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The Asteraceae (Compositae, alternate name) with its approximately 1,620 genera and more than 23,600 species is the largest family of flowering plants (Stevens, 2001). The family is distributed worldwide except for Antarctica but is especially diverse in the tropical and subtropical regions of North America, the Andes, eastern Brazil, southern Africa, the Mediterranean region, central Asia, and southwestern China. The majority of Asteraceae species are herbaceous, yet an important component of the family is constituted by shrubs or even trees occurring primarily in the tropical regions of North and South America, Africa and Madagascar and on isolated islands in the Atlantic and Pacific Oceans. Many species of sunflowers are ruderal and especially abundant in disturbed areas, but a significant number of them, especially in mountainous tropical regions, are narrow endemics. Because of the relentless habitat transformation precipitated by human expansion in montane tropical regions, a number of these species are consequently in danger of extinction.
The family contains several species that are important sources of cooking oils, sweetening agents, and tea infusions. Members of several genera of the family are well-known for their horticultural value and popular in gardens across the world and include zinnias, marigolds, dahlias, and chrysanthemums. The commercial sunflower genus Helianthus has been used as a model in the study of hybridization and its role in speciation (Rieseberg et al., 2003). See list of economically important Asteraceae