what causes auxin to be released
And the mechanism that's thought to be behind it is brilliantly simple. Cells in the root and shoot tips contain little packets of starch, called statoliths. The starch is dense so the statoliths always settle at the bottom of the cell. The position of the statoliths determines which direction auxin is released in. Tilting the plant on its side makes the statoliths roll and settle on the side of the cells, causing auxin to be released in that direction, and the cells there to elongate.