Mention the most significant function of bundle sheath.

Explanation

**Step-by-Step Solution:**

1. **Definition of Bundle Sheath**: The bundle sheath is a layer of compactly arranged cells that surrounds the vascular bundles, which consist of xylem and phloem.

2. **Function of Bundle Sheath**: The most significant function of the bundle sheath is to regulate the movement of substances between the vascular tissues (xylem and phloem) and the surrounding parenchyma cells in the leaves.

3. **Protection of Vascular Tissue**: The bundle sheath also serves to protect the vascular tissues from direct exposure to air, which can be detrimental to their function.

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