The structure of most flowers affords an excellent indication of the device used for the transference of pollen from one flower to another (pollination). Long ago it was assumed that Nature wished no flower to be fertilized by its own pollen, but in the light of present knowledge we know this is not wholly true. The subject of pollination of flowers by insects received a great light through the investigations of Charles Darwin and the publication in 1862 of his well-known book on the fertilization of orchids by the aid of insects.
As we understand the matter today, it appears that flowers are habitually intercrossed (flowers of the same species), and that there are manifold structural adaptations which secure or favor this interchange of pollen. Separation of sexes (stamens and pistils) is a direct adaptation to cross-pollination, rendering it necessary between individuals with dioecious flowers, and favoring it in most plants with monoecious and polygamous flowers. Strictly, close fertilization can take place in hermaphroditic flowers only.
Flowers depend upon certain external agencies for the transference of pollen from one flower to the flower on another plant. These agencies are wind (anemophilous flowers) and insects (entomophilous flowers). Other agencies are of minor importance, although water must be considered in connection with some aquatic plants.
Wind-pollinated flowers are mostly dull in color, destitute of odor and nectar, since these qualities attract insects. Wind-pollinated flowers usually have the sexes separated, the flowers borne in great abundance and have very light pollen. Most of our common trees (the Pines, Oaks, Hickories etc.) depend upon wind for the transference of pollen, as do also the grasses, sedges, Plantain and others.
Insect-pollinated flowers are correlated with showy coloration (including white, which is most showy at dusk), odor or secretion of nectar. Structural adaptations of the flower in reference to insect visitation are wonderfully various, and most of these are found upon investigation to favor, or often to necessitate, cross-pollination. The range of these variations is too extensive to be treated here. Literature upon this subject is easily available and most textbooks of botany contain chapters upon the subject.
After pollination the pollen grain germinates upon the surface of the stigma, sends a tube down through the tissue of the stigma and style and discharges into the ovule a male nucleus which unites with a nucleus in the embryo sac of the ovule, fertilizing the ovule, and stimulating its development into an embryonic plant. By a process of hardening of the coats of the ovule its development is arrested and the seed is produced.