  Un sous-syst\`eme de dimension diff\'erentielle au plus 2 d'une extension
plate est plate. Si un tel syst\`eme plat est stationnaire, il admet des
sorties plates ind\'ependantes du temps.
  A subsystem of a flat system of differential dimension at most 2 is flat.
Furthermore, if such a flat system is stationary, we show that there exist flat
utputs not depending on the time.
