Sexual Reproduction in Flowering Plants — Short Notes

Flower Structure & Pre-Fertilisation

• Angiosperms reproduce sexually; the flower is the reproductive unit.
• Male whorl: androecium of stamens; each stamen = filament + anther.
• Female whorl: gynoecium of one or more carpels; each carpel = stigma + style + ovary.
• Pre-fertilisation events: microsporogenesis (male) & megasporogenesis (female), pollination, then double fertilisation.

Microsporogenesis & Pollen Grain

• Anther is tetragonal, has four microsporangia (later become pollen sacs).
• Each microsporangium contains microspore mother cells (MMC) that undergo meiosis → 4 haploid microspores (tetrad).
• Microspores mature into pollen grains.
• Pollen wall: outer exine (sporopollenin — most resistant biological material; germ pores present) + inner intine (cellulose + pectin).
• Mature pollen has two cells: vegetative cell (large, food-storing, irregular nucleus) + generative cell (small, spindle-shaped, floats in vegetative cytoplasm).
• Pollen viability varies: rice/wheat ~30 min; Rosaceae/Solanaceae/Leguminosae months.
• Pollen banks preserve germplasm in liquid N₂ at −196°C.

Megasporogenesis & Embryo Sac

• Ovule (megasporangium) — attached to placenta via funicle; regions: hilum, integuments, nucellus, micropyle.
• Megaspore mother cell (MMC) in nucellus → meiosis → 4 megaspores; usually only 1 functional.
• Functional megaspore → female gametophyte / embryo sac (monosporic development).
• Embryo sac has 3 mitotic divisions → 8 nuclei → 7-celled, 8-nucleate structure:
• Egg apparatus at micropylar end: 1 egg + 2 synergids (with filiform apparatus).
• 3 antipodal cells at chalazal end.
• Central cell with 2 polar nuclei.

Pollination

• Transfer of pollen from anther to stigma. Types:

1. Autogamy — same flower (chasmogamous = open; cleistogamous = closed, always self).
2. Geitonogamy — different flower, same plant (functionally cross but genetically self).
3. Xenogamy — different plant (true cross-pollination).

• Agents: abiotic (wind, water) & biotic (insects, birds, bats).
• Wind-pollinated (anemophily): light dry pollen, well-exposed stamens, feathery stigma (grasses, maize).
• Water (hydrophily): rare; e.g. Vallisneria, Zostera.
• Insect (entomophily): colourful/fragrant/nectar-rich flowers; e.g. Yucca & moth (obligate mutualism).

Outbreeding Devices

• Pollen release & stigma receptivity at different times.
• Different heights of anther and stigma.
• Self-incompatibility — genetic block on self-pollen germination.
• Dioecy — unisexual flowers on different plants (e.g. papaya).

Pollen–Pistil Interaction

• Compatible pollen grain germinates → pollen tube from vegetative cell.
• Generative cell divides in pollen tube → 2 male gametes.
• Pollen tube enters ovule through micropyle (porogamy), then embryo sac via a synergid.

Double Fertilisation (unique to angiosperms)

• Pollen tube releases 2 male gametes into embryo sac:
• Male gamete 1 + egg → zygote (2n) → embryo → syngamy.
• Male gamete 2 + 2 polar nuclei → PEN (3n) → endosperm → triple fusion.
• Both fusions together = double fertilisation.

Post-Fertilisation

• Endosperm develops first (nutritive tissue). Types: nuclear (coconut milk), cellular, helobial.
• Embryo develops through globular → heart-shaped → mature.
• Dicot embryo: 2 cotyledons, plumule, radicle. Monocot: 1 cotyledon (scutellum), coleoptile, coleorhiza.
• Ovule → seed; ovary → fruit.
• True fruit: ovary only. False fruit (e.g. apple): thalamus also.
• Parthenocarpy: fruit without fertilisation (banana).

Apomixis & Polyembryony

• Apomixis — asexual seed formation without meiosis/fertilisation (e.g. some Asteraceae, grasses).
• Ensures hybrid vigour retention across generations (useful in agriculture).
• Polyembryony — more than one embryo per seed (citrus, mango).

Summary

• Sexual reproduction ensures genetic variation via meiosis + syngamy.
• Angiosperm-specific: double fertilisation producing zygote (2n) + endosperm (3n).
• Outbreeding devices maintain variability; apomixis preserves it.