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Compositae

Compositae The largest family of flowering plants, which represents almost every life-form. The flowers are individually small, but are clustered into heads (capitula, see CAPITULUM) resembling flowers; the florets are seated on receptacles of varied form, and the heads are surrounded by an involucre, or bracts resembling a calyx. The florets often have no calyx, but sometimes a hairy or scaly pappus develops in the fruit. The corolla is tubular or strap-like, and 5-lobed. The 5 stamens are joined into a tube. The ovary is inferior and one-celled, forming an achene in the fruit. Some have all the florets similar, either all tubular or all strap-shaped; in some the outer florets are strap-like (e.g. Bellis, the daisy) and female or neuter, while the inner florets are tubular and usually hermaphrodite. The family is closely related to the Dipsacaceae (which, however, has long, usually free stamens, conspicuous calyx teeth, a cup-like epicalyx to each flower, and the ovule pendulous, instead of basal, in the ovary). The family includes the largest angiosperm genus, Senecio (ragworts and groundsels), of more than 2000 species; and many valuable cultivated plants, e.g. Lactuca sativa (lettuce), Helianthus tuberosus (Jerusalem artichoke), Tragopogon porrifolius (salsify), as well as oil-producing plants, e.g. Helianthus annuus (sunflower), and many garden plants and serious weeds. The family consists of 1317 genera, with about 21000 species, with a cosmopolitan distribution.

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Asteraceae

Asteraceae

Asteraceae (or Compositae), the sunflower family, is the largest family of flowering plants, encompassing some fifteen hundred genera and nearly twenty-five thousand species. Most species are herbs, but some are shrubs and a few are trees. They are of worldwide distribution and are often conspicuous in their habitats. Members of the family share a distinctive flowering structure, consisting of numerous small flowers (often called florets) that are tightly clustered into a head. The five petals of a floret are fused to form a corolla tube that encloses the stamens and pistil . Florets that have all evenly sized petals are called disk florets. Florets that have fused petals expanded greatly on one side into a long strap are called ray florets. A typical head consists of numerous disk florets in the center and a row of ray florets around the margin. The strap-shaped corollas of the ray florets project outward, giving the entire head the appearance of a single, large flower (e.g., sunflowers, Helianthus annuus ). Some members of the family have heads with only disk florets (e.g., ironweeds, Vernonia spp.), and some have all florets with strap shaped corollas (e.g., dandelions, Taraxacum officinale ). A few members have the florets and heads greatly reduced and are wind pollinated (e.g., ragweeds, Ambrosia spp.).

Despite the size of the family and its wide distribution, only a few members have become economically important crops, such as lettuce (Lactuca serriola ), artichokes (Cynara scolymus ), sunflowers (Helianthus annuus ), chicory (Cichorium intybus ), and pyrethrum (Tanecetum cinerariifolium, an insecticide). Some species are notable weeds (e.g., dandelions) and some, like sagebrush (several species of Artemisia ), dominate the landscape where they occur. Many species are cultivated as garden ornamentals, and some use their generic names as common names, for example, aster, chrysanthemum, dahlia, gaillardia, and zinnia.

see also Angiosperms; Dicots; Flowers; Inflorescence.

Theodore M. Barkley

Bibliography

Bremer, K. Asteraceae: Cladistics and Classification. Portland, OR: Timber Press, 1994.

Hind, D. J. N., and H. J. Beetje, eds. Compositae: Systematics. Proceedings from the International Compositae Conference. Kew, UK: Royal Botanic Gardens.

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Compositae

Compositae Family of c.20,000 species of plants in which the ‘flower’ is actually a composite flower-head consisting of a cluster of many, usually tiny, individual flowers (florets). In a typical composite, such as the daisy, the flower-head has a central yellow disc, consisting of a cluster of tiny bisexual florets lacking visible petals. The outer ring of female, ray florets has large white petals. In composites such as the dandelion and endive, the flower-head consists entirely of ray florets. Others, such as thistles, consist entirely of disc florets. Composites make up by far the largest family of plants, and include food plants, such as artichokes, and many popular garden flowers. The Compositae are often known as the Asteraceae.

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