The perianth and stamens are most frequently perigynous on the edge of the widened receptacle; its form varies between a flat cupule and a long tube or a cup (Figs. [495], [496], [498], [499], [500]); the carpels are situated on its base or inner surface, in some instances on a central conical elongation of the floral axis (Fig. [496]). The carpels in Pomaceæ also unite more or less with the hollow receptacle, or this grows in and fills up the space between the carpels, so that a more or less epigynous flower is formed (Fig. [504]).—The following numbers of stamens occur: 5, 10 (in 1 whorl), 15 (10 + 5), 20 (10 + 5 + 5), 25 (10 + 10 + 5), 30-50 (in 10-merous whorls)—compare the diagrams. The theoretical explanation of this relation of the 10-merous whorls and their alternation with the 5-merous whorls is not definitely determined; a splitting of the members of the 5-merous whorls may be supposed, but the development shows no indication of this, and it is not supported in any other way. Several genera have “gynobasic” styles, that is, the style springs from the base of the ovary (Fig. [497] A, B).
The Rosifloræ are on one side closely related to the Saxifragaceæ (especially through Spiræa) from which it is difficult to separate them, and to the Myrtifloræ; on the other side they are allied, through the Mimosaceæ with the large number of stamens, and through the Amygdalaceæ with its single carpel, to the Leguminosæ. The family begins with forms which have many-seeded follicles, and passes on the one side to forms with nuts and drupes in perigynous flowers, and on the other side to the Pomaceæ.
Order 1. Rosaceæ. Herbs or shrubs, generally with compound leaves and persistent (adnate) stipules, flower perigynous, gynœceum formed of many free (therefore oblique) carpels, syncarps with fruitlets of various kinds. The exceptions are noted under the genera.
Fig. 494.—Diagram of Comarum palustre.
Fig. 495.—Flower of Spiræa lanceolata.
1. Spiræeæ (Fig. [495]) has 2–many ovules in each ovary, while in the other groups there is generally only 1, and never more than 2 ovules in each loculus. There are generally 5 cyclic carpels and the fruit is 5 follicles, which are not enclosed by the receptacle. The majority are shrubs. Stipules are often wanting.—Spiræa (Meadow-Sweet). The flowers are generally borne in richly flowered inflorescences of various forms. The carpels, in some species, unite together and form a simple gynœceum with free styles (an approach to the Pomaceæ).—Closely allied to Spiræa are the East Asiatic shrubs: Kerria japonica, which has solitary flowers, in this country nearly always double (the fruit a nut), and Rhodotypos kerrioides which has opposite leaves, a remarkable feature among the Rosifloræ; it has a 4-merous flower, a well developed disc inside the andrœcium, and a drupe. Closely allied also is Gillenia (N. Am.) differing chiefly in the ascending ovules, Spiræa having pendulous ovules, and a more tubular receptacle.
The groups Quillajeæ and Neuradeæ form a transition from Spiræa to Pomaceæ. In the first group, which contains only trees or shrubs with generally simple leaves, the carpels are either free or united (into a capsule); in the second the receptacle unites with the carpels, which are themselves often united together; in this case, too, the fruit is a capsule. Quillaja (S. Am.); Exochorda (China).
2. Potentilleæ (Figs. [494], [496], [497]). The flower has an “epicalyx” (Fig. [494] C) alternating with the sepals and formed by their stipules which are united in pairs, and hence its leaves are often more or less deeply bifid. The receptacle is cupular and often quite insignificant. The sepals are valvate in the bud. The large number of fruitlets are achenes, borne on a well-developed convex portion of the receptacle (the Ranunculeæ resemble the Potentilleæ, but have no epicalyx, no enlarged receptacle, and spirally-placed stamens). Most of the species are herbs with dichasial inflorescences, often arranged in racemes.—Potentilla (Cinquefoil). The achenes are borne on a dry, hairy receptacle; the style is situated towards the apex of the ovary, and is not prolonged after flowering. Herbs with digitate, in some, however, pinnate leaves, and generally yellow flowers.—Comarum (Fig. [494]) (Marsh Cinquefoil) forms, by its fleshy-spongy receptacle, a transition to the next genus.—Fragaria (Strawberry) (Fig. [496]). The receptacle becomes finally fleshy, coloured, and falls off (biologically it is a berry); the numerous fruitlets (drupes with thin pericarp) have basal styles (Fig. [497]); leaves trifoliate; long, creeping runners.—Geum (Avens) has a terminal style which after flowering elongates into a long beak, with the apex (after the uppermost part has been thrown off) bent back into a hook, thus furnishing a means of distribution for the fruits. Leaves pinnate.—Dryas comprises 3 Arctic or Alpine species with simple leaves and solitary flowers, the calyx and corolla 8–9-merous, the fruit resembles that of Geum, but the styles become still longer and feather-like (a flying apparatus).