Final answer:
Phototropism is the bending of seedlings toward light. Darwin discovered that the signal for phototropism travels from the apical meristem to the base of the plant. The signal is a growth stimulant called indole acetic acid (IAA) which causes faster cell elongation on the shaded side, resulting in the plant bending towards the light.
Step-by-step explanation:
Phototropism is the bending of seedlings toward light. Charles Darwin discovered that light is perceived by the tip of the plant (the apical meristem), but the bending response takes place in a different part of the plant. He concluded that a signal travels from the apical meristem to the base of the plant. Later experiments showed that the signal is a growth stimulant called indole acetic acid (IAA) which causes faster cell elongation on the shaded side, resulting in the plant bending towards the light.