Pollen tube. Light micrograph of a section through an evening primrose (Oenthera biennis) stigma. A pollen grain (circular, lower centre) has adhered to the surface of the stigma and produced a pollen tube that is penetrating the surface of the stigma. When the tube is produced, the pollen nucleus divides to produce two nuclei. One is a vegetative nucleus at the tip of the tube (not seen), which controls the growth of the tube. The other is the generative nucleus, it is seen here passing from the grain through the tube (dark spot). It will later divide to form two sperm nuclei (male gametes) which are passed through the tube to the ovary, where fertilisation takes place. Magnification: X234 when printed 10 centimetres wide.