Reproduction - Sexual Reproduction In Flowering Plants

• Introduction to sexual reproduction in flowering plants
• Importance of sexual reproduction
• Structure and function of reproductive organs
  • Flower
  • Stamen
  • Pistil
• Process of pollination
  • Definition of pollination
  • Types of pollination
  • Examples of agents of pollination (wind, insects, birds)
• Process of fertilization
  • Definition of fertilization
  • Steps involved in fertilization
  • Formation of zygote and endosperm
• Development of seed and fruit
  • Definition of seed and fruit
  • Formation of seed and fruit after fertilization
• Formation and development of endosperm
  • Definition of endosperm
  • Types of endosperm (helobial, cellular, nuclear)
  • Stages in the development of helobial endosperm

Stages in the development of helobial endosperm

• The development of helobial endosperm occurs in four stages.
• Stage 1: Early nuclear division
  • Nuclei divide rapidly without cell formation
• Stage 2: Cell formation
  • Cell walls form around individual nuclei
• Stage 3: Cell division
  • Each cell undergoes multiple rounds of division
  • Formation of endosperm tissue
• Stage 4: Maturation
  • Accumulation of reserve food materials
  • Endosperm becomes mature and ready to provide nourishment to the developing embryo

Importance of sexual reproduction in flowering plants

• Genetic variation: Sexual reproduction promotes genetic diversity by combining the genetic material from two different individuals.
• Adaptation: It allows for the mixture of advantageous traits in offspring, increasing their chances of survival and adaptation.
• Hybrid vigor: Offspring produced through sexual reproduction often exhibit hybrid vigor, being stronger and healthier than their parents.
• Evolutionary advantage: Sexual reproduction enables species to adapt and evolve more rapidly over time.
• Increased genetic variation: It helps in the removal of harmful mutations and the promotion of beneficial traits.

Formation and development of endosperm

• The endosperm is formed through the process of double fertilization in flowering plants.
• It is a triploid tissue that provides nutrition to the developing embryo.
• Endosperm formation involves the fusion of the male gamete with the two polar nuclei present in the embryo sac.
• The resulting triploid nucleus divides and forms the endosperm tissue.
• The endosperm develops rapidly and provides nourishment to the developing embryo until seed maturity.

Definition of seed and fruit

• Seed:
  • A seed is a mature ovule containing a dormant plant embryo, protective seed coat, and stored food reserves.
  • It serves as a reproductive structure capable of germinating into a new plant.
• Fruit:
  • Fruit is the mature ovary of a flowering plant.
  • It contains seeds and aids in their dispersal.
  • Fruits can be fleshy (berries, apples) or dry (capsules, nuts) depending on the type of plant.

Formation of seed and fruit after fertilization

• After fertilization, the ovule develops into a seed, and the ovary wall develops into a fruit.
• The seed consists of the embryo, endosperm (in some plants), and seed coat.
• The fruit protects the developing seeds and aids in their dispersal.
• Fruits can develop from a single ovary (simple fruit) or multiple ovaries (aggregate fruit or multiple fruit).

Definition of pollination

• Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
• It is a crucial step in sexual reproduction in plants.
• It can occur within the same flower (self-pollination) or between different flowers (cross-pollination).

Types of pollination

• Self-pollination:
  • Pollen from the anther is transferred to the stigma of the same flower or another flower in the same plant.
  • It leads to little or no genetic variation in the offspring.
• Cross-pollination:
  • Pollen from the anther of one plant is transferred to the stigma of another plant of the same species.
  • It promotes genetic diversity in the offspring.

Examples of agents of pollination

• Wind:
  • Some plants produce large quantities of lightweight, dry pollen grains that are easily carried by the wind.
  • Examples: grasses, conifers, ragweed
• Insects:
  • Many flowers have evolved specific adaptations to attract insects for pollination, such as bright colors, nectar, and fragrances.
  • Examples: bees, butterflies, moths
• Birds:
  • Flowers with long, tubular nectar-filled corollas attract birds for pollination.
  • Examples: hummingbirds, sunbirds

Definition of fertilization

• Fertilization is the fusion of the male and female gametes (sperm and egg) to form a zygote.
• It is a key event in sexual reproduction that combines the genetic material from two parents.
• Fertilization occurs after successful pollination and transfer of pollen to the stigma.

