📝 Chapter Notes & Revision

Sexual Reproduction in Flowering Plants

🏫 MP BoardClass 12Biology

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Class 12 Biology: Sexual Reproduction in Flowering Plants

MP Board

1. Introduction & Flower Structure

  • Flower: Modified shoot meant for sexual reproduction.
  • Essential Whorls: Androecium (Male reproductive organ) and Gynoecium (Female reproductive organ).

2. Pre-fertilization: Structures and Events

Events prior to fertilization include gametogenesis (formation of gametes) and pollination.

A. Stamen, Microsporangium, and Pollen Grain (Male)

  • Stamen: Consists of an anther and a filament.
  • Microsporangium: Usually four-walled:
    1. Epidermis (Protection)
    2. Endothecium (Dehiscence of anther)
    3. Middle layers (Degenerate to provide nourishment)
    4. Tapetum (Innermost layer, nourishes developing pollen grains).
  • Microsporogenesis: Formation of microspores from a Pollen Mother Cell (PMC) through meiosis. 1 PMC (2n) -> 4 Microspores (n).
  • Pollen Grain (Male Gametophyte):
    • Exine: Outer hard layer made of Sporopollenin (extremely resistant to high temperatures, strong acids, and enzymes).
    • Intine: Inner thin layer made of cellulose and pectin.
    • Generative Cell: Small, floats in the cytoplasm of the vegetative cell, divides to form two male gametes.
    • Vegetative Cell: Bigger, has abundant food reserve and a large irregularly shaped nucleus.

B. Pistil, Megasporangium (Ovule), and Embryo Sac (Female)

  • Gynoecium: Can be syncarpous (fused carpels) or apocarpous (free carpels).
  • Parts of Pistil: Stigma (landing platform), Style (elongated stalk), Ovary (basal bulged part).
  • Megasporangium (Ovule): Attached to the placenta by the funicle. The body of the ovule fuses with funicle in the region called hilum.
  • Megasporogenesis: Formation of megaspores from a Megaspore Mother Cell (MMC) through meiosis inside the nucellus. 1 MMC (2n) -> 4 Megaspores (n), out of which 3 degenerate and 1 functional megaspore develops.
  • Female Gametophyte (Embryo Sac):
    • Monosporic development (from a single functional megaspore).
    • Structure (7-celled, 8-nucleate at maturity):
      • Egg apparatus: 1 Egg cell + 2 Synergids (having filiform apparatus).
      • Antipodals: 3 cells at the chalazal end.
      • Central cell: 2 Polar nuclei (fuse to form a single diploid secondary nucleus).

3. Pollination

Transfer of pollen grains from the anther to the stigma of a pistil.

Types of Pollination:

  1. Autogamy: Transfer of pollen grains from anther to stigma of the same flower (e.g., Commelina showing chasmogamous and cleistogamous flowers).
  2. Geitonogamy: Transfer of pollen grains from anther to stigma of another flower of the same plant (functionally cross-pollination, genetically self-pollination).
  3. Xenogamy: Transfer of pollen grains from anther to stigma of different plants of the same species (True cross-pollination).

Agents of Pollination:

  • Abiotic Agents: Wind (Anemophily - e.g., corn cob), Water (Hydrophily - e.g., Vallisneria, Hydrilla).
  • Biotic Agents: Insects (Entomophily), Bats (Chiropterophily), Birds (Ornithophily).
  • Outbreeding Devices: Mechanisms to prevent inbreeding and promote cross-pollination (e.g., unisexuality, dichogamy, self-incompatibility).

4. Double Fertilization

Unique feature of angiosperms discovered by S.G. Nawaschin.

  1. Syngamy: One male gamete (n) fuses with the egg cell (n) to form a zygote (2n, future embryo). n + n = 2n
  2. Triple Fusion: The second male gamete (n) fuses with the diploid secondary nucleus (2n) in the central cell to form the Primary Endosperm Nucleus (PEN, 3n, future endosperm). n + 2n = 3n
  • Double Fertilization = Syngamy + Triple Fusion.

5. Post-fertilization: Structures and Events

  • Endosperm: Triploid (3n) tissue formed after triple fusion, provides nourishment to the developing embryo.
    • Types: Nuclear, Cellular, and Helobial.
  • Embryo (Embryogeny): Develops at the micropylar end of the embryo sac where the zygote is situated.
    • Dicot Embryo: Consists of an embryonal axis and two cotyledons. Parts include epicotyl, hypocotyl, plumule, and radicle.
    • Monocot Embryo: Consists of only one cotyledon (termed Scutellum in grass family). Protective sheath around radicle is coleorrhiza and around plumule is coleoptile.
  • Seed: Fertilized ovule.
    • Albuminous (Non-endospermic): Endosperm completely consumed (e.g., Pea, Groundnut).
    • Ex-albuminous (Endospermic): Endosperm retained (e.g., Wheat, Maize, Castor).
  • Fruit: Ripened ovary.
    • True Fruit: Develops only from the ovary (e.g., Mango).
    • False Fruit: Thalamus also contributes to fruit formation (e.g., Apple, Strawberry).
    • Parthenocarpic Fruit: Formed without fertilization, hence seedless (e.g., Banana) - induced by growth hormones like auxins/gibberellins.

6. Special Modes of Reproduction

  • Apomixis: Production of seeds without fertilization (a form of asexual reproduction mimicking sexual reproduction). E.g., Asteraceae, grasses.
  • Polyembryony: Occurrence of more than one embryo in a seed (e.g., Citrus, Mango). Can be due to adventive embryony or cleavage polyembryony.