Based on the experiments of Charles and Francis Darwin which part of the oat seedling detects the direction of light during phototropism?

He basically covered the tips of the sprouting seeds which would cease the growth of the sprouting seed as the receptors did not receive light, whereas the sprouting seeds uncovered continued to curve and grow towards the light. I'm not sure if he discovered it cause from what i've read it didn't result in any scientific reasoning. I hope that kinda helped answer your question:)