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.

Monocot embryo
Single cotyledon (scutellum) in one side of the embryonal axis
• radical with root cap enclosed by coleorrhizae.
• Above the attachment of scutellum is, coleoptile.( covers plumule)

Residual, persistant, nucellus is called as perisperm
Ex: black pepper and beet The embryo may enter a state of inactivity called dormancy.seed coats near micropyle facilitate entry of oxygen and water during germination.
Embryo is dormant, till favorable conditions are onset.
Fruitis (Ripened Ovary) – the wall of the ovary called Pericarp.

Fruit

False fruit: if the parts other than ovary, becomes fruit it is false fruit.
Ex: Apple,Strawberry, cashew

True fruit: Ovary become fruit
Ex: Mango

Parthenocarpic fruit: Fruit developed without fertilization
Ex: banana

Importance of seeds
Dispersal to new habitat
Nourishing young seeding
Protection of new embryo
Leads to variation

Apomixis
Seed production without fertilization

Special modes of reproduction.
1) polyembryony
2) artificial reproduction involves emasculation, bagging and tagging techniques.

Fertilisation: Process of fusion of sperm with ovum

Site of fertilisation in human female: Ampullary - isthmic junction

Secretion of acrosome helps the sperm entry into cytoplasm of ovum through zona pellucida and plasma membrane. Sperm entry induce the completion of the 2nd meiotic division of secondary Oocyte.

Placenta: An intimate connection between foetus and uterine wall of the mother to exchange materials.

Function of Placenta : Nutrition, Respiration, Excretion, as barrier, Endocrine function.

Placenta as Endocrine tissue: Placenta Produces several hormones such as - Estrogen, hcG, hPL Progesterone and relaxin (in late phase of pregnancy).

Embryonic Development: at various month of Pregnancy After-1 month = Heart, 2 months = Limbs and digits, 3 months = External genital organ. 5 months = First movement, 6 months = body covered with fine hairs, eye lid, eye lashes, 9 months = Fully developed and ready for delivery.

POINTS TO REMEMBER

Blastula: A stage of embryogenesis which comes after morula and has a hollow fluid filled space called blastocoel

Endometrium: Innermost glandular layer lining the uterine cavity.

Foetus: An advanced stage of embryo within the uterus.

Gestation Period: A period between fertilisation of ovum and the birth of baby

Hymen: A thin membrane partially covering the vaginal aperture.

Implantation: Fixing of embryo fertilized egg in uterus. It leads to pregnancy

Menarche: The beginning of first menstron in female on attaining puberty

Menopause: Permanent cessation of mensual cycle in female. It occurs between the age 45 to 50 years in human female.

Oogenesis: Formation and development of ova in ovary

Ovulation: Process of release of mature ovum (Secondary oocyte) from the ovary 

Parturition Process of delivery of the foe Child birth)

Puberty A stage at which immature reproductive system of boy or girl becomes mature

Scrotum: A muscular pouch which houses two totoo

Spermatogenes's Pinces of fromation of snem fime male germ Cell in the tentes

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