The tissue that produces cells that add to the girth of a root is called the vascular cambium.

Here's why:

* Vascular cambium is a lateral meristem, meaning it's a layer of cells that divide to produce new cells laterally, increasing the width of the root.

* It's located between the xylem and phloem, the tissues responsible for water and nutrient transport.

* The vascular cambium produces new xylem cells (towards the inside) and new phloem cells (towards the outside), contributing to the growth of the root's vascular cylinder and its overall girth.

Let me know if you have any other questions!