A cortex is an outer layer of a stem or root in a plant, lying below the epidermis but outside of the vascular bundles. It is also the surface layer or "skin" of the nonfruiting part of the body of some lichens.
The cortex is composed mostly of large thin-walled parenchyma cells of the ground tissue system and shows little to no structural differentiation. The outer cortical cells often acquire irregularly thickened cell walls, and are called collenchyma cells. Some of the outer cortical cells may contain chloroplasts. The cortex forms layers of cells that constitute cork. It is responsible for the transportation of materials into the central cylinder of the root through diffusion and may also be used for food storage in the form of starch. The innermost layer of the cortex is the endodermis.
On a lichen, the cortex is the "skin", or outer layer of thallus tissue that covers the undifferentiated cells of the medulla. Fruticose lichens have one cortex encircling the branches, even flattened, leaf-like forms; foliose lichens have different upper and lower cortices; crustose, placodioid, and squamulose lichens have an upper cortex but no lower cortex; and leprose lichens lack any cortex.