16.3 Sexual reproduction in plants
- Syllabus
- 0610–2026–2027
- Topic
- 16.3
- Level
- —
Learning objectives
16.3.1Draw the following parts• Identify in diagrams and images and draw the following parts of an insect-pollinated flower: sepals, petals, stamens, filaments, anthers, carpels, style, stigma, ovary and ovules16.3.2Functions of the structures listed• State the functions of the structures listed in 16.3.116.3.3Describe the anthers and stigmas• Identify in diagrams and images and describe the anthers and stigmas of a wind-pollinated flower16.3.4Pollen grains of insect-pollinated• Distinguish between the pollen grains of insect-pollinated and wind-pollinated flowers16.3.5Pollination as the transfer of pollen• Describe pollination as the transfer of pollen grains from an anther to a stigma16.3.9Self-pollination as the transfer• Describe self-pollination as the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant16.3.10Cross-pollination as the transfer• Describe cross-pollination as the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species16.3.11Potential effects of self-pollination• Discuss the potential effects of self-pollination and cross-pollination on a population, in terms of variation, capacity to respond to changes in the environment and reliance on pollinators16.3.6Fertilisation occurs when a pollen• State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule16.3.7Structural adaptations• Describe the structural adaptations of insect-pollinated and wind-pollinated flowers16.3.8Environmental conditions that affect• Investigate and describe the environmental conditions that affect germination of seeds, limited to the requirement for: water, oxygen and a suitable temperature16.3.12Growth of the pollen tube and its• Describe the growth of the pollen tube and its entry into the ovule followed by fertilisation (details of production of endosperm and development are not required)
Identify and draw flower parts
Read an insect-pollinated flower from outside inward: sepals surround the bud; petals surround the reproductive organs; stamens are the male parts; the central carpel is the female part.
| Whole part | Component | Recognition cue |
|---|---|---|
| stamen | filament | stalk supporting an anther |
| stamen | anther | pollen-producing structure at the filament tip |
| carpel | stigma | pollen-receiving surface at the top |
| carpel | style | stalk connecting stigma to ovary |
| carpel | ovary | swollen base containing ovules |
| carpel | ovule | structure inside the ovary |
| outer flower | petal | usually large structure around reproductive parts |
| outer flower | sepal | leaf-like structure protecting the bud |
In a drawing, use clear single lines, show anthers on filaments and ovules inside the ovary, and keep label lines separate and touching the named structure.
The stamen is anther plus filament; the carpel includes stigma, style and ovary. An ovule is inside an ovary, not another name for it.
State flower-part functions
| Structure | Function |
|---|---|
| sepal | protects the flower in the bud |
| petal | attracts insect pollinators and helps position them |
| anther | produces pollen grains |
| filament | supports and positions the anther |
| stigma | receives and traps pollen grains |
| style | supports the stigma and provides the route for pollen-tube growth |
| ovary | contains ovules and later develops into the fruit |
| ovule | contains the female nucleus and later develops into a seed |
Name the precise structure for the function: pollen is produced by the anther, while the filament only supports it; ovules form seeds, while the ovary forms the fruit.
Recognise wind-pollinated anthers and stigmas
In a wind-pollinated flower, anthers are exposed outside the flower on long, flexible filaments so wind can carry away pollen. Stigmas are exposed, large and feathery so they intercept airborne pollen.
| Structure | Wind-pollinated cue | Functional result |
|---|---|---|
| anther | large, loosely attached and hanging outside | pollen is released easily into moving air |
| stigma | large, branched or feathery and outside | large surface area catches pollen |
Do not use large petals, nectar or enclosed reproductive organs as wind-pollination cues; those fit insect pollination.
Distinguish pollen grains
| Feature | Insect-pollinated pollen | Wind-pollinated pollen |
|---|---|---|
| surface | sticky or spiky | smooth |
| size and mass | relatively large and heavy | small and light |
| quantity | fewer grains | very many grains |
| transport consequence | attaches to an insect | carried easily by air |
A pollen grain is classified by a combination of features and how they aid transport, not by one sketch detail alone.
Define pollination
Pollination is the transfer of pollen grains from an anther to a stigma.
The anther is the source and the stigma is the destination. Wind or animals may be the transfer agent, but the definition stays the same.
Pollination is transfer, not fusion. Fertilisation happens later, after a pollen tube has grown and nuclei fuse in an ovule.
Define self-pollination
Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant.
Ask whether source and destination belong to the same plant. If yes, the transfer is self-pollination even when two flowers are involved.
‘Self’ means the same plant, not only the same flower.
Define cross-pollination
Cross-pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.
Both conditions matter: the plants are different individuals, and they belong to the same species.
Transfer to another flower on the same plant is self-pollination; transfer between different species is not cross-pollination within this definition.
Discuss effects of self- and cross-pollination
| Population effect | Self-pollination | Cross-pollination |
|---|---|---|
| variation | less genetic variation; homozygosity can increase | more genetic variation from different parents |
| environmental change or disease | lower chance that some individuals have a favourable variant | greater chance that some survive and the population adapts |
| reliance on pollinators | can reproduce when isolated and is less reliant on a transfer agent | relies more on wind or animals moving pollen between plants |
| pollen and energy | less pollen may be wasted | more pollen and attraction structures may be required |
Self-pollination improves reproductive assurance but narrows variation; cross-pollination raises variation and adaptive capacity but makes successful transfer less certain.
Self-pollination is still sexual reproduction and can produce some variation; it does not make offspring genetically identical clones.
Locate fertilisation in a flower
Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule.
The fusion occurs in an ovule, which is inside the ovary. The resulting nucleus is the zygote nucleus.
Pollen landing on the stigma is pollination, not fertilisation; the male nucleus must reach an ovule and fuse with its nucleus.
Compare flower adaptations
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| petals | large, brightly coloured; scent and nectar may attract insects | small or absent; no need for scent or nectar |
| anthers | firm and held inside where insects brush them | exposed on long flexible filaments |
| stigma | sticky and held inside | exposed, large and feathery |
| pollen | fewer, larger, sticky or spiky grains | very many, small, light, smooth grains |
Every feature should be linked to its job: attract or contact an insect, release pollen into air, attach to an animal, or catch airborne pollen.
A feature is an adaptation only when its structure is linked to improved pollen transfer.
Investigate conditions for germination
Seeds require water, oxygen and a suitable temperature to germinate.
| Requirement | Role |
|---|---|
| water | rehydrates tissues and allows enzyme-controlled reactions and transport |
| oxygen | supports aerobic respiration to release energy |
| suitable temperature | allows enzymes to work at an effective rate without denaturing |
Change one condition at a time, keep seed type, number, time and other conditions constant, use repeats, and compare the proportion germinated. A no-water, no-oxygen or unsuitable-temperature treatment is compared with a treatment receiving all three requirements.
Light is not an universal germination requirement. Germination requirements are not the same as the later needs of a growing green plant.
Describe pollen-tube growth and fertilisation
After a pollen grain lands on a compatible stigma, it germinates and grows a pollen tube down through the style into the ovary. The tube enters an ovule; the pollen nucleus travels down it and fuses with a nucleus in the ovule, completing fertilisation.
| Order | Structure or event |
|---|---|
| 1 | pollen grain on stigma |
| 2 | pollen tube grows through style |
| 3 | tube reaches ovary and enters an ovule |
| 4 | pollen nucleus moves down the tube |
| 5 | pollen nucleus fuses with a nucleus in the ovule |
The pollen grain does not itself travel down the style. The pollen tube grows; the pollen nucleus moves through it. Endosperm production and later development are outside this objective.