The dyes safranin and basic fuchsine as well as ferric salts enter the stele when applied as dilute solutions to intact root parts where Casparian strips are well developed. As the entrance is not hampered when the protoplasm has been killed or its activity and permeability diminished by low temperature the most likely way of entrance is through the cell walls. In the Casparian strips no fatty substances could be detected. As soon as traces of fatty substances are found in the ageing exodermis all entrance into cortex and stele of the substances used is blocked.