Class 12 Biology Chapter 2 Notes: Sexual Reproduction in Flowering Plants

Chapter 2 Sexual Reproduction in Flowering Plants Notes

2.1 Flower – A Fascinating Organ of Angiosperms

The flower is the reproductive structure of angiosperms. It contains male and female reproductive organs and is responsible for sexual reproduction.

Reproductive Structures:

Male reproductive part (Androecium)
- Consists of stamens
- Anther contains pollen sacs
- Male gametophyte develops here
Female reproductive part (Gynoecium)
- Consists of carpel (stigma, style, ovary)
- Ovary contains ovules
- Female gametophyte develops inside ovule

2.2 Pre-fertilisation: Structures and Events

Pre-fertilisation includes gametogenesis and pollination.

Development of Gametophytes

Male Gametophyte (Microgametogenesis):

Takes place in pollen sac of anther
Pollen mother cells (2n) undergo meiosis
Produce haploid microspores
Microspores develop into pollen grains (male gametophyte)

Female Gametophyte (Megagametogenesis):

Takes place in nucellus of ovule
Megaspore mother cell (2n) undergoes meiosis
Four megaspores are formed
Only one functional megaspore develops into embryo sac

Microsporogenesis vs Megasporogenesis

Feature	Microsporogenesis	Megasporogenesis
Occurs in	Anther	Ovule
Mother cell	Microspore mother cell (2n)	Megaspore mother cell (2n)
Division	Meiosis	Meiosis
End product	Pollen grains	Embryo sac
Gametophyte formed	Male	Female

Both processes involve meiotic division, producing haploid cells.

Developmental Sequence of Male Gametes

Correct sequence:
Sporogenous tissue → Pollen mother cell → Microspore tetrad → Pollen grain → Male gametes

Structure of a Typical Angiosperm Ovule

Ovule develops as an outgrowth on placenta
Major parts:
- Nucellus – nutritive tissue
- Megaspore mother cell – forms megaspores
- Integuments – protective layers
- Micropyle – opening for pollen tube entry
- Embryo sac – female gametophyte

Monosporic Development of Female Gametophyte

In most flowering plants:
- Only one megaspore is functional
- Remaining three degenerate
Embryo sac develops from a single megaspore
This is called monosporic development

7-Celled, 8-Nucleate Embryo Sac

Functional megaspore undergoes three mitotic divisions
Results in 8 nuclei
Organisation:
- Egg apparatus (3 cells) at micropylar end
- Antipodals (3 cells) at chalazal end
- Central cell with two polar nuclei
Thus, embryo sac is 7-celled and 8-nucleate

2.3 Pollination and Types of Flowers

Chasmogamous Flowers:

Open flowers
Exposed anther and stigma
Allow both self and cross pollination

Cleistogamous Flowers:

Closed flowers
Anther and stigma lie close
Always self-pollinated
No chance of cross-pollination
Ensure assured seed formation

Devices to Prevent Self-Pollination

Self-pollination reduces vigour, hence plants adopt mechanisms to promote cross-pollination.

Common Devices:

Dichogamy
- Anther and stigma mature at different times
Self-incompatibility (Self-sterility)
- Pollen fails to germinate on stigma of same flower
- Controlled by S-genes

Self-Incompatibility

Genetic mechanism preventing self-fertilisation
Even viable pollen cannot fertilise egg
Prevents seed formation
Promotes genetic diversity

Bagging Technique (Artificial Hybridisation)

Emasculated flowers are covered with butter paper or polythene bags
Prevents unwanted pollen contamination
After stigma maturity:
- Desired pollen dusted
- Flower rebagged till fruit develops
Widely used in plant breeding programmes

2.4 Double Fertilisation

Syngamy:

One male gamete + egg → zygote (2n)

Triple Fusion:

Second male gamete + two polar nuclei → primary endosperm nucleus (3n)

Both events together form double fertilisation, unique to angiosperms.

Dormancy of Zygote

Zygote remains dormant initially
Allows endosperm to develop first
Ensures proper nutrition for embryo

2.5 Post-Fertilisation: Structures and Events

Changes After Fertilisation:

Sepals, petals and stamens wither
Ovule → Seed
Ovary → Fruit
Ovary wall → Pericarp
Zygote → Embryo

Differences (Important for Exams)

Hypocotyl vs Epicotyl

Hypocotyl: Region below cotyledons
Epicotyl: Region above cotyledons

Coleoptile vs Coleorrhiza

Coleoptile: Protects plumule
Coleorrhiza: Protects radicle

Integument vs Testa

Integument: Ovule covering
Testa: Seed coat

Perisperm vs Pericarp

Perisperm: Remnant of nucellus
Pericarp: Fruit wall

False Fruit (Apple)

True fruit develops from ovary only
False fruit develops from ovary + other floral parts
In apple:
- Edible part = fleshy thalamus
- Ovary forms true fruit inside

Emasculation

Removal of stamens from bisexual flower
Prevents self-pollination
Used in artificial hybridisation
Followed by bagging

Parthenocarpy

Fruit formation without fertilisation
Produces seedless fruits
Examples:
- Banana
- Grapes
- Orange
Important for commercial fruit production

Role of Tapetum

Tapetum is the innermost layer of anther wall.

