Inter-cellular spaces in "die-back" rhizomes are not excessively low in O2 or high in CO2 or ethanol. The plants show no appreciable unbalance or deficiency of major or minor nutrients, and no accumulation of Na or heavy-metal ions to toxic levels. There is therefore neither direct nor indirect evidence of damage from anaerobiosis. "Die-back" soils are highly reducing with a high sulfide content; the symptoms can be re -produced experimentally by growing rhizomes in full water-culture with added sulfide. It seems virtually certain that "die-back" is induced by a toxic reduced inorganic ion in the substrate, but the evidence is insufficient to implicate sulfide as the immediate cause; insoluble sulfides may be acting as a reducing "reservoir" and may exert their action through another ion.