Hormones in plants

Plants respond to stimuli slowly:

  • Roots grow towards water.
  • Shoots grow away from gravity.
  • Shoots grow towards light – phototropism.


  1. Plant responds to unidirectional light (e.g. plant on a windowsill)
  2. Plant shoot bends towards light.
  3. Advantage of this response: - more light - more photosynthesis - more growth

How does a shoot bend towards unidirectional light?

Plant hormones auxin

  1. The shoot tip is sensitive to light and produces auxin – if the tip is covered phototropism doesn’t occur.
  2. Light causes the uneven distribution of auxin – it moves downwards and accumulates on the shaded side.
  3. Auxin causes cells on shaded side to elongate faster than on the illuminated side – this is known as differential growth.
  4. The stem bends towards light.
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