Roots of Typha latifolia L. exposed to Fe 2 + under reduced conditions in solution culture developed visible coatings (plaques) of an oxidized Fe compound that extended as much as 15-17 /tm into the rhizosphere. Iron concentrations were significantly less and discoloration was not apparent on the surface of roots exposed to Fe-(BPDSh, Fe>, Fe-EDDHA, and Fe-EDT A. The extent of plaque formation increased with the concentration of Fe 2 + in solution and with pH of the solution in the range of 3.0 to 4.6. Above pH 4.6, oxidation of Fe 2 + in the culture solution may have reduced precipitation of Fe on the root surface. Plaque development was most extensive approximately 1.0 cm from the root tip, but all root surfaces showed some Fe staining. Scanning electron micrographs of plaqued roots, grown both in solution culture and in the field, provided support for a model of cast formation by oxidation and precipitation of Fe on external cell surfaces. 1

Roots of Typha latifolia L. exposed to Fe2+ under reduced conditions in solution culture developed visible coatings (plaques) of an oxidized Fe compound that extended as much as 15‐17 μm into the rhizosphere. Iron concentrations were significantly less and discoloration was not apparent on the surface of roots exposed to Fe‐(BPDS)3, Fe3+, Fe‐EDDHA, and Fe‐EDTA. The extent of plaque formation increased with the concentration of Fe2+ in solution and with pH of the solution in the range of 3.0 to 4.6. Above pH 4.6, oxidation of Fe2+ in the culture solution may have reduced precipitation of Fe on the root surface. Plaque development was most extensive approximately 1.0 cm from the root tip, but all root surfaces showed some Fe staining. Scanning electron micrographs of plaqued roots, grown both in solution culture and in the field, provided support for a model of cast formation by oxidation and precipitation of Fe on external cell surfaces.