Pollination and Fertilization

Pollination is the transfer of pollen grains from the anther (the male part) to the stigma (the tip of the female part). It must happen before a flower can make seeds, because the pollen carries the male gametes. In self-pollination, the pollen lands on the stigma of the same flower or another flower on the same plant. In cross-pollination, the pollen is carried to the stigma of a different plant of the same species. Cross-pollination needs a carrier; self-pollination does not.

Cross-pollination mixes the characters of two plants, so the offspring are healthier and more varied. Many flowers are built to encourage it. Some are unisexual (only male or only female parts). In some, the anther and stigma ripen at different times so a flower cannot pollinate itself. Others are self-sterile (their own pollen will not work), or have structural barriers that keep pollen away from their own stigma. These devices are called contrivances.

Pollen cannot move on its own, so an agent carries it. Insect-pollinated flowers (entomophily) are bright, scented and make nectar to attract bees and butterflies. Wind-pollinated flowers (anemophily) are small, dull and make huge amounts of light, dry pollen, like in grasses and maize. Water-pollinated flowers (hydrophily) release pollen onto the water. Some flowers are pollinated by birds (ornithophily). Humans also pollinate by hand to breed better crops - this is artificial pollination.

Once a pollen grain lands on a suitable stigma, it grows a pollen tube down through the style. The tube enters the ovule through a tiny pore called the micropyle and carries two male gametes into the embryo sac. Here a special event called double fertilization happens. One male gamete fuses with the egg cell to form the zygote (2n), which grows into the embryo - this is syngamy. The other male gamete fuses with the two polar nuclei to form the endosperm (3n), the food store - this is triple fusion. Two fusions in one ovule: that is why it is called double fertilization. The ovule then becomes the seed and the ovary becomes the fruit.