Reproduction in Plants
Asexual and sexual reproduction, flowers, pollination, fertilisation and germination. Part A of Chapter 16.
Asexual Reproduction
Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent. There is no fusion of gametes, so the offspring are genetically identical to the parent.
You may be asked to identify examples of asexual reproduction in diagrams, images or information provided. In each example here, a single parent produces new individuals that are genetically identical to it.
ExtendedAdvantages and disadvantages
| Advantages | Disadvantages | |
|---|---|---|
| Wild population | Only one parent is needed; many offspring can be produced quickly; offspring are well adapted if the environment stays the same | No variation, so the population may not cope with a change in the environment or a new disease |
| Crop production | Crops with the desired features can be copied exactly and quickly | All the plants are equally affected by the same disease or change |
Worked example (identical offspring): A strawberry plant produces runners that grow into new plants. Explain why the new plants are genetically identical to the parent. Step 1. The new plants are produced from one parent plant. Step 2. This is asexual reproduction, which produces genetically identical offspring. Answer: one parent; asexual reproduction.
Sexual Reproduction
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, and the production of offspring that are genetically different from each other.
Fertilisation is the fusion of the nuclei of gametes. Because each offspring receives genetic material from two gametes, the offspring are genetically different from each other.
ExtendedHaploid and diploid nuclei
The nuclei of gametes are haploid. The nucleus of a zygote is diploid.
ExtendedAdvantages and disadvantages
| Advantages | Disadvantages | |
|---|---|---|
| Wild population | Variation: some offspring may survive a change in the environment or a disease | Usually two parents are needed (finding a mate or pollinator); takes longer; a successful feature may not be passed on |
| Crop production | Variation can produce new varieties, including disease-resistant ones | Offspring may lose the desired features of the parents |
Worked example (fertilisation): State what happens to the nuclei of two gametes at fertilisation, and name the cell that forms. Step 1. The nuclei of the two gametes fuse. Step 2. The cell formed is a zygote. Answer: nuclei fuse; zygote.
The Flower
You must be able to identify and draw the parts of an insect-pollinated flower and state what each part does.
| Part | Belongs to | Function |
|---|---|---|
| Sepals | — | Protect the flower in the bud |
| Petals | — | Attract insects |
| Anther | Stamen | Produces pollen grains |
| Filament | Stamen | Holds the anther up |
| Stigma | Carpel | Receives pollen grains |
| Style | Carpel | Holds the stigma in position |
| Ovary | Carpel | Contains the ovules |
| Ovule | Ovary | Contains the female gamete (egg cell) |
Worked example (functions of parts): State the function of the anther and of the stigma. Step 1. The anther produces pollen grains. Step 2. The stigma receives pollen grains. Answer: anther: pollen; stigma: receives it.
Pollination
Pollination is the transfer of pollen grains from an anther to a stigma. Pollen can be carried by insects or by the wind.
Adaptations of the flower
The structure of a flower is adapted to its method of pollination.
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| Petals | Large, brightly coloured | Small, dull (or none) |
| Scent and nectar | Present, to attract insects | Absent |
| Anthers | Inside the flower | Hang outside the flower, on long filaments |
| Stigma | Inside the flower; sticky | Feathery, outside the flower |
| Pollen | Larger grains; sticky or spiky | Smaller, lighter, smooth grains; produced in very large amounts |
An insect visits the flower and pollen sticks to its body. When it visits another flower, some pollen rubs off on the stigma. In a wind-pollinated flower the pollen is blown away from the anthers, and some lands on feathery stigmas.
Worked example (adaptations): State two features that help an insect-pollinated flower to be pollinated. Step 1. The petals are large and brightly coloured, which attracts insects. Step 2. The flower has scent and nectar, which also attract insects. Answer: large coloured petals; scent.
Wind-Pollinated Flowers and Pollen
In a wind-pollinated flower the anthers hang outside the flower on long filaments, so the wind can blow the pollen away. The stigmas are feathery and hang outside the flower, giving a large surface to catch pollen from the air.
Pollen grains
You must be able to distinguish between the pollen grains of insect-pollinated and wind-pollinated flowers.
| Pollen grains | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Size | Larger | Smaller and lighter |
| Surface | Sticky or spiky, to stick to insects | Smooth, so they are easily blown by the wind |
Worked example (distinguishing pollen): A pollen grain under the microscope is small, smooth and light. State which method of pollination it suggests, and why. Step 1. Small, smooth, light grains are easily carried by air. Step 2. So the flower is likely to be wind-pollinated. Answer: wind-pollinated.
Self- and Cross-Pollination
ExtendedTwo kinds of pollination
Pollination can be self-pollination or cross-pollination.
ExtendedEffects on a population
| Self-pollination | Cross-pollination | |
|---|---|---|
| Variation | Less variation among offspring | More variation among offspring |
| Response to changes in the environment | Less able to respond, because there is less variation | More able to respond, because some offspring may be better adapted |
| Reliance on pollinators | Does not rely on insects or wind | Relies on pollinators to transfer pollen |
Worked example (discussing pollination): Discuss one advantage and one disadvantage of cross-pollination for a population. Step 1. Advantage: it produces more variation, so the population can respond better to changes in the environment. Step 2. Disadvantage: it relies on pollinators such as insects or wind to carry the pollen. Answer: more variation; relies on pollinators.
Fertilisation in Plants
In plants, fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. This produces a zygote. Pollination must happen first, so that pollen reaches the stigma.
| Process | What happens |
|---|---|
| Pollination | Pollen grains are transferred from an anther to a stigma |
| Fertilisation | A pollen nucleus fuses with a nucleus in an ovule |
ExtendedThe pollen tube
After landing on the stigma, a pollen grain grows a pollen tube. The tube grows down through the style and enters the ovule. The pollen nucleus then fuses with the nucleus of the egg cell: this is fertilisation.
Worked example (pollination and fertilisation): Explain the difference between pollination and fertilisation. Step 1. Pollination is the transfer of pollen grains from an anther to a stigma. Step 2. Fertilisation is when a pollen nucleus fuses with a nucleus in an ovule. Answer: transfer of pollen; fusion of nuclei.
Germination
A seed germinates only when the conditions are right. You must be able to investigate and describe the environmental conditions that affect germination, limited to water, oxygen and a suitable temperature.
| Condition | Why it is needed | Tube |
|---|---|---|
| Water | Water is needed for the reactions of growth to start | B has none |
| Oxygen | Needed for respiration, which releases energy for growth | C has none |
| Suitable temperature | Too cold: reactions in the seed are too slow | D is cold |
To make the investigation fair, keep the same kind and number of seeds, the same amount of light and the same time in every tube. Only one condition differs from tube A, the control. The boiled water is cooled before the seeds are added.
Worked example (investigation): Seeds in tube C, under a layer of oil on boiled water, did not germinate. State the conclusion. Step 1. The boiled water and oil removed dissolved oxygen. Step 2. So oxygen is needed for germination. Answer: oxygen is needed.
Exam advice
Common mistakes
Model answer
Recall checklist
- Define asexual reproduction.
- Define sexual reproduction and fertilisation.
- Label a flower and state each function.
- Define pollination; compare insect and wind pollination.
- State the conditions for germination.
- Discuss pros and cons of each reproduction type (Extended)
- Define self- and cross-pollination (Extended)
- Describe the pollen tube and fertilisation (Extended)
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