Sexual Reproduction in Flowering Plants Class 12 Notes Biology Chapter 2 - CBSE

Chapter:2

What are Sexual Reproduction In Flowering Plants?

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    Flower

    It is the reproductive organ of plants. It consists of 4 whorls:

    • Calyx (outermost)
    • Corolla (next to calyx)
    • Androecium (next to corolla)
    • Gynoecium (innermost)

    Androecium (Male Reproductive Part)

    • Stamen consist of anther and filament.
    • Anther has two lobes (bilobed). Each lobe consists of two theca (dithecous) and filament (long slender stalk). Proximal end of the filament is attached to the thalamus or petal.
    • Sporogenous tissue is compactly arranged homogenous cells present at center of each microsporangium when the anther is young.

    A typical microsporangium is surrounded by 4 walls

    • Epidermis: Protects and help in dehiscence of anther.
    • Endothecium: Layer below epidermis.
    • Middle layers: Layers below endothecium.
    • Tapetum: Nourishes the developing pollen grain.

    Pollen Grains

    Pollen grains/male gametophyte - spherical in shape and made up of 2 layers+ 1 Germ Pore+ 2 Cells

    • Exine
    • Intine
    • Germ Pores
    • Cells
    • ➥ Vegetative Cell: Bigger and have abundant food reserve which is responsible for the development of pollen grain.
    • ➥ Generative Cell: Small and involves in syngamy (fuse with an egg). Its function is formation of male gametes.

    Gynoecium:(Female Reproductive Part Of Flower)

    • Gynoecium with one is pistil – Monocarpellary, Gynoecium with more than one pistil is – Multicarpellary, Fused pistil – Syncarpous , Free pistil – Apocarpous
    • Pistil (Female Reproductive Part) consist of: Stigma, Style, Ovary.
    • Ovule is a small structure attached to placenta. The parts of the ovule are: Funicle, Hilum, Integuments, Micropyle, Chalaza, Nucellus.
    • Process of formation of megaspores from megaspore mother cells is called megasporogenesis.
    • Megaspore mother cells (2n) meiotically
    • ↓
    • 4 megaspores (haploid)
    • Only 1 megaspore is functional and forms gametophyte and the rest 3 degenerates.
    • The embryo sac develops from the functional megaspore (n).
    • Egg appartus-present at the micropylar end and consists of 2 synergids and 1 egg cell.

    Pollination

    The transfer of pollen grains from anther to stigma of a pistil is called pollination.

    Based on the Source of Pollen, Pollination is of 3 types

    • Autogamy: Transfer of pollen grains from anther to stigma of the same flower.
    • Geitonogamy: Transfer of pollen grains from anther to stigma of another flower of the same plant, Genetically similar. Ex: cucurbits
    • Xenogamy: Transfer of pollen grains from anther to stigma of another flower of different plant.

    Agents Of Pollination

    • Abiotic agents: Wind and Water.
    • Biotic agents: Insects, Birds, Bats, Reptiles and Mammals

    Ways to avoid Self-pollination

    • Release of pollen grains and receptivity of stigma are not synchronized.
    • Stigma and anther are placed at different positions.
    • Self-incompatibility.
    • Production of unisexual flowers (prevents autogamy but not geitonogamy). e.g. castor, maize.
    • Male and Female flowers are produced on different flowers (prevents autogamy & geitonogamy). e.g. papaya.

    Steps Of Pollen: Pistil Interaction

    Recognition of compatible pollen→ Germination of pollen →grains Development of Male Gametophyte

    Major Post- Fertilization Events

    • Development of embryo
    • Development of endosperm
    • Maturation of ovule into seed
    • Ripening of ovary into fruit

    Two Types of Endosperm Development

    • Free Nuclear Type: Nuclear division is not followed by cell wall formation and nuclei remain free e.g., coconut water.
    • Cellular Type: Division of nucleus is immediately followed by cell wall formation.

    Embryo

    Zygote → A Proembryo → A Mature embryo (heart-shaped)

    It consists of: Embryonal Axis, Cotyledons, Plumule and Radicle.

    Seed

    Fertilized and mature ovule develops into seed.

    • Seed consists of: Cotyledon(s), Embryonal Axis, Seed coat, Testa (outer coat), Tegmen (inner coat).
    • Seed are of two types: Albuminous and Non-Albuminous.
    • Perisperm : remnants of nucellus that is persistent. Ex: Black pepper.
    • Dormancy: state of inactivity.

    Fruit

    • True Fruit: Developed from ripened ovary.
    • Parthenogenesis: Ovary gets transformed to fruit without fertilization. Ex: Banana.
    • Parthenocarpy: Seedless fruit which is formed due to gibberellins & auxins.
    • False Fruit: Any part of flower other than ovary forms the fruit. Ex: Apple.

    Other Modes of Reproduction

    • Apomixis: Apomixis: Form of asexual reproduction where seed is formed without fertilization. Ex. Citrus and Mango.
    • Polyembryony: Occurrence of more than one embryo in a seed. Ex: Citrus, groundnut.