Sexual Reproduction in Plants: Fertilization and Structure of Seeds and Fruits

Fertilization in Plants

  • Fertilization: Fusion of male and female gametes.

Objectives

  • Define fertilization, germination, and dispersal.
  • Explain the process of fertilization.
  • Describe the formation of seeds and fruits after fertilization.
  • Differentiate between seed and fruit.
  • Relate fruit and seed structure to flower structure.
  • Identify seed parts: testa, hilum, cotyledons, and micropyle.
  • Draw, label, and annotate seed and fruit structures.

The Process of Fertilization

  • After pollination, the pollen grain on the stigma grows a tube.
  • This tube grows down through the style into the ovary.
  • The tube enters the ovule (egg cell).
  • The male gamete (nucleus) from the pollen grain fuses with the female gamete (ovum nucleus).
  • This fusion is called fertilization.

Visual Representation of Fertilization

  • Pollen grains land on the stigma.
  • Each pollen grain grows a pollen tube down to the ovule.
  • The nucleus from one pollen grain reaches the ovule.
  • The male cell joins with the nucleus in the female cell (ovum) inside the ovule.

Events After Fertilization

  • The style and stigma wither, along with the petals and sepals, and drop off.
  • The ovules grow larger and become the seeds.
  • The ovary grows larger and becomes the fruit.

Development of Fruit and Seeds

  • The ovary grows bigger and becomes a fruit.
  • Seeds develop inside the fruit.
  • Petals wither and drop off as the ovary begins to swell.

Fruit vs. Seed

  • Fruit: The developed ovary.
  • Seed: The fertilized ovule.

Fruit Structure

  1. Exocarp: The skin of the fruit.
  2. Mesocarp: The flesh of the fruit.
  3. Endocarp: The inner stony wall of the fruit that contains the seed(s).

External Structure of a Seed

  • Seeds develop from fertilized ovules in plant ovaries.
  • Each seed is surrounded by a protective seed coat called the testa.
  • A tiny hole in the testa, called the micropyle, allows the seed to absorb water for germination.
  • A scar on the testa, called the hilum, indicates where the seed was attached to the ovary.

Examples of External Seed Structure

  • Red Pea: Shows the testa, micropyle, and hilum.
  • Corn/Maize: Shows the testa and hilum.

Internal Structure of a Seed

  • Inside the seed is the embryo, which will grow into a new plant.
  • The embryo is the young plant inside the seed.
  • The embryo consists of three parts:
    • Plumule: Grows into the shoot of the new plant.
    • Radicle: Grows into the root of the new plant.
    • Cotyledon: Stores food for the embryo (seed leaves).

Types of Seeds Based on Cotyledons

  • Dicotyledon: Seeds with two cotyledons (e.g., red pea).
  • Monocotyledon: Seeds with one cotyledon (e.g., corn/maize).
  • Some seeds contain endosperm, another food store (e.g., corn, coconut, castor oil).

Seed Structure Summary

  • Plumule: Becomes the first shoot of the plant.
  • Radicle: Becomes the first root of the plant.
  • Testa (seed coat): Protects the seed from injury and drying out.
  • Cotyledon: Contains stored food for initial growth.
  • Embryo: Made up of the radicle and plumule.
  • Hilum: Area where the seed is attached to the ovary wall.
  • Micropyle.

Plant Reproduction Stages

  • Pollination
  • Fertilization
  • Seed Dispersal

Dispersal

  • The transport of seeds away from the parent plants is called dispersal.
  • Dispersal reduces overcrowding and competition for resources.
  • Seeds travel by wind, animals, water, or bursting.

Germination

  • Germination is the process by which a seed grows into a new plant (seedling).
  • A seedling is a young plant.
  • Seeds need the right conditions to grow, which may vary.
  • The three main conditions for germination are water, oxygen, and warmth.

The Germination Process

  • The testa splits.
  • The radicle (young root) emerges.
  • The plumule (young shoot) emerges.
  • Cotyledons may come above ground.
  • Root hairs take in water and minerals.
  • Foliage leaves develop to make food for the plant.
  • Cotyledons get smaller as the food inside is used up.

Activity #1

  • The coat of a seed is called the testa.
  • The scar on the seed shows where the seed was attached inside the fruit.
  • Seeds need water, air, and warmth to grow.