Which cells fill up the spaces and form ground tissue in roots, stem and leaves of plant?

Explanation

**Step-by-Step Solution:**

1. **Understanding Plant Tissues**: Plant tissues are categorized into three main types: meristematic, permanent, and ground tissues. Ground tissue is crucial as it fills the spaces between other tissues and provides support and storage.

2. **Identifying Ground Tissue**: Ground tissue is primarily composed of three types of cells: parenchyma, collenchyma, and sclerenchyma. Among these, parenchyma is the most abundant and versatile.

3. **Role of Parenchyma Cells**: Parenchyma cells are characterized by their thin cell walls and large intercellular spaces. They serve as filler tissue in various parts of the plant, including roots, stems, and leaves.

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