🧬 Chapter 1: Sexual Reproduction in Flowering Plants – Class 12 Biology Notes | NCERT + NEET Focus

🌸 Chapter 1

 Sexual Reproduction in Flowering Plants

1. Introduction

Reproduction is the biological process by which living organisms give rise to offspring similar to themselves. In flowering plants (angiosperms), reproduction occurs through the sexual process, involving the fusion of male and female gametes to produce seeds and fruits.

Flowering plants reproduce sexually, although some may also reproduce asexually. Sexual reproduction ensures variation and evolution through genetic recombination.

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2. Flower — The Reproductive Organ

The flower is the reproductive structure of angiosperms. It is a modified shoot meant for sexual reproduction.

Parts of a Flower:

1.    Calyx (Sepals): Protects the flower in the bud stage.

2.    Corolla (Petals): Brightly colored to attract pollinators.

3.    Androecium (Stamens): Male reproductive part consisting of:

o   Anther – produces pollen grains.

o   Filament – stalk that supports the anther.

4.    Gynoecium (Carpels): Female reproductive part consisting of:

o   Stigma – receives pollen.

o   Style – connects stigma and ovary.

o   Ovary – contains ovules (female gametophyte).

Diagram

A diagramatic representation of L.S. of a Flower

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3. Pre-Fertilization Events

Pre-fertilization events include microsporogenesis and megasporogenesis, leading to the formation of male and female gametes.

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A. Male Reproductive Part – The Stamen

Each stamen has:

• Anther – usually bilobed with two thecae.
• Filament – slender stalk that attaches the anther.

Structure of an Anther:

A bilobed anther has four microsporangia — two in each lobe.

• The outer wall has 4 layers:

1.    Epidermis

2.    Endothecium (helps in dehiscence)

3.    Middle layers

4.    Tapetum (nourishes developing pollen)

The microsporangium contains microspore mother cells (MMCs) that undergo meiosis to form haploid microspores (pollen grains).

Diagram:

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B. Microsporogenesis

• It is the process of formation of microspores from microspore mother cells through meiosis.
• Each MMC → meiosis → 4 microspores (tetrad).

Each microspore develops into a pollen grain.

Diagram:

a) Transverse section of a young anther b) Enlarged view of one microsporangium showing wall layers  c) A mature dehisced anther

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C. Structure of Pollen Grain

• Outer layer (Exine): Hard, made of sporopollenin (resistant to decay).
• Inner layer (Intine): Thin, made of cellulose and pectin.
• Has pollen apertures (germ pores) through which pollen tube emerges.
• Contains two cells at maturity:

1.    Generative cell → gives rise to two male gametes.

2.    Vegetative cell → forms the pollen tube.

Diagram:

Fig 1.5Stages of a microsspore maturing into a pollen grain

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D. Female Reproductive Part – The Pistil/Carpel

Each pistil has:

• Ovary: Contains one or more ovules.
• Style: Elongated tube.
• Stigma: Receives pollen.

Structure of Ovule:

• Funicle: Stalk attaching ovule to ovary wall.
• Hilum: Junction between ovule and funicle.
• Integuments: Protective layers (usually two).
• Micropyle: Small opening for pollen tube entry.
• Nucellus: Contains nourishment tissue.
• Embryo sac: Female gametophyte within the nucellus.

Diagram: 

a)A dissected flower of Hibiscus showing pistil  b) Multicarpellary, syncarpous pistil of Papaver c) A Multicarpellary, apocarpous gynoecium of Michelia  d) A diagrammatic view of a typical anatropous ovule

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E. Megasporogenesis

It is the formation of megaspore from the megaspore mother cell (MMC) inside the ovule.

• MMC (diploid) → meiosis → 4 haploid megaspores.
• Usually, one megaspore is functional; others degenerate.

a) Parts of ovule b) 2,4 and 8-nucleate stages of embryo sac and a mature embryo sac c) A diagramatic representation of the mature embryo sac 

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F. Development of Female Gametophyte (Embryo Sac)

• The functional megaspore develops into the embryo sac by three mitotic divisions:
• 8 nuclei are formed → organized into:
• 3 antipodal cells (at chalazal end)
• 2 polar nuclei (in central cell)
• 2 synergids and 1 egg cell (at micropylar end)

Hence, a mature embryo sac is 7-celled, 8-nucleate.

