Sexual reproduction in plants: the structure of an insect-pollinated flower; gamete formation; pollination and double fertilisation; seed and fruit formation; and adaptations promoting cross-pollination.

What this dot point is asking

Eduqas wants you to describe the structure of a flower, describe gamete formation, explain pollination and double fertilisation, describe seed and fruit formation, and explain adaptations promoting cross-pollination. This is the plant-reproduction counterpart in Component 2.

Flower structure and gamete formation

An insect-pollinated flower typically has brightly coloured petals, nectaries and scent to attract insects. Its reproductive organs are:

• the stamen (male): the anther (where pollen forms) on a filament;
• the carpel (female): the stigma (catches pollen), the style, and the ovary containing one or more ovules.

Inside the anther, cells divide by meiosis to form pollen grains, each containing a male gamete (which divides into two male nuclei). Inside each ovule, meiosis forms the embryo sac, containing the female gamete (the egg cell) and two polar nuclei.

Pollination and double fertilisation

Pollination is the transfer of pollen from an anther to a stigma (by insects, wind, and so on). After landing on a compatible stigma, the pollen grain germinates:

1. A pollen tube grows down through the style towards the ovule, its growth controlled by the tube nucleus and digesting a path with enzymes.
2. The tube delivers two male gamete nuclei into the embryo sac.
3. Double fertilisation: one male nucleus fuses with the egg cell to form a diploid zygote (the future embryo); the other fuses with the two polar nuclei to form a triploid endosperm (a food store).

Seed and fruit formation

After fertilisation:

• the zygote develops into the embryo (with a young root and shoot);
• the endosperm (and/or the cotyledons) stores food for germination;
• the whole ovule becomes the seed, with a tough seed coat (testa);
• the ovary develops into the fruit, which protects the seeds and often aids dispersal.

Adaptations promoting cross-pollination

Examples in context

Example 1. Wind-pollinated grasses. Grasses have small, dull flowers with large feathery stigmas and anthers that dangle outside the flower, adaptations to catch and release pollen on the wind rather than attract insects, a useful contrast Eduqas may set.

Example 2. Self-incompatibility in fruit trees. Many apple varieties cannot fertilise themselves, so orchards plant compatible varieties together to ensure cross-pollination and a good crop, a real application of the cross-pollination adaptations.

Try this

Q1. Name the male and female reproductive parts of a flower. [2 marks]

• Cue. Male: the stamen (anther and filament). Female: the carpel (stigma, style and ovary).

Q2. State the products of double fertilisation and their ploidy. [2 marks]

• Cue. A diploid zygote (egg plus one male nucleus) and a triploid endosperm (two polar nuclei plus one male nucleus).

Q3. Explain why cross-pollination is advantageous to a plant population. [2 marks]

• Cue. It increases genetic variation by mixing alleles from two parents, so the population can better adapt to change and harmful recessive alleles are less likely to be expressed.