In a mature dicot stem which has undergone seconday growth, youngest layer of secondary xylem is situated

Options

A. in between pith and primary xylem
B. just outside the vascular cambium
C. just inner to the vascular cambium
D. just inner to the phellogen.

Explanation

To answer the question regarding the position of the youngest layer of secondary xylem in a mature dicot stem that has undergone secondary growth, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Secondary Growth**: In dicot plants, secondary growth occurs due to the activity of the vascular cambium, which produces secondary xylem (wood) and secondary phloem.

2. **Identifying the Layers**: The vascular cambium is a layer of meristematic tissue that is responsible for the production of secondary xylem and secondary phloem. The secondary xylem is produced towards the inside of the cambium, while the secondary phloem is produced towards the outside.

3. **Position of Youngest Secondary Xylem**: The youngest layer of secondary xylem is formed closest to the vascular cambium. As the cambium continues to divide and produce new xylem, the newest layers are always found just inside the vascular cambium.

4. **Conclusion**: Therefore, in a mature dicot stem that has undergone secondary growth, the youngest layer of secondary xylem is situated just inside the vascular cambium.

### Final Answer:
The youngest layer of secondary xylem in a mature dicot stem that has undergone secondary growth is situated just inside the vascular cambium.