What is the Difference Between Testa and Tegmen

The main difference between testa and tegmen is that testa is the outer seed coat whereas tegmen is the inner seed coat. Furthermore, testa is smooth, thick, impermeable, and may be colored while tegmen is a thin membranous and hyaline.

Testa and tegmen are the two seed coats found in the bitegmic seed of dicots. They form two distinct layers surrounding the embryo, providing protection.

Key Areas Covered

1. What is the Testa
     – Definition, Structure, Importance
2. What is the Tegmen
     – Definition, Structure, Importance
3. What are the Similarities Between Testa and Tegmen
     – Outline of Common Features
4. What is the Difference Between Testa and Tegmen
     – Comparison of Key Differences

Key Terms

Dicot Seed, Embryo, Integuments, Seed Coats, Tegmen, Testa

What is the Testa

Testa is one of the two seed coats of the dicot seed. It serves as the outer layer of the seed coat. The main function of the testa is to protect the embryo of the seed from mechanical damages and dehydration during the period of seed dispersal. Furthermore, the outer integument of the ovule gives rise to the testa. Generally, the outer integument is made up of four to eight layers organized into three parts: the outer epidermis, the outer pigmented zone of two to five layers containing tannin and starch, and the inner epidermis. Also, there are two layers of the testa derived from different parts of the outer integument. They are the endotesta and the exotesta. Here, the inner epidermis of the outer integument gives rise to the endotesta while the outer epidermis of the outer integument gives rise to the exotesta.

Figure 1: Seed Structure

Furthermore, testa is smooth, thick, and usually has a brownish color. Moreover, it is impermeable. The hard, protective mechanical layer formed by the testa sometimes prevent the penetration of water into the seed. Thus, it leads to the seed dormancy.

What is the Tegmen

Tegmen is the second seed coat of the dicot seed. It only occurs in bitegmic seeds of dicots. That means; some dicot seeds do not contain a tegmen and they are known as unitegmic seeds. However, tegmen is also responsible for the protection of the embryo from mechanical injury and dehydration. Furthermore, tegmen is derived from the different layers of the inner integument of the ovule.

Figure 2: Structure of the Ovule

Also, the two layers of the tegmen are the endotegmen and the exotegmen. Here, the inner epidermis of the inner integument gives rise to the endotegmen while the outer epidermis of the inner integument gives rise to the exotegmen. Moreover, the tegmen is usually whitish or hyaline in color. It is also thin and membranous.

Similarities Between Testa and Tegmen

• Testa and tegmen are the coats of the bitegmic seed of dicots.
• They form two distinct layers surrounding the embryo.
• Also, they form from the two integuments of the ovule. Here, the inner and the outer epidermis of each integument give rise to the inner and the outer layers of each seed coat, respectively.
• Furthermore, they protect the embryo of the seed from predators, mechanical damages, dehydration, etc.
• Hilum, which is the narrow elongated scar of both seed coats, represents the point of attachment of the stalk of the seed.
• Moreover, the minute pore of one end of the hilum is known as the micropyle while the narrow ridge of the opposite end is known as the raphe.

Difference Between Testa and Tegmen

Definition

Testa refers to the protective outer covering of a seed while tegmen refers to the protective inner layer of the seed. Thus, this is the main difference between testa and tegmen.

Derived from

Furthermore, testa is derived from the outer integument of the ovule while the tegmen is derived from the inner integument of the ovule.

Structure

Another difference between testa and tegmen is that testa is smooth, thick, and impermeable while tegmen is thin and membranous.

Color

Moreover, testa is brownish while tegmen is whitish or hyaline.

Layers

The two layers of the testa are the endotesta and the exotesta while two layers of the tegmen are the endotegmen and exotegmen.

Function

More importantly, testa plays a role in seed dormancy by being impermeable to water while tegmen protects the embryo from dehydration and mechanical damages. Hence, this is the functional difference between testa and tegmen.

Conclusion

Testa is the protective outer covering of the seed. Generally, it is smooth, thick, and impermeable. It also has a brownish color. Moreover, due to its impermeable nature, it plays a key role in seed dormancy. On the other hand, tegmen is the protective inner covering of the seed. Usually, it is thin and membranous. Also, it is whitish or hyaline in color. Additionally, the main function of the tegmen is to protect the embryo from dehydration and mechanical damages. Thus, the main difference between testa and tegmen is structure and function.

References:

1. Koirala, Shailesh. “Seed and Its Structure.” The Science Info, 17 July 2018, Available Here.

Image Courtesy:

1. “Dycotyledon seed diagram-en” By LadyofHats – Own work. (Public Domain) via Commons Wikimedia   
2. Ovule-Gymno-Angio-en” By derivative work: Qef (talk)Ovule-Gymno-Angio-fr.svg: Original uploader was Tameeria at en.wikipedia. Translation and vectorisation by Cehagenmerak. (Public Domain) via Commons Wikimedia