ASTERALES

order of flowering plants, containing one family, the Asteraceae, or Compositae, with more than 1,100 genera and almost 20,000 species of herbs, shrubs, and trees, distributed throughout the world. Members of the Asterales are diverse in their habit and habitat, but they tend to be herbs growing in sunlit places in temperate and subtropical regions. Asteraceae is variously known as the aster, daisy, or composite family. It is one of the largest plant families and is important primarily for its many garden ornamentals, such as ageratums, asters, chrysanthemums, cosmos, dahlias, marigolds (Tagetes), and zinnias. Other well-known garden plants and wildflowers belonging to the aster family include Boltonia, Brachycome, burdock (Arctium), butterbur (Petasites), Calendula, cat's ear (Hypochoeris), cudweed (Filago and Gnaphalium), Gerbera, hawksbeard (Crepis), Inula, Matricaria, and Piqueria. Some genera include noxious weeds, such as dandelion (Taraxacum), ragweed (Ambrosia), and thistle (Carduus, Cirsium, and others). Artichoke (Cynara), endive (Cichorium), safflower (Carthamus), salsify (Tragopogon), lettuce (Lactuca), sunflower (Helianthus), and wormwood (Artemisia) are economically important for the products derived from their flowers, seeds, leaves, roots, or tubers. Members of the family have flower heads composed of many small flowers, called florets, that are surrounded by bracts (leaflike structures). Bell-shaped disk florets form the centre of each head; strap-shaped ray florets extend out like petals from the centre and are sometimes reflexed (bent back). Some species have flowers with only disk or only ray florets. The sepals have been reduced to a ring of hairs, scales, or bristles that is called the pappus on the mature fruit. The one-seeded fruit has a hard outer covering. order of dicotyledonous flowering plants, a division of the subclass Asteridae. Its members are diverse in their habits and habitats, but they tend to be herbaceous plants that grow in sunlit places in temperate and subtropical regions. The Asterales consist of a single large family of flowering plants called the Asteraceae, also known as Compositae. The number of species is not accurately known but is estimated to be about 20,000. The only other family with a comparable number of species is the orchid family, Orchidaceae (Liliidae). Asterales is the principal order for the subclass Asteridae. The order includes many familiar garden ornamentals, such as asters, chrysanthemums, dahlias, daisies, marigolds, sunflowers, and zinnias. Some other members of the group, such as dandelions, ragweeds, and thistles, are familiar weeds. Lettuce and safflower are examples of economically important crop plants of the order. Additional reading Arthur Cronquist, Phylogeny and Taxonomy of the Compositae, The American Midland Naturalist, 53:478511 (1955), and his updating of this synthesis, The Compositae Revisited, Brittonia, 29(2):137153 (1977); V.H. Heywood, J.B. Harborne, and B.L. Turner (eds.), The Biology and Chemistry of the Compositae, 2 vol. (1977), a multiauthor symposium.