The outer whorl is the calyx, and is composed of parts called sepals, which may be either united or distinct. The calyx is usually green; but, in some cases, is more or less highly coloured. Sometimes the calyx is quite free from the pistil or central part of the flower, the sides of which are thus left naked, and the calyx is then said to be inferior. If, however, it is united to the surface of the pistil it is superior. When it remains after other parts of the flower have decayed, it is said to be persistent.
The second whorl—the corolla—is usually the whorl that gives most beauty to the flower. It is composed of parts, united or distinct, called petals.
Both calyx and corolla vary very considerably in shape. They may be cup-shaped, tubular, bell-shaped, spreading, funnel-shaped, lipped, &c. If the sepals and petals are arranged symmetrically round a common centre, the calyx and corolla, respectively, are said to be regular; if otherwise, they are irregular.
The third whorl consists of the stamens, each of which, in its most perfect form, is made up of a filament or stalk, and an anther which, when mature, splits and sets free the pollen that is formed within it. Sometimes the stamen has no filament, and the anther is then said to be sessile.
The mode of attachment of the stamens is very variable. They may grow from immediately below the pistil, or from its summit; or they may be attached to either the petals or the sepals. The filaments are usually distinct, but sometimes they are united in such a manner as to form a tube, or grow into two or more bundles. The anthers are usually distinct, even when the filaments are united; but these sometimes grow together.
Longitudinal Section Through the Flower of the Buttercup
Showing the calyx, corolla, stamens and pistil. The pistil consists of several distinct carpels, one of which is represented in section to show its single ovule.
The central part of the flower is the pistil, and this is made up of one or more parts called carpels. Each carpel, when distinct, is a hollow case or ovary, prolonged above into one or more stalks or styles, tipped by a viscid secreting surface called the stigma. The ovary contains the ovules, attached to a surface called the placenta; and these ovules, after having been impregnated by the pollen, develop into seeds which are plants in embryo. The ovary may have no style, and the stigma is then sessile.
Where the pistil consists of more than one carpel, these carpels may unite in such a manner as to form a single cell, or an ovary of two or more cells. In other cases the carpels remain quite distinct, thus forming a number of distinct ovaries, each with its own stigma. For purposes of identification it is often necessary to note the position of the placenta. This may be at the side of the ovary, in which case it is said to be parietal; or it may stand up in the centre of the ovary, without any attachment to the sides, when it is described as free central. If, however, it occupies the centre of the ovary, but is attached by means of radiating partitions to the sides, it is termed axile.
If the ovary is quite free in the centre of the flower, the surrounding parts being attached below it, it is said to be superior; but if the perianth (p. [11]) adheres to it, it is inferior.