A flower may look as though its main purpose is to provide colour or scent, but biologically it is much more important than that. A flower contains structures specialised for sexual reproduction. Some produce the male reproductive cells, some contain the female reproductive cells, and others help pollen reach the correct place.
The whole process can be followed as one connected story. Pollen must first move from an anther to a stigma. A pollen tube then grows through the flower towards an ovule. Only after the male and female gamete nuclei fuse has fertilisation actually occurred. Understanding that sequence makes the rest of this topic much easier.
Try these five questions before reading the chapter. They test whether you can recognise the main reproductive structures of a flower and distinguish pollination from fertilisation.
- 5 diagnostic questions
- Core flower structures and processes
- Instant explanation for every answer option
The parts of a flower and what they do
To understand sexual reproduction in flowering plants, begin with the flower itself. Different structures have different jobs, but they work together as one reproductive system.
Many flowers contain both male and female reproductive structures, although some species have separate male and female flowers.
The male reproductive part: the stamen
The male reproductive structure is the stamen. Each stamen consists of an anther and a filament.
Anther: produces pollen grains. The pollen grains contain the male gametes.
Filament: supports and positions the anther so that pollen can be transferred effectively.
The female reproductive part: the carpel
The female reproductive structure is the carpel. It consists of the stigma, style and ovary.
Stigma: receives pollen grains. Its surface is usually adapted to retain suitable pollen.
Style: connects the stigma to the ovary and provides the route through which a pollen tube grows.
Ovary: contains one or more ovules.
Ovule: contains the female gamete.
Petals and sepals support reproduction
Petals and sepals are not themselves male or female reproductive organs, but they can still play important roles in successful reproduction.
| Flower structure | Main function |
|---|---|
| Anther | Produces pollen grains containing the male gametes. |
| Filament | Supports and positions the anther. |
| Stigma | Receives pollen grains. |
| Style | Connects the stigma to the ovary and provides a route for pollen-tube growth. |
| Ovary | Contains the ovules and later develops into the fruit after fertilisation. |
| Ovule | Contains the female gamete and develops into a seed after fertilisation. |
| Petal | May attract insect pollinators through colour, scent or other features. |
| Sepal | Protects the flower while it is developing as a bud. |
Pollination transfers pollen from anther to stigma
Pollination is the transfer of pollen grains from an anther to a stigma. It is an essential step before fertilisation can occur, but pollination itself is not fertilisation.
Pollen may be transferred within the same plant or between different plants of the same species. This gives us two useful terms: self-pollination and cross-pollination.
Self-pollination
Self-pollination occurs when pollen is transferred from the anther of a flower to the stigma of the same flower, or to another flower on the same plant.
Cross-pollination
Cross-pollination occurs when pollen is transferred from a flower on one plant to a flower on a different plant of the same species.
Self-pollination: both pollen and ovule come from the same individual plant. It does not require pollen to reach another plant.
Cross-pollination: pollen comes from a different plant of the same species. This usually increases the opportunity for alleles from genetically different parents to be combined.
Cross-pollination therefore tends to produce greater genetic variation among offspring. Variation can be useful because, if environmental conditions change or disease appears, individuals within a genetically varied population may differ in their ability to survive and reproduce.
Exam distinction: cross-pollination is still only the transfer of pollen. Genetic material from two gametes is actually combined during fertilisation.
Insect-pollinated and wind-pollinated flowers have different adaptations
A pollen grain cannot move deliberately towards another flower. A flowering plant therefore needs a method of transferring pollen. Two important agents of pollination are insects and wind.
The structures of the flowers reflect the method they use. An insect-pollinated flower benefits from attracting an insect and making pollen stick to its body. A wind-pollinated flower instead benefits from exposing its reproductive structures to moving air.
