MARTIN SHIELDS / SCIENCE PHOTO LIBRARY MARTIN SHIELDS / SCIENCE PHOTO LIBRARY
Phototropism is a directional plant response to light. Plant shoots (stems) exhibit positive phototropism by growing towards a light source. This sunflower seedling, Helianthus annuus, is bending towards the sun because auxin hormones released into the far (dark) side of the stem cause elongation on that side.
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