Endodermis

The endodermis is the innermost layer of the plant root cortex, forming a single cell–thick cylinder that surrounds the vascular cylinder (the stele). It serves as a boundary between the cortex and the stele and plays a key role in controlling what reaches the vascular tissue.

Most endodermal cells develop Casparian strips, bands of wall thickening that are impregnated with lignin and suberin. These strips create a continuous barrier around the cell, restricting the passive flow of water and dissolved minerals through the cell walls (the apoplast). As a result, substances must cross the plasma membranes of endodermal cells, moving through the cytoplasm (the symplast) and undergoing selective transport before entering the stele.

Suberin often forms additional lamellae in endodermal cell walls as the tissue matures, reinforcing the barrier with age. The endodermis also contains specialized cells known as passage cells, which may have incomplete Casparian strips and allow limited apoplastic transport to the stele.

Functionally, the endodermis contributes to ion homeostasis and selective uptake by the root, helping to regulate nutrient acquisition and prevent loss of minerals back into the cortex or soil. Its development and integrity are important for efficient water uptake and for the plant’s overall mineral balance.

In variation, the presence and structure of the endodermis can differ among plant groups and root types, but its basic role as a selective barrier between cortex and stele is widespread.