.............. firmly fix the plant to the soil.

Explanation

**Step-by-Step Solution:**

1. **Identify the Function of the Plant Part**: The question asks about the part of the plant that firmly fixes it to the soil. This indicates we need to think about the structural components of a plant.

2. **Consider the Plant Parts**: The main parts of a plant include roots, stems, leaves, flowers, and fruits. Among these, the part that is primarily responsible for anchoring the plant in the soil is the roots.

3. **Understand the Role of Roots**: Roots not only anchor the plant but also absorb water and nutrients from the soil. They grow downward into the soil, which helps stabilize the plant.

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