Structure of the flower, pollination, fertilization

1. Structure of the Flower

1. A flower is the reproductive organ of an angiosperm (flowering plant).
2. It consists of four main whorls:
   • Calyx: The outermost whorl, made up of sepals, which protect the flower bud.
   • Corolla: The second whorl, consisting of petals that attract pollinators.
   • Androecium: The male reproductive part, made up of stamens (each with a filament and an anther).
   • Gynoecium: The female reproductive part, consisting of carpels (each with a stigma, style, and ovary).
3. The ovary contains ovules, which develop into seeds after fertilization.
4. Flowers can be bisexual (having both male and female parts, e.g., Hibiscus) or unisexual (having only one reproductive organ, e.g., Papaya).

2. Pollination

1. Pollination is the transfer of pollen grains from the anther to the stigma.
2. It is essential for fertilization and seed production.

Types of Pollination

1. Self-Pollination (Autogamy): Pollen from the same flower reaches the stigma.
   • Occurs in cleistogamous flowers (closed flowers, e.g., Pea).
   • Ensures genetic purity but reduces variability.
2. Cross-Pollination (Allogamy): Pollen is transferred from the anther of one flower to the stigma of another flower.
   • Promotes genetic diversity and adaptability.
   • Occurs through different agents (wind, water, insects, birds, etc.).

Agents of Pollination

1. Anemophily (Wind Pollination): Seen in plants like maize and wheat.
2. Hydrophily (Water Pollination): Found in aquatic plants like Vallisneria.
3. Entomophily (Insect Pollination): Common in bright-colored flowers (e.g., Sunflower, Rose).
4. Ornithophily (Bird Pollination): Occurs in brightly colored and nectar-rich flowers (e.g., Hibiscus).

3. Fertilization

1. Fertilization is the fusion of male and female gametes to form a zygote.
2. It occurs after successful pollination.

Steps of Fertilization

1. Pollen Germination: Pollen grains land on the stigma and form a pollen tube.
2. Pollen Tube Growth: The tube extends through the style, carrying two male gametes to the ovule.
3. Double Fertilization:
   • One male gamete fuses with the egg cell to form a zygote (which becomes the embryo).
   • The other male gamete fuses with two polar nuclei to form a triploid endosperm (which provides nourishment to the developing embryo).
4. Seed and Fruit Formation: The ovule develops into a seed, and the ovary transforms into a fruit.

4. Importance of Pollination and Fertilization

1. Pollination is essential for fruit and seed production.
2. Cross-pollination increases genetic variation and adaptation to the environment.
3. Fertilization leads to the formation of new plants and ensures species survival.

5. Special Adaptations in Pollination

1. Herkogamy: Physical separation of stamens and stigma to prevent self-pollination (e.g., Primrose).
2. Dichogamy: Different maturation times for male and female parts (e.g., Sunflower).
3. Xenogamy: Cross-pollination between flowers of different plants.

6. Significance of Double Fertilization

1. Unique to angiosperms, ensuring efficient seed development.
2. Leads to the formation of nutritive tissue (endosperm) that supports embryo growth.
3. Results in the production of viable seeds essential for plant propagation.

7. Conclusion

1. Sexual reproduction in flowering plants ensures diversity and survival.
2. Pollination plays a crucial role in fertilization and seed production.
3. Understanding these concepts is vital for competitive exams and botanical studies.