Seeds and Germination: The Beginning of a New Plant

Introduction

A seed is the final product of sexual reproduction in plants. It is essentially a baby plant (the embryo) in a dormant state, packed with a food supply and protected by an outer coat. When conditions are right, this dormant embryo awakens and grows into a new plant, a process called germination.

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Structure of Seeds

Seeds are broadly classified into two groups based on the number of cotyledons (seed leaves) they possess: dicotyledonous (dicot) seeds and monocotyledonous (monocot) seeds.

1. Structure of a Dicot Seed (Example: Bean Seed)

A bean seed is a typical non-endospermic dicot seed, meaning the food is stored in the large cotyledons, and the endosperm is absent in the mature seed.

• Seed Coat: The tough outer protective layer. It consists of:
  • Testa: The thick, outer layer.
  • Tegmen: The thin, inner layer.
• Hilum: A scar on the seed coat where the seed was attached to the fruit wall.
• Micropyle: A tiny pore near the hilum. It allows water to enter the seed during germination.
• Embryo: The main part of the seed, which consists of:
  • Cotyledons (2): Two thick, fleshy seed leaves that store food for the embryo.
  • Embryonic Axis (Tigellum): The main axis of the embryo, which is attached to the cotyledons. It has two ends:
    • Radicle: The lower end, which develops into the root system.
    • Plumule: The upper end, which develops into the shoot system (stem, leaves, and flowers).

2. Structure of a Monocot Seed (Example: Maize Grain)

A maize grain is a type of fruit called a caryopsis, where the fruit wall (pericarp) is fused with the seed coat (testa). It is an endospermic seed, meaning it has a special food-storing tissue called the endosperm.

• Seed Coat and Fruit Wall: A single protective layer formed by the fusion of the pericarp and testa.
• Endosperm: The bulky, food-storing tissue that makes up the majority of the grain. It is rich in starch and provides nourishment to the embryo.
• Aleurone Layer: A protein-rich layer that surrounds the endosperm.
• Embryo: Small and situated in a groove at one end of the endosperm. It consists of:
  • Scutellum (1 Cotyledon): A single, shield-shaped cotyledon. Its function is to digest and absorb food from the endosperm and transfer it to the growing embryo.
  • Embryonic Axis:
    • Plumule: The embryonic shoot, protected by a sheath called the coleoptile.
    • Radicle: The embryonic root, protected by a sheath called the coleorhiza.

Differences between Dicot and Monocot Seeds

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Seed Germination

Germination is the process by which the dormant embryo within a seed resumes its growth and develops into a seedling, which can grow independently.

Types of Germination

1. Epigeal Germination (e.g., Bean Seed):

   • "Epi" means above, and "geal" means earth.
   • In this type of germination, the hypocotyl (the part of the embryonic axis below the cotyledons) elongates rapidly and forms a hook.
   • This hook pushes the cotyledons above the ground.
   • The cotyledons turn green, perform photosynthesis for a short time, and then wither and fall off as the seedling develops its own leaves.

2. Hypogeal Germination (e.g., Maize Grain, Pea Seed):

   • "Hypo" means below, and "geal" means earth.
   • In this type of germination, the epicotyl (the part of the embryonic axis above the cotyledons) elongates.
   • The cotyledons (or the single cotyledon in monocots) remain below the ground.
   • The cotyledons provide nourishment to the growing seedling until it can photosynthesize, but they never emerge from the soil.

Differences between Epigeal and Hypogeal Germination

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Conditions for Seed Germination

For a seed to germinate, it needs a combination of favorable external conditions. These can be demonstrated by simple experiments.

1. Water (Moisture):

   • Why it's needed: Water softens the seed coat, making it easier for the plumule and radicle to emerge. It also activates the enzymes within the seed, which are necessary to break down the stored food into a soluble form that can be used by the embryo for growth.
   • Experiment: Take two beakers. Place some cotton wool in each. In beaker A, place dry cotton wool with some bean seeds. In beaker B, place moist cotton wool with some bean seeds. Keep both at room temperature. After a few days, the seeds in beaker B will germinate, while those in beaker A will not. This shows that water is necessary for germination.

2. Oxygen (Air):

   • Why it's needed: The growing embryo needs energy to divide and grow. This energy is released through aerobic respiration, which requires oxygen. The seed respires actively during germination.
   • Experiment (Three Bean Seed Experiment): Take a beaker of water. Place three bean seeds on a wooden strip. Tie one seed at the top, well above the water level (gets oxygen, no water). Tie the second seed in the middle, just at the water level (gets both oxygen and water). Tie the third seed at the bottom, fully submerged in water (gets water, no oxygen). After a few days, only the middle seed germinates, proving that both water and oxygen are required.

3. Suitable Temperature (Warmth):

   • Why it's needed: Germination is an enzyme-driven process. Enzymes work best within a specific temperature range, known as the optimum temperature (usually 25°C to 35°C). Very low or very high temperatures will inactivate the enzymes and halt the germination process.
   • Experiment: Take two petri dishes with moist cotton wool and bean seeds. Keep one dish at room temperature (around 25°C) and the other in a refrigerator (around 4°C). The seeds at room temperature will germinate, while the seeds in the refrigerator will not, demonstrating the need for a suitable temperature.