Pollination. Light micrograph of a section through an evening primrose (Oenthera biennis) stigma. Pollen grains (circular) are adhering to the surface of the stigma. Once attached to the surface, each pollen grain produces a tube that penetrates the stigma (dark areas). The tube enables the male sex cells to reach the ovary (not seen), where fertilisation takes place. Magnification: X30 when printed 10 centimetres wide.