Precipitation of ferrous ions by ferricyanide in transpiring leaves ofThemeda triandra Forsk. produced crystalline deposits, which were visible with the light and electron microscope. Prussian blue crystals were formed within the lumina of the tracheary elements and the apoplast, or cell wall continuum of the vascular tissues and bundle-sheath cells. Little if any deposition was noted within the lignified secondary thickenings of the tracheary elements. The localization pattern suggests that the ferrous ions moved from the lumina of the tracheary elements via the exposed primary walls. Prussian blue crystals were abundant in the outer tangential and radial walls of the bundle-sheath cells. By contrast, crystals were lacking in the walls of neighbouring mesophyll cells, suggesting that the suberin lamella in the bundle-sheath walls effectively inhibited the apoplastic movement of ferrous ions and possibly may impede, or restrict the movement of water across the bundle-sheath/mesophyll interface.