Sexual Reproduction in Flowering Plants - Detailed Notes

Flower

  • Androecium: Male reproductive organ.

  • Gynoecium: Female reproductive organ.

Pre-fertilisation

  • Hormonal changes lead to floral development.

  • Inflorescences bear flowers.

  • Androecium and gynoecium develop.

Stamen, Microsporangium, and Pollen Grain

  • Stamen: filament (stalk) and anther (bilobed).

  • Anther is bilobed (ditecous) with two theca in each lobe.

  • Anther has four microsporangia.

  • Microsporangia develop into pollen sacs.

Microsporangium

  • Microsporangium has four wall layers: epidermis, endothecium, middle layers, tapetum.

  • Tapetum nourishes pollen grains.

  • Sporogenous tissue in the center.

  • Microsporogenesis: sporogenous cells undergo meiosis to form microspore tetrads.

Microsporogenesis

  • PMC forms microspores through meiosis.

  • Microspores in tetrads.

  • Microspores develop into pollen grains.

Pollen Grain

  • Pollen grains vary in size and shape.

  • Two-layered wall: exine (sporopollenin) and intine (cellulose, pectin).

  • Exine has germ pores.

  • Mature pollen grain: vegetative and generative cells.

  • Pollen grains shed at 2-celled or 3-celled stage.

  • Pollen can cause allergies.

Pollen Viability

  • Viability depends on temperature and humidity.

  • Stored in liquid nitrogen in pollen banks.

Pollen products

  • Used as food supplements.

Pistil, Megasporangium, Embryo sac

  • Gynoecium: single or multiple pistils.

  • Pistil: stigma, style, ovary.

  • Ovules arise from the placenta.

  • Ovule: funicle, integuments, micropyle, chalaza, nucellus.

  • Embryo sac in the nucellus.

Megasporogenesis

  • MMC forms megaspores.

  • MMC undergoes meiosis to produce four megaspores.

Female Gametophyte

  • One megaspore develops into the embryo sac.

  • Monosporic development.

Embryo sac development

  • Functional megaspore divides mitotically.

  • 2-nucleate, 4-nucleate, 8-nucleate stages.

  • Six nuclei form cells; two polar nuclei remain.

  • Egg apparatus: two synergids and one egg cell.

  • Antipodals at the chalazal end.

  • Mature embryo sac: 8-nucleate, 7-celled.

Pollination

  • Transfer of pollen grains from anther to stigma.

Kinds of Pollination

  • Autogamy: same flower.

  • Geitonogamy: another flower on the same plant.

  • Xenogamy: different plant.

Autogamy

  • Pollination within the same flower.

  • Requires synchrony and proximity.

  • Chasmogamous and cleistogamous flowers.

Geitonogamy

  • Pollination of another flower on the same plant; genetically similar to autogamy.

Xenogamy

  • Pollination of a different plant; brings genetic variation.

Agents of Pollination

  • Abiotic (wind, water) and biotic (animals) agents.

Wind Pollination

  • Light, non-sticky pollen grains.

  • Exposed stamens and feathery stigma.

  • Single ovule in each ovary; numerous flowers in inflorescence.

  • Common in grasses.

Water Pollination

  • Rare; some monocotyledons.

  • Examples: Vallisneria, Hydrilla, Zostera.

  • Pollen protected by mucilaginous covering.

Animal Pollination

  • Bees, butterflies, flies, beetles, wasps, ants, moths, birds, bats.

  • Insect-pollinated flowers: large, colorful, fragrant, nectar-rich.

Animal Attraction and Rewards

  • Attracted by color and fragrance.

  • Rewards: nectar and pollen grains.

  • Some provide safe places to lay eggs.

Floral Visitors

  • Pollen/nectar robbers consume without pollinating.

Outbreeding Devices

  • Prevent self-pollination; encourage cross-pollination.

  • Strategies: asynchronous pollen release, different placement of anther and stigma, self-incompatibility, unisexual flowers.

Pollen-Pistil Interaction

  • Pistil recognizes compatible pollen.

  • Dialogue mediated by chemical components.

  • Pollen grain germinates, pollen tube grows.

  • Pollen tube carries male gametes.

  • Pollen-pistil interaction: pollen recognition, promotion or inhibition.

Artificial Hybridisation

  • Emasculation (removal of anthers) and bagging to prevent contamination.

  • Pollen dusted on the stigma; flowers rebagged.

Post-Fertilisation

  • Endosperm and embryo development, ovule to seed, ovary to fruit.

Double Fertilisation

  • Pollen tube releases two male gametes.

  • One gamete fuses with egg (syngamy) forming zygote.

  • Other gamete fuses with polar nuclei forming triploid PEN (triple fusion).

  • Double fertilization: syngamy and triple fusion.

Endosperm

  • Endosperm development precedes embryo development.

  • PEN divides to form triploid endosperm.

  • Free-nuclear endosperm becomes cellular.

  • Coconut water is free-nuclear endosperm.

  • Endosperm consumed by embryo or persists in mature seed.

Embryo

  • Develops at micropylar end.

  • Zygote forms proembryo, then globular, heart-shaped, mature embryo.

Dicotyledonous Embryo

  • Embryonal axis and two cotyledons.

  • Epicotyl (plumule) and hypocotyl (radicle, root cap).

Monocotyledonous Embryo

  • One cotyledon (scutellum).

  • Coleorrhiza encloses radicle and root cap.

  • Epicotyl (shoot apex, leaf primordia) enclosed in coleoptile.

Seed

  • Fertilized ovule; seed coat, cotyledons, embryo axis.

  • Non-albuminous or ex-albuminous (no endosperm).

  • Albuminous (retains endosperm).

  • Perisperm (remnants of nucellus).

  • Micropyle facilitates entry of oxygen and water.

  • Water content reduced; embryo dormant.

Fruit

  • Ovary develops into fruit; pericarp (fruit wall).

  • Fleshy or dry.

  • Mechanisms for seed dispersal.

  • True fruits develop from the ovary; false fruits include thalamus.

  • Parthenocarpic fruits develop without fertilization (e.g., banana).

Seed Advantages

  • Independent of water.

  • Adaptive dispersal strategies.

  • Nourishment for young seedlings.

  • Protection by seed coat.

  • New genetic combinations.

  • Basis of agriculture.

Seed Viability

  • Varies from months to years.

  • Lupinus arcticus viable for 10,000 years.

  • Phoenix dactylifera viable for 2000 years.

Reproductive Capacity

  • Orchid fruits contain thousands of tiny seeds.

Apomixis and Polyembryony

  • Apomixis: seeds without fertilization.

  • Nucellar cells develop into embryos.

  • Polyembryony: more than one embryo in a seed.

  • Hybrid seed industry benefits from apomixis.

Summary

  • Flowers: sexual reproduction in angiosperms.

  • Anther: bilobed, dithecous, tetrasporangiate; pollen grains develop inside.

  • Pistil: stigma, style, ovary; ovules inside.

  • Embryo sac: 7-celled, 8-nucleate.

  • Pollination: transfer of pollen.

  • Double fertilization: syngamy and triple fusion.

  • Zygote develops into the embryo; primary endosperm cell forms endosperm.

  • Ovary develops into fruit; ovules into seeds.

  • Apomixis: seeds without fertilization; polyembryony: multiple embryos.