5. Astereæ, Aster Group (or Radiatæ, Ray-flowered). The flowers are of two forms and different sexes; the ray-flowers are ♀ (sometimes neuter), most frequently with irregular, falsely ligulate, radiating corollas; the disc-flowers are ☿, regular, with tubular corollas (Fig. [610]). Sometimes only tubular flowers are present, as e.g. in Senecio vulgaris (Groundsel), and the exterior of the capitulum is then as in the Eupatorieæ. The stylar branches are straight, more or less flat and short (Fig. [610]).
A. Anthemideæ. Involucral leaves imbricate, generally membranous at the edge; pappus wanting, or at most a membranous margin to the calyx, but without hairs.
[+]. Chaff-like bracts on the receptacle are found in Anthemis (Chamomile), Anacyclus (A. officinarum), Achillea (Milfoil, Fig. [610]), Santolina, etc.
[++]. A naked receptacle is found in the following: Bellis (Daisy) has solitary capitula on leafless stalks with white ray-flowers.—Matricaria (Wild Chamomile) has a conical receptacle. (M. chamomilla has a very high, hollow receptacle; M. inodora has large, odourless capitula, and the receptacle is not hollow.)—Chrysanthemum (Ox-eye) most frequently large, solitary capitula; flat receptacle.—Pyrethrum; pappus scanty.—With these are classed Tanacetum (Tansy) and Artemisia (Wormwood) with tubular corollas only.
B. Heliantheæ. Most frequently a bract to each flower is found on the receptacle. The pappus is never exactly hairy, but consists of scales, spines, etc., and the fruits are most frequently compressed (Fig. [606] c).—Helianthus (Sun-flower); H. tuberosus (Jerusalem Artichoke) has tuberous underground stems. Dahlia has tuberous roots (Am.). Bidens (Bur-marigold, Fig. [606] c); the fruits are compressed with 2 (or more) spines provided with reflexed barbs.—Calliopsis; Rudbeckia; Zinnia; Tagetes has united involucral leaves, and yellow, transparent oil-glands. Spilanthes, Galinsoga, Melampodium, Silphium (Compass-plant), Helenium, Gaillardia.
C. Calenduleæ have 1–2 rows of involucral leaves, a naked receptacle, and large, crescent-shaped, irregularly warted fruits, of different forms in the same capitulum; pappus absent (Fig. [605]).—Calendula (Marigold); ray-flowers ♀, disc-flowers ♂.
D. Senecioneæ, have a fine, hairy, white pappus; no bracts, otherwise as in Anthemideæ. The involucral leaves are most frequently in 1–2 rows.—Senecio (Groundsel) has two whorls of involucral leaves, which most frequently have black tips, the external being much shorter than the internal ones (S. vulgaris has all flowers ☿ and alike).—Cacalia, Doronicum, Cineraria, Ligularia, Arnica (A. montana; large, long-stalked capitula; leaves opposite, forming a kind of rosette).
E. Astereæ have a bristle-like, unbranched pappus, often of a dingy brown; receptacle naked; involucral leaves numerous, imbricate.—Solidago (Golden-rod); capitula small, yellow-flowered, borne in panicles. Aster; disc-flowers most frequently yellow, ray-flowers violet; Callistephus; Erigeron (Flea-bane)—Inula.—All the corollas are tubular in: Gnaphalium (Cud-weed); involucral leaves dry, rattling, often coloured; the foliage-leaves and stem often white with woolly hairs; ray-flowers ♀, with narrow, tubular corolla; disc-flowers ☿ (few). Antennaria (Cat’s-foot; diœcious), Filago, Helichrysum, Ammobium, Rhodanthe and others. Leontopodium (L. alpinum, “Edelweiss”).
F. Ambrosieæ, a very reduced type of Compositæ, differing from the others in having free anthers; the capitula are generally unisexual, monœcious, the ♂ borne in a terminal inflorescence, the ♀ in the leaf-axils. In other respects they are most closely related to Heliantheæ.—Xanthium. In the ♂-capitula there are many flowers without calyx, but with tubular corolla and free involucral leaves. In the ♀-capitula there are only 2 flowers, which are entirely destitute of both calyx and corolla; involucral leaves 2-spined, united to form an ovoid, bilocular envelope, each compartment containing one flower. The envelope of involucral leaves unites with the fruits, enclosing them at maturity with a hard covering from which numerous hook-like spines project, assisting very greatly in the distribution of the fruit. The whole structure thus finally becomes a 1- or 2-seeded false nut.—Ambrosia, the ♀ capitulum 1-flowered.
Pollination. The flowers are somewhat insignificant, but become very conspicuous owing to a number being crowded together in one inflorescence. The corollas of the ray-flowers, being often very large (Astereæ; Centaurea), frequently render the capitula still more conspicuous. The capitula display many biological phenomena similar to those often shown by the individual flowers in other orders, e.g. by periodically opening and closing, in which the involucral leaves resemble the calyx in their action. (The name “Compositæ” originates from the term “flos compositus,” composite flower). An abundance of honey is formed, which to some extent fills up the corolla-tube, and since insects may visit a number of flowers in the course of a short period they are very frequently visited, especially by butterflies and bees. The pollination has been described on page [567]. Protandry is universal. In the bud the tips of the styles, covered by the sweeping-hairs, lie closely enveloped by the anther-tube; in the next stage the style grows through the tube and sweeps out the pollen as it proceeds; ultimately the stylar branches expand and the stigma is then prepared to receive the pollen. In many, the sensitiveness of the filaments assists in sweeping out the pollen at the exact moment of the insect visit. Regular self-pollination is found e.g. in Senecio vulgaris; wind-pollination e.g. in Artemisia and the plants related to it.