What word describes the movement of a plant, directed toward a light source?

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trophyhunter1 | College Teacher | (Level 1) Educator Emeritus

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The word you are looking for is phototropism. When a part of the plant grows toward a light source, this is known as positive phototropism. Usually, this can be seen as the stem and leaves bend toward the light. Auxins, or growth hormones, play a role in the elongation and growth toward the light source. Negative phototropism is seen in the roots, which grow in the opposite direction of the light, however, gravity acts on the growth of roots in a downward direction. Phototropism is one of many responses plants can have to external stimuli. Other tropisms are responses to gravity, water, etc.

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sid-sarfraz | Student, Graduate | (Level 2) Salutatorian

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Plants respond to their surroundings to give them a better chance of survival. Plant responses are called tropisms and are controlled by a hormone. Plants respond to light, gravity and water.

A plant's response to light is called phototropism. 

Plants need light to make food during photosynthesis. Plants will grow towards the light.


  1. Normally light shines from above. Auxin is spread evenly and the shoot grows upward.
  2. If Light comes from one side, auxin accumulates down the shaded side. Auxin makes these cells grow faster.
  3. The result is that the shoot bends towards the light.

Keep in mind that unequal distribution of auxin speeds up growth in shoots and slows down growth in roots. 

  • Hormone gathers on lower side and increases growth in the shoot - it curves upwards
  • Hormone collects on lower side and decreases growth in the root - it curves downwards.

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