Sexual Reproduction in Flowering Plants
Flowering plants reproduce sexually using flowers, structures specialised for producing and bringing together male and female gametes. This chapter covers flower structure, how pollen grains and the embryo sac form, pollination and fertilisation, and how the resulting seeds and fruits develop, are dispersed, and eventually germinate into new plants.
Because each stage depends on the one before it, it helps to trace the whole sequence, flower, pollen and embryo sac, pollination, fertilisation, seed and fruit, dispersal, germination, as a single connected story rather than isolated facts.
Labelling a flower, describing double fertilisation, and comparing self- and cross-pollination are common exam tasks, so precise vocabulary for each floral part and each stage of the process is essential. A common mark-losing habit is switching freely between 'pollen' and 'seed' or between 'ovule' and 'ovary', so checking these terms carefully before an exam is worthwhile.
After a seed forms inside the fruit, it usually needs to be carried away from the parent plant before it can grow successfully, since dispersal reduces competition between parent and offspring for light, water and soil nutrients and lets the species colonise new areas. Fruits and seeds show specific adaptations for wind, water, animal or explosive dispersal that match how far and by what method they travel.
Self-pollination, the transfer of pollen within the same flower or plant, and cross-pollination, the transfer between different plants of the same species, have opposite advantages: self-pollination is reliable even without an external agent but produces less genetic variation, while cross-pollination increases genetic variation but depends on wind or an animal to carry pollen between plants, so many flowers have structures that favour one method over the other.
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Content standards in this chapter
- 21.1 The Structure of a Flower
- 21.2 Development of Pollen Grains and the Embryo Sac
- 21.3 Pollination and Fertilisation in Flowering Plants
- 21.4 Development of Seeds and Fruits
- 21.5 Importance of Seeds for Survival
Key concepts
- Structure of a flower
- The stamen, made up of the anther and filament, is the male reproductive part of a flower, while the carpel, made up of the stigma, style and ovary, is the female part; a single flower may contain one or more of each, and the number and arrangement of these parts is often used to identify a species. Some flowers, described as incomplete, lack one or more of these parts, such as petals or stamens, yet can still reproduce successfully.
- Development of gametes
- Pollen grains, each containing a male gamete, form inside the anther, while the embryo sac, containing the female gamete, forms inside the ovule within the ovary; both are produced by meiosis, which creates genetic variation among the gametes even before fertilisation combines two parents' genes.
- Pollination
- Pollination is the transfer of pollen grains from the anther to the stigma, either of the same flower (self-pollination) or of a different flower on a different plant of the same species (cross-pollination); the transfer is carried out by an external agent such as wind or an insect, and floral features often reveal which agent a flower relies on. A flower can receive its own pollen and pollen from another plant at the same time, so both types of pollination are not always mutually exclusive events.
- Fertilisation
- After pollination, a pollen tube grows down through the style towards the ovule, carrying two male gametes; in double fertilisation, one male gamete fuses with the female gamete to form a diploid zygote and the other fuses with two nuclei in the embryo sac to form a triploid tissue called the endosperm, which stores food for the developing embryo. Because two separate fusions occur from the same pollen tube, double fertilisation is unique to flowering plants and is not seen in animals or in non-flowering plants such as ferns.
- Seeds and fruits
- After fertilisation, the ovule develops into a seed containing the embryo, its food store and a protective seed coat, while the ovary wall develops into the fruit, which protects the seed or seeds inside and, in many species, also assists with dispersal.
- Importance of seeds
- Seeds allow a plant species to survive conditions unfavourable for growth, such as drought or cold, by remaining dormant until conditions improve, and to be carried away from the parent plant to colonise new areas with less competition for resources.
- Seed dispersal
- Seeds and fruits are dispersed by wind, using light weight and structures such as wings or feathery plumes; by water, using a buoyant, waterproof fruit wall; by animals, either through hooks and spines that cling to fur or through a fleshy, edible fruit that is eaten and the seeds later deposited elsewhere; or explosively, when a dry pod twists and splits suddenly, flinging seeds away from the parent plant. A single fruit sometimes combines more than one feature, such as being both lightweight and having a hook, so a real specimen may not fit neatly into only one dispersal category.
- Germination requirements
- A seed generally needs three conditions to germinate: water, to soften the seed coat and activate enzymes; oxygen, to allow aerobic respiration that releases the energy needed for growth; and a suitable temperature, at which these enzymes work efficiently; light is not usually required for germination itself, although the seedling needs it soon afterwards for photosynthesis. These three conditions are often tested through a controlled experiment comparing seeds given different combinations of water, air and temperature to show which factor is limiting.
- Self-pollination versus cross-pollination
- Self-pollination reliably produces seed even when no pollinating agent is available and preserves a combination of genes already well suited to the environment, but it produces offspring with little genetic variation and can lead to weaker offspring over many generations; cross-pollination increases genetic variation, generally producing hardier offspring, but it depends on an external agent and on the pollen source being compatible. Structural features that reduce self-pollination, such as the anther and stigma maturing at different times, are themselves valid exam answers when asked how a flower favours cross-pollination.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Labelling a flower and giving the function of each part.
- Comparing wind and insect pollination.
- Describing fertilisation and the fate of the ovule and ovary.
- Explaining seed dispersal mechanisms and linking each fruit or seed feature to its dispersal method.
- Stating and explaining the three conditions needed for a seed to germinate.
- Comparing the advantages and disadvantages of self-pollination and cross-pollination.
Common mistakes
What students write: Confusing pollination and fertilisation.
What earns the mark: Pollination is the transfer of pollen to the stigma; fertilisation is the fusion of the male and female gametes. Students sometimes use the two terms interchangeably in an answer, which a marker will read as a genuine misunderstanding rather than a slip.
What students write: Saying the ovary becomes the seed.
What earns the mark: The ovule becomes the seed and the ovary becomes the fruit.
What students write: Mixing up self and cross pollination.
What earns the mark: Self-pollination is within the same flower or plant; cross-pollination is between different plants of the same species.
What students write: Writing that the stigma makes pollen.
What earns the mark: The anther makes pollen; the stigma receives it.
What students write: Confusing seed dispersal with pollination.
What earns the mark: Pollination happens before fertilisation and moves pollen to the stigma; seed dispersal happens after fertilisation and moves the finished seed away from the parent plant. Keeping the two processes in the correct order on a timeline avoids losing marks on structured questions about the plant life cycle.
What students write: Saying light is essential for a seed to germinate.
What earns the mark: Most seeds can germinate in complete darkness; only water, oxygen and a suitable temperature are needed, though light becomes essential once the seedling starts photosynthesising. A seed placed in a dark cupboard with enough water, air and warmth will still germinate normally.
What students write: Assuming self-pollination cannot successfully produce seeds.
What earns the mark: Many species, such as peas, self-pollinate successfully and produce viable seeds; the drawback of self-pollination is reduced genetic variation, not failure to reproduce.
What students write: Describing any small, light fruit as wind-pollinated.
What earns the mark: A small, light fruit or seed with wings or plumes shows an adaptation for wind dispersal, which is separate from and can occur in a flower that was insect-pollinated. The safest approach is to describe wind and dispersal separately from insect and pollination, using each pair of terms only where it belongs.
Study this chapter
Processes in this chapter
Structures in this chapter
Key terms
Frequently asked questions
What is the difference between pollination and fertilisation?
What happens to the flower after fertilisation?
How do wind-pollinated and insect-pollinated flowers differ?
What conditions are needed for a seed to germinate?
What is the difference between self-pollination and cross-pollination, and why does cross-pollination happen?
Source:SRC-DSKP-EN, SRC-FORMAT
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