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4. Pollination

Pollination is the transfer of pollen grains from the anther to the stigma.

Diagram :

a)pollination by water in Vallisneria   b) Insect pollination

Types of Pollination:

1.    Autogamy: Pollen transfer within the same flower.

2.    Geitonogamy: Between different flowers of the same plant.

3.    Xenogamy: Between flowers of different plants.

Diagram:

a)Self-pollinated flowers   b) Cross Pollinated flowers  c) Cleistogamous flowers

Agents of Pollination:

• Abiotic agents: Wind (anemophily), Water (hydrophily).

Diagram:

fig 1.10: A wind-pollinated plant showing compact inflorescence and well-exposed stamens

Fig 1.11 (a) Pollination by Water in Vallisneria

• Biotic agents: Insects (entomophily), Birds (ornithophily), Bats (chiropterophily).

Fig 1.11 (b) Insect Pollination 

5. Outbreeding Devices:

🌸 1. What Are Outbreeding Devices?

Plants naturally prefer genetic variation, because variation helps them survive changing environments, resist diseases, and evolve better traits.

To increase variation, plants avoid self-pollination and promote cross-pollination.
Outbreeding devices = special mechanisms in flowers that prevent self-pollination and encourage pollen from another flower/plant.

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🌸 2. Why Are Outbreeding Devices Necessary?

Self-pollination → no new gene combinations → weak offspring, less adaptability.

Cross-pollination →
✔ Genetic diversity
✔ Stronger offspring
✔ Better survival
✔ Evolution of new traits

Flowers use different methods to prevent self-pollination and promote cross-pollination.

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🌼 3. Major Outbreeding Devices (As per NCERT)

three major mechanisms:

1. Dichogamy

The male and female reproductive parts mature at different times.
Because timing doesn’t match, self-pollination is not possible.

Two types:

a) Protandry

• Anthers mature first
• Pollen is released before the stigma becomes receptive
• Example: Sunflower

b) Protogyny

• Stigma matures first
• It becomes receptive before the anthers release pollen
• Example: Gloriosa, custard apple

📌 Key point: Because the male and female parts are not ready at the same time, self-pollination cannot occur.

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2. Herkogamy

A physical or structural barrier exists between anther and stigma.
This prevents pollen from directly reaching the stigma of the same flower.

Examples:

• In Hibiscus, stigma is positioned above anthers
• In Calotropis, presence of waxy pollinia prevents self-pollination
• In some flowers, the stigma is hidden or placed far away

📌 Key point: Distance or barrier prevents self-contact.

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3. Self-Incompatibility (Genetic Mechanism)

This is a genetic mechanism, not structural.

Even if pollen of the same flower lands on its stigma, it fails to germinate.

• Pollen tube does not grow, so fertilization doesn’t occur
• Controlled by S-gene (self-incompatibility gene)

Examples: Tobacco, Mustard (Brassica)

📌 Key point: The stigma chemically rejects its own pollen.

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4. Unisexuality (Dioecy)

When plants bear either male flowers or female flowers, self-pollination becomes impossible.

Two types of plants:

a) Dioecious Plants

Male and female flowers are on different plants.
Example: Papaya, Date palm

Self-pollination = impossible
Cross-pollination = compulsory

b) Monoecious Plants

Male and female flowers on the same plant, but separate flowers
Example: Cucumber, Maize

Sometimes still outbreeding because male and female flowers mature at different times.

📌 Key point: Separation ensures transfer from one flower to another.

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5. Heterostyly (Different Style Lengths)

Some flowers produce styles (female part) and stamens (male part) of different lengths.

This promotes cross-pollination because only pollen from a flower with matching height can reach the stigma.

Examples: Primrose (Primula)

📌 Key point: Height differences prevent self-contact.

 

Plants use several mechanisms to prevent self-pollination:

• Dichogamy (anther & stigma mature at different times)
• Herkogamy (physical barriers)
• Self-incompatibility
• Unisexuality (separate male & female flowers)

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7. Double Fertilization

After pollination, pollen germination occurs on the stigma, forming a pollen tube that carries two male gametes to the embryo sac.

Steps:             

1.    One male gamete fuses with the egg cell → zygote (syngamy).

2.    Other male gamete fuses with two polar nuclei → triploid primary endosperm nucleus (PEN).

This process is called double fertilization — unique to angiosperms.

