The region lying between epidermis and pericycle is called cortex. It can be distinguished into hypodermis, general cortex and endodermis in dicots.
Hypodermis is the layer of cells just beneath the epidermis. It is common in stems, but not found in roots. Hypodermis is a narrow band of either sclerenchymatous cells as in monocot stems or a continuous or discontinuous band of collenchymatous cells as in dicot stems. Hypodermis provides mechanical strength against radial pressure for above ground parts of the plant. As roots do not experience radial pressure and the cells of hypodermis are thick walled, roots do not possess hypodermal zone. Its thick walls will prevent the inflow of water.
Exodermis is a layer of few cells thick found in roots at the position of hypodermis. The cells are thick walled and this layer helps in protection.
General cortex or cortex (in roots) is a parenchymatous zone. Cells are loosely arranged with intercellular spaces. Cortex is narrow in stems but very broad in roots and acts as storage zone of water and food materials in roots. Resin ducts, oil ducts, laticiferous cells and laticiferous ducts, etc. may be seen in the general cortex. In some plants the outer cortical layers are photosynthetic due to presence of chloroplast in the cells.
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In monocot stems, except hypodermis, cortex is not distinguished with general cortex and endodermis. The whole mass of tissues is called ground tissue.
The primary function of cortex is protection. However, it acts as a storage region in roots.
Endodermis is the inner most layer of cortex. Usually it is a single layer of compactly arranged cells. According to Eames and Mac Daniels (1947) endodermis “is a uniseriate sheet of cells separating the stele from the cortex, without intercellular spaces and with structural features unlike those of other cells”. Endodermis is present in most of the roots, while in stems it is present in pteridophytes and in a number of angiosperms; rhizomes and leaves of gymnosperms. Endodermal cells are elliptical or barrel or oval shaped. The radial and inner walls of endodermal cells are thick and the outer walls are thin.
Sometimes, the thickening of the radial walls is in the form of a strip called Casparian Strip (after Caspary, 1865). The thickness is believed to be due to lignin or suberin deposit. In roots, some thin walled rounded cells are found in the endodermis at points very close to the protoxylem points of the vascular region. These cells are called passage cells. While passage cells allow water flow from cortex to the xylem, the thickening of the radial wall and outer walls prevent the back flow of water from the inner zone to cortex and act as a water tight jacket. In some plants, the endodermal cells contain starch and hence known as starch sheath.
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The function of the endodermis is to control the movement of water, regulate root pressure and to protect the inner tissues of the organs.