MYRTALES

the myrtle order of flowering plants, belonging to the class called dicotyledon (q.v.; characterized by two seed leaves). It is composed of 14 families, more than 400 genera, and 10,400 species and is distributed throughout the tropics and warmer regions of the world. The majority of these belong to just two families, Melastomataceae and Myrtaceae. Myrtales includes many trees (notably Eucalyptus), shrubs such as the classic myrtle, several food and spice genera, and many ornamental plants. Features of the myrtle order include opposite leaves, flower parts in cycles rather than in spirals, the union of flower parts basally to form a cup around the ovary, and the differentiation of xylem (water-conducting region) around isolated strands of phloem (food-conducting tissues) in the stem. Woodiness and flower parts arising above the ovary characterize the 150 genera and 3,300 species of the Myrtaceae family. Its dominant genus is Eucalyptus, with more than 600 species and many varieties. Nearly all species of Eucalyptus are Australian, where they are classed largely on the appearance and durability of their bark. E. regnans, which grows to more than 90 m (300 feet), and E. diversicolor furnish much construction timber and quality hardwood. The wood of E. botryoides, a relatively small tree, is used in shipbuilding. In addition to lumber, species of Eucalyptus provide resins, essential oils, and wood pulp. Those genera of the myrtle order that produce berry fruits are centred in tropical America. Prized edible fruits include the Surinam cherry (Eugenia uniflora) and the jaboticaba (Myrciaria cauliflora), both of tropical America, and the rose apple (Syzygium jambos) and Java plum (S. cumini), both of Southeast Asia. Many are cultivated. The guava (Psidium guajava), also ornamentally important, furnishes food that is eaten fresh or made into jelly. Well-known spices of the Myrtales include the clove (dried flower-buds) of Syzygium aromaticum, native to the Old World, and allspice, the unripe berry of Pimenta dioica, originally of Central America. The related P. racemosa yields oil of bay rum, used in perfumery. The myrtle of antiquity, Myrtus communis, is the best-known ornamental of the myrtle family. Originally from the Mediterranean region, it is planted extensively on many continents. Other horticultural favourites are the tea trees (Leptospermum) and the wax-flower (Chamelaucium), both of Australia. The crimson bottlebrush (Callistemon citrinus) of Australia is prized for its silky-haired branches and showy flowers. Evening primrose (Oenothera biennis) Other well-known ornamentals of Myrtales are loosestrife and crape myrtle, both of the Lythraceae family; and the evening primrose (see photograph), the day-flowering sundrops, and Fuchsia of the Onagraceae family. The common fireweed (Epilobium), which covers mountains following forest fires, propagates by rhizomes in addition to copious production of seed. The swamp loosestrife (Decadon verticillatus) of the eastern and central United States populates swamps when its long stems arch downward and root at the tip. Vegetative reproduction, however, is rare in the myrtle order. Inflorescences of the Myrtales usually are much-branched, either in a panicle (in which flowers open first on lower and outer branches) or in a cyme (in which the earliest flowers are terminal and therefore central as other, lower branches arise). Flowers are most commonly bisexual (stamens and pistil in one flower), and petals are alike in size and shape (radially symmetrical) and usually separate. Sepals of the calyx and petals of the corolla are in fours or fives depending on the genus. Technically, the bases of sepals, petals, and stamens form a cup (hypanthium); the showy upper portions of these flower parts are displayed from the rim of this structure. In some members of this order, the ovary wall is also joined to the basal portion so that the ovary is inferior in position. Flowers of a few genera are wind-pollinated but the most common means of pollination is by insects, including bees. Birds pollinate the flowers in some genera. After pollination and fertilization, the ovule becomes the seed and the ovary the fruit. The most common type of fruit is a dry capsule that splits at maturity to release the seeds. Exceptions include the pomegranate, a leathery fruit consisting of distorted ovary chambers filled with juicy seeds. Many genera produce fleshy berries. myrtle order of dicotyledonous flowering plants, a division of the subclass Rosidae. Included are trees and shrubs and herbaceous plants; some are important as sources of food and as ornamentals. Additional reading Articles in Annals of the Missouri Botanical Garden, vol. 71, no. 3 (1984), authoritative reviews of the botanical characters used in the classification of all myrtalean families; Lindsay D. Pryor, The Biology of Eucalypts (1976), descriptions of distribution, uses, and characteristics of the genus Eucalyptus; Stan Kelley, G.M. Chippendale, and R.D. Johnston, Eucalypts, new rev. ed., 2 vol. (1983).