Diagram:

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8. Post-Fertilization Events

Following double fertilization, events of endosperm and embryo development, maturation  of ovule(s) into seed(s) and ovary into fruit , are collectively called as Post-Fertilization Events.

After fertilization occurs inside the ovule, the flower undergoes a series of changes to develop into a seed and a fruit.

Post-fertilization events = changes that occur after the fusion of male and female gametes.

These events include:     

1.    Formation of endosperm

2.    Formation of embryo

3.    Development of seed

4.    Formation of fruit

5.    Degeneration of floral parts

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1.    Formation of endosperm

The primary endosperm nucleus (PEN) is triploid (3n) and divides to form endosperm, a nutritive tissue.

Functions of Endosperm

• Provides nutrition to developing embryo (carbohydrates, proteins, fats).
• Supports early seed germination in many plants.

Types of Endosperm Development (NCERT-Based)

1.    Nuclear Endosperm

o   Most common type

o   Nucleus divides repeatedly without cell wall formation

o   Example: Coconut water = free nuclear endosperm

2.    Cellular Endosperm

o   Each nuclear division is followed by cell wall formation

o   Example: Datura

3.    Helobial Endosperm

o   Intermediate type

o   First division → two chambers

o   Example: Some monocots

📌 NCERT Note: Endosperm development begins earlier than embryo development.

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🌼 2. Development of Embryo

The zygote (2n) divides mitotically to form the embryo.

Embryo development differs indicots and monocots, but basic steps are similar.

General Stages of Embryo Development

1.    Zygote divides → Pro-embryo

2.    Suspensor formation

o   Pushes embryo into nutritive endosperm

3.    Globular stage

o   Spherical embryo

4.    Heart-shaped stage (in dicots)

5.    Torpedo stage

6.    Mature embryo

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Dicot Embryo Structure (NCERT)

Examples: Bean, Pea

Mature dicot embryo has:

• Two cotyledons (embryonic leaves)
• Radicle → root
• Plumule → shoot
• Hypocotyl → stem-like region
• Epicotyl → upper stem region
• Suspensor → shrinks later

diagram :

A typical dicot embryo

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Monocot Embryo Structure

Example: Grass, Rice, Maize

Monocot embryo has:

• Single cotyledon called scutellum
• Coleoptile → protective sheath over plumule
• Coleorhiza → protective sheath over radicle
• Epiblast (sometimes present)

L.S. of an embryo of grass

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🌼 3. Development of Seed

After embryo and endosperm form, the ovule develops into a seed.

Parts of a Seed

1.    Seed coat (testa + tegmen)

o   Develops from integuments of ovule

o   Protection

2.    Embryo

o   Future plant

3.    Endosperm

o   Present in some seeds (coconut, castor)

o   Absorbed in others (pea, bean)

(a)Structure of some seeds (b) False fruits of apple and strawberry

Types of Seeds

1.    Endospermic/Albuminous seeds

o   Endosperm remains

o   Example: wheat, maize, coconut

2.    Non-endospermic/Exalbuminous seeds

o   Endosperm completely absorbed

o   Example: pea, groundnut

Seed Dormancy

Some seeds enter dormancy, where metabolic activity slows down.
Helps seeds survive harsh conditions.

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🌼 4. Development of Fruit

While ovule → seed, the ovary develops into a fruit.

Changes in the Flower After Fertilization

• Petals, sepals, stamens dry and fall off
• Ovary enlarges
• Ovary wall becomes pericarp

Types of Fruit

1.    True fruit

o   Develops from ovary only

o   Example: Mango, Coconut

2.    False fruit

o   Develops from ovary + other floral parts

o   Example: Apple (thalamus forms edible part)

3.    Parthenocarpic fruit

o   Formed without fertilization

o   Seedless

o   Example: Banana

8. Apomixis and Polyembryony

⭐ 1. Apomixis

🌸 What is Apomixis?

Apomixis is a type of reproduction in plants where seeds are formed without fertilization.
It is a special form of asexual reproduction through seeds.

NCERT says:
👉 Apomixis produces seeds without the involvement of gamete fusion.

So, even though the plant forms seeds, meiosis and fertilization do not occur.

