Auxin and acid growth.
<P>
<Li> Auxin stimulates proton pumps in the plasma membrane.
<Li> The proton pumps lower the pH in the cell wall.
<Li> The  acidity  activates  expansin  enzymes  that loosen the wall's
fabric.
<Li> Cell wall-loosening enzymes cut cross-linking polysaccharides.
<Li> With  the  cellulose  in  the  cell  wall  loosened,  the cell can
elongate under turgor pressure.