Reproduction in Plants: Sexual Reproduction in Flowering Plants

Introduction to Sexual Reproduction in Plants

• Sexual reproduction involves the fusion of male and female gametes.
• Flowering plants have a unique reproductive system.
• Reproduction in flowering plants involves various structures and processes.

Parts of a Flower

• Sepals: Protect the flower during bud stage.
• Petals: Attract pollinators.
• Stamens: Male reproductive organs.
• Carpels: Female reproductive organs.

Male Reproductive System of Flowering Plants

• Stamens consist of anther and filament.
• Anther produces pollen grains containing male gametes.
• Filament supports the anther.

Female Reproductive System of Flowering Plants

• Carpels consist of stigma, style, and ovary.
• Stigma receives pollen grains.
• Style connects the stigma to the ovary.
• Ovary contains ovules, which house the female gametes.

Pollination

• Pollination is the transfer of pollen grains from anther to stigma.
• Can be achieved by wind, water, or animals.
• Self-pollination occurs within the same flower, while cross-pollination occurs between different flowers.

Fertilization

• Fertilization is the fusion of the male and female gametes.
• Pollen grains germinate on the stigma and produce a pollen tube.
• The pollen tube grows through the style and reaches the ovary.
• Male gametes are released and fertilize the female gametes in the ovule.

Seed Formation

• After fertilization, the ovule develops into a seed.
• The ovary develops into a fruit to protect the seeds.
• Seeds contain an embryo and stored food for germination.

Germination

• Germination is the process by which a seed develops into a new plant.
• Conditions required for germination include water, oxygen, and suitable temperature.
• The seed absorbs water, swells, and germinates.
• A new plant emerges from the seed, usually with roots and shoots.

Significance of Sexual Reproduction in Flowering Plants

• Genetic variation is introduced through sexual reproduction.
• It leads to the creation of new combinations of traits.
• Enables plant survival through adaptation to changing environments.
• Ensures the continuity of plant species.

Evolution of Reproduction in Plants

• Sexual reproduction evolved in plants for various reasons:
  • Increased genetic variation due to the mixing of genes from different individuals.
  • Enhanced adaptability to changing environmental conditions.
  • Increased chances of survival and propagation of the species.
• The evolution of flowering plants led to the development of specialized reproductive organs and mechanisms for efficient sexual reproduction.

Types of Pollination

• Self-pollination:
  • Pollen from the anther of a flower is transferred to the stigma of the same flower or another flower on the same plant.
  • Examples: Pea plants, wheat, rice.
• Cross-pollination:
  • Pollen is transferred from the anther of one flower to the stigma of another flower on a different plant of the same species.
  • Examples: Most flowering plants, such as roses, sunflowers, and lilies.
• Cross-pollination increases genetic diversity and promotes healthy plant populations.

Agents of Pollination

• Wind pollination (Anemophily):
  • Pollen grains are lightweight, small, and smooth.
  • Examples: Grasses, corn, pine trees.
• Insect pollination (Entomophily):
  • Flowers are brightly colored and produce nectar to attract insects.
  • Examples: Butterflies, bees, moths.
• Bird pollination (Ornithophily):
  • Flowers are brightly colored and produce copious amounts of nectar.
  • Examples: Hummingbirds.
• Animal pollination:
  • Flowers produce scent and have specialized floral features to attract specific animals.
  • Examples: Bats, beetles.

Structure of a Flower

• The flower is a reproductive structure in flowering plants.
• It consists of four main parts: sepals, petals, stamens, and carpels.
• The arrangement and characteristics of these parts vary among different species.
• The presence or absence of any of these parts can determine the flower type.

Structure of a Staminate Flower

• Staminate flowers:
  • Also known as male flowers.
  • Lack carpels and have only stamens.
  • Produce and release pollen grains.
• Example: Male flowers of maize (corn) plant.

Structure of a Carpellate Flower

• Carpellate flowers:
  • Also known as female flowers.
  • Lack stamens and have only carpels.
  • Contain ovules in the ovary.
• Example: Female flowers of maize (corn) plant.

Floral Diagram and Floral Formula

• Floral diagram:
  • Represents a flower’s structure and arrangement of its floral parts.
  • Shows the number of sepals, petals, stamens, and carpels, along with their arrangement.
• Floral formula:
  • A shorthand representation of the floral diagram.
  • Uses symbols and numbers to represent the floral parts and their arrangement.

Double Fertilization

• Double fertilization is a unique feature of flowering plants.
• It involves the fusion of two separate fertilization events:
  1. One sperm fertilizes the egg, forming a zygote that develops into the embryo.
  2. The other sperm combines with two polar nuclei to form the endosperm, providing nourishment to the developing embryo.
• The endosperm provides nutrients to the growing embryo and is consumed by seed-eating animals.

