Discussion
The growing plant bends towards the source of light only on one side. Identify the best explanation for this phenomenon.
Answer: D
The best explanation is that auxin accumulates towards the darker side to induce greater cell elongation.
This bending is phototropism. Light falling on one side of the shoot tip causes the growth hormone auxin to move towards the shaded side. The cells on the shaded side, with more auxin, elongate faster than those on the lit side, so the shoot curves towards the light. Darwin's and Went's experiments on oat coleoptiles established this.
The illuminated side does not grow more, it grows less, so the first option has it backwards. Shoot tips are not 'attracted' to light, and the need for light in photosynthesis is the benefit of bending, not its mechanism.
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