Annual rings are seen in temperate zone trees because -

Options

A. Xylem cell size varies with season
B. Heart wood cells alternate with sap wood cells
C. Xylem activity varies with season
D. Resin is deposited in the rings in the stem

Answer

**Step-by-Step Solution:**

1. **Understanding Annual Rings**: Annual rings, also known as growth rings, are layers of wood produced by the vascular cambium in trees. They indicate the growth of the tree over a year.

2. **Role of Vascular Cambium**: The vascular cambium is a layer of tissue in trees that is responsible for the production of secondary xylem (wood) and secondary phloem. Its activity is crucial for the formation of annual rings.

3. **Seasonal Variation**: In temperate zones, there are distinct seasons (spring, summer, autumn, and winter). The activity of the vascular cambium varies with these seasons:
- **Spring**: The vascular cambium becomes highly active due to favorable conditions (temperature, moisture), leading to the production of a large amount of secondary xylem. This results in wider xylem cells.
- **Autumn/Winter**: The activity of the vascular cambium decreases due to less favorable conditions, resulting in the production of fewer and narrower xylem cells.

4. **Formation of Annual Rings**: The contrast between the wide cells produced in spring and the narrow cells produced in autumn/winter creates visible rings in the wood. Each ring corresponds to one year of growth.

5. **Conclusion**: Therefore, annual rings are more pronounced in temperate zone trees due to the unequal activity of the vascular cambium influenced by seasonal changes, leading to variations in xylem cell size.

**Final Answer**: Annual rings are seen in temperate zone trees because the size of xylem cells varies with the seasons, resulting from the unequal activity of the vascular cambium.

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