Reproduction - Sexual Reproduction in Flowering Plants - Fertilization

• Fertilization is the process of fusion of male and female gametes
• It occurs after the pollination and leads to the formation of a zygote
• Different steps involved in fertilization:

  • Pollination

    • Transfer of pollen grains from anther to stigma
    • Can be self-pollination or cross-pollination
    • Pollen grains carry male gametes (sperm cells)

  • Germination of Pollen Grain

    • Pollen grain lands on the stigma
    • Absorbs moisture and germinates
    • Forms a pollen tube that grows towards the ovule

  • Entry of Pollen Tube into the Ovule

    • The pollen tube penetrates through the tissues of stigma and style
    • Reaches the ovary and enters the ovule through a tiny pore called the micropyle
    • Male gametes are present in the pollen tube

  • Fusion of Male and Female Gametes

    • The pollen tube releases male gametes into the embryo sac
    • One male gamete fuses with the egg cell to form a zygote
    • The other male gamete fuses with the polar nuclei to form endosperm (nutrition for the developing embryo)

  • Formation of Zygote

    • The fertilized egg cell develops into a zygote
    • Zygote is the first cell of the new plant
    • Zygote undergoes divisions and further development to form an embryo

  • Development of Seed

    • The ovule develops into a seed after fertilization
    • Seed contains the future plant embryo and endosperm
    • Protective seed coat is also formed around the embryo

  • Development of Fruit

    • The ovary surrounding the ovules develops into a fruit
    • Fruit provides protection and aids in the dispersal of seeds
    • Fruits can be dry or fleshy, depending on the plant species

11. Double Fertilization

• Only in flowering plants
• Two fusions occur during fertilization:
  • Fertilization of the egg cell with one male gamete to form a zygote
  • Fertilization of the polar nuclei with another male gamete to form endosperm
• This unique process is called double fertilization
• Leads to the formation of both embryo and endosperm
• Ensures efficient utilization of resources for the developing embryo

12. Significance of Double Fertilization

• Ensures nutrition for the developing embryo
• Provides a high chance of successful fertilization
• Promotes the diversity of plant species
• Helps in the formation of seeds and fruits
• Facilitates seed dispersal
• Enhances successful germination of seeds

13. Post-Fertilization Changes in Ovary

• After fertilization, the ovary undergoes several changes:
  1. Ovary wall thickens and becomes the fruit wall.
  2. Ovary tissues differentiate to form various fruit structures.
  3. Development of seed coat and endosperm.
  4. Ripening of the fruit, which involves changes in color, texture, and taste.
  5. Dispersal of mature seeds from the fruit.

14. Types of Fruits

• Fruits can be categorized into different types based on their characteristics:
  1. Simple fruits: Develop from a single ovary of a single flower (e.g. mango, apple).
  2. Aggregate fruits: Develop from several ovaries of the same flower (e.g. raspberry).
  3. Multiple fruits: Develop from the fusion of ovaries of multiple flowers (e.g. pineapple).
  4. Accessory fruits: Develop with additional tissues derived from floral parts other than the ovary (e.g. strawberry).

15. Seed Germination

• The process by which a seed develops into a new plant
• Requires favorable conditions like water, oxygen, and suitable temperature
• Steps involved in seed germination:
  1. Imbibition: Absorption of water by the seed, causing it to swell.
  2. Activation of enzymes: Triggered by imbibition, enzymes break down stored food reserves in the seed.
  3. Growth of the embryo: The embryo grows and develops into a seedling.
  4. Emergence of the seedling: The seedling emerges from the seed coat and starts photosynthesis.

16. Factors Affecting Seed Germination

• Several factors influence the germination of seeds:
  • Water availability: Seeds require sufficient water for imbibition.
  • Temperature: Optimum temperature range required for enzyme activity and growth.
  • Oxygen availability: Seeds need oxygen for respiration during germination.
  • Light: Some seeds require light for germination, while others need darkness.
  • The availability of suitable soil conditions: Nutrient availability and proper pH.

17. Seed Dormancy

• Seed dormancy is a condition where a seed fails to germinate even under favorable conditions.
• It allows seeds to survive adverse conditions and germinate when conditions are favorable.
• Types of dormancy:
  • Physical dormancy: Hard seed coat prevents water absorption.
  • Physiological dormancy: Internal factors inhibit germination.
  • Morphological dormancy: Embryo growth inhibited due to an underdeveloped anatomy.

18. Breaking Seed Dormancy

• Seed dormancy can be overcome by:
  • Scarification: Mechanical or chemical treatment to break the hard seed coat.
  • Stratification: Exposing seeds to cold temperature to mimic winter conditions.
  • Gibberellic acid treatment: Application of plant hormone to stimulate germination.
  • Light exposure: Providing light or darkness as per the seed’s requirement.
  • Natural processes: Fire, freezing, or digestion by animals can break seed dormancy.

19. Significance of Seed Dispersal

• Seed dispersal is the process by which seeds are spread away from their parent plant.
• Importance of seed dispersal:
  • Reduces competition among plants.
  • Colonizes new habitats.
  • Increases genetic diversity.
  • Promotes long-distance dispersal.
  • Helps in the survival of plant species.

