secondary xylem is

Options

A. endarch
B. exarch
C. mesarch
D. none of the above.

Explanation

### Step-by-Step Solution

1. **Understanding Secondary Xylem**:
- Secondary xylem is a type of xylem that forms after the primary xylem during the secondary growth of plants. It is produced by the vascular cambium, which is a type of lateral meristem.

2. **Distinguishing Between Primary and Secondary Xylem**:
- Primary xylem is formed during primary growth from the procambium, which is located at the apical meristem. It includes types such as endarch, exarch, and mesarch.
- Secondary xylem, on the other hand, does not fall into these categories as it develops later in the plant's life cycle.

3. **Characteristics of Secondary Xylem**:
- Secondary xylem is composed of larger vessels and tracheids, which are essential for water conduction in plants.
- The cell walls of secondary xylem are typically thickened due to lignin deposition, which provides mechanical support to the plant.

4. **Conclusion**:
- Given the options provided in the question, none of the terms (endarch, exarch, mesarch) apply to secondary xylem. Therefore, the correct answer is "none of the above."