The endodermis, with its associated Casparian strip, is generally believed to act as an effective barrier to the passive movement of ions from the cortex to the xylem in young roots. However, several workers have suggested that the functional integrity of the endodermis might be somewhat impaired with the emergence of branch roots from the pericycle, thus providing pathways for the mass flow of water and ions into the stele. The present work was undertaken to examine the validity of this hypothesis.Sections of lateral roots embedded in glycol methacrylate were stained and examined by fluorescence microscopy, and a general picture of the relationship between branch root development and concomitant changes in the endodermis emerged. The endodermal cells of the parent root were found to maintain a continuous, unbroken, suberized layer over the surface of a very young lateral root, but with continued elongation there is a period when formation of the Casparian strip lags behind division of endodermal cells. It appears likely that, at this stage, water and ions can enter the stele of the parent root by mass flow.