...................................is the number of stomata per square mm. Of leaf suface.

Explanation

**Step-by-Step Solution:**

1. **Understanding Stomata**: Stomata are small openings on the surface of leaves that facilitate gas exchange. They allow carbon dioxide to enter the leaf for photosynthesis and oxygen to exit as a byproduct. Additionally, stomata play a crucial role in transpiration, which is the process of water vapor loss from the plant.

2. **Location of Stomata**: Stomata are primarily located on the lower epidermis (underside) of leaves, although they can also be found on the upper surface in some plant species. The distribution and density of stomata can vary depending on the plant species and environmental conditions.

3. **Measurement of Stomata**: The density of stomata is quantified as the number of stomata per square millimeter (mm²) of leaf surface. This measurement helps in understanding the plant's ability to perform gas exchange and transpiration.

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