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.

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