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Pollination and fertilisation

Sexual reproduction is a way of making a new individual by joining two special sex cells, called gametes. In the sexual reproduction of animals and plants, the male and female gametes join to form a single fertilised cell called a zygote, which develops into a new individual with a unique collection of genetic material. In a population, this genetic variation is important because, if conditions become hard, there is a chance that some individuals will survive.

Many flowering plants are also able to reproduce asexually through various structures such as stolons, rhizomes and tubers. However, all individuals produced like this are clones of their parent, with no genetic variation. This means that, if something happens that is fatal to one individual, such as a disease outbreak, it is likely to be fatal to all.

Pollination

Pollination is a very important part of the life cycle of a flowering plant. It is part of the sexual reproduction process of flowering plants, which results in seeds that will grow into new plants. Flowers are the structures of flowering plants that contain all the specialised parts needed for sexual reproduction.

Plants have gametes, which contain half the normal number of chromosomes for that plant species. Male gametes are found inside tiny pollen grains on the anthers of flowers. Female gametes are found in the ovules of a flower. Pollination is the process that brings these male and female gametes together.

Pollen can’t get from the anthers to the ovules on its own, so pollination relies on other things to move the pollen. The wind or animals, especially insects and birds, pick up pollen from the male anthers and carry it to the female stigma. Flowers have different shapes, colours and smells, and often sugary nectar and nutritious pollen, to encourage animals to visit and pollinate them. Wind-pollinated flowers are shaped to make it easy for the wind to pick up or deposit pollen.

Many flowers can be pollinated by their own pollen – a process called self-pollination. However, this does not always result in the genetic variation needed for species to survive. Many plants have ways to make sure they are only pollinated by pollen from a flower on a different plant, which is called cross-pollination. Some have the male and female parts in separate flowers on the same plant, while others have male and female flowers on different plants. Many have the stigmas and anthers ripening at different times to prevent self-pollination.

Fertilisation

Only after pollination, when pollen has landed on the stigma of a suitable flower of the same species, can a chain of events happen that ends in the making of seeds. A pollen grain on the stigma grows a tiny tube, all the way down the style to the ovary. This pollen tube carries a male gamete to meet a female gamete in an ovule. In a process called fertilisation, the two gametes join and their chromosomes combine, so that the fertilised cell contains a normal complement of chromosomes, with some from each parent flower.

The fertilised ovule goes on to form a seed, which contains a food store and an embryo that will later grow into a new plant. The ovary develops into a fruit to protect the seed. Some flowers, such as avocados, only have one ovule in their ovary, so their fruit only has one seed. Many flowers, such as kiwifruit, have lots of ovules in their ovary, so their fruit contains many seeds.

Useful links

Find out how the artificial control of pollination plays a part in the breeding of new fruit cultivars.
www.biotechlearn.org.nz/focus_stores/.../breeding_a_new_apple_cultivar external link

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