The barrier between stamens and pistils prevents self-pollination.

Plants have developed a variety of strategies to prevent self pollination, including:

Heterostyly is the presence of two or more types of flowers within the same plant, with differing lengths of styles and stamens.

Herkogamy is a condition in which the flower structure is such that the pollen grains fail to reach the stigma despite being bisexual. Some flowers have a mechanical barrier between the compatible pollen and the stigma, making self-pollination impossible. There is sometimes a hood-like covering that covers the stigma.