The formation of distinct annual rings in stem mainly depends upon

Options

A. Formation of cork cambia
B. Contrasting seasonal variations
C. Uniform climatic conditions
D. Formation of unequal phloem and xylem

Answer

To solve the question regarding the formation of distinct annual rings in the stem, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Structure of the Stem**:
- The stem of a plant consists of various tissues, including vascular tissues (xylem and phloem) and cambium, which plays a crucial role in secondary growth.

2. **Identifying the Cambium**:
- In dicotyledonous plants, there are two types of cambium: intrafascicular cambium (primary meristem) and interfascicular cambium (secondary meristem). Together, they form a continuous ring known as the vascular cambium.

3. **Role of Vascular Cambium**:
- The vascular cambium is responsible for the production of secondary xylem (wood) and secondary phloem. Its activity is influenced by environmental factors, particularly seasonal changes.

4. **Formation of Wood Types**:
- During the growing season (spring), the cambium produces larger, lighter-colored xylem cells, known as spring wood. In contrast, during the late growing season (autumn), it produces smaller, darker xylem cells, known as autumn wood.

5. **Annual Rings Formation**:
- The alternating layers of spring wood and autumn wood contribute to the formation of distinct annual rings. Each complete cycle of spring and autumn wood represents one year of growth, thus forming an annual ring.

6. **Conclusion**:
- The distinct annual rings in the stem mainly depend on **contrasting seasonal variations**. This is because the differences in climate and environmental conditions throughout the year affect the growth rate and characteristics of the wood produced.

### Final Answer:
The formation of distinct annual rings in the stem mainly depends on **contrasting seasonal variations**.

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