The background noise was estimated to be less than 0.001 mm for the xylem spectra and less than 0.01 mm for the stem spectra. Figure 2 . Square roots of power spectra are shown. The tidal spectrum shows the influence of both the Sun and the Moon. For the xylem and stem diameters, only the diurnal frequency is shown.
Higher frequencies correspond to harmonics of the fundamental frequencies.
Diameter variations caused by such a small change due to the direct physical force are many orders of magnitude too small to be observed independently. We also considered the possibility of a biological response. Therefore gravitropic stimuli are measured in terms of g multiplied by presentation time. In plants, the most sensitive known response to g is exhibited by roots of seedlings. According to Perbal et al.