When the pollen is ripe the pollen sacs of another burst, usually by longitudinal slits, and so expose the pollen. The ovules within the carpels cannot develop into flower seeds unless the female cells they contain are fused with certain male cells developed by the pollen. Before this can take place pollen must be transferred from the stamen to stigma of the carpel. This process is known as pollination. If the pollen from one flower is carried to the stigma of the same flower, the flower is said to be self-pollinated. If the pollen from of a flower is carried to the stigma of another flower, cross-pollination takes place.
A survey of the floral world reveals that many plants possess natural devices which ensure , or at least favour, cross-pollination. It may be that cross-pollination in these flowers, bringing together characteristics from two individuals, results in healthier plant seeds and more virile seedling: though in some of the most successful of flowers, including certain cereals, self-pollination is general.