Curriculum
Course: Biology Class 12 CBSE
time left:
:
:
Login

Curriculum

Biology Class 12 CBSE

Chapters

0/10
Quiz

Chapter 1 – Sexual Reproduction in Flowering Plants

Submit quiz
Once you submit, you will no longer be able to change your answers. Are you sure you want to submit the quiz?
Cancel Submit

We are going to start the Quiz below. Before that, read these notes carefully and Sign Up to check your score.

The Flower and Pre-fertilization Structures

  • Flowers are morphological and embryological marvels and the sites of sexual reproduction.
  • Stamen consists of two parts: the long slender stalk called the filament and the terminal bilobed structure called the anther.
  • A typical angiosperm anther is bilobed and dithecous (two thecae per lobe).
  • The anther is a four-sided structure consisting of four microsporangia located at the corners.
  • Microsporangia develop further and become pollen sacs.
  • The microsporangium wall has four layers: epidermis, endothecium, middle layers, and tapetum.
  • The tapetum nourishes the developing pollen grains and possesses dense cytoplasm and generally more than one nucleus.
  • Sporogenous tissue occupies the center of each microsporangium in young anthers.
  • Microsporogenesis is the process of formation of microspores from a Pollen Mother Cell (PMC) through meiosis.
  • Microspores are arranged in a cluster of four cells called the microspore tetrad.

Pollen Grain (Male Gametophyte)

  • Pollen grains represent the male gametophytes.
  • The two-layered wall of pollen consists of a hard outer exine and an inner intine.
  • Exine is made of sporopollenin, one of the most resistant organic materials known.
  • Germ pores are apertures in the exine where sporopollenin is absent.
  • The intine is a thin, continuous layer made of cellulose and pectin.
  • A mature pollen grain contains two cells: the vegetative cell (larger, abundant food reserve) and the generative cell (smaller, floats in cytoplasm).
  • In over 60% of angiosperms, pollen is shed at the 2-celled stage.
  • Pollen allergy (e.g., from Parthenium or carrot grass) causes chronic respiratory disorders.
  • Pollen viability varies (e.g., 30 minutes in rice/wheat; months in Rosaceae).

The Pistil and Megasporogenesis

  • The Gynoecium represents the female reproductive part.
  • A pistil consists of the stigma (landing platform), style (elongated slender part), and ovary (basal bulged part).
  • Megasporangium is the technical term for the ovule.
  • The ovule is attached to the placenta by a stalk called the funicle.
  • The junction where the body of the ovule fuses with the funicle is called the hilum.
  • Integuments are protective envelopes that encircle the nucellus except at the micropyle.
  • The chalaza represents the basal part of the ovule, opposite the micropyle.
  • Megasporogenesis is the formation of megaspores from the Megaspore Mother Cell (MMC).
  • Only one megaspore remains functional, while the other three degenerate (Monosporic development).
  • A mature female gametophyte (embryo sac) is 7-celled and 8-nucleate.
  • The egg apparatus consists of two synergids and one egg cell.
  • Filiform apparatus in synergids guides the entry of the pollen tube.

Pollination

  • Autogamy is pollination within the same flower.
  • Cleistogamous flowers never open and ensure seed-set even without pollinators.
  • Geitonogamy is functionally cross-pollination but genetically autogamy.
  • Xenogamy brings genetically different types of pollen grains to the stigma.
  • Anemophily (wind pollination) requires light, non-sticky pollen and well-exposed stamens.
  • Hydrophily (water pollination) is rare, seen in Vallisneria and Hydrilla.
  • Entomophily (insect pollination) involves large, colorful, fragrant, and nectar-rich flowers.
  • Emasculation (removal of anthers) and bagging are techniques used in artificial hybridization.

Fertilization and Post-Fertilization

  • Pollen-pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition.
  • Syngamy is the fusion of one male gamete with the egg cell to form a diploid zygote.
  • Triple fusion is the fusion of the second male gamete with two polar nuclei to form the triploid Primary Endosperm Nucleus (PEN).
  • Double fertilization (Syngamy + Triple Fusion) is a unique event in flowering plants.
  • The PEN develops into the endosperm, which provides nutrition to the embryo.
  • Embryogeny stages: Zygote → Proembryo → Globular → Heart-shaped → Mature embryo.
  • Dicot embryo has two cotyledons; Monocot embryo has one (called scutellum).
  • Albuminous seeds retain endosperm (e.g., wheat, maize, castor); Non-albuminous consume it (e.g., pea, groundnut).
  • Perisperm is the persistent residual nucellus (e.g., black pepper, beet).
  • Apomixis is a form of asexual reproduction that mimics sexual reproduction (seed formation without fertilization).
  • Polyembryony is the occurrence of more than one embryo in a seed (common in citrus).

Now, Start Quiz and Submit.

  • Questions count: 10
  • Passing grade: 4
  • Time limit: 10 minutes
Start Quiz
1.

Describe the structure of a typical microsporangium.

A microsporangium is generally surrounded by four wall layers: the epidermis, endothecium, middle layers, and the tapetum.
The outer three wall layers perform the function of protection and help in the dehiscence of the anther to release pollen.
The tapetum is the innermost layer and is responsible for providing nourishment to the developing pollen grains.
The tapetum is involved in secreting precursors of sporopollenin, not the endothecium; the endothecium helps in dehiscence.
Cells of the tapetum typically possess dense cytoplasm and often have more than one nucleus.
In a young anther, a group of compactly arranged homogenous cells called sporogenous tissue occupies the center.
The endothecium layer is composed of specialized cells that secrete sporopollenin to form the pollen exine.
2.

