14.7 - Plant Hormones & Tropisms
Phototropism and gravitropism
- Plants respond to their surroundings by growing in particular directions.
- A tropism is a growth response in which part of a plant grows towards or away from a stimulus.
- Phototropism - The response to light.
- Gravitropism (also called geotropism) - The response to gravity.
A response is positive if the plant grows towards the stimulus, and negative if it grows away from it.
How shoots and roots respond
| Plant part | Response to light | Response to gravity |
|---|---|---|
| Shoot | Grows towards light - positively phototropic | Grows away from gravity (upwards) - negatively gravitropic |
| Root | Grows away from light - negatively phototropic | Grows towards gravity (downwards) - positively gravitropic |
Why these responses help the plant survive
- Shoots bend towards light - This lets the shoot and its leaves absorb more light for photosynthesis, helping the plant make food and grow.
- Roots grow downwards - Growing down anchors the plant and helps the root reach water and dissolved minerals.
- Roots grow away from light - This keeps them in the soil.
TermDefinition
TropismA growth response in which a plant grows towards or away from a stimulus.
PhototropismA plant's growth response to light.
GravitropismA plant's growth response to gravity.