Stomata are usually open

Options

A. whenever there is excess water in the soil
B. at night, when the plant requires a supply of oxygen
C. during the day, when the plant requires a supply of carbon dioxide
D. all of the above

Answer

**Step-by-Step Solution:**

1. **Understanding Stomata Function**: Stomata are small openings on the surface of leaves that allow for gas exchange. They play a crucial role in photosynthesis by allowing carbon dioxide (CO2) to enter the leaf and oxygen (O2) to exit.

2. **Turgor Pressure in Guard Cells**: The opening and closing of stomata are controlled by the turgor pressure of guard cells. When guard cells take up water, they become turgid (swollen), causing the stomata to open. Conversely, when they lose water, they become flaccid (soft), leading to the closure of the stomata.

3. **Stomatal Behavior During the Day and Night**: Generally, stomata are open during the daytime when photosynthesis occurs, as plants need a supply of carbon dioxide. At night, stomata typically close to conserve water and because photosynthesis does not take place in the absence of light.

4. **Evaluating the Options**:
- **Option A**: "Whenever there is excess water in the soil" - This is not a direct reason for stomatal opening.
- **Option B**: "At night" - This is incorrect as stomata usually close at night.
- **Option C**: "During the day when the plant requires a supply of carbon dioxide" - This is correct as stomata are usually open during the day for gas exchange.
- **Option D**: "All of the above" - This cannot be correct since not all options are true.

5. **Conclusion**: The correct answer is **Option C**: "During the day when the plant requires a supply of carbon dioxide."