Compositæ. This is the richest of all natural orders in species, these numbering about 10,000; it is also the most widely distributed through all regions of the globe, although most abundant in temperate and subtropical climates; and, furthermore, the richest in individuals, it having been reckoned that about every tenth plant on the earth's surface is a composite. The rationale of this may be broadly seen (at least if we grant an individual constitution and structure especially well adapted to both vegetation and reproduction), for we can see that the consequent great number of healthy individuals must be associated with active local competition and survival of the fittest, and with distribution of these over a wide area; while this again is at least one condition of considerable differentiation of varieties, and so ultimately of species.
While popularly a daisy or dandelion is regarded as a simple flower, the roughest examination suffices to analyse this into an orderly assemblage, technically a capitulum of small but distinct florets; hence the beginner is apt, without more ado, to refer all similar aggregates, say a head of Scabious (Dipsacææ) or Seapink (Plumbaginææ) to the Compositæ. We find, however, such aggregates arising in all alliances, and naturally so; racemes, spikes, or umbels of flowers wholly distinct in structure may all be shortened down into heads or capitula, since reproductive development is tending everywhere to check vegetative growth. We are thus led to inquire of what forms are the composites to be regarded as the reduced members, and the evidence of comparative anatomy goes to prove their relationship to Campanulææ and their irregular forms the Lobeliææ; in a word, to view the yellow 'tubular' florets of our daisy as a head of tiny bells, while the white strap-shaped or 'ligulate' ones of its ray, like all those of a dandelion, resemble the flowers of Iobelia.
In shortening down, or less figuratively, in arresting the inflorescence into a head, the bract of each separate flower remains in place; and in a Zinnia, or sunflower, we find each floret with its separate bract throughout the whole capitulum; in most cases, however, these disappear. Those of the outermost florets, however, together with the immediately lower leaves of the flower-axis (which bear no florets, and are thus in strictness not entitled to their common name of bracts) become usually crowded into an involucre. This subserves in bud the protective purposes of a calyx to the whole inflorescence at once, and thus the calyx of the separate florets becomes unnecessary. In its place we find at most a circle of fine downy hairs, which may be characteristically serrated or feathered, and which only reach full development and usefulness when the fruit has to be distributed. On account of its merely epidermic nature and late appearance, its calycine nature has been denied, and the term pappus substituted; the evolutionist need, however, feel little hesitation in regarding the pappus as simply representing the epidermic fringe of a reduced calyx, nor is verificatory evidence wanting. The stamens grow upon the united corolla and themselves unite by the anthers, thus forming a ring or rather pollen-bearing tube, up through which the style grows, all much as in bells. The style bears two stigmas, indicating an originally two-carpelled arrangement, but the ovary is one-celled, and contains only a single ascending ovule. The ovary hardens as a nutlet, which is commonly floated away upon the wind when ripe by help of its pappus; it may be anchored where it descends by its minute grappling-hooks or serrations. In this way it is conveyed to new soil, it may be at a great distance, an obvious advantage, alike to the species or the individual, when germination takes place.
The classification of so many closely related genera and species into larger groups is one of the most difficult problems of the systematist, and all attempts must as yet be admitted to be largely artificial. The method of Jussieu is to separate (1) those in which the florets are all tubular, as in the thistles (even though the outer be enlarged as in corn-flowers—see CENTAURY), as Cynarocephale; (2) those in which, while the inner florets are tubular, the outer are ligulate as in daisy, sunflower, &c., as Corymbiferae, and (3) those in which all the florets are ligulate, as in dandelion or chicory, as Cichoraceæ. The more recent method of De Candolle is now more generally adopted: his distinctions are (1) Tubulifloræ, including those which have mainly tubular florets, although the ray be ligulate, thus abandoning the attempt to separate the first two divisions of Jussieu; (2) Ligulifloræ, with florets all ligulate, corresponding to Cichoraceæ; (3) Labiatifloræ, with florets all bi-labiæ, a small South American group. But while this last group has unquestionable distinctness, the value of both preceding classifications in other respects is shown to be somewhat superficial by the familiar fact that in cultivation tubular florets tend to become ligulate (in inaccurate popular phrase, double). Thus we see the wild daisy, dahlia, chrysanthemum, &c., alike practically passing into the ligulate group on cultivation.
Although many composites are cultivated and useful plants, none attain the highest economic importance: yet the artichoke, and Jerusalem artichoke, salsafy, lettuce, endive, &c., are familiar inmates of the kitchen-garden, while chicory is extensively cultivated as a substitute for coffee, and even sometimes, as well as Jerusalem artichoke, for the purpose of feeding domestic animals. A very few, like safflower and saw-wort, yield dyestuffs; from the seed of others—e.g. sunflower, a bland oil is expressed, while many are of time-honoured repute for their medicinal properties—e.g. chamomile, arnica, wormwood, elcampane, &c. A still greater number—e.g. headed by dahlias and sunflowers, asters and chrysanthemums—are esteemed ornaments of our flower-gardens, particularly in the latter part of summer and in autumn. Being mostly herbs, or rarely shrubs, the order is quite unimportant as regards timber; the Siriehout (Tarchonanthus camphoratus), a small tree of the Cape of Good Hope, is, however, close-grained and beautiful.
For detailed information, see systematic works such as Luerssen, Med. Pharm. Botanik; Baillon, Histoire des Plantes; or Engler's Pflanzen-familien.