20. Methods of Seed Dispersal

• Different mechanisms involved in seed dispersal:
  • Wind: Seeds with specialized structures or lightweight seeds are dispersed by wind (e.g. dandelions).
  • Water: Buoyant seeds are carried away by water currents (e.g. coconuts).
  • Animals: Seeds may attach to animal fur or be ingested by animals and later excreted (e.g. burdock).
  • Self-propulsion: Seeds have mechanisms to physically disperse themselves (e.g. exploding seed pods).
  • Gravity: Heavy seeds fall directly beneath the parent plant (e.g. horse chestnut).

21. Methods of Asexual Reproduction in Plants

• Asexual reproduction does not involve the fusion of gametes and results in offspring that are genetically identical to the parent plant.
• Different methods of asexual reproduction in plants include:
  • Vegetative propagation: New plants are formed from vegetative parts like stems, roots, and leaves.
  • Fragmentation: The parent plant breaks into fragments, and each fragment develops into a new plant.
  • Apomixis: Formation of seeds without fertilization, resulting in genetically identical offspring.
  • Adventitious plantlets: Plantlets develop from specialized tissues like the leaf margins or stems.
  • Bulbils: Buds present in the axils of leaves or in the flowers develop into new plants.

22. Vegetative Propagation

• Vegetative propagation is the process of asexual reproduction where new plants are formed from vegetative parts.
• Types of vegetative propagation:
  • Rhizomes: Underground stems that grow horizontally and give rise to new plants (e.g., ginger).
  • Runners or stolons: Above-ground stems that establish roots at nodes and form new plants (e.g., strawberry).
  • Tubers: Modified underground stems that store nutrients and develop new plants (e.g., potato).
  • Offset: Small lateral shoots that develop into new plants (e.g., Bryophyllum).
  • Suckers: New shoots growing from the base of the plant that can be detached to form new plants (e.g., banana).

23. Fragmentation

• Fragmentation is a form of asexual reproduction where the parent plant breaks into fragments, and each fragment develops into a new plant.
• Common examples of fragmentation:
  • Spirogyra: A filamentous alga that fragments into smaller pieces, each capable of growing into a new filament.
  • Filamentous algae: Filamentous green algae break into fragments due to physical disturbances or wave actions.
  • Mosses: Moss plants can break into fragments due to environmental factors, such as desiccation or physical disturbances.

24. Apomixis

• Apomixis is a type of asexual reproduction where seeds are formed without the process of fertilization.
• In apomixis, the ovule develops into a seed without the need for pollination or fertilization.
• Example: Pseudogamy in citrus plants, where ovules develop into seeds without fertilization, but the ovary is stimulated by pollen for fruit development.

25. Adventitious Plantlets

• Adventitious plantlets are formed from specialized tissues, such as the leaf margins, stem or root, without the involvement of flowers or seeds.
• These plantlets develop roots and shoots while still attached to the parent plant and can be separated to grow into new independent plants.
• Example: Production of plantlets from the leaf margins of Bryophyllum plants.

26. Bulbils

• Bulbils are small, bulb-like structures that develop in the axils of leaves or flowers and give rise to new plants.
• They can detach from the parent plant and grow into new individuals.
• Example: Production of bulbils in garlic plants, where bulbils develop in the flower head and can be planted to grow new garlic plants.

27. Advantages of Asexual Reproduction

• Asexual reproduction offers several advantages to plants:
  • Faster reproduction: Plants reproduce quickly without the need for pollination or seed formation.
  • Faster colonization: Asexual reproduction allows plants to colonize new areas rapidly.
  • Guaranteed traits: Offspring are genetically identical to the parent, ensuring that desirable traits are maintained.
  • Less energy requirement: Asexual reproduction does not require the production of flowers or pollination.
  • Suitable for stable environments: Asexual reproduction is advantageous in stable environments where conditions for growth and survival remain consistent.

28. Disadvantages of Asexual Reproduction

• Asexual reproduction also has some limitations:
  • Lack of genetic diversity: Offspring are genetically identical, which reduces their ability to adapt to changing environments.
  • Vulnerability to the same diseases and pests: If the parent plant is susceptible to a particular disease, the offspring will also be vulnerable.
  • Limited dispersal: Asexual reproduction can result in offspring that are clustered around the parent plant, reducing their ability to disperse and colonize new areas.
  • Accumulation of harmful traits: If the parent plant has harmful mutations or genetic variations, these traits can accumulate in the offspring through asexual reproduction.

29. Comparing Sexual and Asexual Reproduction

• Sexual Reproduction:
  • Involves the fusion of male and female gametes.
  • Creates genetic diversity through recombination.
  • Allows offspring to adapt to changing environments.
  • Requires pollinators or wind for pollination.
  • Seeds are formed as a result of fertilization.
  • Longer time required for maturity.
• Asexual Reproduction:
  • Offspring are genetically identical to the parent.
  • Rapid reproduction without fertilization or seed formation.
  • Suitable for stable environments.
  • No need for pollinators or external agents.
  • Does not require the production of seeds.
  • Faster maturity compared to sexual reproduction.

30. Conclusion

• The process of fertilization plays a crucial role in the sexual reproduction of flowering plants, leading to the formation of seeds and fruits.
• Double fertilization ensures the development of both the embryo and the endosperm.
• Post-fertilization changes result in the formation of a fruit and the development of seed dormancy.
• Asexual reproduction methods offer advantages such as faster reproduction and guaranteed traits, while also having limitations like reduced genetic diversity.
• Understanding the concepts of sexual and asexual reproduction is essential for comprehending the diversity and survival strategies of plants.