This is the zone that lies in between epidermis and pericycle, and is made up of primary tissues. It varies in thickness from a few layers to many layers.
In monocot stems, owing to the presence of scattered vascular bundles, the cortex is undifferentiated and the zone is predominantly parenchymatous. In roots the cortex consists of many layers of thin-walled parenchyma and a distinct endodermis.
In the leaves the ground tissue, consisting of thin-walled chlorenchyma is called mesophyll. In the dicotyledon stems the cortex is usually differentiated into:
(a) Hypodermis: or outer cortex (b) General cortex and (c) endodermis
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Hypodermis lies just below the epidermis (absent in roots) constituted of few layers of continuous or discontinuous band of collenchymas (as in dicot stems) or sclerenchyma (as in monocot stems). The general cortex or cortical parenchyma is a few layered thin-walled parenchymatous zones with or without chloroplasts.
The cortex is composed of thin-walled, uniform, round and spherical parenchymatous cells. The cells have copious intercellular spaces but the angular parenchymatous cells are without interring cellular space. In some cases chlorenchyma or parenchyma may be differentiated from these cells to help in the adaptation of the plant.
The endodermis is the innermost layer, often wavy and is single layered. It often contains numerous starch grains. In a transverse section it appears barrel shaped without inter cellular spaces.
The cells show deposition of waxy materials- Suberin. On their radial walls, these appear in the form of strips or bands which are called casparian strips. In roots, some thin-walled, rounded cells are are seen in endodermis in the proximity of protoxylem which area known as passage cells.
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The passage cells allow flow of water from cortex to xylem but prevent the back flow of water due to the thickening of radial walls and outer walls.
Function: The cortex has several primary and secondary functions. In the stem it basically serves as a protective zone. Besides, the cortex in stem also store food. In roots it mainly stores food and also helps in absorption and translocation of water.