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COMPOSITAE
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COMPOSITAE , the name given to the largest natural See also:order of flowering See also:plants, containing about one-tenth of the whole number and characterized by the crowding of the See also:flowers into heads. The order is See also:cosmopolitan, and the plants show considerable variety in See also:habit. The See also:great See also:majority, including most See also:British representatives, are herbaceous, but in the warmer parts of the See also:world shrubs and arborescent forms also occur; the latter are characteristic of the See also:flora of oceanic islands. In herbaceous plants the leaves are often arranged in a rosette on a much shortened See also:stem, as in See also:dandelion, See also:daisy and others; when the stem is elongated the leaves are generally alternate. The See also:root is generally thickened, sometimes, as in See also:dahlia, tuberous; root and stem contain oil passages, or, as in See also:lettuce and dandelion, a milky See also: A single See also:fruit. protect the head of flowers in the bud See also:stage, performing the usual See also:function of a calyx. The enlarged See also:top of the See also:axis, the receptacle, is See also:flat, See also:convex or conical, .and the flowers open in centripetal See also:succession. ` In many cases, as in the See also:sunflower or daisy, the See also:outer or See also: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 See also:present are usually pistillate, but neuter in some genera (as See also: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 See also:form of hairs or bristles which enlarge in the fruiting stage to form the pappus by means of which the See also:seed is dispersed. The corolla, of five See also:united petals, is See also: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-See also:tube; the filaments are See also:free; the anthers are joined (syngenesious) to form a tube See also:round the single See also:style, which ends in a pair of stigmas. The inferior ovary contains one ovule (attached to the See also: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 example of an See also:adaptation for See also:pollination by aid of See also:insects. The crowding of the flowers in heads ensures the pollination of a large number as the result of a single See also:insect visit. See also:Honey is secreted at the base of the style, and is protected from See also:rain or See also:dew and the visits of See also:short-lipped insects by the corolla-tube, the length of which is correlated with the length of See also: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 See also: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 See also:surface which has hitherto been hidden, the two being pressed together. Thus the See also:life See also:history of the flower falls into two stages, an earlier or male and a later or See also:female. This favours See also: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 See also:touch any pollen grains which have been See also:left on the style, thus ensuring self-pollination if cross-pollination has not been effected. The devices for See also:distribution of the fruit are very varied. Frequently there is a hairy or silky pappus forming a tuft of hairs, as in See also:thistle or coltsfoot, or a See also:parachute-like structure as in dandelion; these render the fruit sufficiently See also:light to be carried by the See also: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 See also:bear no See also:special appendage and remain-on the head until jerked off. Compositae are generally considered to represent the most Aighly See also:developed order of flowering plants. By the massing of the flowers in heads great See also:economy is effected in the material required for one flower, as conspicuousness is ensured by the association; economy of See also:time on the See also:part of the pollinating insect is also effected, as a large number of flowers are visited at one time. The floral mechanism is both See also: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 (See also:chicory; q.v.), Cynara (See also:artichoke and See also:cardoon; q.v.), Helianthus (See also:Jerusalem artichoke). Many are cultivated as See also:garden or greenhouse plants, such as Solidago (See also:golden See also:rod), Ageratum, See also:Aster (q.v.) (Michaelmas daisy), Hellchrysum (See also:everlasting), See also:Zinnia, Rudbeckia, Helianthus (See also:sun-flower), Coreopsis, Dahlia (q.v.), Tagetes (See also:French and Africanmarigold), Gaillardia, Achillea (See also:yarrow), Chrysanthemum, See also:Pyrethrum (feverfew; now generally included under Chrysanthemum), Tanacetum (tansy), See also:Arnica, Doronicum, See also:Cineraria Calendula (See also:common marigold) (fig. i), Echinops (globe thistle), Centaurea (cornflower) (fig. 2). Some are of medicinal value, such as Anthemis (See also:chamomile), See also:Artemisia (See also:wormwood), Tussilago (coltsfoot), Arnica. Insect See also:powder is prepared from See also:species of Pyrethrum. The order is divided into two suborders:—Tubuliftorae, characterized by See also:absence of latex, and the florets of the disk 2' i. Disk-floret. 3. Ray-floret. 2. Same cut vertically. 4. Fruit with pappus. 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 See also:Britain, in which See also:forty-two genera are native. These include some of the commonest weeds, such as dandelion (See also:Taraxacum See also:Dens-leonis), daisy (Bellis perennis), See also: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-See also:eye daisy and See also:corn-marigold), Arctium (burdock), Centaurea (knapweed and cornflower), Carduus and Cnicus (thistles), Hieracium (hawkweed), Sonchus (sow-thistle), Achillea (yarrow, or milfoil, and sneezewort), Eupatorium (See also:hemp-See also:agrimony), Gnaphalium (cudweed), Erigeron (fleabane), Solidago (golden-rod), Anthemis (may-See also:weed and chamomile), Cichorium (chicory), Lapsana (nipplewort), Crepis (See also:hawk's-See also:beard), Hypochaeris (See also:cat's-See also:ear), and Tragopogon (See also:goat's-beard). Additional information and CommentsThere are no comments yet for this article.
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