Periderm, also known as bark is the outermost layer of stems and roots of woody plants.

Secondary growth in cortical region forms secondary cortex inside and periderm outside the cortical cambium

What role does the periderms play in

The periderm is the secondary protective (dermal) tissue that replaces the epidermis during growth in thickness of stems and roots of gymnosperms and dicotyledons (i.e., secondary growth). Unlike the epidermis, the periderm is a multilayered tissue system, the bulk of which usually constitutes the cork, or phellem.

Its the Periderm.