When in flooded soil, soybean, Glycine max, produces aerenchyma and hypertrophic stem lenticels. Shimamura et al. (pp. 277–284) investigate the oxygen dynamics in these tissues and find that hypertrophic lenticels on the stem of soybean, just above the water surface, are entry points for O2, and these connect to aerenchyma and enable O2 transport into roots in flooded soil. Stems that develop aerenchyma thus serve as ‘snorkels’ that enable O2 movement from air to the submerged roots.
S. Shimamura, R. Yamamoto, T. Nakamura, S. Shimada, S. Komatsu, 2010, 'Stem hypertrophic lenticels and secondary aerenchyma enable oxygen transport to roots of soybean in flooded soil', Annals of Botany, vol. 106, no. 2, pp. 277-284 http://dx.doi.org/10.1093/aob/mcq123