Oxygen Exchange
Emergent and floating plants obtain oxygen directly from the atmosphere through stomata on the leaves.
Passive diffusion of oxygen along a concentration gradient
Reverse flow due to concentration gradient of CO2 and CH4
Figure from Brix, H. 1993. Macrophyte-Mediated Oxygen Transfer in Weltands: Transport Mechanisms and Rates in Constructed Wetlands for Water Quality Improvement, Moshiri, ed. p. 393.