Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots contained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsiflorus Fingerh., which has little root porosity, was much more affected. Ethylene had an even stronger effect on root elongation than hypoxia, since very small concentrations (0.1 cm3 m−3) reduced root extension in the two species, and higher concentrations inhibited elongation more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growth. An exception may be the highly aerenchymatous, adventitious roots of R. palustris. Aerenchyma in these roots provides a low-resistance diffusion pathway for both endogenously produced ethylene and shoot-derived oxygen. This paper shows that extension by roots of R. palustris in flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.