Ch 2 details sexual reproduction in angiosperms — flower structure, microsporogenesis and megasporogenesis, pollination mechanisms, double fertilisation, and seed/fruit development.
Stamen (microsporophyll): anther produces pollen (microsporogenesis — meiosis). Each pollen grain = 2-celled (vegetative + generative) or 3-celled. Pistil (megasporophyll): ovary → ovule → megaspore mother cell → megasporogenesis → embryo sac (7 cells, 8 nuclei: 1 egg + 2 synergids + 3 antipodals + 2 polar nuclei).
Self-pollination: same flower/plant. Cross-pollination: different plant (anemophily — wind, entomophily — insects, hydrophily — water). Devices to prevent self-pollination: dichogamy, herkogamy, self-incompatibility. Double fertilisation: one sperm + egg → zygote (2n), second sperm + polar nuclei → endosperm (3n). Unique to angiosperms.
Zygote → embryo (embryogeny). Endosperm provides nutrition. Ovule → seed (integuments → seed coat). Ovary → fruit. Apomixis: seed formation without fertilisation. Polyembryony: multiple embryos in one seed (citrus).
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Double fertilisation is unique to angiosperms: one sperm fuses with the egg to form the zygote (2n), and the second sperm fuses with two polar nuclei to form the triploid endosperm (3n). The endosperm provides nutrition to the developing embryo. This ensures that endosperm develops only when the egg is fertilised, saving resources — no nutrients are wasted if fertilisation fails.
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