in response to light. Thus, shoots show positive phototropism by growing towards a light source. Work with oat (Avena) coleoptiles has shown that the stimulus is received in the tip of the seedling and that the response is effected by increased growth on the relatively shaded side, triggered by *auxins
, which pass from the tip more readily on the shaded side. Removal of the tip destroys the phototropic response. Most roots are aphototropic.