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Bignonia (family Bignoniaceae) pollen. Computer illustration of pollen grains from a bignonia flower. Pollen grains are the male sex cells of a flowering plant. Their characteristic surface is used by botanists to recognise and classify plants. Pollen in plants is used for transferring male genetic material from the anther of a single flower to the stigma of another in cross-pollination. In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAURIZIO DE ANGELIS)