The influence of stem lacunar structure on the potential of diffusion and mass flow to meet estimated root O2 demands was evaluated and compared in four submersed aquatic plant species. Internodal lacunae formed large continuous gas canals which were constricted at the nodes by thin, perforated diaphragms. Gas transport studies showed that nodes had little effect on diffusion, but significantly reduced mass flow. Measured diffusive resistances approximated those predicted by Fick's first law, ranged from 203 to 5107 × 108 s m−4 and increased as lacunar area decreased in Potamogeton praelongus, two Myriophyllum species and Elodea canadensis. Our analysis suggested that diffusion could satisfy estimated root O2 demands given the development of relatively steep O2 gradients (0.15–0.35 mol O2 mor−1 per 0.5 m stem) between shoots and roots. Plants with high resistances (e.g. > 750 × 108 s m−4) and long lacunar pathlengths may be unable, even during active photosynthesis, to support the O2 demands of a large root system by diffusion alone. Measured nodal resistances to mass flow approximated those predicted by Hagen-Poiseuille law and ranged from 46 to 2029 × 108 Pa s m−3. Our analysis suggested that these resistances were quite low and that relatively small pressure differentials (< 150 Pa per 0.5 m stem) could drive mass flow at rates which would support root O2 demands. Possible mechanisms whereby plant architecture may serve to maintain these pressure differentials are proposed.