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Controlling plant growth


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A 'tropism' is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow.

Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weedkillers, rooting powder and to control fruit ripening.


The direction of plant growth

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.

There are different types of tropisms:

Cilantro seedlings bending towards light

Positive phototropism in plant stems

  • Tropism – growth in response to a stimulus
  • Positive tropism towards the stimulus
  • Negative tropismaway from the stimulus
  • Phototropism – growth in response to the direction of light
  • Geotropism – growth in response to the direction of gravity

Responses of different parts of the plant

ResponsePart of plantDirection of growthAdvantage
Positive phototropismStem tipGrowth towards lightTo get maximum light for photosynthesis
Negative phototropismRoot tipGrowth away from lightLess chance of drying out
Positive geotropismRoot tipTowards gravityMore chance of finding moisture
Negative geotropismStem tipAway from gravityMore chance of finding light

Controlling the direction of growth

Auxin is a plant hormone responsible for controlling the direction of growth of root tips and stem tips in response to different stimuli including light and gravity.

Auxin is made at the tips of stems and roots. It's moved in solution to older parts of the stem and root where it changes the elasticity of the cells. More elastic cells absorb more water and grow longer, causing bending in the stem or root. It's thought that light and gravity can interfere with the transport of auxin causing it to be unevenly distributed.

Auxin experiment

3 groups of seeds in a box with a hole cut at one end. Shoots A are short, shoots B are tall with foil hats, shoots C are curving towards the light.

3 groups of seeds are grown in a cardboard box.

A - when the tips are removed, no auxin is made so the stems do not grow

B – when the tips are covered, auxin moves to all parts of the stem causing all parts to grow

C - when the tips are lit from one side only auxin accumulates on the shaded side causing it to grow more than the illuminated side


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