For instance, the regulation of polar auxin transport is central in a process such as gravitropism. This process, which ensures that the root grows downwards, relies on the redistribution of auxin by the columella cells (the cells located at the very tip of the root). These cells respond to gravity by special organelles, the statoliths, that redistribute auxin from the vasculature to the root epidermis and the lateral root cap. These tissues (which form the external cell layers of the root) transport auxin back to the elongation zone where it regulates cell elongation. When the gravitational gradient is not aligned with the axis of the columella cells (because the root is not vertical), the PIN proteins move to the side of the cell membrane that is gravitationally lowest. This causes more auxin to flow to the lower side of the root. Once in the elongation zone, the extra auxin inhibits cell elongation and cause the root to re-orient downwards.