Upper left panel: mean granule horizontal velocities (in one direction) over the cells versus the granule sizes. The dashed line indicates the law considered in this paper. Upper right panel: same for the vertical velocity . Lower left panel: dispersion of the mean granule horizontal velocities (in one direction) over the cells in each size bin. The dashed line indicates the law considered in this paper. Lower right panel: same for the vertical velocity . The justification for the chosen law is described in Appendix A.