Asteraceae

Asteraceae, commonly known as the aster, daisy, composite, or sunflower family, and formerly and still validly called Compositae, is the largest family of flowering plants, comprising more than 1,900 genera and roughly 32,000 accepted species found on every continent except Antarctica. The family is defined above all by its distinctive inflorescence, the capitulum or flower head, a highly condensed structure that appears to be a single flower but is in fact composed of many individual florets. Members range from tiny annual herbs to shrubs and montane trees and include major food crops, leading ornamentals, medicinal plants, industrial raw materials, and some of the world's most notorious weeds. Together with its relatives in the order Asterales, Asteraceae represents one of the most evolutionarily successful lineages of the angiosperms, accounting for roughly one in ten of all described flowering-plant species.

Etymology and Nomenclature

The scientific name Asteraceae is derived from the type genus Aster, itself from the Greek aster ("star"), in reference to the star-like arrangement of ray florets in many species; the name was introduced by the Russian botanist Ivan Martinov in 1820. The alternative name Compositae, attributed to Karl Friedrich von Giseke (1792), emphasizes the composite nature of the flower head and was the prevailing name in older literature. Under the International Code of Nomenclature, both names are conserved and may be used interchangeably. Carl Linnaeus placed the family's genera in his class Syngenesia (1753), characterized by stamens whose anthers are united into a tube, a hallmark of the family. The foundations of the family's internal classification were laid in the early nineteenth century by the French botanist Henri Cassini, who defined many of its tribes, and refined later by George Bentham and by twentieth-century comparative studies.

Description

Most members of Asteraceae are herbaceous—annual, biennial, or perennial—but the family also includes subshrubs, shrubs, climbers, and, in certain montane and island floras, true trees. Leaves are usually alternate or opposite, simple to deeply divided, and consistently lack stipules. Several groups, notably the tribe Cichorieae (lettuces and dandelion allies), contain laticifers that exude milky latex when wounded.

The capitulum, or head, is borne at the tip of a peduncle and consists of numerous sessile florets attached to a flat, convex, or conical receptacle. The whole structure is subtended by an involucre of one to several series of bracts (phyllaries) that protect the developing florets, and the receptacle itself may be naked, covered with chaffy bracts (paleae), or bristly. Heads may be solitary or aggregated into corymbs, panicles, racemes, or other secondary inflorescences. Three principal head types are recognized: discoid heads, bearing only tubular, radially symmetrical disc florets (as in thistles and tansy); radiate heads, in which outer, strap-shaped ray florets surround a central disc (as in daisies and sunflowers); and ligulate heads, composed entirely of strap-shaped, usually bisexual florets (as in dandelion and chicory).

Each individual floret typically has a five-lobed, fused corolla and five stamens whose filaments are free but whose anthers are fused into a tube (a condition termed syngenesious). As the style grows through the anther tube, sweeping hairs brush pollen outward—a mechanism of secondary pollen presentation—before the two style branches spread apart to expose the stigmas. The gynoecium consists of two fused carpels forming an inferior ovary with a single basal ovule. The calyx is modified into the pappus, a ring of bristles, scales, or awns that functions in seed dispersal rather than in floral