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.
Pollination – the transfer of pollen grains from the male anther to the female stigma. In Britain most pollen transport is carried out by insects but many trees and grasses rely on wind pollination. In the tropics insects, birds, bats and some mammals are important pollinators. Worldwide 87% of flowering species are pollinated by animals with 65% by insects.
Cross-pollination – the transfer of pollen from the anthers of one individual to the stigma of another individual of the same species. This is known as outcrossing and maximises genetic variability and prevents inbreeding.
Self-pollination – the transfer of pollen from the anthers to the stigma of the same flower or to a flower on the same plant. This results in inbreeding and reduction of variability but may still result in viable seeds. Some flowering plant species can switch from cross pollination to self pollination if there are few pollinators around. Often found in pioneer species.
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 ovary becomes a fruit and the ovules become the seeds within it.