Home › Learning Hub › IGCSE Biology › 16a Reproduction and flowers
Topic 16 · 16.1–16.3

Asexual reproduction, sexual reproduction and flowers

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe asexual and sexual reproduction and fertilisation; discuss their advantages and disadvantages, and state haploid and diploid EXTENDED.
  • Identify, draw and state the functions of the parts of an insect-pollinated flower; describe wind-pollinated flowers and pollen grains.
  • Describe pollination and fertilisation, the adaptations of insect- and wind-pollinated flowers, and the conditions for germination.
  • Describe self- and cross-pollination and their effects, and pollen tube growth EXTENDED.

📚The biology

Asexual reproduction

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent.

Three examples. A potato plant with underground tubers, each of which can grow into a new plant. A strawberry plant sending out runners along the ground, with new plants forming along them. A bacterium dividing into two identical cells.
Each offspring is a clone of its parent.
EXTENDED Advantages: only one parent is needed; it is fast, so a species can spread rapidly into a good habitat; crops with desirable features stay the same
Disadvantages: no genetic variation, so the whole population may be killed by a new disease or a change in the environment; overcrowding and competition near the parent

Sexual reproduction

Sexual reproduction involves the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other. Fertilisation is the fusion of the nuclei of gametes.

EXTENDED The nuclei of gametes are haploid (one set of chromosomes); the nucleus of a zygote is diploid (two sets). Sexual reproduction produces variation, so some individuals may survive a change in the environment or a new disease; but it is slower, needs two parents (or pollinators), and a good crop variety will not breed true.

Flowers

A half-flower of an insect-pollinated plant. Green sepals at the base; large coloured petals; stamens, each an anther on a filament; in the centre the carpel, made of the stigma at the top, the style, and the ovary containing ovules; a nectary at the base.
The male parts are the stamens (anther + filament); the female part is the carpel (stigma + style + ovary with ovules).
sepals: protect the flower bud
petals: large and brightly coloured to attract insects
anther: makes pollen grains (containing the male gamete)
filament: holds the anther up
stigma: receives pollen grains
style: holds up the stigma; the pollen tube grows down it
ovary: contains the ovules; becomes the fruit
ovules: contain the female gamete; become seeds

Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule.

A wind-pollinated grass flower. Large feathery stigmas hang outside the flower to catch pollen from the air; anthers dangle outside on long filaments so pollen is shaken out; the petals are small and green, with no scent or nectar.
Wind-pollinated flowers do not need to attract insects, so they have no bright petals, scent or nectar.
Two pollen grains. An insect-pollinated grain: larger, with spikes, which sticks to insects. A wind-pollinated grain: small, smooth and light, sometimes with air sacs, carried by the air.
Wind-pollinated plants make far more pollen, because most of it never reaches a stigma.

Self- and cross-pollination EXTENDED

  • EXTENDED Self-pollination: pollen goes from the anther of a flower to the stigma of the same flower, or of a different flower on the same plant.
  • Cross-pollination: pollen goes to the stigma of a flower on a different plant of the same species.
  • Cross-pollination produces more variation, so a population is better able to respond to changes in the environment; but it relies on pollinators (or wind) and wastes more pollen. Self-pollination does not depend on pollinators but gives less variation.
A pollen grain on the stigma grows a pollen tube down through the style into the ovary; the tube enters an ovule through a small hole, the micropyle, and the pollen nucleus fuses with the ovule nucleus: fertilisation.
EXTENDED After fertilisation the ovule becomes a seed and the ovary a fruit.

Germination

Seeds need water, oxygen and a suitable temperature to germinate (light is not needed). Water activates the enzymes and makes the seed swell; oxygen is needed for aerobic respiration; a suitable temperature lets the enzymes work.

Four test-tubes of seeds. With water, air and warmth: the seeds germinate. Dry, with no water: no germination. In boiled water covered with oil, so no oxygen: no germination. In a fridge, cold: no germination.
Each tube lacks one condition; the first tube is the control.

✏️Worked example

(a) Name the parts of the flower that become the seeds and the fruit. [2] (b) Give two features of a flower that would show it is wind-pollinated. [2] (c) In the germination experiment, explain why the water in one tube was boiled and covered with oil. [2] (d) EXTENDED A farmer grows a crop from cuttings (asexual). Suggest one advantage and one disadvantage. [2]

(a) Ovules become seeds; the ovary becomes the fruit.

(b) Any two: small, dull or green petals; no scent or nectar; anthers hanging outside on long filaments; large, feathery stigmas outside the flower; large amounts of small, light pollen.

(c) Boiling removes dissolved oxygen; the oil stops oxygen from the air dissolving back in, so the seeds have water but no oxygen.

(d) Advantage: all the plants have the same desirable features (e.g. high yield). Disadvantage: they are genetically identical, so a disease could wipe out the whole crop.

Check it. In the germination experiment only one factor changes in each tube, so the result shows which condition was needed.
Pollination is not fertilisation. Pollination is pollen arriving on a stigma; fertilisation is the fusion of nuclei inside the ovule, after the pollen tube has grown.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which part of a flower receives pollen grains? A: anther. B: stigma. C: ovary. D: sepal.
B.
2. (Theory.) Define asexual reproduction. [2]
A process resulting in the production of genetically identical offspring from one parent.
3. (Theory.) State the function of the petals and of the sepals in an insect-pollinated flower. [2]
Petals: large and brightly coloured to attract insects. Sepals: protect the flower in bud.
4. (Theory.) Describe two differences between the pollen of insect-pollinated and wind-pollinated flowers. [2]
Insect: larger, sticky or spiky, fewer grains. Wind: small, smooth and light, produced in very large numbers.
5. (Practical.) State the three conditions seeds need for germination. [3]
Water, oxygen, a suitable temperature.
6. (Theory.) EXTENDED A human body cell has 46 chromosomes. State the number in a sperm cell and in a zygote, and use the terms haploid and diploid. [3]
Sperm: 23, haploid. Zygote: 46, diploid.
7. (Theory.) EXTENDED Explain why cross-pollination may help a species survive a change in its environment. [2]
It produces more genetic variation, so some individuals are more likely to have features that let them survive the new conditions.
8. (Theory.) EXTENDED Describe what happens between a pollen grain landing on a stigma and fertilisation. [3]
A pollen tube grows from the grain down through the style to the ovary; it enters an ovule through the micropyle; the pollen nucleus fuses with the ovule nucleus.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Science and Plants for Schools (SAPS) — growing pollen tubes in the lab
  • Royal Botanic Gardens, Kew — pollination and pollinators