3. Rhamnus L. (buckthorn)

Plants shrubs or small trees, sometimes dioecious (or incompletely so). Trunks or main stems erect or ascending, branched, not twining, usually relatively tough, the bark and twigs various, the winter buds naked or with several overlapping scales. Leaves alternate or opposite, short-petiolate. Leaf blades variously shaped, the margins sharply and finely toothed, the teeth gland-tipped when young, the surfaces glabrous to densely short-hairy, shiny or not, the venation pinnate with a single midvein and 3–11 or rarely more pairs of lateral veins, these relatively straight to strongly arched toward the tip. Inflorescences axillary on present year’s growth, either along the main branchlets or along short axillary branches, of small clusters or umbels, sometimes reduced to a solitary flower. Flowers perfect or imperfect, with very short to relatively long stalks. Hypanthium minute to small, variously 1.5–3.0 mm in diameter at fruiting. Sepals 4 or 5, triangular. Petals 5, longer in staminate flowers than in pistillate ones, greenish yellow. Stamens 4 or 5 (abortive and shed early in pistillate flowers), not exserted. Ovary 2–4-locular (reduced and rudimentary in staminate flowers), unlobed, the style unbranched or 2–4-branched toward the tip. Fruits drupes, globose to broadly oblong-ovoid, with 2–4 stones, the outer surface thin, sometimes somewhat leathery, red or black, not glaucous, the stones indehiscent. Stones wedge-shaped in cross-section, asymmetric in outline with a convex to broadly dorsal (outer) side and a more or less straight ventral angle. About 200 species, North America, Central America, Europe, Asia, Africa.

There has been growing momentum in recent years to split the genus Rhamnus into two or more genera. Among the potential segregates, the genus Frangula Mill. has received the most support, which would affect just one of the Missouri species, R. caroliniana. The approximately 50 species of Rhamnus sect. Frangula (Mill.) DC. are distributed in North America, Europe, Asia, and Africa. They differ fairly consistently from the remainder of Rhamnus in their thornless stems, naked winter buds, leaf blades with pinnate, relatively straight secondary veins, perfect flowers, and seeds possessing a terminal, beaklike projection but lacking a longitudinal furrow. In spite of the morphological differences, the two groups have been considered close relatives, even by those botanists who have chosen to treat them as separate genera. Molecular studies (Richardson et al., 2000; Bolmgren and Oxelman, 2004) have confirmed that each group represents a natural, discrete lineage, but that the two groups are each other’s closest relatives. Thus the taxonomic level at which to recognize the distinctions remains largely a matter of taxonomic opinion (lumping vs. splitting). A survey of the recent floristic literature shows that most North American authors have accepted a broadly circumscribed Rhamnus, as has the Flora of China Project (Y. Chen and Schirarend, 2007). Also, the last comprehensive taxonomic survey of the genera of Rhamnaceae did not accept the genus Frangula (Medan and Schirarend, 2004). A few authors have segregated two genera, notably Kartesz and Meacham (1999) in their annotated checklist of temperate North American plants, as well as the forthcoming treatment in the Flora of North America series (Guy Nesom, personal communication). For the present, it seems most prudent to continue to recognize a single genus.

A number of species of Rhamnus native to Asia are the overwintering hosts of the soybean aphid, Aphis glycines Matsum. (Homoptera: Aphididae) (Venette and Ragsdale, 2004). This Asian insect was first found to have invaded the United States in 2000 in Wisconsin fields, and by 2005 had spread to portions of 22 states (including Missouri). During the warmer portions of the year, A. glycines feeds and reproduces on soybeans and related legumes, reducing agricultural yields. These crop pests also are a vector for the spread and transmission of viral diseases of these economically important legumes.