Convectional air flow in intercellular spaces improves the gas exchange of plant tissues. This thermo-osmosis, in combination with aerenchyma formation, is often put forward as a mechanism to improve gas flow in tissues of helophytes, mangroves and plants of river basins. Thermo-osmosis includes several phenomena which are not dependent on biological activity but are of a purely physical nature. Easiest to understand is the efflux of gas from the tissue caused by evaporation of water from those cell surfaces bordering intercellular spaces; the more water evaporates, the greater is the partial pressure of the intercellular water vapour, but the volume of that gas efflux is very small. Gas fluxes arising from differences in temperature, e.g. between the leaves floating on the water surface and the rhizomes in the anoxic bottom of the pond, are more effective, as is also gas exchange caused by the differences in molecular mass of the gases involved. Such gas fluxes are called thermo-osmosis, as they require gas-filled pores or cavities, the dimensions of which are smaller than the mean free path of the gas molecules in Brownian motion (ca. 100 nm at atmospheric pressure and 20 °C). Therefore, not only plant tissues, but also ceramic filters or other porous materials with pore diameters of similar size to the intercellular spaces of plant material are used as thermo-osmotic active layers in studies (Table).
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