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| Petals | Usually relatively large and conspicuous, helping to attract insects. | Usually small, inconspicuous or reduced because attracting insects is unnecessary. |
| Scent and nectar | Often present and help attract insect pollinators. | Usually absent because wind does not need to be attracted. |
| Anthers | Usually held firmly inside the flower where visiting insects can brush against them. | Often exposed outside the flower on long filaments so that moving air can shake pollen free. |
| Stigma | Usually sticky and positioned where visiting insects are likely to touch it. | Usually large, feathery and exposed, giving a large surface area for trapping airborne pollen. |
| Amount of pollen | Relatively smaller quantities are needed because transfer by insects is more directed. | Very large quantities are produced because much of the pollen never reaches another flower. |
| Pollen grains | Often sticky or rough so that they attach to insects. | Usually small, light and smooth so that they can be carried easily by air. |
Why wind-pollinated flowers produce so much pollen
Wind pollination is relatively undirected. A pollen grain released into the air may land on the ground, on a leaf, on a flower of another species or somewhere else entirely. Only a small proportion reaches a suitable stigma.
Producing a very large number of small, light pollen grains increases the probability that at least some will reach flowers of the same species.
Why insect-pollinated pollen is often sticky
When an insect visits a flower, pollen may become attached to its body. If the same insect later visits another flower of the same species, some of that pollen may be transferred to the stigma. Sticky or rough pollen is therefore useful because it is less likely to fall from the insect during transport.
Pollination is followed by pollen-tube growth
Successful pollination places a suitable pollen grain on a stigma, but the male gamete is still some distance from the female gamete inside an ovule. The next stage solves that problem.
The pollen grain germinates on the stigma and forms a pollen tube. The pollen tube grows down through the style towards the ovary.
A male gamete nucleus travels down the pollen tube. The pollen tube eventually reaches an ovule and enters it through a small opening called the micropyle.
The sequence is:
Pollen reaches stigma → pollen grain germinates → pollen tube grows down the style → male gamete nucleus travels through the pollen tube → pollen tube reaches an ovule → fertilisation can occur.
Fertilisation is the fusion of the male and female gamete nuclei
Once the male gamete nucleus reaches the female gamete inside the ovule, the two nuclei can fuse. This fusion is called fertilisation.
Fertilisation: the fusion of the nucleus of a male gamete with the nucleus of a female gamete to form a zygote.
Notice the difference from pollination. Pollination happens at the stigma. Fertilisation takes place later, inside an ovule.
| Process | What happens? | Where? |
|---|---|---|
| Pollination | Pollen grains are transferred from anther to stigma. | At the stigma. |
| Pollen-tube growth | A pollen tube grows from the pollen grain and carries the male gamete nucleus towards an ovule. | From the stigma through the style towards the ovary. |
| Fertilisation | The male gamete nucleus fuses with the female gamete nucleus. | Inside an ovule. |
After fertilisation, seeds and fruits develop
Fertilisation produces a zygote, but the zygote does not simply “turn into a seed”. Several parts of the flower develop in different ways.
Zygote → embryo
Ovule → seed
Ovary → fruit
The zygote divides repeatedly by mitosis and develops into the embryo of the new plant. Meanwhile, the fertilised ovule develops into a seed. The ovary develops into a fruit, which surrounds the seed or seeds.
These changes complete the connection between sexual reproduction and the next generation. The gametes have fused, an embryo has formed and the new plant is packaged inside a seed.
Common Mistakes
| Common mistake | Why it is wrong | Correct idea |
|---|---|---|
| “Pollination and fertilisation are the same process.” | Pollination only transfers pollen to a stigma. No gamete nuclei have fused yet. | Pollination happens first. Fertilisation occurs later when the male and female gamete nuclei fuse. |
| “The pollen grain is the same thing as the male gamete.” | The pollen grain is a structure that contains the male gamete. | The male gamete nucleus is carried towards the ovule through the pollen tube. |
| “The ovary contains the male gametes.” | The ovary is part of the female reproductive structure. | The ovary contains ovules, and each ovule contains a female gamete. |
| “The stigma produces pollen.” | The stigma receives pollen rather than producing it. | Pollen grains are produced by the anthers. |
| “A pollen tube carries a pollen grain down the style.” | The pollen grain remains at the stigma while a tube grows out from it. | The male gamete nucleus travels down the pollen tube. |
| “Wind-pollinated flowers have large colourful petals so the wind can find them.” | Wind is not attracted by colour, scent or nectar. | Wind-pollinated flowers invest instead in exposed anthers, feathery stigmas and large quantities of light pollen. |
| “The zygote becomes the whole seed.” | The zygote forms only the embryo within the developing seed. | The zygote develops into the embryo, while the ovule develops into the seed. |
| “The ovule becomes the fruit.” | The ovule and ovary have different fates after fertilisation. | The ovule develops into a seed, while the ovary develops into a fruit. |
What You Should Be Able to Do
Identify flower structures
Recognise the anther, filament, stigma, style, ovary, ovule, petals and sepals, and state their functions.
