Rhizome is a modification of underground ............

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the nature of rhizomes and what they are a modification of.

2. **Identifying the Type of Structure**: Rhizomes are classified as a type of underground structure.

3. **Determining the Category**: In plant morphology, structures can be categorized into roots, stems, and leaves. Since rhizomes are specifically mentioned as underground structures, we need to identify whether they belong to the root or stem category.

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