[en] General equations of motion are derived for a thin untwisted magnetic flux tube embedded in a non-magnetic compressible fluid, in the presence of gravity. Special equations governing small perturbations in purely horizontal or vertical flux tubes are derived from these. There is in general a longitudinal and a transversal wave mode. The transversal wave in a vertical tube is studied in detail. This wave propagates under the combined influences of buoyancy and magnetic tension along the tube. In an exponential atmosphere it has a cut off frequency which is at least twice as low as the acoustic cut off frequency. It can transport a significant amount of energy from the convection zone to the chromosphere. The transversal motions seen in the magnetic field of the chromosphere are interpreted as transversal tube waves generated in the convection zone. (orig.)