Girth of stem increases due toapical meristemlateral meristemintercalary meristemvertical meristem

The girth of a stem increases due to the lateral meristem.

Here's a step-by-step explanation:

1. The lateral meristem is a type of meristematic tissue in plants that is responsible for lateral growth.

2. This growth increases the thickness of the plant, which is also known as secondary growth.

3. The lateral meristem includes two types of tissues: the vascular cambium and the cork cambium.

4. The vascular cambium produces secondary xylem (wood) and secondary phloem, which contribute to the increased girth of the stem.

5. The cork cambium produces cork, a tough protective material.

So, the lateral meristem is responsible for the increase in girth of the stem.