• Tropism: a growth movement in response to a stimulus. Plants grow towards or away from stimuli.
  • A growth movement towards a stimulus is called a positive tropism. A growth movement away from a stimulus is called a negative tropism.
    • Light; phototropism
    • Gravity; geotropism or positive gravitropism
    • Water; hydrotropism


  • Plant roots are negatively phototrophic while plant shoots are positively phototrophic. This is so they can get enough light for photosynthesis.
  • Plants also produce hormones (or plant growth substances), like humans. Positive phototropism is caused by the plant hormones called auxins.
  • Auxins are produced at the tip of a plant’s shoot where they cause the elongation of the cells. If a shoot is grown with light coming from only one direction, auxins move to the shaded side of the shoot.
  • As a result, the auxins cause the cells on the shaded side of the shoot to elongate more, which in turn causes the shoot to grow towards the light.


  • Auxins are also found in the root tips of plants where they have the opposite effect. In roots the cells at the top elongate faster, and the root bends downwards.
  • This helps plant roots to anchor the plant in place and to reach water and minerals underground.


  • When a seed germinates, roots and a shoot start to grow. Some seeds need periods of darkness or cold before they will germinate. After this period is complete, the seed releases plant hormones called gibberellins.
  • These hormones cause the starch stored in the seed to be turned into sugars that the seed uses for energy to grow.
  • In some species, Gibberellins also stimulate flower and fruit production.

Other plant hormones

  • Cytokinins – induces mitosis (causes plants to grow and flower and stimulates seed germination).
  • Ethylene – Ripening of fruit and dropping of leaves.
  • Abscisic Acid – inhibits growth during winter so plants can hibernate and not waste energy