Read Pollen Acceptance and Pollination
Pollen-pistil interactions determine whether pollen germinates and reaches the ovule.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Accept or reject pollen
A compatible pollen grain germinates and its tube grows through the style. Self-incompatibility can reject genetically self pollen from the same plant, including a different flower. In the figure, tube elongation is driven by the vegetative cell, not generative-cell activity.
Use pollination knowledge
Pollen-pistil recognition enables controlled hybridisation. Emasculation is used for bisexual flowers when needed, but is not required for unisexual female flowers. In Vallisneria, male flowers reach the water surface and female flowers are brought up by a long stalk.
Separate the three pollination modes
Autogamy transfers pollen within one flower. Geitonogamy transfers pollen between flowers on the same plant: it uses a pollination vector but is genetically like selfing. Xenogamy transfers pollen between different plants of the same species and introduces genetically distinct pollen. Dioecious plants require xenogamy.
Flower timing and opening control selfing
Homogamy synchronises anther and stigma maturity; dichogamy separates their timing. Chasmogamous flowers open and expose their reproductive organs. Cleistogamous flowers remain closed and assure autogamy, but limit variation and can increase inbreeding depression.
Why bisexual flowers use outbreeding devices
Self-incompatibility, dichogamy and other mechanisms promote outcrossing in many bisexual flowers. Outcrossing reduces inbreeding depression and increases genetic variation, which can improve population resilience in changing environments.
OlympIQ exam lens
- ✓Compatibility controls germination and tube growth.
- ✓Self-incompatibility can reject pollen from another flower on the same plant.
- ✓Emasculation is not needed for unisexual female flowers.
- ✓Vallisneria: male flowers float; female flower reaches surface.
- ✓Autogamy: same flower; geitonogamy: different flowers on same plant; xenogamy: different plants.
- ✓Geitonogamy is functionally cross-pollination but genetically selfing.
- ✓Cleistogamy assures autogamy; chasmogamy opens the flower.
- ✓Outbreeding reduces inbreeding depression and increases variation.
The generative cell forms sperm; vegetative-cell activity drives pollen-tube growth.
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