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This thesis describes an investigation into the aeration pathways, resistances to gas-movement, mechanisms of internal aeration and the locations and quantities of oxygen efflux from the underground parts of the common reed Phragmites australis. The "ventilating pressure concept" was also tested and reappraised. Well-developed interconnecting gas-spaces within the culm, rhizome and roots offered relatively small resistance to either diffusive or convective gas transport, and the porosity of root-rhizome junctions was unusually high. Radial channels located at the nodes proved to be the only connecting points between cortex and pith in culms and rhizomes. Rhizome and old adventitious root surfaces were impermeable to oxygen but the young parts of adventitious roots and the numerous laterals readily released oxygen to anaerobic agar media and soil, markedly raising the redox potential of the rhizosphere. It was discovered that callus readily forms in Phragmites in response to wounding and senescence, blocking the gas-spaces of root-rhizome junctions, leaf-sheath-culm junctions, and rhizome nodal diaphragms. However, the culm-rhizome junctions normally remain callus-free despite senescence of the culms, and gaseous connexion between the underground parts and the atmosphere is thus maintained thoughout the year. A major discovery was that pressurised gas-flows are an important feature in Phragmites' aeration: Venturi- and/or Humidity-induced convections produced much higher rhizome oxygen concentrations and radial oxygen loss from the roots than when rhizome aeration was chiefly diffusive. Both experiments and mathematical modelling demonstrated that comparatively slow rates of convection are sufficient to achieve this. The humidity-induced convection, the first reported in a grass, was shown to be initiated chiefly in living leaf sheaths, the convected gases being transmitted via gas-spaces in the culm to those of the underground rhizome, and vented via old broken culms. The flows are particularly rapid at low atmospheric humidities and increase with increasing PAR. The major mechanism promoting the convection appears to be a humidity- induced diffusion or transitional Knudsen diffusion of atmospheric gases into the plant, the concentration gradient being maintained by the difference in humidity between the interior of the plant and the outer air. The process was mimicked and further investigated using Nuclepore membranes providing important insights into the mechanism and its modelling, and it was shown that pore diameters within the Knudsen regime are not essential to produce the static pressure differentials and convective flows found in Phragmites. The Venturi-induced convection, the first reported example in a plant, is created by the action of winds blowing across tall, dead, broken culms; air is drawn into the rhizome system via short broken culms in more sheltered positions. Mathematical models and polarographic measurements of radial oxygen loss from roots were used to estimate the quantities of oxygen which might be released by Phragmites in the root-zone process of sewage treatment. It was concluded that 5 to 12 g 02 m⁻² day⁻¹ would be a conservative estimate but that the amount could be greater or less depending upon root numbers and their physiological condition, as well as upon soil oxygen demand and diffusivities.