The exchange of considerable amounts of gases, particularly oxygen and carbon dioxide, between the soil and the atmosphere (in the range of several to several tens milligrams per hectare per year [cf. Chapter 1]) takes place under the influence of both the pressure gradient (mass flow) and the concentration gradient (diffusion flow). Both these kinds of flow may take place in the soil pores and also in the tissue of plants (internal exchange). The relative contribution of the two fundamental mechanisms of gas exchange, i.e., mass and diffusion flow, as well as of the two flow pathways, i.e., through the soil and through the plant, may vary within relatively broad limits; however, the diffusive flow through the soil is of primary significance for mesophytes, and through the plant for aquatic plants.