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_. A leaf or scale will often be observed at the foot of a flower stalk or at the base of a sessile flower. This is termed a _bract_, and a flower possessing a bract is said to be _bracteate_. The bract is sometimes so large that it almost completely encloses the flower, or even a cluster of flowers. [Illustration: INFERIOR (1) AND SUPERIOR (2)OVARY.] The flower is the reproductive part of the plant, being concerned in the production of the seeds; but the organs directly connected with the seed-formation are the pistil and the stamens, the former containing the ovules, and the latter producing the pollen cells by means of which the ovules are impregnated. Thus the stamens and the pistil are the _essential_ parts of the flower, though the corolla and the calyx may perform some subsidiary function in connexion with the reproduction of the species. This being the case, a flower may be described as _perfect_ if it consists of stamens and pistil only, without any surrounding calyx or corolla; and _imperfect_ if it possesses no pistil _or_ no stamens, regardless of the presence or absence of calyx and corolla. [Illustration: UNISEX FLOWERS OF THE NETTLE 1. Pistillate. 2. Staminate.]