cell elongation
the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypothesesinvolving the entire cell in the gravitropism effecthave been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.