Steps involved in fertilization

1. Pollen grain germinates on the stigma, forming a pollen tube.

2. The pollen tube grows through the style towards the ovary.

3. The two male gametes present in the pollen grain move down the pollen tube.

4. One male gamete fuses with the egg cell, forming a zygote (2n).

5. The other male gamete fuses with the two polar nuclei, forming the triploid endosperm (3n).

6. Fertilization is complete, and the ovule develops into a seed.

Role of pollinators in plant reproduction

• Pollinators play a crucial role in plant reproduction by transferring pollen from the anther to the stigma of flowers.
• They facilitate cross-pollination, which increases genetic diversity in the offspring.
• Examples of pollinators:
  • Bees: Honeybees, bumblebees, solitary bees
  • Butterflies: Monarch butterflies, painted lady butterflies
  • Birds: Hummingbirds, sunbirds
  • Bats: Fruit bats, nectar bats

Coevolution between plants and pollinators

• Coevolution is the reciprocal evolutionary change between two interacting species.
• Plants and their pollinators often exhibit coevolutionary relationships.
• Examples of coevolution:
  • Long-tongued insects and flowers with deep corolla tubes.
  • Birds with curved bills and flowers with long, tubular corollas.
  • Moths and bats with long proboscis and flowers that open at night.

Factors affecting pollination success

• Availability of pollinators: The presence and abundance of pollinators in the environment.
• Flower characteristics: Color, fragrance, shape, and nectar availability can attract specific pollinators.
• Timing of flowering: Flowers that bloom during the active season of their specific pollinators have higher pollination success.
• Distance between plants: Proximity of plants of the same species increases chances of cross-pollination.

Mechanisms of self-incompatibility

• Self-incompatibility is a mechanism that prevents self-pollination and promotes cross-pollination.
• It ensures genetic diversity and reduces the likelihood of inbreeding.
• Different mechanisms of self-incompatibility:
  • Gametophytic self-incompatibility (GSI): Controlled by the genes in the pollen grain.
  • Sporophytic self-incompatibility (SSI): Controlled by the genes in the style of the pistil.

Advantages of self-incompatibility

• Promotes outcrossing: Self-incompatibility ensures cross-pollination, leading to genetic diversity and healthier offspring.
• Reduces inbreeding depression: Inbreeding depression is the loss of fitness due to breeding between closely related individuals. Self-incompatibility helps avoid this.
• Preserves genetic variation: Self-incompatibility maintains a diverse gene pool within a population, increasing the chances of survival in changing environments.

Role of hormones in plant reproduction

• Hormones play important roles in regulating various reproductive processes in plants.
• Examples of hormones involved in plant reproduction:
  • Gibberellins: Promote pollen tube growth and stimulate the development of stamen and pistil.
  • Auxins: Influence flower development, promote fruit growth, and stimulate fruit ripening.
  • Ethylene: Regulates fruit ripening and senescence.
  • Abscisic acid: Inhibits seed germination and promotes seed dormancy.

Factors influencing fruit and seed development

• Fertilization: Successful fusion of gametes initiates fruit and seed development.
• Hormones: Auxins, gibberellins, and cytokinins influence fruit growth, seed development, and seed dormancy.
• Environmental conditions: Temperature, light, moisture, and nutrients affect fruit and seed development.
• Genetic factors: The genetic makeup of the plant determines the characteristics of the fruit and seed.

Dispersal mechanisms in fruits and seeds

• Dispersal is the process of transporting fruits and seeds away from the parent plant.
• It reduces competition among offspring and increases the chances of successful colonization.
• Different dispersal mechanisms:
  • Wind dispersal: Fruits or seeds equipped with structures (e.g., wings, parachutes) are carried by the wind.
  • Animal dispersal: Fruits or seeds consumed by animals and later dispersed through their feces.
  • Water dispersal: Fruits or seeds that float on or in water and are carried to new locations.

Germination of seeds

• Germination is the process where a seed develops into a young seedling under favorable conditions.
• Requirements for seed germination:
  • Water: It activates enzymes and triggers metabolic processes.
  • Oxygen: Required for cellular respiration.
  • Optimal temperature: Varied for different plant species.
  • Light (in some cases): Some seeds require light for germination, while others need darkness.

Factors affecting seed germination

• Dormancy: Some seeds have physiological dormancy, requiring specific conditions (e.g., temperature, light) or exposure to certain chemicals (e.g., gibberellins) to break dormancy.
• Environmental conditions: Adequate moisture, temperature, and oxygen availability are critical for seed germination.
• Seed coat thickness: Thick seed coats may require scarification (abrasion or treatment) to allow water penetration.
• Allelopathy: The presence of certain chemicals released by plants can inhibit or stimulate seed germination.