Stigma
(from stigma, a mark; in allusion to the Stigma being a mark or spot on the style). That part of the pistil of a flower which is fitted to receive the pollen when mature, and to permit the passage of pollen tubes for the fertilisation of the ovules. It is peculiar in having its surface covered with long cells, attached loosely by one end to the cells below, with the other end free. These cells secrete a sticky fluid, which retains the pollen grains when they touch it, and also stimulates them to emit tubes (see Pollen). The tubes can readily pass in between the loosely-arranged cells of the Stigma, and then down the loose "conducting tissue" of the style and ovary. The Stigma has no epidermis upon it, in which it differs from all other parts of flowering plants. It is usually situated on the tip or along one side of the style; or the latter may be absent, in which case the Stigma is situated on the tip of the ovary. It may be displaced from this position by inequality of growth in the two sides of the ovary. Each carpel has a Stigma; but where two or more carpels are closely united, the Stigmata may also be so united as to appear like one. In those flowers that are pollinated by wind (e.g., Grasses), the Stigma is frequently covered with long, spreading hairs; while in those pollinated by insects, and in cleistogamous flowers, the Stigma is usually small, and confined to the tip of the style, or to a narrow line on one side of it.