A theory, originally proposed in 1970 by R. Cleland, A. Hager, and co-workers, that describes how auxins stimulate cell expansion in certain plant tissues, notably in coleoptiles of cereals and other grasses, such as oat (Avena). It asserts that the auxins induce acidification of the immediate cell-wall environment, thereby activating enzymes called expansins that loosen load-bearing bonds within the cell wall. This permits expansion of the walls by the cell’s internal turgor pressure, and thus enlargement of the cell. It is thought that the auxin binds with auxin-binding proteins in the plasma membrane and stimulates the synthesis of proton pumps (also in the cell’s plasma membrane), which excrete protons (H⁺) into the cell wall from the cytoplasm. The consequent lowered cell-wall pH activates the wall enzymes, while the increased membrane potential enhances ion uptake by the cell, which causes water to enter the cell, thereby increasing turgor pressure. Experimental evidence broadly supports the theory.