Explanation:
The correct answer is D: lateral meristem. Here's why:
Lateral meristem:
- Lateral meristem is responsible for the increase in the girth or width of the stem.
- It is located in the cambium region of the stem and is responsible for secondary growth.
- The lateral meristem produces new cells that contribute to the increase in width of the stem.
- The two types of lateral meristem are vascular cambium and cork cambium.
Apical meristem:
- Apical meristem is responsible for the primary growth of the stem, which includes the increase in length.
- It is located at the tips of the stem and the roots.
- The apical meristem produces new cells that contribute to the increase in length of the stem.
Intercalary meristem:
- Intercalary meristem is found in certain plant parts, such as the nodes of grasses.
- It is responsible for the growth and regeneration of these specific plant parts.
- It does not play a significant role in the overall increase in the width of the stem.
Primary meristem:
- Primary meristem is responsible for the primary growth of the plant, including the development of the basic plant organs.
- It is located at the root and shoot tips.
- The primary meristem helps in the increase in length of the stem, but not in the width.
In conclusion, the correct option is D: lateral meristem, as it is responsible for the increase in the width or girth of the stem.

The Size of the Stem Increases in Width due to Lateral Meristem

Lateral meristem, also known as the cambium, is responsible for the increase in girth or width of the stem. It is a type of meristematic tissue that is located in the cylindrical region of the stem and root called the vascular cambium. The vascular cambium is a thin layer of cells that lies between the xylem and phloem tissues in the stem and root.

Functions of Lateral Meristem:
Lateral meristem is responsible for two important functions in plants:
1. Secondary Growth: Lateral meristem is responsible for the secondary growth of the stem. While the apical meristem is responsible for the primary growth (lengthening) of the stem, the lateral meristem is responsible for the secondary growth (thickening) of the stem. It adds additional layers of cells to the stem, increasing its girth or width.
2. Production of Secondary Tissues: Lateral meristem produces secondary tissues, namely secondary xylem and secondary phloem. Secondary xylem, also known as wood, is responsible for the structural support of the stem and transport of water and minerals. Secondary phloem is responsible for the transport of organic nutrients, such as sugars, from the leaves to other parts of the plant.

Process of Secondary Growth:
The process of secondary growth occurs in the stem and root of woody plants, such as trees and shrubs. It is a continuous process that takes place throughout the life of the plant.
1. Vascular Cambium Activity: The vascular cambium, a type of lateral meristem, is responsible for the production of secondary xylem and secondary phloem. It is a thin layer of cells that divides and differentiates into two new cells: one cell becomes part of the secondary xylem and the other becomes part of the secondary phloem.
2. Formation of Secondary Xylem: The cells produced by the vascular cambium towards the center of the stem differentiate into secondary xylem. These cells are dead and provide structural support to the stem.
3. Formation of Secondary Phloem: The cells produced by the vascular cambium towards the outer side of the stem differentiate into secondary phloem. These cells are alive and responsible for the transport of organic nutrients.
4. Increase in Stem Girth: The continuous activity of the vascular cambium adds new layers of secondary xylem and secondary phloem, leading to an increase in the girth or width of the stem.

Therefore, the correct answer is option 'D' - lateral meristem is responsible for the increase in width or girth of the stem.