POLLINATION – the process by which pollen grains are transferred from the male anther to the female stigma. Cross-pollination occurs when pollen grains arrive from another plant of the same species. Self-pollination occurs within a flower or between flowers of one plant. Pollination is the first step in a complex process that results in the production of seeds. The Narcissus (daffodil) flower, shown here in cross-section, illustrates the process.
Pollen grains are produced on the walls of the anthers. Each grain contains a pollen tube cell and two haploid sex cells (each has one set of male chromosomes). For cross-pollination to take place pollen grains from another Narcissus flower must be transported to the stigma. On reaching it the pollen grain germinates and produces a pollen tube (shown in red) which then grows down the style to reach an ovule in the ovary. The pollen tube then enters the ovule and discharges the 2 sex cells into the embryo-sac. One male sex cell fuses with the egg cell to become an embryo and the other fuses with a second cell (with two haploid nuclei) and the triple haploid cell becomes the endosperm that provides nutrients for the growing embryo. This process known as ‘double fertilisation’, involves 2 sperm fertilising 2 egg cells. The process must be repeated for each ovule in the ovary. After fertilsation the ovule becomes a seed.