It is established that auxin, indoleacetic acid, increases the rate of cell elongation in Avena coleoptile by causing the cell walls to become more easily stretched, more plastic. This was first demonstrated by Heyn (16), who showed that coleoptiles treated with auxin were more readily stretched by an applied weight than coleoptiles not so treated. Heyn's observation has been confirmed and extended by Tagawa and Bonner (33).