What increases in plants during secondary growth?

Explanation

**Step-by-Step Solution:**

1. **Understanding Plant Growth**: Plants grow due to the activity of meristematic cells, which are specialized cells capable of division. These cells are found in specific regions of the plant known as meristems.

2. **Types of Meristems**: There are three main types of meristems in plants:
- **Apical Meristem**: Located at the tips of roots and shoots, responsible for primary growth (length).
- **Intercalary Meristem**: Found in the internodes (between nodes) of certain plants, also contributing to primary growth.
- **Lateral Meristem**: Located along the sides of the plant, responsible for secondary growth (girth).
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