During seed germination, the part of the embryo that grows into the root is called the radicle. The radicle is the embryonic root of the plant, and it is the first structure to emerge from the seed during germination. It is responsible for anchoring the plant in the soil and absorbing water and nutrients from the soil.

Option 1: Radicle - This is the correct answer. The radicle develops into the primary root of the plant.

Option 2: Plumule - The plumule is the part of the embryo that gives rise to the shoot system, including the stem and leaves. It is not involved in root formation.

Option 3: Cotyledon - Cotyledons are seed leaves that provide nutrients to the developing embryo. They do not directly grow into the root.

Option 4: Epicotyl - The epicotyl is the portion of the embryo between the cotyledons and the plumule. It develops into the upper part of the stem and leaves, not the root.

In conclusion, during seed germination, the radicle is the part of the embryo that grows into the root.