Seed structure and types of germination (epigeal, hypogeal)

1. Introduction to Seeds

1. A seed is a fertilized and matured ovule that gives rise to a new plant.
2. It consists of essential structures necessary for plant growth.
3. Seeds are the primary means of reproduction in most plants.

2. Structure of a Seed

1. A seed typically has three main parts:

a) Seed Coat

1. The outer protective layer of the seed.
2. Consists of two layers:
   • Testa: The outer, thick protective layer.
   • Tegmen: The inner, thin layer.
3. Protects the seed from damage, dehydration, and infections.

b) Embryo

1. The developing plant inside the seed.
2. Consists of:
   • Radicle: The part that develops into the root.
   • Plumule: The part that develops into the shoot.
   • Cotyledons: The seed leaves that store food for the embryo.

c) Endosperm (in some seeds)

1. Provides nutrients to the growing embryo.
2. Present in monocot seeds (e.g., maize) but absent in most dicot seeds (e.g., bean).

3. Types of Seeds

1. Monocotyledonous (Monocot) Seeds: Contain a single cotyledon (e.g., maize, wheat, rice).
2. Dicotyledonous (Dicot) Seeds: Contain two cotyledons (e.g., pea, bean, gram).

4. Germination of Seeds

1. Germination is the process by which a seed develops into a new plant.
2. It requires water, oxygen, and suitable temperature.
3. There are three stages of germination:
   • Imbibition: Absorption of water by the seed.
   • Enzyme Activation: Breakdown of stored nutrients.
   • Embryo Growth: Expansion of radicle and plumule.

5. Types of Germination

1. There are two main types of germination:

a) Epigeal Germination

1. The cotyledons are pushed above the soil due to rapid elongation of the hypocotyl.
2. Common in dicots (e.g., bean, castor, cotton).
3. Advantages:
   • Exposure of cotyledons to sunlight for photosynthesis.
   • Faster initial growth.

b) Hypogeal Germination

1. The cotyledons remain below the soil as the epicotyl elongates.
2. Common in monocots (e.g., maize, pea, gram).
3. Advantages:
   • Cotyledons remain protected underground.
   • Better resistance to adverse weather conditions.

6. Factors Affecting Germination

1. Water: Softens the seed coat and activates enzymes.
2. Oxygen: Essential for cellular respiration to provide energy.
3. Temperature: Optimum temperature (20-30°C) is required.
4. Light: Some seeds require light (e.g., lettuce), while others germinate in darkness.
5. Dormancy: Some seeds undergo a period of inactivity before germination.

7. Importance of Germination

1. Ensures the continuation of plant species.
2. Allows for crop production and agriculture.
3. Essential for forestry and ecosystem balance.

8. Conclusion

1. Seeds are crucial for plant reproduction and survival.
2. Understanding the types of germination helps in better agricultural practices.
3. Proper environmental conditions are necessary for successful germination.