The spatial display of stem and leaves is crucial for the functioning of plants and their mutual interactions. It involves complex movements : stem change curvature and orientation to grow vertically (gravitropism) and these movements are continuously regulated by the perception of the current shape (autotropism). If a lot is known about the Physiology, very few studies have dealt with the kinematics and dynamics of these movements, and the corresponding relevant physical and biological aspects. In this work we focused on the reaction of a wheat coleoptiles using a novel high spatio-temporal kinematical method called "Rod PIV" combining Particle Image Velocimetry and contour and median line detection of the organ : When its orientation vs the gravity was modified by tilting, the organ bent through local variation of curvature due to differential growth, and displayed a complex spatio-temporal pattern including oscillatory and propagative aspects. Moreover elongative and differential growth displayed coupled propagative oscillation. The insights of these results in terms of the modelling of the biomechanical and regulatory processes of tropic bending are discussed as well as the possible adaptive significance of these coupled oscillators for the gravitropic performance.