plant hormones

• Homeostasis and Response


  Plant Hormones
• Functions of Plant Hormones

  • Plants can respond to changes in the external environment
  • Responses are usually slower than animal responses
  • Includes roots and shoots growing towards or away from a particular stimulus
  • Plants flowering at a particular time
  • Ripening of fruits
• Tropism


  When part of a plant responds by growing in a particular direction
• Types of tropisms


  • Gravitropic (geotropic) - growth of shoots away from gravity, growth of roots towards gravity
  • Phototropic - growth of shoots towards light, growth of roots away from light
• Tropisms are controlled by

  Auxins
• Response to light


  1. More light reaches one side of the shoot

  2. More auxin is sent down the shaded side of the shoot

  3. Cells on the shaded side elongate more

  4. Shoot grows towards the light
• Response to gravity


  1. Auxin moves to the bottom of the root

  2. Causes less elongation

  3. Root grows downwards
• Gibberellins


  Another group of plant hormones, important in initiating seed germination
• Ethene


  A gas and also a plant hormone, controls cell division and ripening of fruits
• Sample Method


  1. Take plastic boxes with holes on one side

  2. Fill with soil and plant cress seeds

  3. Put containers in even lighting with holes pointing in different directions

  4. Water and leave for several days
• It is important to ensure the room is evenly lit with no windows
• The holes in the containers are pointed in different directions to show that light entering is causing the shoots to bend
• Selective weed killers


  Can be used on crops to kill weeds but not the crop
• Ripe fruits give off ethene. Green tomatoes are often kept with a red tomato to promote ripening.