In bean seeds, the hypocotyl grows faster.
During the germination process of a bean seed, rapid growth occurs in specific parts of the embryo. The primary engine for upward movement and emergence from the soil is the hypocotyl. As stated in our reference, "In bean seeds, hypocotyl grows faster and the cotyledons are brought above ground." This vigorous elongation of the hypocotyl forms a hook shape that pushes through the soil surface. Once above ground, the hook straightens, pulling the cotyledons (seed leaves) and the developing epicotyl (shoot tip) upwards. This type of germination, where the cotyledons are lifted above the soil, is known as epigeal germination, which is common in beans.
Understanding Bean Seed Parts and Germination
To understand why the hypocotyl's rapid growth is crucial, let's look at the main parts of a bean seed embryo:
- Radicle: The embryonic root, which emerges first and anchors the seedling.
- Hypocotyl: The embryonic stem below the cotyledons. Its rapid elongation is key to epigeal germination.
- Epicotyl: The embryonic stem above the cotyledons, developing into the shoot and true leaves.
- Cotyledons: Seed leaves that store food (starch, proteins, oils) to nourish the seedling until it can perform photosynthesis.
During germination, the radicle emerges first, growing downwards into the soil. Following this, the hypocotyl begins to grow rapidly. This fast growth creates the hypocotyl hook.
The Role of the Hypocotyl Hook
The hook shape of the growing hypocotyl is a protective mechanism. It is the bend of the hook, rather than the delicate shoot tip (epicotyl) or cotyledons, that is pushed through the rough soil particles. This prevents damage to the critical growing points. Once the hook emerges into the light above the soil surface, hormonal signals trigger it to straighten. This straightening action pulls the cotyledons and the epicotyl upright, completing the emergence process.
Comparing Growth Rates
While all parts grow during germination, the hypocotyl exhibits the most significant and rapid elongation responsible for emergence in epigeal germinators like beans. The cotyledons, primarily food storage organs, generally do not grow in size; they shrink as their reserves are used. The epicotyl begins significant growth after emergence, developing the true leaves for photosynthesis.
Here's a simple comparison of the primary role during emergence:
| Part | Primary Role During Emergence | Growth Rate Relative to Hypocotyl |
|---|---|---|
| Radicle | Anchoring the seedling | Initial, but less rapid upward |
| Hypocotyl | Lifting cotyledons above ground | Fastest upward elongation |
| Epicotyl | Developing true leaves (after emergence) | Slower initially |
| Cotyledons | Providing food (decrease in size) | Do not grow, they shrink |
Practical Insight: Observing Bean Germination
You can easily observe this phenomenon by planting a bean seed in a pot. Within a few days, you'll see the radicle emerge downwards. Shortly after, a hook will appear, pushing upwards through the soil. This hook is the elongating hypocotyl. As it emerges, it will pull the two cotyledons and the folded epicotyl behind it. The hook then straightens, and the first true leaves unfold from the epicotyl between the cotyledons. This visual process clearly demonstrates the hypocotyl's rapid growth rate being essential for successful seedling emergence.
This rapid elongation ensures the seedling quickly reaches sunlight, enabling it to begin photosynthesis and become self-sufficient, no longer relying solely on the stored food in the cotyledons.
