Why do roots grow downwards?
Plant cells do not have all contents fixed in place. Mitochondria are free to migrate to locations that need energy and plastids that store starch, amyloplasts are free to settle to the lowest point in the cell. It is the settling of the amyloplast in the cells of the root cap that determines down for the growing root tip by altering growth hormone concentrations. One side of the root grows faster with respect to the other causing the root to bend with respect to gravity. Auxin inhibits cell elongation on one side while the other continues to grow producing a bend. The signal between root tip and the region of elongation is not known.
