myrtle, common name for the Myrtaceae, a family of shrubs and trees almost entirely of tropical regions, especially in America and Australia. The family is characterized by leaves (usually evergreen) containing aromatic volatile oils. Many have showy blossoms. Although of lesser importance in the United States, the family is of considerable economic value throughout the world for timber, gums and resins, oils, spices, and edible fruits.

The true myrtle genus (Myrtus) is predominantly of the American tropics, but the classical myrtle (M. communis) is native to the Mediterranean area. It is a strongly scented bush whose glossy leaves and blue-black berries were made into wreaths for victors in the ancient Olympic games. (In America several unrelated plants are also called myrtles, e.g., the sand myrtle of the heath family, the periwinkles of the dogbane family, and several species of the bayberry family.) Among the many trees of the myrtle family yielding edible fruit, only the guava (genus Psidium), native to tropical America, is grown commercially in the United States.

The most important spice plants of the family are the clove tree (Syzygium aromaticum or Eugenia caryophyllata), native to the Moluccas and the Spice Islands, and the tropical American Pimenta genus that includes the pimento or allspice (P. officinalis or dioica) and the bay rum tree (P. racemosa), source of an oil used as an ingredient of bay rum. Some hardwood members of the myrtle family are among the many trees known as ironwood, e.g., Eugenia confusa, of Florida and tropical America.

Eucalyptus, a large genus of evergreen shrubs and trees, is a characteristic component of the flora in its native Australia, where its species are the leafy haunt and sole food source of the koala, often associated with it in story. Among its many species' common names are ironbark, stringybark, and gum, ash, and box (names also applied to many unrelated trees). Numerous species, especially the Tasmanian blue gum (E. globulus), are now naturalized in the W United States and have become the distinctive vegetation of many California areas that were previously treeless. Several species are among the tallest trees known, e.g., E. regnans, known as the mountain ash or giant ash, the tallest flowering tree, which reaches a height of over 300 ft (91 m). The generally accepted record for the tallest mountain ash is 375 ft (114.3 m), for a tree that was felled in 1881. Eucalyptus trees are a valuable source of timber, of kinos (a resinous substance used in medicines and tanning), and of eucalyptol and other essential and medicinal oils.

The myrtle family is classified in the division Magnoliophyta, class Magnoliopsida, order Myrtales.

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