Flower structure and function

A flower is the reproductive structure of a flowering plant. Its parts work together to produce pollen and ovules, to bring about pollination, and to protect and nourish the seed and fruit that follow fertilisation.

Parts and functions

How structure suits function

The stamen and the carpel are the working parts of the flower, while the sepals and petals support pollination. Insect-pollinated and wind-pollinated flowers solve the same problem, moving pollen from anther to stigma, with opposite sets of adaptations, which is exactly what an examiner asks you to compare.

A structure-to-function answer needs a feature, a consequence and a link to pollination. The adaptations below follow that pattern.

• Insect-pollinated flowers have large, brightly coloured, scented petals and nectaries, so insects are attracted to visit and carry pollen between flowers.
• Insect-pollinated flowers make sticky or spiky pollen and have a sticky stigma held inside the flower, so pollen clings to a visiting insect's body and is left on the stigma of the next flower.
• Wind-pollinated flowers have small, dull, scentless petals or none at all and no nectar, so no energy is spent attracting animals that are not needed.
• Wind-pollinated flowers produce large amounts of light, smooth, dry pollen, so it is easily lifted and carried on air currents to other flowers.
• Wind-pollinated flowers have long filaments that hang the anthers outside the flower and large feathery stigmas that also hang out, so pollen is released into the wind and airborne pollen is caught over a wide surface.

Related processes

Pollination is the transfer of pollen grains from an anther to a stigma. In self-pollination the pollen lands on a stigma of the same flower or plant; in cross-pollination it is carried to a different plant of the same species, which increases variation.

This is where the petal, stamen and stigma adaptations above come into play.

Fertilisation follows pollination. A pollen grain on the stigma germinates and grows a pollen tube down through the style; the tube enters the ovule through a small opening called the micropyle and delivers the male gamete, which fuses with the female gamete (egg cell) to form a zygote.

Because pollination and fertilisation are separate steps, a flower can be pollinated without being fertilised.

After fertilisation the flower changes into a fruit. The zygote grows into an embryo, the ovule becomes the seed, and the ovary wall becomes the fruit, while the petals, stamens and stigma wither and drop off.

This links flower structure directly to seed structure and to germination studied later in the same topic.

Common labelling errors

Worked question

Question. Diagram 1 shows a section through an insect-pollinated flower. Structure P produces pollen grains, structure Q is a sticky surface at the top of a stalk, and structure R contains the ovules.

(a) Name structures P, Q and R. (b) State the name of the part made up of P and its stalk.

(c) Explain two ways this flower is adapted for insect pollination. (d) After pollen lands on Q, describe what happens until fertilisation occurs.

Model answer. (a) P is the anther; Q is the stigma; R is the ovary. (b) P and its stalk (the filament) make up the stamen.

(c) The flower has large, brightly coloured, scented petals and nectar, which attract insects to visit; its pollen is sticky or spiky and the stigma is sticky, so pollen clings to the insect and is deposited on the stigma of another flower. (d) The pollen grain germinates and grows a pollen tube down through the style; the tube enters the ovule through the micropyle and releases the male gamete, which fuses with the female gamete (egg cell) to form a zygote, this is fertilisation.

Source:SRC-DSKP-EN

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