Formation of Fruits and Seeds

• After fertilization, the ovary develops into a fruit.
• The fruit protects the developing seeds and aids in their dispersal.
• The seeds contain the embryo, along with stored food reserves for germination.
• Examples: Apples, oranges, watermelons.

Importance of Fruits in Seed Dispersal

• Fruits play a crucial role in seed dispersal.
• They attract animals through color, aroma, and taste.
• Animals eat the fruits and disperse the seeds through their feces or by dropping them in new locations.
• Seed dispersal strategies help reduce competition for resources and enable plants to colonize new areas.

Methods of Pollination

• Self-pollination:
  • Transfers pollen from the anther to the stigma of the same flower or another flower on the same plant.
  • Ensures that plants can reproduce even when pollinators are scarce.
  • Example: Pea plants.
• Cross-pollination:
  • Involves the transfer of pollen from the anther of one flower to the stigma of another flower on a different plant.
  • Increases genetic diversity and promotes healthy plant populations.
  • Example: Roses.
• Modes of cross-pollination:
  • Insect pollination
  • Wind pollination
  • Bird pollination
  • Animal pollination
  • Water pollination

Advantages and Disadvantages of Different Modes of Pollination

• Insect pollination:
  • Advantages:
    • Efficient and targeted transfer of pollen.
    • Synchronization of flowering and pollinator activity.
  • Disadvantages:
    • Dependence on specific pollinators.
    • Vulnerability to pollinator decline.
• Wind pollination:
  • Advantages:
    • Large quantities of pollen produced for successful fertilization.
    • Less dependence on specific pollinators.
  • Disadvantages:
    • Wasteful and inefficient due to random pollen dispersion.
    • Limited control over pollen destination.

Coevolution Between Plants and Their Pollinators

• Plant-pollinator interactions often result in coevolution.
• Plants evolve floral structures and adaptations to attract specific pollinators.
• Pollinators coevolve with plants, developing specialized behaviors and traits.
• This coevolutionary relationship benefits both the plants and their pollinators.
• Examples:
  • Long tubular flowers and hummingbirds.
  • Strong scent and nocturnal moths.

Fertilization in Flowering Plants

• Pollen germination:
  • Pollen grains land on the stigma.
  • Pollen germinates, producing a pollen tube.
• Pollen tube growth:
  • The pollen tube grows through the style, guided by chemical signals.
  • It reaches the ovary and enters the ovule.
• Double fertilization:
  • In the ovule, one sperm fertilizes the egg to form a zygote.
  • Another sperm combines with polar nuclei to form endosperm.

Types of Fruits

• Simple fruits:
  • Develop from a single ovary.
  • Examples: Apple, orange, cherry.
• Aggregate fruits:
  • Develop from multiple ovaries in a single flower.
  • Example: Raspberry.
• Multiple fruits:
  • Develop from multiple ovaries of multiple flowers on the same inflorescence.
  • Example: Pineapple.

Seed and Fruit Dispersal

• Methods of dispersal:
  • Wind dispersal (anemochory)
  • Animal dispersal (zoochory)
  • Water dispersal (hydrochory)
  • Self-dispersal (automychorry)
• Seed adaptations for dispersal:
  • Wings (maple seeds)
  • Hooks (burdock seeds)
  • Floatation devices (coconut seeds)

Importance of Seed and Fruit Dispersal

• Prevents competition:
  • Allows plants to colonize new habitats with less competition for resources.
• Population spread:
  • Dispersal ensures reestablishment of populations in case of local extinction.
• Genetic diversity:
  • Dispersal enhances gene flow between populations, increasing genetic diversity.
• Ecosystem enrichment:
  • Dispersed seeds contribute to the overall biodiversity and functioning of ecosystems.

Seed Dormancy

• Seed dormancy:
  • A period of arrested growth and development in seeds.
  • Ensures that seeds do not germinate under unsuitable conditions.
• Types of dormancy:
  • Physical dormancy: Seed coat impermeability.
  • Physiological dormancy: Internal biochemical factors.
  • Morphological dormancy: Immature embryo.
• Seed dormancy can be broken by favorable environmental conditions.

Germination Process

• Germination requirements:
  • Water: Activates enzymes and softens the seed coat.
  • Oxygen: Supports cellular respiration during germination.
  • Suitable temperature: Provides optimal conditions for enzyme activity.
• Steps of germination:
  1. Water absorption and seed swelling.
  2. Activation of enzymes and resumption of metabolic activity.
  3. Emergence of the radicle (embryonic root) and subsequent growth.

Conclusion

• Sexual reproduction in flowering plants involves pollination, fertilization, and seed formation.
• Different modes of pollination ensure successful reproduction.
• Coevolution between plants and pollinators is a result of their mutualistic relationship.
• Seed and fruit dispersal aids in colonization, genetic diversity, and ecosystem enrichment.
• Seed dormancy and germination are crucial for the survival and propagation of plants.