In a five year-old stem the oldest layer of secondary xylem is situated just outside the primary xylem.

Answer

To solve the question regarding the arrangement of secondary xylem in a five-year-old stem, we can break down the explanation step by step:

### Step-by-Step Solution:

1. **Understanding Secondary Growth**:
- Secondary growth refers to the increase in thickness or girth of the plant stem and roots, primarily due to the activity of the vascular cambium and cork cambium. This growth occurs in dicotyledonous plants.

2. **Identifying the Structure of the Stem**:
- In a typical dicot stem, the internal structure consists of various layers. At the center, there is pith, followed by primary xylem, secondary xylem, cambium, secondary phloem, and finally primary phloem.

3. **Position of Primary and Secondary Xylem**:
- The primary xylem is formed during the initial growth of the plant and is located closer to the center of the stem. As the plant ages and undergoes secondary growth, new layers of secondary xylem are formed outside the primary xylem.

4. **Locating the Oldest Layer of Secondary Xylem**:
- In a five-year-old stem, the oldest layer of secondary xylem will be the one that was formed first during the secondary growth process. This layer is situated just outside the primary xylem.

5. **Conclusion**:
- Therefore, the statement that "in a five-year-old stem, the oldest layer of secondary xylem is situated just outside the primary xylem" is true.