovate, obovate-elliptic, or oblong-lanceolate, sometimes ovate to obovate, rarely oblanceolate-elliptic, adaxial surfaces glabrous or sparsely pilose on veins.	> 14
14	Leaf blades green abaxially, abaxial surface with straight, colorless hairs on veins, secondary veins 3–5(–6) pairs; pedicels 10–20 mm.	Rhamnus davurica
14	Leaf blades yellowish green abaxially, abaxial surface with wavy or curled, yellow hairs along veins or at least in vein axils, secondary veins (4–)5–7(–8) pairs; pedicels 5–10 mm.	Rhamnus utilis