The Structure of a Flower
A flower is the reproductive structure of a flowering plant, typically made of sepals, petals, stamens and carpels attached to a receptacle. Its structure differs depending on whether it relies on insects or wind for pollination.
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The main parts of a flower
- Sepal, usually green, protects the flower bud before it opens.
- Petal, often brightly coloured and scented in insect-pollinated flowers, to attract pollinators.
- Stamen, the male part, made of an anther, which produces pollen grains, on a filament that supports it.
- Carpel (pistil), the female part, made of a stigma that receives pollen, a style connecting it to the ovary, and an ovary containing one or more ovules.
- Receptacle, the base of the flower where all the other parts are attached.
Insect-pollinated versus wind-pollinated flowers
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| Petals | Large, brightly coloured, often scented | Small, dull or absent |
| Nectar | Usually present, to reward pollinators | Absent |
| Pollen | Fewer grains, larger and sticky or spiky to cling to insects | Huge numbers of grains, small, light and smooth to be carried by wind |
| Stigma | Small, often inside the flower | Large, feathery, hanging outside the flower to catch airborne pollen |
| Anther | Fixed firmly inside the flower | Loosely attached, hanging outside the flower to release pollen easily into the wind |
Why structure suits the pollination method
Insect-pollinated flowers invest in colour, scent and nectar because they need to attract and reward a visiting insect, which then carries pollen directly and efficiently to another flower. Wind-pollinated flowers do not need to attract anything, so they save resources on petals and nectar and instead produce vast quantities of light pollen and expose large stigmas, since wind pollination is far less precise and most pollen never reaches a stigma.
How it is examined
You may be given a labelled flower diagram and asked to name a part and its function, or given a flower's features (such as small dull petals and a feathery stigma) and asked to identify and justify whether it is insect-pollinated or wind-pollinated.
From pollination to fertilisation and seed formation
Pollination is only the first step towards producing seeds. Once a pollen grain lands on a compatible stigma, it germinates and grows a pollen tube down through the style towards the ovary; the tube carries a male nucleus to an ovule, where it fuses with the egg nucleus in a process called fertilisation.
After fertilisation, each fertilised ovule develops into a seed containing an embryo and a food store, while the ovary wall around it develops into a fruit that protects the seeds and, in many species, later helps disperse them away from the parent plant.
Worked exam-style question
Question. A student examines an unknown flower and records these features: small, dull green petals; no nectar or scent; anthers hanging loosely outside the flower on long filaments; a large, feathery stigma exposed outside the flower. (a) State, with a reason, whether this flower is insect-pollinated or wind-pollinated.
(b) Explain why this type of flower produces very large numbers of pollen grains. (c) Name the structure that receives pollen grains during pollination.
(d) Describe what happens immediately after a pollen grain lands on a compatible stigma.
Model answer. (a) Wind-pollinated, because it has small, dull, unscented petals with no nectar, features that do not attract insects, and both the anthers and stigma hang outside the flower to release and catch pollen carried by air. (b) Wind pollination is very inefficient, since most pollen released into the air never lands on a stigma of the same species, so the plant must produce huge numbers of pollen grains to make it likely that at least some reach a stigma.
(c) The stigma. (d) The pollen grain germinates and grows a pollen tube down through the style towards the ovary, carrying a male nucleus towards an ovule.
Practice question
Try this. A gardener notices that a particular flower has large, brightly coloured petals and produces nectar, but its anthers are held rigidly inside the flower rather than hanging outside. State whether this flower is more likely to be insect-pollinated or wind-pollinated, and give two reasons for your answer.
Exam tip
Key terms
These Chapter 21 terms cover the structures and process examined alongside flower structure:
- Pollination, the transfer of pollen from an anther to a stigma, the step before fertilisation.
- Anther, the part of the stamen that produces pollen grains.
- Stigma, the part of the carpel that receives pollen during pollination.
- Germination, the process by which a seed, formed after fertilisation, begins to grow into a new plant.
Source:SRC-DSKP-EN
Frequently asked questions
What is the difference between a stamen and a carpel?
How can you tell if a flower is wind-pollinated just by looking at it?
What happens to the ovary and ovule after fertilisation?
More for Sexual Reproduction in Flowering Plants
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