Functions:

Provides nutrition to developing pollen
Secretes:
- Callase enzyme – separates pollen tetrads
- Ubisch bodies – helps exine formation
- Pollen kit – sticky coating in insect-pollinated plants

2.5 Apomixis and Polyembryony

Apomixis:

Asexual seed formation
No fertilisation
Seen in grasses and Asteraceae
Maintains hybrid characters
Useful in hybrid seed industry

2.6 Summary (Exam Ready)

Flower is reproductive organ of angiosperms
Gametophytes develop before fertilisation
Double fertilisation is unique to angiosperms
Seeds and fruits develop after fertilisation
Apomixis helps maintain hybrid vigour

📝 Long Answer Questions

Q1. Describe the structure of a typical angiosperm ovule.

Answer:
A typical angiosperm ovule develops as an outgrowth on placenta and consists of:

Nucellus: Nutritive tissue
Megaspore mother cell: Produces megaspores
Integuments: Protective layers
Micropyle: Opening for pollen tube entry
Embryo sac: Female gametophyte

Q2. Explain double fertilisation in flowering plants.

Answer:
Double fertilisation is a characteristic feature of angiosperms and involves two processes:

Syngamy:
One male gamete fuses with egg to form diploid zygote (2n)
Triple Fusion:
Second male gamete fuses with two polar nuclei to form triploid primary endosperm nucleus (3n)

Both together constitute double fertilisation.

Q3. Describe devices that prevent self-pollination in flowering plants.

Answer:
Plants adopt mechanisms to promote cross-pollination:

Dichogamy:
Anther and stigma mature at different times
Self-incompatibility:
Viable pollen fails to fertilise ovule of same flower
Controlled by S-genes

These mechanisms increase genetic variation.

📝 PYQs (Previous Year Questions)

🔹 1️⃣ Very Short Answer Questions (1 Mark)

What is the reproductive organ of angiosperms?
Name the male reproductive part of a flower.
Where does microsporogenesis occur?
What is the ploidy of pollen mother cell?
What is monosporic development?
How many cells are present in a mature embryo sac?
Name the opening through which pollen tube enters ovule.
What is emasculation?
Which layer of anther provides nutrition to pollen grains?
What type of fertilisation is unique to angiosperms?

🔹 2️⃣ Short Answer Questions (2–3 Marks)

Define microsporogenesis and megasporogenesis.
Differentiate between microsporogenesis and megasporogenesis.
Describe the structure of a typical angiosperm ovule.
What is double fertilisation? Name its two components.
Why is embryo sac described as 7-celled and 8-nucleate?
What are cleistogamous flowers? Mention one advantage.
Explain any two devices to prevent self-pollination.
What is bagging technique and why is it used?
Write any two functions of tapetum.
What is parthenocarpy? Give two examples.

🔹 3️⃣ Long Answer Questions (3–5 Marks)

Describe development of male gametophyte in flowering plants.
Explain development of female gametophyte with diagram.
Describe double fertilisation in angiosperms.
Explain post-fertilisation changes in a flowering plant.
Describe devices for preventing self-pollination.
Explain emasculation and bagging in artificial hybridisation.
Differentiate between true fruit and false fruit with example.

🔹 4️⃣ Difference-Based PYQs (Very Important)

Microsporogenesis vs Megasporogenesis
Chasmogamous flowers vs Cleistogamous flowers
Hypocotyl vs Epicotyl
Coleoptile vs Coleorrhiza
Integument vs Testa
Perisperm vs Pericarp

🔹 5️⃣ Assertion–Reason / Concept PYQs

Double fertilisation is an adaptation of angiosperms. Justify.
Self-pollination reduces vigour in plants. Explain.
Zygote remains dormant after fertilisation. Give reason.
Apomixis is important in hybrid seed industry. Explain.

❓ FAQs (5)

FAQ 1. Why are cleistogamous flowers important?

They ensure assured seed formation as they are always self-pollinated.

FAQ 2. Why is double fertilisation unique to angiosperms?

Because both syngamy and triple fusion occur only in flowering plants.

FAQ 3. What is the role of tapetum?

It provides nutrition to pollen and secretes callase enzyme, ubisch bodies and pollen kit.

FAQ 4. Why does zygote remain dormant initially?

To allow endosperm to develop first for proper nutrition of embryo.

FAQ 5. How is false fruit different from true fruit?

False fruit develops from ovary plus other floral parts, while true fruit develops from ovary only.

Ai Radhe Notes

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