Explain the process of Microsporogenesis.  

Microsporogenesis is the process by which microspores are formed from a pollen mother cell (PMC).
As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
The PMC undergoes meiosis, resulting in haploid pollen grains, not diploid.
Each cell of the sporogenous tissue is a potential pollen or microspore mother cell.
The microspores are arranged in a cluster of four cells and are technically referred to as a microspore tetrad.
As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
During this process, the PMC undergoes mitosis to ensure that each pollen grain remains diploid.
3.

Detail the structure of a Pollen Grain.  

The pollen grain represents the male gametophyte and has a prominent two-layered wall.
The hard outer layer called the exine is made up of sporopollenin, which is extremely resistant to high temperatures and strong acids.
Germ pores are specific apertures in the exine where sporopollenin is absent, allowing for pollen tube emergence.
The inner wall of the pollen grain is called the intine, which is a thin and continuous layer made of cellulose and pectin.
A mature pollen grain contains a large vegetative cell and a small generative cell that floats in its cytoplasm.
The generative cell divides mitotically to give rise to the two male gametes, not the vegetative cell.
The vegetative cell is responsible for producing the male gametes through further meiotic division.
4.

Describe the structure of a Megasporangium (Ovule).  

The ovule is a small structure attached to the placenta by means of a stalk called the funicle.
The hilum represents the junction where the body of the ovule fuses with the funicle.
The nucellus is located inside the integuments and contains reserve food materials for the embryo sac.
Each ovule has one or two protective envelopes called integuments which encircle the nucellus.
The micropyle is a small opening at the tip of the ovule where the integuments are absent.
Opposite the micropylar end is the chalaza, which represents the basal part of the ovule.
The nucellus is a mass of cells located outside the integuments that functions to attract insect pollinators.
5.

Explain Megasporogenesis and the development of the Female Gametophyte.  

Megasporogenesis is the process of formation of megaspores from the megaspore mother cell (MMC).
The MMC undergoes meiosis to produce a linear tetrad of four haploid megaspores.
The mature embryo sac is 7-celled and 8-nucleate.
In a majority of flowering plants, three megaspores degenerate and only one remains functional.
The functional megaspore develops into the female gametophyte, also known as the embryo sac.
This method of embryo sac formation from a single megaspore is termed monosporic development.
The mature angiosperm embryo sac is typically 8-celled and 7-nucleate at the time of fertilization.
6.

Discuss the different types of Syngamy (Autogamy, Geitonogamy, Xenogamy).  

Autogamy is the transfer of pollen grains from the anther to the stigma of the same flower.
Chasmogamous flowers have exposed anthers and stigma, similar to flowers of other species.
Cleistogamous flowers do not open at all, ensuring that only self-pollination can occur.
Geitonogamy involves the transfer of pollen from the anther to the stigma of another flower on the same plant.
Xenogamy is the only type of pollination which brings genetically different types of pollen grains to the stigma.
Geitonogamy is genetically identical to xenogamy because the pollen comes from a different flower.
Geitonogamy is genetically identical to autogamy because the pollen comes from the same plant.
7.

Explain the process of Double Fertilization.  

After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm.
Double fertilization is unique to angiosperms (flowering plants)).
One male gamete moves toward the egg cell and fuses with its nucleus to form a diploid zygote (Syngamy).
The other male gamete moves toward the two polar nuclei in the central cell and fuses with them.
This fusion involves three haploid nuclei and is therefore termed triple fusion.
The result of triple fusion is the formation of a triploid primary endosperm nucleus (PEN).
Double fertilization is a common phenomenon observed in all plants, including bryophytes and gymnosperms.
8.

Describe the development of Endosperm and Embryo.  

Endosperm development usually precedes embryo development to ensure a steady food supply.
The primary endosperm cell divides repeatedly to form a triploid endosperm tissue.
In the free-nuclear endosperm, the PEN undergoes successive nuclear divisions without immediate wall formation.
The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped, and mature embryo.
A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
In monocots, the single cotyledon is called the scutellum; the epiblast is a rudimentary second cotyledon.
In monocots, the single cotyledon is called the epiblast and is situated laterally to the embryonal axis.
9.

Detail the structure of Seeds and Perisperm.  

The seed is the final product of sexual reproduction and is often described as a fertilized ovule.
Non-albuminous seeds have no residual endosperm as it is completely consumed during embryo development.
Albuminous seeds retain a part of the endosperm as it is not completely used up (e.g., wheat, maize).
Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent.
This residual, persistent nucellus is known as the perisperm.
The micropyle remains as a small pore in the seed coat to facilitate the entry of oxygen and water during germination.
Integuments of ovules harden as strong protective seed coats, while the micropyle closes permanently to prevent water entry.
10.

Explain Apomixis and Polyembryony.  

Apomixis is a form of asexual reproduction that mimics sexual reproduction by producing seeds without fertilization.
Apomictic embryos are genetically identical to the parent plant because there is no fertilization or meiosis involved.
In some species, the diploid egg cell is formed without reduction division and develops into an embryo without fertilization.
In many Citrus and Mango varieties, nucellar cells start dividing and protrude into the embryo sac to form embryos.
The occurrence of more than one embryo in a seed is referred to as polyembryony.
Hybrid seeds produced through apomixis are preferred because they do not segregate characters in the offspring.
Apomictic embryos are genetically different from the parent plant because they are produced within the fruit.
Previous
Next
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10