Secondary Growth in Plants and Increase in Girth

Secondary growth is the increase in girth or thickness of the plant body in plants that exhibit secondary growth, such as woody plants. It occurs in the lateral meristems, namely the vascular cambium and cork cambium. Secondary growth contributes to the increase in girth by adding new cells to the plant's vascular tissues and outer protective layers.

1. Vascular Cambium and Secondary Xylem Formation
The vascular cambium is a lateral meristem that is responsible for the production of secondary xylem and secondary phloem. It is located between the primary xylem and primary phloem in the stem and root of the plant.

- The vascular cambium actively divides and produces new cells both towards the inner side (secondary xylem) and the outer side (secondary phloem) of the stem or root.
- The secondary xylem, also known as wood, contributes significantly to the increase in girth. As the vascular cambium produces new secondary xylem cells, the older cells towards the center of the stem or root become non-functional and provide structural support to the plant.

2. Secondary Phloem Formation
The secondary phloem is another tissue formed during secondary growth, and it is responsible for the transport of organic nutrients from the leaves to other parts of the plant.

- The vascular cambium produces new secondary phloem cells towards the outer side of the stem or root, contributing to the increase in girth.
- The older secondary phloem cells, located towards the outside of the stem, become non-functional and eventually form part of the bark.

3. Cork Cambium and Periderm Formation
The cork cambium is another lateral meristem that is responsible for the production of cork cells. It is located in the outer bark of woody stems and roots.

- The cork cambium actively divides and produces new cork cells towards the outside of the stem or root.
- As new cork cells are produced, the older cork cells on the surface of the stem or root become non-functional and are sloughed off, contributing to the increase in girth.

4. Bark Formation
The outer protective layer of the stem or root, known as the bark, is formed as a result of secondary growth.

- The bark includes the secondary phloem, cork cambium, and cork cells.
- The increase in the production of secondary phloem and cork cells by the vascular cambium and cork cambium, respectively, contributes to the overall increase in girth.

In conclusion, secondary growth in plants contributes to the increase in girth by the production of secondary xylem, secondary phloem, cork cells, and the formation of bark. These processes are facilitated by the vascular cambium and cork cambium, which actively divide and produce new cells towards the inner and outer sides of the stem or root. The increase in girth provides structural support, allows for the transport of nutrients, and forms a protective outer layer for the plant.