Myriophyllum heterophyllum Michx. is a rhizomatous submersed aquatic plant that produces a short, emergent floral spike. We hypothesized that lacunar pressures in emergent spikes should be at or near atmospheric pressure and that a mass flow of gases from submersed stems through the rhizome to emergent stems may occur as lacunar O2 concentrations and pressures in submersed stems increase during photosynthesis. We examined the potential for a pressure gradient ([delta]P) to develop along this pathway by measuring diurnal changes in lacunar gas composition and pressure in submersed stems of nonflowering plants and emergent stems of flowering individuals. Methane release from emergent spikes was also monitored during three diurnal cycles to evaluate the hypothesis that the [delta]P is maintained by the release of lacunar gases to the atmosphere. Lacunar O2 concentrations and pressures in submersed stems increased at sunrise and reached maximum levels by midday. Although O2 fluctuated similarly in emergent stems, lacunar pressures remained at or near atmospheric pressure, indicating that a [delta]P is generated between submersed and emergent stems during photosynthesis. Methane release from emergent spikes increased as lacunar pressures increased, indicating that rhizome gases are transported through emergent stems by mass flow and the [delta]P is maintained by venting lacunar gases from emergent spikes. The potential for mass flow in both flowering and nonflowering individuals is discussed.