3.1 Structure of Seeds and Germination

Introduction

  • A seed is a dormant embryo with a food supply, protected by a seed coat.
  • Germination: The process where a dormant embryo resumes growth and develops into a seedling.

Structure of Seeds

1. Dicot Seed (e.g., Bean Seed)

  • Seed Coat: Testa (outer), Tegmen (inner).
  • Hilum: Scar where seed attached to fruit.
  • Micropyle: Pore for water entry.
  • Embryo:
    • Cotyledons (2): Store food.
    • Embryonic Axis: Radicle (root), Plumule (shoot).

Structure of Seeds (Continued)

2. Monocot Seed (e.g., Maize Grain)

  • Seed Coat & Fruit Wall: Fused protective layer.
  • Endosperm: Bulky, starch-rich food-storing tissue.
  • Aleurone Layer: Protein-rich layer surrounding endosperm.
  • Embryo: Small, in a groove.
    • Scutellum (1 Cotyledon): Shield-shaped, absorbs food from endosperm.
    • Embryonic Axis: Plumule (protected by coleoptile), Radicle (protected by coleorhiza).

Differences between Dicot and Monocot Seeds

Feature Dicot Seed (Bean) Monocot Seed (Maize)
Cotyledons Two One (scutellum)
Endosperm Absent in mature seed Present in mature seed
Food Storage In cotyledons In endosperm
Protective Sheaths Absent Coleoptile & coleorhiza present

Types of Germination

  1. Epigeal Germination (e.g., Bean Seed):
    • Hypocotyl elongates, pushing cotyledons above ground.
    • Cotyledons may become photosynthetic.
  2. Hypogeal Germination (e.g., Maize Grain, Pea Seed):
    • Epicotyl elongates, cotyledons remain below ground.
    • Cotyledons provide nourishment, do not photosynthesize.

Differences between Epigeal and Hypogeal Germination

Feature Epigeal Germination Hypogeal Germination
Fate of Cotyledons Pushed above the soil Remain below the soil
Function of Cotyledons May become photosynthetic Solely provide nutrition
Elongating Part Hypocotyl Epicotyl

Conditions for Seed Germination

  1. Water (Moisture): Softens seed coat, activates enzymes.
    • Experiment: Seeds germinate in moist cotton wool, not dry.
  2. Oxygen (Air): Needed for aerobic respiration to release energy for embryo growth.
    • Experiment: Three bean seeds (only one at water level germinates).
  3. Suitable Temperature (Warmth): Optimum range (25°C-35°C) for enzyme activity.
    • Experiment: Seeds germinate at room temp, not in refrigerator.