What causes an increase in the diameter of woody dicot stems?

An increase in the diameter of woody dicot stems, also known as secondary growth, is primarily caused by the activity of two lateral meristems: the vascular cambium and the cork cambium. The vascular cambium is responsible for the production of secondary xylem towards the center of the stem, and secondary phloem towards the outside. The cork cambium produces the cork cells that make up the outer bark.

To explain the process in more detail, here are the steps involved in the increase in diameter of woody dicot stems:

1. Formation of vascular cambium: During the primary growth of a woody dicot stem, the ground meristem forms the primary xylem and primary phloem, which are responsible for water and nutrient transport. In the next step, the vascular cambium is formed in between the primary xylem and primary phloem.

2. Activity of vascular cambium: The vascular cambium is a thin layer of meristematic cells that divide and multiply. These cells produce new xylem cells towards the center of the stem and new phloem cells towards the outside. This continuous division and differentiation of cells lead to an increase in the diameter of the stem.

3. Development of secondary xylem: The new xylem cells produced by the vascular cambium on the inner side of the stem accumulate, forming a ring of secondary xylem. These cells are responsible for water and mineral transportation. As more cells are added, the diameter of the stem increases, causing the stem to thicken.

4. Development of secondary phloem: Simultaneously, the vascular cambium also produces new phloem cells towards the outer side of the stem. The secondary phloem transports sugars and other organic compounds produced in the leaves to other parts of the plant. As the secondary phloem cells are added, the stem diameter further increases.

5. Formation of cork cambium: As the stem continues to thicken, another lateral meristem called the cork cambium appears in the outermost layer of the stem, just outside the phloem. The cork cambium is responsible for the production of cork cells.

6. Development of outer bark: The cells produced by the cork cambium towards the outside differentiate into cork cells, which form a protective layer known as the outer bark. The outer bark offers protection from physical damage, pathogens, and water loss.

Overall, the combined activity of vascular cambium, which produces secondary xylem and secondary phloem, and cork cambium, which produces cork cells, leads to the increase in diameter of woody dicot stems.