Which of the following statements is correct about a woody dicot stem which shows extensive secondary growth?

Options

A. Primary xylem persists in the centre of the axis.
B. Priamry and the older secondary phloem get crushed.
C. Secodnary xylem forms the bulk of the stem
D. all of these

Answer

To answer the question about which statement is correct regarding a woody dicot stem that exhibits extensive secondary growth, we can analyze each option provided:

1. **Primary xylem persists in the center of the axis.**
- In woody dicots, the primary xylem does remain in the center of the stem. This is because, during secondary growth, new layers of secondary xylem are formed outside the primary xylem, which allows the primary xylem to persist.

2. **Primary xylem and older secondary phloem get crushed.**
- As the stem undergoes secondary growth, the older secondary phloem is pushed outward and can become crushed due to the pressure from the expanding secondary xylem. This is a characteristic of extensive secondary growth in woody plants.

3. **Secondary xylem forms the bulk of the stem.**
- The secondary xylem, which is produced by the vascular cambium during secondary growth, indeed forms the bulk of the woody stem. This is what constitutes the wood of the plant.

4. **All of these.**
- Since all the previous statements are correct regarding a woody dicot stem with extensive secondary growth, this option is also correct.

Based on the analysis of the statements, the correct answer to the question is **"All of these."**

### Step-by-Step Solution:
1. **Identify the characteristics of a woody dicot stem.**
- Understand that woody dicots undergo secondary growth, which increases the girth of the stem.

2. **Analyze each statement provided in the options.**
- Check if each statement about the primary xylem, secondary phloem, and secondary xylem is accurate.

3. **Confirm the correctness of each statement.**
- Verify that the primary xylem persists, older secondary phloem gets crushed, and secondary xylem forms the bulk of the stem.

4. **Conclude that all statements are correct.**
- Since all individual statements are true, the correct answer is "All of these."