Secondary growth involves activity of the ....... meristem.

Explanation

**Step-by-Step Solution:**

1. **Understanding Secondary Growth**: Secondary growth refers to the increase in thickness or girth of a plant, primarily due to the activity of certain meristems.

2. **Identifying the Type of Meristem**: The type of meristem responsible for secondary growth is known as lateral meristem. This is in contrast to primary meristems, which are responsible for the elongation of the plant.

3. **Types of Lateral Meristem**: Lateral meristems are classified into two main types:
- **Vascular Cambium**: This is responsible for the formation of secondary xylem (wood) and secondary phloem (part of the bark).
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