Internal Factors:
* Mature Embryo: The embryo within the seed must be fully developed and ready to grow. This means it has undergone sufficient cell division and differentiation.
* Stored Food Reserves: The seed must have sufficient food reserves (endosperm or cotyledons) to sustain the embryo during the early stages of growth before it can produce its own food through photosynthesis.
* Hormonal Signals: Certain hormones, like gibberellic acid, play a crucial role in triggering the processes of germination.
External Factors:
* Water: Water is essential for seed germination. It rehydrates the seed, activates enzymes, and allows for the transport of nutrients.
* Oxygen: Oxygen is needed for respiration, which provides the energy required for seed germination.
* Temperature: Each plant species has an optimal temperature range for germination. Some seeds require warm temperatures, while others prefer colder conditions.
* Light: Some seeds require exposure to light to germinate (photoblastic seeds), while others are indifferent to light (non-photoblastic seeds).
* Other Factors: Factors like mechanical scarification (breaking the seed coat), fire, or passage through an animal's digestive system can also stimulate germination by weakening the seed coat or promoting hormone production.
The Process:
1. Imbibition: Water is absorbed by the seed, causing it to swell and soften.
2. Activation of Enzymes: Water activates enzymes within the seed, initiating metabolic processes.
3. Cell Division and Growth: The embryo starts to divide and grow, using stored food reserves.
4. Radicle Emergence: The radicle (embryonic root) emerges first, anchoring the seedling in the soil and absorbing water and nutrients.
5. Hypocotyl Growth: The hypocotyl (stem below the cotyledons) grows, lifting the cotyledons above the soil.
6. Shoot Emergence: The shoot emerges, with leaves unfolding, and begins photosynthesis.
In summary: Seed germination begins when the right combination of internal (mature embryo, food reserves, hormones) and external factors (water, oxygen, temperature, light, etc.) are present. These factors work together to activate the metabolic processes necessary for the embryo to grow and emerge from the seed.
