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Tropisms are growthg responses in plants in which the direction of growth depends on the direction of the stimulus. Phototropism is the response to light. The shoots of plants are positively phototropic (grow towards the light) while the roots are negatively phototropic (grow away from light).
Experiments carried out in 1880 by Charles Darwin and his son Francis showed that the stimulus was detected by the tip of the shoot but the bending response took place just below the tip.
In 1913 the Dutch plant physiologist Boysen-Jensen showed that a chemical substance produced in the tip of the shoot passed to the bending region to cause the response and that this substance passed down the shaded side of the shoot.
F. W.Went extracted the growth substance which was identified as auxin. He showed that the phototropic response was due to the unequal distribution of auxin in the shoot. More auxin accumulates on the shaded side of the shoot and causes more rapid cell elongation on that side, so that the shoot bent over towards the light.
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