Uptake and binding of uranyl ions by barley roots.

After undergoing the processing for electron microscopy, bound uranyl ions are revealed as characteristic electron-opaque crystals. Meristematic walls and associated vesicles become heavily labeled, while pinocytotic accumulation into vacuoles seems probable in cap cells and those just behind the meristem. The endodermal Casparian strip and suberinized lamella are effective barriers to the passage of uranyl ions.