The hypocotyl is the stem of a germinating seedling found below the cotyledons (seed leaves) and above the radicle (embryonic root).
Essentially, the hypocotyl is the transitional part of a plant embryo that connects the root to the shoot. It's a vital structure in seedling development, particularly in epigeal germination where the cotyledons are carried above the ground.
Role of the Hypocotyl
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Elongation and Emergence: The hypocotyl elongates to push the seedling above the soil surface, exposing the cotyledons to sunlight for photosynthesis (in epigeal germination).
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Support: It provides initial support to the developing seedling.
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Transport: It acts as a conduit for transporting nutrients and water from the root (radicle) to the cotyledons and developing leaves.
Epigeal vs. Hypogeal Germination
The behavior of the hypocotyl differs in two main types of germination:
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Epigeal Germination: In this type, the hypocotyl elongates significantly, pulling the cotyledons above the soil surface. Examples include beans and sunflowers.
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Hypogeal Germination: Here, the hypocotyl remains short, and the cotyledons stay underground. Instead, the epicotyl (the stem above the cotyledons) elongates. Examples include peas and corn.
| Feature | Epigeal Germination | Hypogeal Germination |
|---|---|---|
| Hypocotyl | Elongates significantly, pulling cotyledons upwards | Remains short; cotyledons stay underground |
| Cotyledons | Carried above ground | Remain underground |
| Epicotyl | Elongates after cotyledons emerge (if applicable) | Elongates, forming the main stem above ground |
| Examples | Beans, sunflowers, pumpkins | Peas, corn, walnuts |
Factors Affecting Hypocotyl Growth
Several factors can influence hypocotyl elongation, including:
- Light: Darkness often stimulates hypocotyl elongation (etiolation) as the seedling searches for light. Once exposed to light, elongation slows down.
- Temperature: Optimal temperature ranges promote healthy hypocotyl development.
- Water availability: Adequate moisture is necessary for germination and hypocotyl growth.
- Plant Hormones: Plant hormones like gibberellins play a key role in regulating hypocotyl elongation.
The hypocotyl is therefore a critical structure for the successful establishment of a seedling, ensuring its access to sunlight and nutrients.
