Sexual Reproduction in Flowering Plants for CBSE Class 10 Science

While asexual reproduction allows organisms to multiply rapidly, it produces offspring that are exact genetic copies of the parent. Over evolutionary time, changing planetary conditions demand genetic diversity and novel combinations of traits. This vital diversity is made possible by sexual reproduction—a biological mechanism where genetic material from two distinct individuals is combined.

In angiosperms (flowering plants), the flower is the specialized reproductive organ.

Important: <u>After fertilisation, the ovule always develops into the seed, while the surrounding ovary ripens into the fruit.</u> designed to execute this genetic exchange. In CBSE Class 10 Science, Chapter 7 (How do Organisms Reproduce?) covers flower anatomy, the mechanics of pollination, the journey of the pollen tube, and post-fertilisation transformations into seeds and fruits.

What You Will Learn

• Why sexual reproduction provides evolutionary advantages over asexual reproduction
• Anatomical structure of a typical flower: Sepals, Petals, Stamens, and Carpel/Pistil
• Unisexual vs. Bisexual flowers
• Pollination: Self-pollination vs. Cross-pollination and pollinating agents
• The germination of pollen on the stigma and the path of the pollen tube
• The event of fertilisation (syngamy)
• Post-fertilisation transformations: How ovules become seeds and ovaries become fruits
• Board exam diagrams, structural labels, and common misconceptions

1. Why Sexual Reproduction?

1. Generation of Diversity: Sexual reproduction combines DNA from two different parental genomes, generating unique combinations of variations in every generation. This genetic variability is the engine of natural selection and evolution.
2. Preserving Chromosome Number (Meiosis):
   • If two normal body cells combined, the offspring would inherit double the number of chromosomes ($4n$), leading to cellular chaos.
   • To prevent this, specialized reproductive tissues undergo a reduction division (meiosis) to form gametes (germ cells) containing only half the normal chromosome number ($n$, haploid).
   • When male and female gametes fuse during fertilisation ($n + n$), the original diploid chromosome number ($2n$) is perfectly restored!

2. Anatomy of a Flower

A complete flower consists of four concentric rings or whorls attached to the swollen tip of the flower stalk (the receptacle):

                               Cross-Section of a Flower
                                         STIGMA
                                           ||
                                           || <-- STYLE (Pistil / Carpel:
            ANTHER --+                     ||            Female Organ)
                     | <-- STAMEN          ||
           FILAMENT -+    (Male Organ)     ||
                                        +======+
                                        | OVARY|
                                        +======+ (Contains Ovules)
                                       /                                           PETAL        SEPAL

1. Accessory (Non-Reproductive) Whorls:

• Calyx (Sepals): The outermost green, leaf-like whorl that protects the flower during its delicate bud stage.
• Corolla (Petals): Brightly colored, scented petals that attract animal and insect pollinators (bees, butterflies, birds).

2. Essential (Reproductive) Whorls:

• Androecium (The Male Reproductive Part - Stamens): Each stamen consists of:
  • Filament: A slender, flexible supporting stalk.
  • Anther: A bilobed terminal sac that produces yellowish powdery particles called pollen grains. Each pollen grain encloses male germ cells (gametes).
• Gynoecium / Pistil (The Female Reproductive Part - Carpel): Located in the center of the flower. Each carpel consists of three distinct parts:
  1. Stigma: The sticky, terminal landing platform that traps pollen grains.
  2. Style: An elongated, slender neck through which the pollen tube grows downward.
  3. Ovary: The swollen basal chamber containing one or more ovules. Inside each ovule lies the female germ cell (the egg cell).

Unisexual vs. Bisexual Flowers:

• Unisexual Flowers: Flowers containing either stamens or carpels, but not both.
  • Examples: Papaya, Watermelon.
• Bisexual Flowers: Flowers containing both stamens and carpels in the same blossom.
  • Examples: Hibiscus (China rose), Mustard.

3. Pollination: The Transfer of Pollen

Pollination is the transfer of pollen grains from the anther of a stamen to the sticky stigma of a carpel.

                            Types of Pollination
                                     |
       +-----------------------------+-----------------------------+
       |                                                           |
Self-Pollination                                            Cross-Pollination
- Transfer within the SAME flower,                          - Transfer from anther of one flower
  or between flowers of the SAME plant                      to stigma of a DIFFERENT plant
- No external agents mandatory                              - Requires agents: Wind, Water, Insects, Animals

Pollinating Agents:

• Biotic Agents: Bees, butterflies, moths, birds, bats.
• Abiotic Agents: Wind (anemophily - lightweight, powdery pollen in grasses/corn), Water (hydrophily in aquatic plants).

4. Fertilisation: The Journey of the Pollen Tube

Once compatible pollen grains land upon the sticky stigma, the biochemical events of fertilisation commence:

    Pollen Grain lands on Stigma
       ↓
    Pollen grain germinates → Emits POLLEN TUBE
       ↓
    Pollen tube grows down through the STYLE (Chemotropic growth)
       ↓
    Reaches Ovary and enters OVULE through micropyle
       ↓
    Releases Two Male Gametes
       ↓
    Male Gamete fuses with Female Egg Cell → FERTILISATION (Zygote formed!)

Step-by-Step Mechanism (NCERT Figure 7.8):

1. The pollen grain absorbs sugary secretions on the stigma and germinates, putting forth a slender pollen tube.
2. Guided by chemical attractants (positive chemotropism), the pollen tube grows downward through the cellular tissue of the style toward the ovary.
3. The pollen tube penetrates the ovary, enters the ovule, and bursts at its tip to discharge two male gametes.
4. Syngamy (Fertilisation): One male gamete fuses with the female egg cell inside the embryo sac to form a single diploid cell called the Zygote.

5. Post-Fertilisation Transformations (Seed and Fruit Development)

After fertilisation is complete, dramatic structural transformations take place inside the flower:

                            Post-Fertilisation Changes
                                        |
       +--------------------------------+--------------------------------+
       |                                                                 |
Inside the Ovule                                                  In the Surrounding Flower
- Zygote divides → EMBRYO                                         - Ovary wall expands → FRUIT
- Ovule forms tough protective coat → SEED                        - Sepals, Petals, Stamens, Style
                                                                    wither and fall off!

1. Fate of the Zygote:

The single-celled zygote divides repeatedly by mitosis to form an embryo enclosed within the ovule.

2. Fate of the Ovule $\longrightarrow$ The Seed:

The ovule develops a tough, resistant protective outer coat (testa) and gradually transforms into a seed. The seed contains the future miniature plant (the embryo) along with stored food reserves in the form of cotyledons.

• The part of the embryo that will develop into the future root is the radicle.
• The part that will develop into the future shoot (stem and leaves) is the plumule.

3. Fate of the Ovary $\longrightarrow$ The Fruit:

The ovary grows rapidly, accumulates sugars and nutrients, and ripens into the fruit.

4. Fate of the Accessory Whorls:

The petals, sepals, stamens, style, and stigma dry up, shrivel, and fall off. (In some fruits like brinjal and tomato, the green sepals remain persistently attached).

6. Summary and Examination Tips

Exam Tip: When drawing the "Germination of Pollen on Stigma" diagram in Section C/D: Clearly label the: (1) Pollen grain, (2) Stigma, (3) Pollen tube, (4) Style, (5) Ovary, (6) Ovule, and (7) Female germ cell. This diagram carries 3 full marks!

Common Mistake: Confusing the ovule and the ovary. The ovule becomes the seed, while the ovary becomes the fruit! Remember: You eat the fruit (ovary) and spit out the seeds (ovules)!