HAMAMELIDALES

order of woody flowering plants, belonging to the class known as dicotyledon (characterized by the presence of two seed leaves). Its three families are composed of 30 genera and 150 species, most of which are medium-sized trees or shrubs. They are distributed worldwide. The Hamamelidaceae family (witch hazels) constitutes the bulk of the genera and species of the order bearing its name; its major distribution centres are eastern Asia and the Malaysian region, and it extends to Australia, North America, Central America, tropical and southern Africa, and Madagascar. Members of this group thrive in moist woodlands and even on steep slopes in forested areas. Some are understory plants in forests, where they seldom form solid stands. Trunks of sweet gum (Liquidambar styraciflua), however, rise to heights of 36 m (120 feet). Several genera grow in the mountains of tropical and subtropical regions. The nine species of Platanus, or plane trees, which make up the Platanaceae family, are native to eastern Asia, the eastern Mediterranean region, North America, and Central America. They occupy habitats similar to those of the Hamamelidaceae. Specimens of the American plane tree, or sycamore (P. occidentalis), attain heights of 50 m (165 feet). Myrothamnus, with two species, represents the family bearing its name. These bushy shrubs are found only in Madagascar and the southern half of Africa. Their leaves are folded, persistent, and long-living, an adaptation to the extremely dry conditions of their habitat. None are cultivated. The American sycamore, the plane tree, and the sweet gum furnish construction timbers and wood for furniture. They are handsome, hardy trees and are often planted extensively along streets. The crimson-tinted leaves of the sweet gum are striking in autumn; this tree also yields the resinous balsam styrax, used in perfumery and in medicine. Extracts of witch hazel (Hamamelis) continue to be used cosmetically and as household liniments. Leaves of the Hamamelidales are mainly deciduous; a few species are evergreen. Plants of the witch hazel and the plane tree families produce one leaf per node (joint) and consist of a leaf-stalk (petiole), with a pair of leaflike bracts (stipules) basally and a simple, lobed blade. Sycamores and plane trees develop large stipules. Many species have hairs on the blade and petiole of the leaves. Reproduction by seeds is highly successful in all three families of the Hamamelidales. Inflorescences (flower clusters) range from a few flowers (even single ones) arising in the axil of a leaf (angle between stem and base of leaf-stalk) to pendant, spherical heads composed of many flowers. Most flowers in the witch hazel family are perfect (bisexual), but in the plane tree and the Myrothamnus families the stamens (male) and pistils (female) are borne in separate flowers. Sepals and petals of most plants in the order are diminutive. When present, depending upon the genus, three to seven sepals form a short tube, and a similar number of petals remain separate. Stamens vary in number from two to eight. The witch hazel family produces a single pistil, but in the other families as many as nine pistils may be present in each flower. In the witch hazels, the perianth (sepals and petals) may be merged with the lower portion of the ovary, but in the other members, the ovary is completely superior. One or two ovules are differentiated within each ovary (many in Myrothamnus); they develop into seeds following pollination and fertilization. Pollination is accomplished by bees, flies, and wind; birds pollinate Rhodoleia. In the witch hazel family and in Myrothamnus, fruits split open (capsule) to release seeds, but in plane trees the small fruits remain closed (achene). The unifying features of the Hamamelidales include woody structure, angular vessel (conducting tube) elements of the xylem, simple leaves, stipules (with one exception), superior ovary, and few carpels in the pistil. Features distinguishing the families from each other include type of inflorescence, pistil structure and form, and arrangement of leaves.