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RHAMNACEAE BUCKTHORN FAMILY

John O. Sawyer, Jr., except as noted

[Perennial herb] shrub, tree, generally erect, often thorny. Leaf: simple, generally alternate, often clustered on short-shoots; stipules generally present, occasionally modified into spines; generally petioled; blade pinnate-veined or 1–5-ribbed from base. Inflorescence: cyme, panicle, umbel, or flowers 1 or clustered in axils or on short-shoots. Flower: generally bisexual, radial; hypanthium subtending, surrounding, or partly fused to ovary; sepals 4–5; petals 0, 4–5, generally clawed; stamens 0, 4–5, alternate sepals, attached to hypanthium top, each generally fitting into a petal concavity; disk (0 or) between stamens, ovary, thin to fleshy, entire or lobed, free from ovary, adherent or fused to hypanthium; ovary superior or ± inferior, chambers [1]2–4, 1–2-ovuled, style 1, stigma entire or 2–3-lobed. Fruit: capsule, drupe.
50–52 genera, 950 species: especially tropics, subtropics some cultivated (Ceanothus; Frangula; Rhamnus; Ziziphus). [Richardson et al. 2000 Amer J Bot 87:1309–1324] —Scientific Editors: Steve Boyd, Thomas J. Rosatti.
Unabridged references: [Brizicky 1965 J Arnold Arbor 45:439–463; Richardson et al. 2000 Kew Bull 55:311–341]

Key to Rhamnaceae

RHAMNUS BUCKTHORN
Shrub, small tree, < 10 m. Stem: branches alternate, stiff or flexible; twigs generally not thorn-tipped; winter bud scales present, generally ± 3 mm. Leaf: scattered along branches or clustered on short-shoots, deciduous or evergreen; stipules generally deciduous; petioles generally glabrous; blade veins prominent or not. Inflorescence: flowers 1 or in cyme-like clusters in axils. Flower: unisexual (bisexual), generally on separate plants, generally < 3 mm; hypanthium bell-shaped to cup-like, 2–3 mm wide; sepals 4–5, thin, spreading, not keeled adaxially; petals 0 or 4–5; disk thin, adhering to hypanthium; ovary appearing superior or partly inferior, chambers 2–4, each 1-ovuled, style 1, stigma 2–4-lobed. Fruit: drupe, 2–3[4]-stoned.
110 species: temperate, few tropics; some of value in medicine or as dyes. (Greek: name for plants of this genus) [Bolmgren & Oxelman 2004 Taxon 53:383–390] W.H. Brewer collected Rhamnus cathartica L., considered invasive in parts of United States, in 1861, but it apparently never naturalized. Other taxa in TJM (1993) moved to Frangula.
Unabridged references: [Wolf 1938 Rancho Santa Ana Bot Gard Monogr 1]

Key to Rhamnus

R. pilosa (Curran) Abrams
NATIVE
Shrub, < 2 m. Stem: bark gray; branches few, generally flexible, generally ascending; twigs densely hairy. Leaf: evergreen; petiole 2–5 mm, hairy; blade 15–20 mm, ovate to round, thick, soft-hairy, abaxially flat to concave, base, tip rounded to acute, margin finely toothed, flat to rolled under, veins not prominent. Inflorescence: 1–6-flowered, soft-hairy; pedicel 2–4 mm. Flower: generally unisexual; hypanthium ± 2 mm wide; sepals 4; petals 0, 4. Fruit: 2-stoned, 6 mm, red.
Uncommon. Chaparral; 75–1650 m. s Peninsular Ranges (San Diego Co.). Jan–Mar [Online Interchange]
Unabridged synonyms: [Rhamnus crocea subsp. pilosa (Trel. ex Curran) C.B. Wolf; Rhamnus crocea Nutt. var. pilosa Trel. ex Curran]
Unabridged note: Sp. not known from Baja California or Arizona as reported in USDA Plants Database based on note in Abrams, L. 1960. An illustrated flora of the Pacific States. No specimens support the claim, but some specimens from RSA-POM approach Rhamnus pilosa morphologically. Expanded author citation: Rhamnus pilosa (Trel. ex Curran) Abrams

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Citation for the whole project: Jepson Flora Project (eds.) 2013. Jepson eFlora, http://ucjeps.berkeley.edu/IJM.html, accessed on Apr 18 2014
Citation for this treatment: [Author of taxon treatment] 2012. Rhamnus, in Jepson Flora Project (eds.) Jepson eFlora, http://ucjeps.berkeley.edu/cgi-bin/get_IJM.pl?tid=41071, accessed on Apr 18 2014

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CCH collections by month

Duplicates counted once; synonyms included.
Species do not include records of infraspecific taxa.
Blue line denotes eFlora flowering time.