REPRODUCTION IN PLANTS

REPRODUCTION

Definition of Reproduction

  • Reproduction refers to the perpetual continuation of life for the survival of a species, defined as organisms that share similar characteristics and can interbreed to produce fertile offspring.

Types of Reproduction

  1. Asexual Reproduction
       - Involves the formation of new organisms from a single parent without the involvement of gametes.
       - Characterized by:
           - No fertilization, i.e., no fusion of gametes.
           - Offspring are genetically identical to each other and to the parent organism, known as CLONES.

Examples of Asexual Reproduction in Plants and Animals:

  • a) Vegetative Propagation
       1. Budding
          - New organism produced as an outgrowth from the parent.
          - The outgrowth eventually separates and becomes an independent, identical copy.
          - Example: Most roses propagated by budding on suitable stocks; hydra is an animal example.
       2. Cutting
          - A cutting of about 20 cm is made just below a joint and inserted in moist soil or water.
          - Example: Commonly used for rose propagation.
       3. Grafting
          - A bud or shoot from one plant is inserted under the bark of another closely related variety.
          - The bud grows using water and nutrients from the host plant.
          - This method is used for apple trees, pear trees, and rubber trees.
       4. Layering
          - A young branch still attached to the parent plant is bent down to the ground and secured until it forms roots.
          - Rooting powder may enhance root development.
          - Once rooted, the branch is detached to become independent.

  • b) Fragmentation / Regeneration
       - Involves breaking an organism into two or more parts, with each part growing into a new individual.
       - Common in less differentiated animals with strong regenerative abilities.
       - Examples: Starfish and flatworms undergo accidental fragmentation.

Commercial Application of Asexual Reproduction in Flowering Plants

  • Includes techniques such as grafting and cutting.

Advantages of Asexual Reproduction:

  • Simple and quick method of reproduction.

  • High chances of offspring survival.

  • Eliminates the risk of gamete destruction before fusion.

  • Ideal for maintaining favorable traits in the population.

  • Capable of producing numerous offspring quickly.

Disadvantages of Asexual Reproduction:

  • Genetic defects from parents easily passed to offspring.

  • Limits evolutionary changes.

Sexual Reproduction in Plants

Functions of Flower Parts:

  • Stigma: Receives pollen grains.

  • Style: Allows pollen tube to reach the ovary.

  • Ovary: Produces female sex cells (ovules).

  • Anther: Produces male sex cells (pollen grains).

  • Filament: Supports the anther.

  • Petals: Protects flower parts.

  • Sepals: Protects the ovary and flower bud.

  • Flower stalk: Attaches the flower to the plant.

  • Receptacle: Base where all flower parts attach.

Pollination

  • Defined as the transfer of pollen grains from the anther to the stigma.

Types of Pollination:

  1. Self Pollination
       - Transfer of pollen from the anther to the stigma in the same flower or from one flower to another on the same plant.

  2. Cross Pollination
       - Transfer of pollen from anther of one flower to the stigma of a different flower of the same species.

Agents of Pollination:

  • Insects

  • Wind

Characteristics of Flowers Based on Pollination:

  • Wind Pollinated Flowers:
      - Dull petals
      - Small or no petals
      - Feathery stigma
      - Light pollen grains
      - No scent
      - No nectar

  • Insects Pollinated Flowers:
      - Bright colored petals
      - Large petals
      - Plain stigma
      - Sticky pollen grains
      - Scented petals
      - Have nectar

Fertilization

  • Requires prior pollination.

  • Defined as the fusion of the male nucleus (from pollen) with the female nucleus (from ovules).

Growth of the Pollen Tube

  • After fertilization, the fertilized egg (ovule) develops into an embryo.

  • The embryo contains:
      - Plumule: Develops into the shoot.
      - Radicle: Develops into the root system.

Seed Development

  • After fertilization, petals dry up and fall off, leading to the ovary developing into fruit and ovules into seeds.

Non-Endospermic Seed (Dicot Example)

  • Example: Bean seed.
      - Functions of Parts:
        1. Testa (seed coat): Tough outer protective layer.
        2. Cotyledon: Food storage for the embryo (2 in dicots).
        3. Micropyle: Opening for water and oxygen entry during germination.
        4. Embryo Components:
           - Plumule: develops shoot during germination.
           - Radicle: forms the root system.

Environmental Conditions for Germination:

  • Moisture: Essential for initiating and sustaining germination. Excess water can prevent germination by excluding oxygen.

  • Oxygen: Required for aerobic respiration. Provides energy for chemical reactions in embryo cells during growth.

  • Suitable Temperature: Activates enzymes for optimal chemical reactions; generally optimal range is 5 °C–40 °C for germination.

Role of Enzymes in Seed Germination

  • In early germination, food reserves (mainly starch and proteins) in cotyledons are converted by enzymes into soluble products used by actively growing regions.
    - Glucose from starch is utilized in synthesizing cellulose, incorporated into cell walls, and used in respiration for energy.

Seed Dispersal

  • Process of transferring seeds from the parent plant to new sites.

Advantages of Seed Dispersal:

  • Reduces competition for nutrients and space.

  • Increases chances of finding suitable areas for colonization, boosting species population over time (enhances plant diversity).

Disadvantages of Seed Dispersal:

  • Risk of not finding a suitable place for germination.

Mechanisms of Seed Dispersal:

  1. Animal Dispersed Seeds
       - Examples: Seeds with hooks/spines attach to animal fur (e.g., goose grass).
       - Contained in edible, fleshy fruits (e.g., tomatoes, strawberries) that survive digestion and are excreted with feces in fertile soil.

  2. Wind Dispersed Seeds
       - Examples: Parachute fruits (willow herb, dandelion) with feathery hairs that increase floating capability.
       - Winged fruits (sycamore, ash) that spin as they fall, and utilize air currents.
       - Pepper pot effect: (e.g., poppy capsules release seeds by wind-induced shaking).

  3. Water Dispersed Seeds
       - Example: Coconuts with fibrous ovarians allowing them to float on water for dispersal.

  4. Self Dispersed Seeds
       - Example: Explosive fruits (e.g., pea family) that dry, shrivel, and split open, propelling seeds outward.