Fig. 446.—Diagram of Althæa rosea: i the epicalyx.
An epicalyx is often found formed by floral-leaves placed close beneath the calyx, in some 3, in others several. The median sepal is posterior in the species without epicalyx, often anterior in those which have an epicalyx.—The petals are twisted either to the right or to the left in accordance with the spiral of the calyx; they are most frequently oblique, as in the other plants with twisted corollas, so that the portion covered in the æstivation is the most developed. The corolla drops off as a whole, united with the staminal tube.—Only the 5 petal-stamens are developed, but they are divided into a number of stamens, placed in 2 rows, and provided only with half-anthers (leaf-segments, see Fig. [446]; the sepal-stamens are completely suppressed); these 5 staminal leaves are then united into a tube, frequently 5-dentate at the top, and bearing the anthers on its external side. The pollen-grains are specially large, spherical and spiny. There are from 3 to about 50 carpels united into one gynœceum and placed round the summit of the axis which most frequently projects between them. There is only 1 style, which is generally divided into as many stigma-bearing branches as there are carpels (Figs. [445], [448]). The fruit is a schizocarp or capsule. Endosperm (Figs. [447] A, [451]) scanty, often mucilaginous round the embryo, which is rich in oil.
The order is the most advanced type of Columniferæ; it stands especially near to the Sterculiaceæ, but is separated from these and from the Tiliaceæ, among other characters, by its 2-locular (ultimately 1-chambered) anthers.
The sub-orders may be arranged as follows:—
I. Carpels in one whorl.
A. The fruit a capsule, most frequently with loculicidal dehiscence, and many seeds in each loculus.
1. Gossypieæ. The staminal-column is naked at the apex, blunted, or 5-dentate.—Gossypium (the Cotton plant) has an epicalyx of 3 large ovate-cordate leaves, an almost entire, low and compressed calyx. Solitary flowers. Large, most frequently yellow, corollas. A 3–5-valved capsule with many spherical seeds. “Cotton” is the seed-hairs developed upon the entire surface of the seeds (Fig. [447]), and consists of long, 1-cellular hairs, filled with air (and therefore white); these are thin-walled, with a large lumen, and during drying twist spirally, and come together more or less in the form of bands. They consist of cellulose, and have a cuticle.—Hibiscus has several, most frequently narrow, epicalyx-leaves, a distinct 5-toothed or 5-partite calyx.—Abutilon; Modiola.
Fig. 447.—A Seed of Gossypium with hairs; B the same in longitudinal section.
2. Bombaceæ. The staminal tube is more or less deeply cleft into bundles, sometimes almost to the base; pollen smooth, style simple with capitate, lobed stigma. Almost all plants belonging to this group are trees, and in many instances have large barrel-shaped stems, that is, swollen in the centre, and sometimes covered with large warts. The wood is exceptionally light and soft. The flowers are often enormously large, and have beautiful petals; in some they unfold before the leaves. The capsule-wall is sometimes closely covered on its inner service with long, silky, woolly hairs, while the seeds themselves are generally without hairs. These hairs, however, on account of their brittle nature, cannot be used like those of the Cotton-plant. Digitate leaves are found in the Baobab-tree (Adansonia) from Africa, noted for its enormously thick, but short stem, and in the American Silk-cotton trees (Bombax, Eriodendron, Chorisia). Ochroma, Cheirostemon, Durio, and others also belong to this group. Durio is noted for its delicious fruits, which have a most unpleasant smell.