The cell type most commonly found in the cortex of stems and roots is parenchyma.

Here's why:

* Parenchyma cells are the basic, undifferentiated cells of plants. They are thin-walled and have a large central vacuole.

* Cortex is the region of tissue between the vascular tissue (xylem and phloem) and the epidermis (outer layer) in stems and roots.

* Parenchyma cells in the cortex are involved in:

* Storage: They store food (like starch) and water.

* Photosynthesis: In stems, cortical parenchyma can contain chloroplasts and participate in photosynthesis.

* Gas exchange: They can help facilitate gas exchange between the vascular tissue and the environment.

While other cell types might be present in smaller amounts, parenchyma is the dominant cell type in the cortex.