14.5) Tropic responses

14.5) Tropic responses


Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water.


Gravi(geo)tropism: is a response in which plant grows towards or away from gravity.

Phototropism: is a response in which a plant grows towards or away from the direction from which light is coming.


There are two main types of tropisms:

  • positive tropisms – the plant grows towards the stimulus
  • negative tropisms – the plant grows away from the stimulus


Seedlings are good material for experiments on sensitivity because their growing roots (radicals) and shoots respond readily to the stimuli of light and gravity.


Advantages of positive phototropism:

  • Leaves exposed to more sunlight and are able to do more photosynthesis,
  • Flowers can be seen by insects for pollination.
  • The plant gets higher for better seed dispersal.


Advantages of positive geotropism:

  • By growing deeply into the soil, the root fixes the plant into the ground firmly,
  • Roots are able to reach more water,
  • Roots have a larger surface area for more diffusion and osmosis.



  • Auxins are a family of plant hormones.
  • They are mostly made in the tips of the growing stems and roots.
  • Diffuse to other parts of the stems or roots.
  • Is unequally distributed in response to light and gravity.

Auxins change the rate of elongation in plant cells, controlling how long they become.


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