Differentiate between the following pair of the terms testa and tegmen?

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Hint: 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 the mechanical damages and dehydration during the period of the seed dispersal. Tegmen is the second seed coat of the dicot seed it occurs only in bitegmic seeds dicot. Tegmen is derived from the different layers of the inner integument of the ovule.

Complete answer:
- The outermost covering of the seed is the seed coat. The seed coat has two layers: the outer testa and inner tegmen.
Tabular Column: Difference between Testa and Tegmen
StructureThe protective outer coveringThe protective inner layer of the seed
Derived structureDerived from the outer integument of the ovuleDerived from the inner integument of the ovule
ThicknessSmooth and thickThin
Made of two layers calledEndotesta and exotestaEndotegmen and exotegmen
FunctionPlays a role during seed dormancy by not allowing water to enter and protect the seed.Protects the embryo from dehydration and mechanical damage

Figure 1: Diagram showing different parts of a seed

Additional information:
The hilum is a scar on the seed coat through which the developing seeds were attached to the fruits. Above the hilum is the small pores called the micropyle. Within the seed coat is the embryo consisting of the embryo axis and the two cotyledons. The cotyledons are fresh and are full of reserve food materials. At the two ends of the embryo, the axis is present to the radicle and the plumule. In testa after fertilization, the outer integument of the ovule develops into the testa. In tegmen the nucleus of the ovule is covered and protected by one or more membranes called tegmen.

In some seeds such as castors the endosperm formed as the result of the double fertilization is the food storing tissue. In plants such as bean gram and the pea, the endosperm is not present in the mature seeds and such seeds are called non-endospermous. Testa is the outermost covering of the seed which protects the seeds from the external injury and bacterial infection whereas, tegmen is the covering of the seed that lies just under the testa.