When the anther is mature, each of its grooves splits near the edge, and allows the fine powdery granules which fill the anthers to be removed by insects or by other means. This fine powder is named the pollen, and each of the granules composing it is named a pollen grain. If the stamens are now removed the centre of the flower alone is left. If the lower part of the centre of the flower be cut across, it will be found to be divided into a large number of cavities containing the minute rudiments of future seeds. It will be seen that there are ten cavities, though they may vary in number. The central organ of the flower is named the pistil (from Lat. _pistillum_, pestle). The pistil is usually composed of united leaves. The separate leaves of the pistil are termed carpels (from Greek _karpos_, fruit). These leaves are sometimes not combined, as they are in the orange. The style belongs to the carpel, and varies considerably in length, as well as in stoutness, in different flowers. Although the carpels may be united, the styles may remain completely separate, as, for example, in the pink, or, as in the fuchsia, they may be combined into a single rod. The pollen grains (Lat. fine flour) contained in the anther are composed of very rich protoplasm (Greek _protos_, first; _plasma_, formative matter), which usually has in it small drops of oil and small starch granules. The pollen grains are bounded by two principal layers, an outer and an inner; the purpose of the outer layer (which is often provided with thickenings in the shape of knots, spines, etc.) being to preserve the contents of the grain from evaporation. The inner layer is living and capable of growth, and at certain spots it possesses thickenings which project into the protoplasm. Opposite to these the external cuticle is frequently thinner, and this eventually is lifted off as a sort of lid, and through this the inner substance can grow out, and is then named the pollen tube. When the anther lobes open to discharge their pollen grains, these grains are completely developed. The grains fall on the part of the ovary named the stigma (Greek _stigma_, a puncture made with a sharp instrument; here it means a sharp point or apex) and the inner layer begins to force its way out. The tube is produced from the contents of the pollen grain, and is formed by growth, just as any other part of the plant. The pollen tube passes down to the ovules, the route depending on the length of the style. The time taken by the pollen tube to reach the ovary may amount to a few hours in certain plants, while it needs months in others. It is necessary that at least one pollen tube should enter the mouth of the ovule before it can develop into a seed. The seed, when mature, contains the embryo plant. It is not possible for an ovule in numerous cases to be fertilized by pollen from stamens that grow near it in the same flower.