CBSE Class 12 Biology Sexual Reproduction In flowering Plants Notes Set B

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Topics:

Flowers

Pre- fertilization- structure and events

Pollination

Fertilization

Double fertilization

Post fertilization- events

Development of seed and fruits

Apomixes

Polyembryony

Types of fruits

Introduction

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cell into haploid gametophytic cells by meiosis.Fusion of haploid gametes of opposite sex to form diploid zygote.The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants all these steps of sexual reproduction occur within specialized reproductive organs called the flowers.

Pre- fertilization: Structures and Events

The parts of a stamen are filament and anther.It is bilobed and dithecous. The anther is a four- lobed structure consisting of four micro sporangia located at the corner, two in each lobe. Developed micro sporangia are called pollen sacs.

Structure of the pollen grains or male gametophyte

1. Wall layers: the outer hard layer-exine is made up of the most resistant organic material Sporopollenin which is resistant to high temperature, pH, and enzymatic action.

2. Exine has apertures called germ pores

3. Inner wall- intine is made up of cellulose and pectin 

Structure of an ovule or megasporangium
The ovule has a stalk-funicle- Hilum is the junction between ovule and funicle- two integuments envelop the ovule except at micropyle. Opposite end is called chalazal end. It encloses mass of cells called nucellus, with an embryo sac.

Megasporogenesis:
                             Nucellus
                                   ↓
                       Megaspore mother cell
                                   ↓
                             (One cell enlarges)
                                   ↓
                                 meiosis
                                   ↓
                             4-Megaspores
           3- degenerate and one functional megaspore remains.
                                   ↓ Nucleus divides mitotically
                                   ↓
           2 - Nucleate (move to opposite poles)
                                   ↓
                          II Mitosis
                      4 -Nucleate
                                   ↓
                                  III Mitosis

2) Geitonogamy
Transfer of pollen grain from the anther to the stigma of another flower in the same plant.

3) Xenogamy
Transfer of pollen grain from the anther to the stigma of a different plants.

Outbreeding devices
• Dichogamy – pollen release and stigma receptivity are not synchronized.
• Heterostyly – anther and stigma are placed at different positions.
• Self- incompatibility
• Production of unisexual flowers.

Pollen- pistil interaction
1) The pollen grains germinate in pollen tubes and the pollen tube passes through style, reaches ovary.
2) Enters ovule through micropyle with the help of synergids
3) Two male gametes are released.
(Artificial hybridization is achieved by emasculation and bagging techniques (removal of anther and dusting stigma with anther of desired type to prevent contaminations of its stigma with unwanted pollen)

Double fertilization
.One male gametes (n) + egg → (n) Syngamy (2n) → Zygote
Another male gamete (n) + polar nuclei (2n) → PEN(3X)
Primary endosperm nucleus develops into endosperm.

Post- fertilization events
A) Development of endosperm and embryo B) maturation of ovule into seed C) ovary into fruit.

Endosperm development
PEN undergoes repeated nuclear divisions to form into free- nuclear endosperm and if the cell wall formation occurs, forms to cellular- endosperm

Embryo development (Embryogeny)
Zygote → pro- embryo → globular embryo
                                             ↓
                                     Heart shaped embryo
                                             ↓
                                     Matured embryo

Dicot embryo consist of an embryonal axis and two cotyledons.

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