Rhamnus is a genus of about 140 accepted species of shrubs or small trees, commonly known as buckthorns, in the family Rhamnaceae. Its species range from tall (rarely to ) and are native mainly in east Asia and North America, but found throughout the temperate and subtropical Northern Hemisphere, and also more locally in the subtropical Southern Hemisphere in parts of Africa and South America. One species, the common buckthorn (Rhamnus cathartica), is able to flourish as an invasive plant in parts of Canada and the U.S., where it has become naturalized. Both deciduous and evergreen species occur. The leaves are simple, long, and arranged alternately, in opposite pairs, or almost paired (subopposite). One distinctive character of many buckthorns is the way the veination curves upward towards the tip of the leaf. The plant bears fruits which are black or red berry-like drupes. The name is due to the woody spine on the end of each twig in many species. One species is known to have potential to be used medicinally. Description Rhamnus species are shrubs or small to medium-sized trees, with deciduous or rarely evergreen foliage. Branches are unarmed or end in a woody spine. The leaf blades are undivided and pinnately veined. Leaf margins are serrate or rarely entire. Rhamnus species are generally dioecious, with male and female flowers on separate plants. Most species have yellowish green, small, unisexual or rarely polygamous flowers; which are produced singly or in axillary cymes, cymose racemes, or cymose panicles containing a few flowers. Calyx tube campanulate to cup-shaped, with 4 or 5 ovate-triangular sepals, which are adaxially ± distinctly keeled. Petals 4 or 5 but a few species may lack petals. The petals are shorter than the sepals. Flowers have 4 or 5 stamens which are surrounded by and equal in length the petals or are shorter. The anthers are dorsifixed. The superior ovary is free, rounded, with 2-4 chambers. Fruits are a 2-4 stoned, berrylike drupe, which is obovoid-globose or globose shaped. Seeds are obovoid or oblong-obovoid shaped, unfurrowed or abaxially or laterally margined with a long, narrow, furrow. The seeds have fleshy endosperm. Species , Plants of the World Online accepted the following species: Rhamnus alaternus L. – Italian buckthorn Rhamnus alnifolia L'Hér. – alderleaf buckthorn, alder-leaved buckthorn Rhamnus alpina L. Rhamnus arguta Maxim. – sharp-tooth buckthorn Rhamnus arnottiana Gardner ex Thwaites Rhamnus aurea Heppeler Rhamnus baldschuanica Grubov Rhamnus