The big idea: A flower is the reproductive organ of a flowering plant. It contains male and female parts that make and bring together gametes (sex cells).
The male part is the stamen — an anther (which makes pollen) held up on a filament.
The female part is the carpel — a sticky stigma that catches pollen, a style that joins it to the ovary, and ovules inside the ovary that hold the female gametes.
- Flower
- The reproductive structure of a flowering plant; it contains the male and female reproductive parts.
- Stamen (male part)
- The male reproductive organ of a flower, made of an anther and a filament.
- Anther
- The part of the stamen that produces and holds pollen grains (which carry the male gametes).
- Carpel (female part)
- The female reproductive organ of a flower, made of a stigma, a style and an ovary containing ovules.
- Stigma
- The sticky tip of the carpel that catches and holds pollen grains.
- Ovule
- The structure inside the ovary that contains the female gamete; after fertilization it becomes the seed.
| Reproductive organ | Made up of | Role |
|---|---|---|
| Stamen (male) | Anther + filament | Produces pollen, which carries the male gametes |
| Carpel / pistil (female) | Stigma + style + ovary (with ovules) | Receives pollen and contains the female gametes in the ovules |
A quick way to keep the parts straight: Male = stamen = anther on a filament (the a in anther makes the pollen).
Female = carpel = stigma → style → ovary with ovules.
Pollen lands on the stigma, then travels down the style to reach an ovule in the ovary.
Making a seed happens in a clear order: pollination brings the pollen to the stigma, then fertilization joins the gametes, and finally the fertilised ovule grows into a seed while the ovary becomes a fruit.
The two words students mix up most are pollination and fertilization — they are not the same step.
- Pollination
- The transfer of pollen from an anther to a stigma.
- Fertilization
- The fusion of a male gamete (from the pollen) with a female gamete (the egg) inside the ovule, forming a zygote.
- Pollen tube
- A tube that grows from a pollen grain down through the style, carrying the male gamete to the ovule.
- Seed
- A fertilised ovule; it contains the embryo plant and a food store.
- Fruit
- The ripened ovary; it surrounds and helps disperse the seeds.
Pollination — pollen reaches the stigma: In pollination, a pollen grain is moved from an anther onto a stigma.
It may be carried by insects (which are attracted by bright petals, scent or nectar), by the wind, or by other animals.
At this point the gametes have not yet joined — pollination only delivers the pollen.
Fertilization — the gametes fuse: Once pollen lands on the stigma, a pollen tube grows down the style to an ovule in the ovary.
The male gamete travels down this tube and fuses with the female gamete (egg) inside the ovule. This fusion is fertilization, and it forms a zygote.
So pollination comes first (pollen onto stigma); fertilization comes second (gametes fuse inside the ovule).
| Feature | Pollination | Fertilization |
|---|---|---|
| What happens | Pollen is transferred from an anther to a stigma | A male gamete fuses with a female gamete (egg) in the ovule |
| Where | On the outside, onto the stigma | Inside the ovule, after the pollen tube grows down |
| Gametes fuse? | No — gametes have not met yet | Yes — this is the actual joining of the two gametes |
| Comes first? | First — it delivers the pollen | Second — it follows once the pollen tube reaches the ovule |
| Result | Pollen is now on the stigma | A zygote forms; the ovule becomes a seed |
After fertilization — seed and fruit: Once an ovule is fertilised:
the ovule becomes a seed (holding the embryo plant and a food store), and the ovary becomes a fruit (which surrounds the seeds and helps spread them).
A handy pair to remember: ovule → seed, ovary → fruit.
A memory hook: Pollination = Pollen Put on stigma (delivery only).
Fertilisation = gametes Fuse (the real joining, inside the ovule).
Pollination is the postman; fertilization is the gametes actually meeting.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
Try Practice Free7-day free trial • No card required
How this is tested: On Paper 1A you may see a cross-section of a flower and have to identify a labelled structure — a common answer is the stigma (the sticky tip that catches pollen).
On Paper 1B a 1-mark Define question can ask you to define pollination (transfer of pollen from anther to stigma), and a 2-mark Outline question can ask for the mechanisms that encourage cross-pollination.
Keep pollination and fertilization strictly separate — confusing the two is the most common way to lose the mark.
IB-style question — define pollination
Define the term pollination. [1]
How to score the mark
- Name what moves. Pollination is about pollen — say where it comes from and where it goes.
- Give the transfer. Pollination is the transfer of pollen from an anther to a stigma. (Award the mark for 'transfer of pollen from anther to stigma'. Do not describe gametes fusing — that is fertilization, not pollination.)
Final answer
Pollination is the transfer of pollen from an anther to a stigma.
✓ Why this scores the mark: The answer names pollen, its source (anther) and its destination (stigma).
Writing 'the male and female gametes join' would score zero — that is fertilization. A definition of pollination must stop at the pollen reaching the stigma.
IB-style question — outline how plants encourage cross-pollination
Outline two mechanisms by which flowering plants encourage cross-pollination rather than self-pollination. [2]
How to score both marks
- First mechanism — self-incompatibility. Many plants have self-incompatibility alleles: the plant's own pollen is rejected by its own stigma, so only pollen from a different plant can fertilise it.
- Second mechanism — separating the sexes in time or space. The male and female parts may mature at different times (so a flower's own pollen is not ready when its stigma is), or male and female parts may be on separate flowers or separate plants. (Award 1 mark per distinct mechanism, up to 2.)
Final answer
Self-incompatibility (the plant rejects its own pollen) and separating the sexes in time or space (anthers and stigmas maturing at different times, or on different flowers/plants).
| Feature | Self-pollination | Cross-pollination |
|---|---|---|
| Pollen comes from | The same flower (or same plant) | A different plant of the same species |
| Genetic variation | Low — offspring very similar to the parent | Higher — offspring mix two parents' alleles |
| Effect on offspring | Less variation, more risk of inbreeding | More variation, healthier offspring over time |
| Plants more likely to do it | Plants whose own pollen can fertilise their own ovules | Plants with mechanisms that block their own pollen |