MAVO Year 4 · Plants · Section 9.5

Pollination, fertilisation and dispersal

Pollination

Pollination is the transfer of pollen to a stigma of a compatible flower, normally of the same species. It may occur through insects, wind or other agents.

Insect-pollinated flowers often attract visitors with colour, scent or nectar. As an insect feeds, pollen can stick to its body and be carried to another flower's stigma. Wind-pollinated flowers are often small and inconspicuous and produce much lightweight pollen. Their exposed anthers release pollen, and large or feathery stigmas help intercept it.

Fertilisation and development

A compatible pollen grain on a stigma may germinate and produce a pollen tube growing towards an ovule. Male gametes travel through this tube. One combines with the egg nucleus: fertilisation. In flowering plants another male gamete contributes to the formation of nutritive endosperm; this is part of double fertilisation.

The fertilised egg develops into an embryo. The ovule becomes a seed, enclosing the embryo, a protective coat and a source of stored food. The ovary usually develops into a fruit containing the seeds. Petals and stamens commonly wither or fall away.

Why disperse seeds?

Seed dispersal can reduce competition between seedlings and the parent and allow colonisation of new places. Different plants have different mechanisms.

• Wind: light fruits or seeds have hairs or wings, as in dandelions and maples.
• Animals eating fruit: seeds may survive digestion and be deposited elsewhere in droppings, as with rowan fruits.
• Attachment: hooks or sticky surfaces attach fruits or seeds to fur or clothing, as in burdock.
• Self-dispersal: ripe fruits split or spring open, throwing seeds away, as in balsam.

Sometimes the structure being dispersed is a whole fruit with its seed inside rather than the seed alone.