Introduction:
Dicot stems are characterized by the presence of vascular bundles, which consist of xylem and phloem tissues. The secondary growth in dicot stems is responsible for the increase in girth and is facilitated by the activity of vascular cambium. The vascular cambium is a lateral meristem that gives rise to secondary xylem and phloem.

Explanation:
The production of secondary xylem and phloem in dicot stems is primarily carried out by the vascular cambium. Let's understand the process in detail:

1. Vascular Cambium:
The vascular cambium is a thin layer of meristematic tissue located between the primary xylem and phloem in the vascular bundles of dicot stems. It is derived from the procambium, which is the primary meristematic tissue that gives rise to the primary vascular tissues.

2. Secondary Growth:
The vascular cambium is responsible for the secondary growth of dicot stems, which leads to an increase in the diameter or girth. It accomplishes this by producing new cells both towards the inner and outer sides of the stem.

3. Formation of Secondary Xylem:
The vascular cambium divides to produce new cells towards the inner side of the stem. These cells differentiate into secondary xylem, also known as wood. The secondary xylem cells are responsible for the conduction of water and minerals from the roots to the leaves.

4. Formation of Secondary Phloem:
The vascular cambium also divides to produce new cells towards the outer side of the stem. These cells differentiate into secondary phloem, which is responsible for the conduction of organic materials, such as sugars, from the leaves to other parts of the plant.

5. Phellogen:
The phellogen, also known as cork cambium, is a secondary lateral meristem that arises from the outer layers of the cortex or the epidermis. It produces the outer protective layer of the stem, called cork.

Conclusion:
In summary, the vascular cambium in dicot stems is responsible for the production of secondary xylem and phloem. The secondary xylem forms towards the inner side of the stem, while the secondary phloem forms towards the outer side. This secondary growth leads to an increase in the girth of the stem and is essential for the transport of water, minerals, and organic materials throughout the plant.