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• Auxins


  Plant hormones that control growth at the ends of shoots and roots
• How auxins work

  1. Produced in tips

  2. Dissolve in cell solution

  3. Diffuse backwards along shoot or root
• Auxins in shoots


  Stimulate growth, causing shoots to grow
• Auxins in roots


  Inhibit growth
• Phototropism


  • Response to light
• Geotropism/Gravitropism


  • Response to gravity
• Auxins accumulate on


  • Shaded side rather than sunny side
  • Lower side rather than upper side
• Shoots


  • Positively phototropic (grow towards light)
  • Negatively geotropic (grow away from ground)
• Roots


  • Negatively phototropic (grow away from light)
  • Positively gravitropic (grow down towards ground)
• Normally all plant roots are underground so don't detect light
• If roots get exposed to light

  Auxins accumulate on lower shaded side, upper side grows faster, causing root to curve downwards
• If root is horizontal

  Auxins accumulate on lower side, inhibit growth, upper side grows faster, causing root to curve downwards