Define pollination
State accurately that pollination is the transfer of pollen grains from anther to stigma.
Compare self- and cross-pollination
Decide whether pollen transfer occurs within the same plant or between different plants of the same species.
Compare pollination adaptations
Relate the structures of insect-pollinated and wind-pollinated flowers to how pollen is transferred.
Describe pollen-tube growth
Explain how a pollen grain germinates and how the male gamete nucleus reaches an ovule.
Explain fertilisation and development
State what fuses during fertilisation and relate the zygote, ovule and ovary to the embryo, seed and fruit respectively.
These ten questions test whether you can connect flower structure with pollination, pollen-tube growth, fertilisation and the formation of seeds and fruits.
- 10 mastery questions
- Application and misconception testing
- Instant explanation for every answer option
Frequently Asked Questions
Questions Students Often Ask About Sexual Reproduction in Flowering Plants
The most common difficulties in this topic come from mixing up structures or treating pollination and fertilisation as though they happen at the same time. These questions connect the stages together.
Is a pollen grain the male gamete?
Not exactly. A pollen grain is a structure that contains the male gamete. After a suitable pollen grain reaches the stigma, it forms a pollen tube, and the male gamete nucleus travels through this tube towards an ovule.
What is the simplest way to remember the difference between anther and stigma?
Think about the direction of pollen movement. The anther produces and releases pollen, while the stigma receives pollen. Pollination therefore goes from anther to stigma.
Does pollination mean that a seed will definitely form?
No. Pollination only places pollen on a stigma. For a seed to develop, a suitable pollen grain must germinate, a pollen tube must reach an ovule and fertilisation must occur. If any of these stages fails, successful seed formation may not occur.
Where exactly does fertilisation occur in a flower?
Fertilisation occurs inside an ovule. The pollen tube grows through the style towards the ovary and enters an ovule through the micropyle. The male gamete nucleus can then fuse with the female gamete nucleus.
Why does a wind-pollinated flower need a feathery stigma?
A feathery stigma has a large surface area exposed to the air. This increases the probability of intercepting pollen grains that are being carried past the flower by wind.
Why do wind-pollinated flowers usually not need brightly coloured petals?
Bright petals are useful mainly for attracting animal pollinators such as insects. Wind cannot see colour or be attracted by scent or nectar, so investing heavily in these features does not improve wind pollination.
Does cross-pollination itself create genetic variation?
Cross-pollination transfers pollen between different plants of the same species. Its importance is that it can bring gametes from genetically different individuals together. The actual combination of genetic material occurs when the gametes fuse during fertilisation.
What is the difference between an ovary and an ovule?
The ovary is the larger structure at the base of the carpel and contains one or more ovules. After fertilisation, the ovary develops into the fruit, while each fertilised ovule develops into a seed.
Does the zygote develop directly into the seed?
No. The zygote develops into the embryo. The entire ovule develops into the seed. This distinction is important in examination questions because the zygote is only one part of the developing seed.
What is the full sequence from pollination to seed formation?
Pollen is transferred from anther to stigma. A suitable pollen grain germinates and forms a pollen tube. The pollen tube grows through the style towards an ovule, carrying the male gamete nucleus. The male and female gamete nuclei fuse during fertilisation to form a zygote. The zygote develops into an embryo, while the ovule develops into a seed.
The easiest way to master sexual reproduction in flowering plants is to follow the journey of the male gamete. Start at the anther, move with the pollen to the stigma, follow the pollen tube down the style and finish inside an ovule, where fertilisation can occur. Once that pathway is clear, the structures and adaptations of the flower begin to fit together as one connected process.