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🌿 Why is Apomixis Important?

• Produces genetically identical (clonal) seeds
• Preserves hybrid vigour (heterosis) for many generations
• Helps in plant breeding and crop improvement

Example:
Hybrid seeds are expensive because they need to be produced every year.
If a hybrid could produce apomictic seeds → same hybrid quality every year without buying new seeds.

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🌻 Types of Apomixis (NCERT Based)

NCERT explains the main types under two categories:

1. Parthenogenesis

• The embryo develops from the egg cell without fertilization.
• Egg cell behaves as if fertilized.
• Endosperm may develop normally or by pseudogamy.

2. Apospory / Adventive Embryony

• The embryo arises from diploid cells of nucellus or integuments, not from the egg.
• No meiosis → embryo is diploid and identical to the parent.
• Example: Citrus, Mango

Here, multiple embryos may form → leads to polyembryony.

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🍋 Examples of Apomixis (Important for NCERT–CBSE)

• Citrus (Orange family)
• Mango
• Grasses (Poaceae family)
• Some species of Asteraceae

In many of these plants, nucellar cells directly develop into embryos.

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🌼 Advantages of Apomixis (Exam Points)

1.    Produces uniform and true-to-type plants.

2.    Hybrid vigour is maintained for many generations.

3.    Farmers do not need to buy new hybrid seeds every year.

4.    Seeds are formed even without pollination → useful when pollinator population is low.

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🌺 How is Apomixis Useful in Agriculture?

• Hybrid seeds lose vigour if reused because of segregation in sexual reproduction.
• If hybrid plants show apomixis →
  ✔ Hybrid character stays intact
  ✔ No need to recreate hybrids every season
  ✔ Huge economic advantage

NCERT mentions that crop breeders are trying to introduce apomixis into hybrid crops like maize, wheat, etc.

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🌟 2. Polyembryony

🌸 What is Polyembryony?

Polyembryony means the presence of more than one embryo within a single seed.

NCERT definition:
👉 The occurrence of more than one embryo in a seed is called polyembryony.

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🌿 How Does Polyembryony Occur?

Polyembryony can happen in several ways:

1. From the zygote

• The fertilized egg splits into two or more parts.
• Each part forms a separate embryo.
• Similar to identical twins in humans.

2. From nucellar cells (nucellar embryony)

• Extra embryos develop from the nucellus (a diploid tissue).
• Common in citrus fruits.

3. From synergids or antipodal cells

• Sometimes accessory embryo sacs may form embryos.

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🍊 Examples of Polyembryony (NCERT)

• Citrus (Orange, Lemon)
  → Most famous example; often shows nucellar polyembryony
• Mango
• Onion
• Opuntia

In citrus seeds, you may find 2–10 embryos in one seed.

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🌻 Significance / Advantages of Polyembryony

1.    More than one seedling per seed

2.    Nucellar embryos are genetically identical → helpful for cloning

3.    Disease-free plants can be produced

4.    Helps confirm that a plant is apomictic

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🌼 Relationship Between Apomixis and Polyembryony

Often, polyembryony is a result of apomixis.

For example:
In Citrus and Mango, nucellar cells (asexual) form several embryos → nucellar polyembryony.
So, the seed contains one sexual embryo + many asexual embryos.

This makes the offspring identical to the mother plant, even though seeds are involved.

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📝 NCERT Summary Table

Topic	Definition	Key Feature	Example
Apomixis	Seed formation without fertilization	Asexual reproduction through seeds	Citrus, Mango, Grasses
Polyembryony	More than one embryo in a seed	May be sexual or asexual	Citrus, Onion, Mango
Nucellar Embryony	Embryo from nucellar cells	A type of apomixis	Citrus
Parthenogenesis	Embryo from unfertilized egg	Type of apomixis	Some grasses

 

9. Significance of Sexual Reproduction

• Introduces genetic variation.
• Ensures species continuity.
• Helps in evolution through natural selection.

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Summary Chart

Stage	Process	Result
Microsporogenesis	Formation of pollen grains	Male gametes
Megasporogenesis	Formation of embryo sac	Female gamete
Pollination	Transfer of pollen to stigma	Fertilization possible
Double Fertilization	Fusion of gametes	Zygote + Endosperm
Post-Fertilization	Seed and fruit formation	New plant generation

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