Assertion :- Vascular cambium activity is not uniform throughout the year in temperate regions
Reason :- Vascular cambium activity is under the control of many physiological and environmental factors.

Options

A. If both Assertion & Reason are True & Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but the Reason is a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Vascular cambium activity is not uniform throughout the year in temperate regions." This means that the growth and activity of the vascular cambium, which is responsible for secondary growth in plants, varies with the seasons.

2. **Understanding the Reason**: The reason provided is that "Vascular cambium activity is under the control of many physiological and environmental factors." This indicates that various factors such as temperature, light, and moisture influence the activity of the vascular cambium.

3. **Linking Assertion and Reason**: Both the assertion and reason are true. The activity of the vascular cambium does indeed vary with the seasons, particularly in temperate regions where there are distinct seasonal changes.

4. **Seasonal Activity of Vascular Cambium**:
- In **spring**, the vascular cambium is highly active due to favorable conditions (warm temperatures, adequate moisture), leading to the production of early wood (spring wood) characterized by wider xylem vessels.
- In **autumn**, the activity of the vascular cambium decreases as conditions become less favorable (cooler temperatures, reduced moisture), resulting in the formation of late wood (autumn wood) with narrower xylem vessels.

5. **Conclusion**: Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that the correct option is A.