Compositae

Compositae, the name given to the largest natural order of flowering plants, containing about one-tenth of the whole number and characterized by the crowding of the flowers into heads. The order 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 succession. In many cases, as in the sunflower or daisy, the outer or ray-florets are larger and more conspicuous than the inner, or disk-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 disk-florets are hermaphrodite. 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 as in the disk-florets, or irregular when it is either strap-shaped (ligulate), as in the ray-florets of daisy, &c., or all the florets of dandelion, or more rarely two-lipped. The five stamens are attached to the interior of the corolla-tube; the filaments are free; the anthers are joined (syngenesious) to form a tube round the single style, which ends in a pair of stigmas. 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.

Fig. 1.
1. Flower head of Marigold.	3. Head of fruits, nat. size.
2. Same in vertical section.	4. A single fruit.

The flower-heads are an admirable example of an adaptation for pollination by aid of insects. 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 gradually 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 history 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 stigmas finally curl back so that they touch any pollen grains which have been left on the style, thus ensuring self-pollination 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 parachute-like structure as in dandelion; these render the fruit sufficiently light to be carried by the wind. In Bidens the pappus consists of two or more stiff-barbed bristles which cause the fruit to cling to the coats of animals. Occasionally, as in sunflower or daisy, the fruits bear no special appendage and remain on the head until jerked off.

Fig. 2.—Flowering shoot of Cornflower. 1. Disk-floret in vertical section.

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), Helichrysum (everlasting), Zinnia, Rudbeckia, Helianthus (sunflower), Coreopsis, Dahlia (q.v.), Tagetes (French and African marigold), Gaillardia, Achillea (yarrow), Chrysanthemum, Pyrethrum (feverfew; now generally included under Chrysanthemum), Tanacetum (tansy), Arnica, Doronicum, Cineraria Calendula (common marigold) (fig. 1), Echinops (globe thistle), Centaurea (cornflower) (fig. 2). Some are of medicinal value, such as Anthemis (chamomile), Artemisia (wormwood), Tussilago (coltsfoot), Arnica. Insect powder is prepared from species of Pyrethrum.

Fig. 3.—Groundsel (Senecio vulgaris).
1. Disk-floret.	3. Ray-floret.
2. Same cut vertically.	4. Fruit with pappus.

The order is divided into two suborders:—Tubuliflorae, characterized by absence of latex, and the florets of the disk being not ligulate, and Liguliflorae, characterized by presence of latex and all the florets being ligulate. The first suborder contains the majority of the genera, and is divided into a number of tribes, characterized by the form of the anthers and styles, the presence or absence of scales on the receptacle, and the similarity or otherwise of the florets of one and the same head. The order is well represented in Britain, in which forty-two genera are native. These include some of the commonest weeds, such as dandelion (Taraxacum Dens-leonis), daisy (Bellis perennis), groundsel (fig. 3) (Senecio vulgaris) and ragwort (S. Jacobaea); coltsfoot (Tussilago Farfara) is one of the earliest plants to flower, and other genera are Chrysanthemum (ox-eye daisy and corn-marigold), Arctium (burdock), Centaurea (knapweed and cornflower), Carduus and Cnicus (thistles), Hieracium (hawkweed), Sonchus (sow-thistle), Achillea (yarrow, or milfoil, and sneezewort), Eupatorium (hemp-agrimony), Gnaphalium (cudweed), Erigeron (fleabane), Solidago (golden-rod), Anthemis (may-weed and chamomile), Cichorium (chicory), Lapsana (nipplewort), Crepis (hawk’s-beard), Hypochaeris (cat’s-ear), and Tragopogon (goat’s-beard).