The primary difference between testa and tegmen lies in their position and function as seed coverings; the testa is the outer protective layer, while the tegmen is the inner layer.
Detailed Comparison
| Feature | Testa | Tegmen |
|---|---|---|
| Position | Outermost covering of the seed. | Inner covering of the seed, directly under the testa. |
| Function | Protects the seed from external injury and bacterial infection. | Provides additional protection to the seed. |
| Description | Usually thick and hard. | Typically thin and membranous. |
Key Differences Explained
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Testa (Seed Coat): The testa, derived from the integuments of the ovule, is the outer covering of the seed. It is often hard and robust to provide a first line of defense against physical damage, harsh environmental conditions, and microbial attacks. Think of it as the seed's armor.
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Tegmen (Inner Seed Coat): The tegmen is the inner layer of the seed coat, located directly beneath the testa. It originates from the inner integument of the ovule. While also offering some protection, its role is more supplementary to the testa. It is typically much thinner and more delicate compared to the testa.
Practical Insights
- Protection: The testa's strength is crucial for seed survival, especially during dispersal. It protects the delicate embryo inside from the elements.
- Barrier: The testa acts as a barrier to water loss, which is critical for seed longevity in dry conditions.
- Germination: The testa often has to be broken or weakened for germination to occur. Certain seeds have very thick testas that require scarification (a process of abrasion or chemical treatment) before germination can commence.
- Development: Both the testa and tegmen develop alongside the seed, providing layers of protection throughout the seed development process.
Example
- Consider a bean seed; the hard outer shell you see is the testa. Inside, right beneath this shell is a thin, papery layer, which represents the tegmen.
In summary, the testa and tegmen are both protective layers of a seed, but they differ in their position, thickness, and primary function, with the testa being the outer, stronger protector and the tegmen being the inner, thinner layer providing supplementary protection.
