Frangula Mill.
Trees or shrubs, erect, plants unarmed, bud scales absent. Leaves alternate (rarely some opposite, not in Mesoamerica), petiolate, venation pinnate, lateral veins without stripes, without glands at junction of leaf blade and petiole, margins generally toothed, each tooth with a small apical gland (sometimes easily dislodged), stipules free, borne laterally at base of petiole, un-lobed. Inflorescences axillary, pedunculate simple or a briefly compound cyme with first branch strongly divaricate or appearing more or less umbelliform or a compound cyme with one arm of cyme umbelliform and the other arm continuing as a rachis which develops another umbelliform cluster, or sessile fascicles of flowers, or flowers solitary, flowers pedicellate, bracteoles at base of pedicels, usually lost before anthesis. Flowers bisexual, 5-merous. Hypanthium tubular, funnelform or campanulate, circumscissile or not. Sepals triangular, adaxially keeled, sepals, petals and stamen filaments lost with apical half of circumscissile hypanthium or, less frequently, hypanthium not circumscissile and sepals, petals and stamen filaments tardily and irregularly dehiscing along rim of hypanthium. Petals usually smaller or equal to sepals in length, concave, claw equal or shorter than limb, limb apically emarginated to deeply bi-lobed, white or greenish-white, forming hoods over anthers or folded laterally around stamens at anthesis. Stamens opposite petals, slightly longer than petals at maturity.Disk lining the inner surface of the hypanthium, hypogynous, relatively thin, nectiferous, glabrous, entire. Ovary superior, 3- (rarely 2-) locular, style entire, stigmas 3 (rarely 2), small. Fruit fleshy drupe, globose or obovoid,with 3 (rarely 2) lines often obvious when young, with 3 (rarely 2) indehiscent stones,seeds 1 per stone, the seed with a hard cartilaginous thickening that protrudes through a basal opening in the stone with two inward curving horns as a beak-like rostrum.
Aprox. 50 species, Canada to S. America, Europe, and Asia; 1 species with 2 varieties is known from Nicaragua and another species is expected. Johnston and Johnston (1978) treated Frangula as a subgenus in the genus Rhamnus. Frangula is recognized at the generic level here based on molecular studies (Bolmgren and Oxelman, 2004) and the numerous morphological differences summarized in Johnston and Johnston (1978). The most consistent distinguishing characteristics are the indehiscent stones and unfurrowed seeds with basal thickenings exserted from base of stone of Frangula and the dehiscent stones and furrowed seeds totally included in stone before dehiscence and without thickened bases of Rhamnus. Other characters include naked buds of Frangula, thorns never present, leaves alternate (some leaves opposite in F. granulosa (Ruiz & Pav.) Grubov), flowers always bisexual, 5-merous, sepals fleshy and distinctly keeled, petals present, style simple with stigma 3-lobed (usually briefly so), ovary usually 3-locular vs. in Rhamnus bud scales present,thorns sometimes present, leaves opposite or alternate, flowers often unisexual, flowers 4 or 5 merous, sepals chartaceous and not distinctly keeled, petals often absent or reduced in pistillate flowers, style usually deeply 2—4 cleft, and ovary 2---4 locular. All but four species of Frangula in the New World have the hypanthium circumscissile, a feature not seen in the New World species of Rhamnus. Added AP 19 Aug. 2013.
M.C. Johnston y L.A. Johnston. Rhamnus. Fl. Neotrop. 20: 1–96. 1978;A. Pool. New Species and combinations in Neotropical and northern Mexican species of Frangula Miller (Rhamnaceae). Novon In press.