Pollination and fertilisation

Pollination and fertilisation are the two key steps in plant sexual reproduction. Pollination occurs when the male pollen lands on the sticky, female stigma. Fertilisation occurs when the pollen has moved down through the female style, entered the ovary and fused with the female ovule (the egg) which then forms the embryo.

Pollination can either involve pollen and an ovule from the same plant (“self-pollination”) or from a different plant (“cross-pollination”). Cross-pollination is the process used for the hybridisation of plants, which can occur naturally or by human intervention. To some extent plants can control pollination, often to encourage cross-pollination. They may do this, for example, by the stigma ripening and becoming receptive to pollen before the pollen in the same plant is ripe.

There are many different ‘carriers’ which can transfer pollen onto a flower’s stigma:

• Flying insects – the bright colours and scents of flowers aren’t there to entertain us, they’re used to attract insects for pollination. Insects are enticed into the flower with the promise of sweet nectar and, upon entering the flower, they brush against the anthers and pick up pollen on their bodies. They then rub this against the stigma of the same plant, or neighbouring plants, and pollination has taken place. Bees and flies are commonly involved in pollination (if you notice the ‘rotting flesh’ smell of ivy flowers, this is to attract flies for pollination!).
• Other creatures – it isn’t just flying insects which carry out pollination. Slugs, birds, bats and rodents can all carry out the same process for plants which have specialised in attracting them. Often these plants are red coloured and produce copious amounts of nectar (particularly to attract larger animals such as birds and bats) without the sweet scent which attracts bees.
• Wind – plants which are pollinated by wind-carried pollen tend to have small, green flowers as they do not need bright colour to attract insects. They produce a large amount of pollen, which is blown on the wind and captured by the flowers’ stigma, which often protrude from the flower for this purpose. Grasses, trees with catkins and conifers are examples of plants which use wind pollination.

Exceptions do occur, whereby a fruit (with or without seed) may be produced without pollination and/or fertilisation:

• Apomixis – this is similar to parthenogenesis (although not the same process) and results in the production of a seed without pollination or fertilisation of the ovule. The offspring is genetically identical to the parent. This is also known as agamospermy (where it applies to angiosperms – plants bearing seeds within fruits).
• Parthenocarpy – the production of a fruit without seeds, where neither pollination nor fertilisation has taken place.