Tropisms

Tropisms

Plants produce hormones called auxins to coordinate and control growth in response to light (phototropism) and gravity (gravitropism or geotropism).
Auxins are produced in the tips of shoots and roots.
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.

Phototropism:
- Plant shoots grow towards the light (positive phototropism) to maximise the light absorbed for photosynthesis.
- In shoots, auxin causes cell elongation.
- Auxins will concentrate on the shaded side of the shoot, causing unequal growth that makes the shoot bend towards the light.
Gravitropism:
- Plant roots grow downwards towards water (positive geotropism) in response to gravity, while plant shoots are negatively gravitropic.
- In roots, auxin slows cell growth on the lower side, causing curvature downwards.

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