Reproduction: Question 7
Syllabus 16.3
A pea plant can be pollinated in two ways: self-pollination, in which pollen transfers from an anther to a stigma of the same flower (or another flower on the same plant), or cross-pollination, in which pollen transfers from the anther of one plant to the stigma of a flower on a genetically different plant of the same species.
(a) State one difference between self-pollination and cross-pollination, in terms of where the pollen comes from. [1]
(b) Describe what happens after a pollen grain lands on a compatible stigma, up to and including fertilisation. [3]
(c) After fertilisation, the ovule develops into a seed and the ovary develops into a fruit. State what happens to the petals and stigma of the flower once fertilisation is complete, and suggest one function of the fruit that forms around the seeds. [3]
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Worked solution
Part (a): Self-pollination vs cross-pollination
Self-pollination is the transfer of pollen from an anther to a stigma on the same plant (the same flower, or another flower on that plant). Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of a flower on a different, genetically different plant of the same species.
Part (b): From pollination to fertilisation
Once a pollen grain lands on a compatible stigma, it does not travel to the ovule as a whole grain. Instead, it germinates and grows a pollen tube, which grows down through the style towards an ovule inside the ovary. A male nucleus passes down this pollen tube and enters the ovule. Fertilisation occurs when this male nucleus fuses with the nucleus of the egg cell (the female gamete) inside the ovule, producing a zygote with a full set of chromosomes from both parent plants.
Part (c): What happens to the flower, and the role of the fruit
Once fertilisation is complete, the flower has fulfilled its purpose of attracting pollinators (if insect-pollinated) and receiving pollen, so the petals and stigma are no longer needed: they wither and fall off. Meanwhile, the fertilised ovule develops into a seed, and the ovary around it develops into a fruit.
The fruit that forms around the seed(s) helps with seed dispersal: it may be eaten by an animal that later deposits the undigested seed elsewhere, or it may have adaptations (such as wings or hooks) that help carry the seed away by wind or by catching onto an animal’s fur. Dispersing seeds away from the parent plant reduces competition between the parent and its offspring for light, water, and mineral nutrients, and allows the species to colonise new areas.
Final answers
- (a) Self-pollination: pollen moves within the same plant. Cross-pollination: pollen moves between two genetically different plants.
- (b) A pollen tube grows from the pollen grain down through the style to an ovule; the male nucleus travels down the tube and fuses with the egg cell’s nucleus, forming a zygote.
- (c) The petals and stigma wither and fall off; the ovule becomes a seed and the ovary becomes a fruit, which helps disperse the seeds away from the parent plant.