Fig. 124.—Anisocarpous sympetalæ (Aggregatæ). A, flowering branch of Houstonia purpurea, × 1 (Rubiaceæ). B, vertical section of a flower, × 2. C, fruit of bluets (Houstonia cœrulea), × 1. D, cross-section of the same. E, bedstraw, Galium (Rubiaceæ), × ½. F, a single flower, × 2. G, flower of arrow-wood, Viburnum (Caprifoliaceæ), × 2. H, the same, divided vertically. I, flowering branch of trumpet honeysuckle, Lonicera (Caprifoliaceæ), × ½. J, a single flower, the upper part laid open, × 1. K, diagram of the flower. L, part of the inflorescence of valerian, Valeriana, (Valerianeæ), × 1. M, young; N, older flower, × 2. O, cross-section of the young fruit; one division of the three contains a perfect seed, the others are crowded to one side by its growth. P, inflorescence of teasel, Dipsacus (Dipsaceæ), × ¼. fl. flowers. Q, a single flower, × 1. R, the same, with the corolla laid open.
The various species of Lobelia, of which the splendid cardinal-flower (L. Cardinalis) ([Fig. 123], E) is one of the most beautiful, represent the very characteristic family Lobeliaceæ. Their milky juice contains more or less marked poisonous properties. The last family of the order is the gourd family (Cucurbitaceæ), represented by a few wild species, but best known by the many cultivated varieties of melons, cucumbers, squashes, etc. They are climbing or running plants, and provided with tendrils. The flowers are usually unisexual, sometimes diœcious, but oftener monœcious ([Fig. 123], I).
Fig. 125.—Anisocarpous sympetalæ (Aggregatæ). Types of Compositæ. A, inflorescence of Canada thistle (Cirsium), × 1. B, vertical section of A. r, the receptacle or enlarged end of the stem, to which the separate flowers are attached. C, a single flower, × 2. o, the ovary. p, the “pappus” (calyx lobes). an. the united anthers. D, the upper part of the stamens and pistil, × 3: i, from a young flower; ii, from an older one. an. anthers. gy. pistil. E, ripe fruit, × 1. F, inflorescence of may-weed (Maruta). The central part (disc) is occupied by perfect tubular flowers (G), the flowers about the edge (rays) are sterile, with the corolla much enlarged and white, × 2. G, a single flower from the disc, × 3. H, inflorescence of dandelion (Taraxacum), the flowers all alike, with strap-shaped corollas, × 1. I, a single flower, × 2. c, the split, strap-shaped corolla. J, two ripe fruits, still attached to the receptacle (r). The pappus is raised on a long stalk, × 1. K, a single fruit, × 2.
The last and highest order of the Sympetalæ, and hence of the dicotyledons, is known as Aggregatæ, from the tendency to have the flowers densely crowded into a head, which not infrequently is closely surrounded by bracts so that the whole inflorescence resembles a single flower. There are six families, five of which have common representatives, but the last family (Calycereæ) has no members within our limits.
The lower members of the order, e.g. various Rubiaceæ ([Fig. 124], A, E), have the flowers in loose inflorescences, but as we examine the higher families, the tendency for the flowers to become crowded becomes more and more evident, and in the highest of our native forms Dipsaceæ ([Fig. 124], P) and Compositæ ([Fig. 125]) this is very marked indeed. In the latter family, which is by far the largest of all the angiosperms, including about ten thousand species, the differentiation is carried still further. Among our native Compositæ there are three well-marked types. The first of these may be represented by the thistles ([Fig. 125], A). The so-called flower of the thistle is in reality a close head of small, tubular flowers ([Fig. 125], C), each perfect in all respects, having an inferior one-celled ovary, five stamens with the anthers united, and a five-parted corolla. The sepals (here called the “pappus”) (p) have the form of fine hairs. These little flowers are attached to the enlarged upper end of the flower stalk (receptacle, r), and are surrounded by closely overlapping bracts or scale leaves which look like a calyx; the flowers, on superficial examination, appear as single petals. In other forms like the daisy and may-weed ([Fig. 125], F), only the central flowers are perfect, and the edge of the inflorescence is composed of flowers whose corollas are split and flattened out, but the stamens and sometimes the pistils are wanting in these so-called “ray-flowers.” In the third group, of which the dandelion ([Fig. 125], H), chicory, lettuce, etc., are examples, all of the flowers have strap-shaped, split corollas, and contain both stamens and pistils.
The families of the Aggregatæ are the following: I. Rubiaceæ of which Houstonia ([Fig. 124], A), Galium (E), Cephalanthus (button-bush), and Mitchella (partridge-berry) are examples; II. Caprifoliaceæ, containing the honeysuckles (Lonicera) ([Fig. 124], I), Viburnum (G), snowberry (Symphoricarpus), and elder (Sambucus); III. Valerianeæ, represented by the common valerian (Valeriana) ([Fig. 124], L); IV. Dipsaceæ, of which the teasel (Dipsacus) ([Fig. 124], P), is the type, and also species of scabious (Scabiosa); V. Compositæ to which the innumerable, so-called compound flowers, asters, golden-rods, daisies, sunflowers, etc. belong; VI. Calycereæ.
