Tropisms

• Define tropism:

  Directional plant responses that occur in response to a stimulus.
• Where do plants grow?

  In their root tips and shoot tips. Growth hormone factors are made here.
• What are the 3 main types of plant growth:

  1. Cell division.
  2. Cell elongation.
  3. Cell differentiation.
• Where is IAA produced?

  In root tips and shoot tips.
• What does IAA cause in a plant?

  In shoots, it stimulates cell elongation, and in roots it inhibits cell elongation.
• How does IAA move throughout a plant?

  It diffuses from cell to cell. It is transported long distances via the phloem.
• Describe positive phototropism when sunlight is directly above the shoot tip:

  • IAA is made in cells in the shoot tip.
  • It is evenly distributed.
  • Cells elongate at the same rate.
  • The shoot grows upwards.
• Describe positive phototropism when sunlight is facing one part of the shoot tip:

  • Uneven light causes IAA to diffuse to the shaded side of the shoot.
  • Higher concentration on the shaded side causes cells to elongate more than on the light side.
  • Shoot bends towards the light.
• Describe geotropism in shoots:

  • High IAA concentration at the bottom of the shoot where it stimulates cell elongation.
  • Low IAA concentration on the top of the shoot inhibits cell elongation.
  • Shoot bends upwards- positive geotropism.
• Describe geotropism in roots:

  • High IAA concentration at the bottom of the root inhibits cell elongation.
  • Low IAA concentration at the top of the root leads to elongation of cells.
  • Shoot bends downwards- negative geotropism.