Phototropic curvature is the result of uneven distribution of

Darwin and his son Francis used germinating oat (Avena sativa) and canary grass (Phalaris canariensis) seedling in their experiments and hypothesized that when shoots were illuminated from one side, they bent forward the light in response to an 'influence', that was transmitted downward from its source at the tip of the shoot. Paal concluded that the tip secretes a substance, which promotes the growth of part below it. In 1926, Frits Went discovered that some unidentified compound probably caused curvature of oat coleoptile towards light, ie, phototropism. The compound (auxin) found by Went is relatively abundant in coleoptile tips.