Endodermis

It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances (Casparian strip) to restrict apoplastic flow of water to the inside.

In many seedless plants, such as ferns, the endodermis is a distinct layer of cells immediately outside the vascular cylinder (stele) in roots and shoots.

Endodermis is often made visible with stains like phloroglucinol due to the phenolic and lipid nature of the Casparian strips or by the abundance of amyloplasts.

The endodermis does not allow gas bubbles to enter the xylem and helps prevent embolisms from occurring in the water column.

Experimental evidence suggests that passage cells function to allow transfer of solutes such as calcium and magnesium into the stele, in order to eventually reach the transpiration system.