COMPOSITAE, the name given to the largest family of flowering plants. It includes approximately 90o genera and over 13,00o species comprising about one-tenth of the flowering plants. It is characterized by the crowding of the flowers into heads. The family is cosmopolitan, and the plants show considerable variety in habit. The great majority, including most British representatives, are herbaceous, but in the warmer parts of the world shrubs and arborescent forms also occur; the latter are characteristic of the flora of oceanic islands. In herbaceous plants the leaves are often arranged in a rosette on a much shortened stem, as in dandelion, daisy and others; when the stem is elongated the leaves are generally alternate. The root is generally thickened, sometimes, as in dahlia, tuberous ; root and stem contain oil passages, or, as in lettuce and dandelion, a milky white latex. The flowers are crowded in heads (capitula) which are surrounded by an involucre of green bracts, these protect the head of flowers in the bud stage, performing the usual function of a calyx. The enlarged top of the axis, the receptacle, is flat, convex or conical, and the flowers open in centripetal suc cession. In many cases, as in the sunflower or daisy, the outer or ray-florets are larger and more conspicuous than the inner, or disc-florets; in other cases, as in dandelion, the florets are all alike. Ray-florets when present are usually pistillate, but neuter in some genera (as Centaurea) ; the disc-florets are hermaphro dite. The flower is epigynous ; the calyx is sometimes absent, or is represented by a rim on the top of the ovary, or takes the form of hairs or bristles which enlarge in the fruiting stage to form the pappus, by means of which the seed is dispersed. The corolla, of five united petals, is regular and tubular in shape in the disc-florets, or irreg ular, when it is either strap-shaped (ligu late), as in the ray-florets of daisy, etc., or all the florets of dandelion, or more rarely two-lipped. The five stamens are attached to the interior of the corolla-tube; the fila ments are free ; the anthers are joined (syngenesious) to form a tube round the single style, which ends in a pair of stig mas. The inferior ovary contains one ovule (attached to the base of the chamber), and ripens to form a dry one-seeded fruit ; the seed is filled with the straight embryo. The flower-heads are an admirable exam ple of an adaptation for pollination by in sects. The crowding of the flowers in heads ensures the pollination of a large number as the result of a single insect visit. Honey is secreted at the base of the style, and is protected from rain or dew and the visits of short-lipped insects by the corolla-tube, the length of which is correlated with the length of proboscis of the visiting insect. When the flower opens, the two stigmas are pressed together below the tube formed by the anthers, the latter split on the inside, and the pollen fills the tube ; the style grad ually lengthens and carries the pollen out of the anther tube, and finally the stigmas spread and expose their receptive surface which has hitherto been hidden, the two being pressed together. Thus the life his tory of the flower falls into two stages, an earlier or male and a later or female. This favours cross-pollination as compared with self-pollination. In many cases there is a third stage, as in dandelion, where the stig mas finally curl back so that they touch any pollen grains which have been left on the style below, thus ensuring self-pollina tion if cross-pollination has not been effected.
The devices for distribution of the fruit are very varied. Frequently there is a hairy or silky pappus forming a tuft of hairs, as in thistle or coltsfoot, or a para chute-like structure as in dandelion ; these render the fruit sufficiently light to be carried by the wind. In Bidens the pap pus consists of two or more stiff-barbed bristles which cause the fruit to cling to the coats of animals. Occasionally, as in sun flower or daisy, the fruits bear no special appendage and remain on the head until jerked off.
Compositae are generally considered to represent the most highly developed order of flowering plants. By the massing of the flowers in heads great economy is effected in the material required for one flower, as conspicuousness is ensured by the association ; economy of time on the part of the pollinating insect is also effected, as a large number of flowers are visited at one time. The floral mechanism is both simple and effective, favouring cross pollination, but ensuring self-pollination should that fail. The means of seed-distribution are also very effective.
A few members of the order are of economic value, e.g., Lactuca (lettuce; q.v.), Cichorium (chicory; q.v.), Cynara (artichoke and cardoon; q.v.), Helianthus (Jerusalem artichoke). Many are cultivated as garden or greenhouse plants, such as Solidago (golden rod), Ageratum, Aster (q.v.) (Michaelmas daisy), Heli chrysum ("everlasting"), Zinnia, Rudbeckia, Helianthus (sun flower), Coreopsis, Dahlia (q.v.), Tagetes (French and African marigold), Gaillardia, Achillea (yarrow), Chrysanthemum, Tana cetunr (tansy), Arnica, Doronicum, Cineraria, Calendula (common marigold), Echinops (globe thistle), Centaurea (q.v.). Some are of medicinal value, such as Anthemis (chamomile), Artemisia (wormwood), Tussilago (coltsfoot), Arnica. Insect powder is prepared from species of Pyrethrum now included in the genus Chrysanthemum.
The order is divided into two subfamilies :—Tubuliflorae, characterized by absence of latex, and the florets of the disc being not ligulate, and Liguliflorae, characterized by presence of latex and all the florets being ligulate. The first subfamily con tains the majority of the genera, and is divided into a number of tribes. The family is well rep resented in Britain, in which 42 of the 90o genera are native. These include some of the com monest weeds, such as dandelion (Taraxacum officinale), daisy (Bellis perennis), groundsel (Senecio vulgaris) and ragwort (S. Jacobaea) ; coltsfoot (Tussi lago Farfara) is one of the earliest plants to flower, and other genera are Chrysanthemum (ox-eye daisy and corn-marigold), Arc tium (burdock), Centaurea (knap weed and cornflower), Carduus and Cnicus (thistles), Hieracium (hawkweed), S o n c h u s (sow thistle), Achillea (yarrow, or mil f oil, and sneezewort), Eupatorium (hemp-agrimony), Gnaphalium (cud-weed), Erigeron (fleabane), Solidago (golden-rod), Anthemis (may-weed a n d chamomile), Cichorium (chicory), Lapsana (nipplewort) , Crepis (hawk's beard), Hypochaeris (cat's-ear), and Tragopogon (goat's beard).
In North America over 200 genera are represented by many species distributed over the whole country.