In botany, acortex is an outer layer of a stem or root in avascular plant, lying below theepidermis but outside of thevascular bundles.[1] The cortex is composed mostly of large thin-walledparenchyma cells of theground tissue system and shows little to no structural differentiation.[2] The outer cortical cells often acquire irregularly thickenedcell walls, and are calledcollenchyma cells.[3]
In the three dimensional structure of herbaceous stems, theepidermis, cortex andvascular cambium form concentric cylinders around the inner cylindrical core ofpith. Some of the outer cortical cells may containchloroplasts, giving them a green color. They can therefore produce simple carbohydrates through photosynthesis.[4]
In woody plants, the cortex is located between theperiderm (bark) and the vascular tissue (phloem, in particular). It is responsible for the transportation of materials into the central cylinder of the root through diffusion and may also be used for storage of food in the form ofstarch.[5]
In the roots ofvascular plants, the cortex occupies a larger portion of the organ's volume than in herbaceous stems. The loosely packed cells of the root cortex allow movement of water and oxygen in the intercellular spaces.[4]
One of the main functions of the root cortex is to serve as a storage area for reserve foods.[4] The innermost layer of the cortex in the roots of vascular plants is theendodermis. The endodermis is responsible for storing starch as well as regulating the transport of water, ions andplant hormones.[2]
On alichen, the cortex is also the surface layer or "skin" of the nonfruiting part of the body of somelichens.[6] It is the "skin", or outer layer ofthallus tissue, that covers the undifferentiated cells of themedulla.Fruticose lichens have one cortex encircling the branches, even flattened, leaf-like forms.Foliose lichens have different upper and lower cortices.Crustose,placodioid, andsquamulose lichens have an upper cortex but no lower cortex, andleprose lichens lack any cortex.[citation needed]