01Class 12 Biology
Sexual Reproduction in Flowering Plants short notes
Short notes on flower structure, pollination, double fertilisation and seed formation. Know the embryo sac structure especially well.
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Stamen and pollen
- An anther is usually bilobed and dithecous, with four microsporangia.
- The tapetum is the innermost wall layer, and it nourishes developing pollen.
- The pollen wall has an outer exine of sporopollenin and an inner intine. Exine is one of the most resistant organic materials known.
- Pollen is usually shed at the 2-celled stage: a vegetative cell and a generative cell.
Pistil and ovule
- One megaspore mother cell gives four megaspores by meiosis, and usually one survives.
- The embryo sac is 7-celled and 8-nucleate, a type called monosporic.
- It has 3 antipodals, 2 synergids and an egg cell, and a central cell with 2 polar nuclei.
- The filiform apparatus on synergids guides the pollen tube.
Pollination
- Autogamy is within the same flower, geitonogamy is between flowers of one plant, and xenogamy is between plants.
- Cleistogamous flowers never open, so they self-pollinate.
- Wind-pollinated flowers have light pollen and feathery stigma. Water pollination occurs in Vallisneria and Zostera.
- Outbreeding devices include dichogamy, herkogamy, self-incompatibility and unisexual flowers.
Fertilisation and seeds
- Double fertilisation: one male gamete joins the egg (syngamy), and the other joins polar nuclei (triple fusion), forming triploid endosperm.
- Non-albuminous seeds use up the endosperm, as in pea. Albuminous seeds keep it, as in wheat and maize.
- The ovary becomes fruit, and the ovule becomes seed.
- Apomixis forms seeds without fertilisation, and polyembryony gives more than one embryo, as in citrus.
Where students slip up
- Counting 8 cells in the embryo sac. It's 7 cells and 8 nuclei.
- Thinking geitonogamy is genetically cross-pollination. It's functionally cross, but genetically self.
- Saying endosperm is diploid. It's triploid in flowering plants.
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