Phototropism

phototropism [ foh-TO-truh-piz-uhm, foh-toh-TROH-piz-uhm ] noun: growth or movement of an organism stimulated by light either toward or away from the light source

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You have observed phototropism if you grow houseplants–they bend toward the window. If you want them to grow vertically, then you have to routinely rotate the pots. That is because plant shoots grow toward blue wavelengths of light. When the sun is overhead, the plant’s growth hormone, auxin, is evenly distributed at the stem tip. When the light source is to one side, auxins move to the shaded side of the stem where they stimulate growth, which elongates cells causing the plant stems to bend toward the light.

Not all plant movement is toward the light (positive phototropism). Roots in the soil grow away from light sources (negative phototropism) and some vines can grow toward solid dark objects so that they can grab a hold and climb.

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Reference

Kantharaj GR. Physiology of Plant Movements. Plant Growth and Development. Plant Cell Biology for Masters. (accessed December 10, 2020).