Pollination is part of sexual reproduction in plants. It describes how the pollen grains get to the female parts of a plant. Pollen grains, which contain the male gametes, need to get to where the female gamete(s) are.
What happens is basically the same as sexual reproduction in animals. Each pollen grain is haploid: it has half of the DNA (genetic information) that is needed to make a new plant. During fertilization this combines with the DNA that is in the egg of the female part and a zygote is formed. In seed plants a seed is started.
Ways of pollinating
In flowering plants, pollen has to get from one flower to another. There are two main ways that this can happen: by non-living things like wind or water, or by living things such as insects or birds.
Maize and the wind
Maize (called corn in some parts of the world) is pollinated by wind. The male anthers let go of their pollen and it blows over to a nearby female flower on another corn plant. Most of the flowers are either male or female on a corn plant (monoecious), rather than both sexes in one flower (hermaphrodite).
Maize flowers have evolved (changed over time) to use wind for pollination. They do not need pretty petals. The pollen is light so it can blow around, and the ends of the female parts (stigma) are fluffy to catch all the tiny pollen grains.
Tomatoes and bees
With tomato plants, bees move the pollen from the male parts of one flower (anthers), to the female parts of another flower (stigma). The bee moves between flowers as it collects the nectar that the flowers make. The bees take the nectar and some pollen back to their hive, and the tomato plants get to reproduce (make new tomato plants).
Because the tomato flowers have evolved to attract bees, they have spread-out petals and are white to human eyes (bees, like most insects, can see into the ultraviolet range as well as our visual range of wavelengths). The pollen is often stuck together in clumps called pollinia, which in turn get stuck to the bee. Bees are extremely hairy, and carry tiny electric charges which attract the pollen onto their bodies. Honey bees have special pollen baskets, usually on their rear legs; they groom the pollen off their bodies into these pockets.
Much of the pollen gets taken back to the nest or hive, where it is used as a source of protein, most needed by the larvae. Some gets rubbed off on the next flower, where the female stigma is sticky. A pollen tube grows down to permit the male gamete to fertilize an egg and make a seed.
90% of flowering plants are pollinated by animals, and only 10% use abiotic (non-living) pollination. Of these abiotic pollinations, 98% is done by wind and just 2% by water.
What happens after pollination
What happens after pollination is fertilisation. In plants it is a double fertilisation in which two sperm cells fertilize cells in the plant ovary. One of these is a normal fertilisation, which produces the embryo. The other is a unique kind of fertilisation which produces the seed endosperm.
The process begins when a pollen grain sticks to the stigma of the pistil (female reproductive structure). Then it germinates, and grows a long pollen tube. While this pollen tube is growing, a haploid cell travels down the tube behind the tube nucleus. This cell divides by mitosis into two haploid sperm cells.
As the pollen tube grows, it makes its way from the stigma, down the style and into the ovary. Here the pollen tube reaches the ovule and releases its contents (which include the sperm cells). One sperm makes its way to fertilize the egg cell, producing a diploid (2n) zygote. The second sperm cell fuses with two cell nuclei, producing a triploid (3n) cell.
As the zygote develops into an embryo, the triploid cell develops into the endosperm, which serves as the embryo's food supply. The ovary now will develop into a fruit and the ovule will develop into a seed.
Gymnosperms
There is evidence that some gymnosperms were insect-pollinated in the Triassic period, but pollination by animals is not the main method in this group. Most are wind-pollinated. Some gymnosperms and their insect pollinators are co-evolved for pollination. The best-known examples are members of the order Cycadales and their associated species of beetle.