RHAMNALES

the buckthorn order of flowering plants, belonging to the class called dicotyledon (characterized by two seed leaves). It consists of 67 genera and 1,700 species in three families and is distributed worldwide in the tropics and subtropics, with significant extensions into temperate regions. Its most economically important member is the grape (genus Vitis). Members of the Rhamnales are characterized by woodiness, stamens (male) alternating with sepals (opposite petals, when present), a disk of tissue developing under or around the ovary, and joined bases of flower parts that form a cup (hypanthium) surrounding the ovary. The order's main constituents, the Rhamnaceae (buckthorn) and Vitaceae (grape) families, are scientifically interesting in that many of their genera are native to both the Old World and New World tropics or subtropics. In contrast to these widely distributed families, the Leeaceae family, comprising a single genus of about 70 species, is restricted to tropical areas from southern Asia to Africa. Trees, shrubs, and woody vines predominate in the order. Many members thrive best in rich soil with adequate moisture and a mild climate, but some species, particularly among the Rhamnaceae, thrive in drier environments. The Rhamnaceae family is characterized by simple (blade is one unit) leaves, well-developed sepals, stamens opposite petals, a prominent hypanthium, and a two- to four-chambered ovary in which ovules arise basally. Most fruits are either berrylike (with an inner hard layer) or dry capsules that shed their seeds. A notable tendency in the family is its members' adaptations to dry climates: small and crowded leaves, shortened branches, thorniness or spininess, and a low, shrubby, intricately branched habit. The Rhamnaceae family contains 55 genera and about 900 species. Its best-known member is the fruit-bearing jujube (Ziziphus jujuba) of China, which grows in dry, alkaline soil. Valuable timber is derived from several buckthorn genera, including Maesopsis eminii, Ziziphus mauritiana, Z. jujuba, Z. spina-christi, and Pomaderris apetala. Most members of the Rhamnaceae are not large enough to supply lumber, however. They yield various products, such as cascara sagrada (a cathartic made from the bark of Rhamnus purshiana, the California bearberry) and the green dye, lokao, the only natural dye that colours cotton a true green. Other members of Rhamnaceae are components of chaparrala plant association essential to erosion and flood control. R. californica and R. crocea, the redberry evergreen shrubs, are used as ornamentals. Distinctive features of the Vitaceae, the other major family of the Rhamnales, include terminal buds that develop into tendrils lateral to the stem, inflorescences (flower clusters) that arise opposite a leaf, small sepals (or none), and stamens positioned opposite the petals. Ovules arise centrally in the two-chambered ovary, which matures into a juicy berry. The most notable growth tendency among the Vitaceae is that toward trailing and twining, and generally a vinelike habit. Of the 11 genera and 700 species in the Vitaceae family, none is more important than the grape genus (Vitis). Large, lobed leaves and shreddy bark are characteristic of grapes, which inhabit wooded areas in warmer regions of the Northern Hemisphere. Grapes are a source of both fresh and dried fruit and have been used in winemaking in Mediterranean lands since prehistoric times. The culture of table grapes is a major commercial enterprise in central Europe, New York, California, and other regions. From antiquity, winemaking has been based on the Caucausian wine grape (Vitis vinifera). Its derivatives are adapted to the European climate and to California. The fox grape of the eastern United States, with its purple-black berry, is the dominant parent of the Concord and many other cultivars. The muscadine grape (V. rotundifolia) is the ancestor of the scuppernong grape, a fruit with musk-flavoured pulp. Its derivatives are cultivated in the Cotton Belt of the United States. At least five genera of the Vitaceae, including Parthenocissusthe genus of both the Virginia creeper (P. quinquefolia) and the Japanese, or Boston, ivy (P. tricuspidata)are ornamentally important. Many species of the Rhamnales produce root sprouts to form extensive colonies. The cape grape produces globose tubers (short underground stems). Nearly all species produce abundant seed. Flower parts arise in fours or fives depending upon the family. Perfect flowers, in which both male and female reproductive organs are present, are typical of many genera. The Asiatic Tetrastigma, however, produces male and female flowers on separate plants. In the grape genus, flowers are functionally unisexual. In the Vitaceae stamens arise from the disk, which is nectar-producing. Petals are united at their apex and fall as one unit when the flower opens. Two carpels constitute the pistil, which is differentiated into the basal ovary, a style, and the pollen-receptive stigmas. No more than two ovules arise in each ovary chamber, and some of these may abort. Insects are the pollinators.