Reproduction - Sexual Reproduction In Flowering Plants

  • Topic: Flower Evolution

Introduction

  • Sexual reproduction in flowering plants
  • Importance of flowers in the process
  • Evolution of flowers

Characteristics of Flowers

  • Specialized reproductive structures
  • Composed of modified leaves
  • Attractive colors and fragrances
  • Various shapes and sizes

Parts of a Flower

  • Sepals: protect the flower in bud stage
  • Petals: attract pollinators
  • Stamens: produce pollen
  • Pistil: contains the ovary

Evolution of Flowers

  • Theory of coevolution
  • Gradual development over millions of years
  • Co-evolution with pollinators
    • Mutualistic relationship
    • Behavioral and structural adaptations

Fossil Evidence

  • Fossils provide insights into the evolution
  • Oldest fossilized flower: Archaefructus
  • Lack of complete transitional forms

Types of Flowers

  • Perfect flowers: contain both male and female reproductive structures
  • Imperfect flowers: contain either male or female structures
  • Monoecious plants: have both types of flowers on the same plant
  • Dioecious plants: have male and female flowers on separate plants

Pollination

  • Transfer of pollen from stamen to pistil
  • Different modes of pollination:
    • Self-pollination
    • Cross-pollination
    • Wind pollination
    • Insect pollination

Adaptations for Different Pollinators

  • Flowers have different structures based on the pollinator:
    • Bee-pollinated flowers: bright colors, sweet fragrance
    • Bird-pollinated flowers: tubular shape, bright colors
    • Bat-pollinated flowers: pale colors, strong fragrance
    • Wind-pollinated flowers: small and inconspicuous

Reproduction in Flowering Plants

  • Pollination process
  • Fertilization
  • Development of seeds and fruits

Modes of Pollination

  • Self-pollination: pollen transferred from anther to stigma of the same flower or another flower on the same plant
  • Cross-pollination: pollen transferred from anther to stigma of flowers on different plants
  • Agent-mediated pollination:
    • Insect pollination: bees, butterflies, moths
    • Bird pollination: hummingbirds, sunbirds
    • Bat pollination: fruit bats

Self-pollination

  • Occurs in plants with closed flowers
  • No need for external agents
  • Advantages:
    • Ensures reproductive success
    • Maintains favorable traits in a population
  • Disadvantages:
    • Decreased genetic diversity
    • Accumulation of harmful recessive traits

Cross-pollination

  • Advantageous for genetic diversity
  • Dependent on external agents for pollen transfer
  • Factors promoting cross-pollination:
    • Dichogamy: male and female reproductive structures mature at different times
    • Herkogamy: physical separation of reproductive structures
    • Self-incompatibility: plants reject own pollen

Wind Pollination

  • Occurs in plants with inconspicuous flowers
  • Large amount of pollen production
  • Lack of attractants like scent or nectar
  • Examples: grasses, conifers
  • Disadvantages:
    • Wastage of pollen
    • Lack of targeted pollen delivery

Insect Pollination

  • Bees, butterflies, and other insects are important pollinators
  • Bees are more efficient due to their specialized structures
  • Attracting insects:
    • Brightly colored petals
    • Sweet fragrances
  • Examples: orchids, sunflowers

Bird Pollination

  • Birds are attracted to brightly colored, tubular flowers
  • Examples: hummingbirds, sunbirds
  • Flowers adapted for bird pollination:
    • Tubular shape for beak insertion
    • Bright colors for visibility
    • Abundant nectar as a reward

Bat Pollination

  • Occurs primarily in tropical regions
  • Pale-colored, musky-scented flowers
  • Large amounts of nectar
  • Flowers adapted for bat pollination:
    • Bowl-shaped or bell-shaped
    • Large and durable

Fertilization in Flowering Plants

  • After pollination, pollen grain germinates on the stigma
  • Pollen tube grows down to the ovary
  • Male gametes are released inside the embryo sac
  • Fusion of male and female gametes results in fertilization
  • Double fertilization in angiosperms

Double Fertilization in Angiosperms

  • Unique reproductive feature of angiosperms
  • Simultaneous fertilization of two female gametes by two male gametes
  • One male gamete fuses with the egg to form the zygote (2n)
  • The other male gamete fuses with the two polar nuclei to form the endosperm (3n)

Seed and Fruit Development

  • After fertilization, ovule develops into a seed
  • Ovary develops into a fruit
  • Seed development:
    • Embryo formation
    • Storage of nutrients
    • Protective seed coat
  • Fruit development:
    • Ripening of ovary wall
    • Dispersal of seeds

Reproduction in Flowering Plants: Seed Dispersal

  • Seed dispersal is the process by which plants spread their seeds to new locations.
  • Advantages of seed dispersal:
    • Prevents competition among parent and offspring
    • Increases chances of survival and colonization
  • Different mechanisms of seed dispersal:
    • Wind dispersal
    • Water dispersal
    • Animal dispersal

Wind Dispersal

  • Some plants have adaptations for wind dispersal:
    • Lightweight seeds
    • Wing-like structures (samara)
    • Hairs or plumes (pappus) for buoyancy
  • Examples: dandelions, maple trees

Water Dispersal

  • Seeds of some plants are dispersed by water:
    • Lightweight or buoyant seeds
    • Seeds with fibrous or air-filled appendages
    • Seeds contained in buoyant fruits
  • Examples: water lilies, coconut trees

Animal Dispersal

  • Many plants rely on animals for seed dispersal:
    • Edible fruits are consumed by animals
    • Seeds are excreted in a different location
    • Seeds might stick to fur or feathers
  • Examples: berries, apples

Bird Dispersal

  • Birds play a major role in seed dispersal:
    • Swallow seeds whole and excrete them
    • Consume fruits and regurgitate seeds
    • Stick seeds to their feathers
  • Examples: mistletoe, berries

Mammal Dispersal

  • Some mammals disperse seeds through their feeding habits:
    • Store seeds in their burrows for later consumption
    • Drop seeds while foraging
    • Swallow seeds and excrete them
  • Examples: squirrels, rodents

Insect Dispersal

  • Insects can also play a role in seed dispersal:
    • Collect seeds for food storage
    • Transport seeds within their bodies
    • Bury or hide seeds for future consumption
  • Examples: ants, beetles

Seed Dormancy

  • Seed dormancy is a mechanism that prevents seeds from germinating immediately.
  • Advantages of seed dormancy:
    • Allows seeds to survive adverse conditions
    • Prevents germination during unfavorable times
    • Ensures better chances of seedling survival
  • Factors that break seed dormancy:
    • Temperature
    • Light
    • Moisture

Germination Process

  • Germination is the process by which a seed sprouts and begins to grow.
  • Required conditions for germination:
    • Adequate water availability
    • Suitable temperature range
    • Sufficient oxygen supply
  • Germination stages:
    1. Seed imbibition
    2. Activation of enzymes
    3. Emergence of the radicle
    4. Growth of the shoot

Germination Factors

  • Temperature: affects seed dormancy and germination rate
  • Water availability: activates enzymes and promotes imbibition
  • Oxygen supply: required for aerobic respiration during germination
  • Light: some seeds require light for germination (photoblastic)
  • Seed coat permeability: allows water and oxygen to enter the seed