Answer:
Auxins
Step-by-step explanation:
Phototropism is a growth response caused by unilateral light. When a plant shoot is subjected to light from one side, it forms a growth curvature towards the source of light. Shoots exhibit positive phototropism. Unidirectional light causes lateral migration of auxins from the lit side to the dark side. Higher concentration of auxins on the dark side stimulates rapid cell elongation in the stem. Hence the dark side elongates faster than the lit side resulting in a growth curvature towards the source of light.