Sunflower seedlings don't simply grow toward the brightest spot. They convert light from each side of the stem into a signal, then add those signals up as directions, so the growth angle can differ from where the light is strongest. Equal light from opposite sides cancels out, and the seedling grows straight up.
1
A model where light is sensed locally around the shoot, transduced nonlinearly, and summed as vectors predicted sunflower seedling growth under unilateral, bilateral, and out-of-plane lighting.
2
Seedlings respond to the vector sum of transduced signals, not the physical sum of incident light, so growth direction can deviate systematically from the direction of maximal illumination, similar to an optical illusion.
3
Symmetrical opposing light cues cancel each other, and a weaker symmetry-breaking light source was experimentally shown to shift growth direction as predicted.
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