Tropic responses
How plants respond
- A tropism is a plant's growth response to a direction.
- gravitropism — growth in response to gravity (roots grow down, shoots grow up).
- phototropism — growth towards the direction of light (shoots grow to the light).
Practice
Phototropism is growth in response to:
Phototropism is a growth response to the direction of light (shoots grow towards it); gravitropism responds to gravity.
Practice
Because of gravitropism, plant roots grow:
Roots show positive gravitropism (grow down); shoots grow up, away from gravity.
Auxin controls it (Supplement)
- A chemical called auxin controls these responses:
- auxin is made in the shoot tip,
- it diffuses down the plant,
- light or gravity makes it spread unequally to one side,
- auxin makes cells elongate, so the side with more auxin grows faster and the shoot bends.
Practice
Auxin makes a shoot bend towards light because it:
Auxin (from the shoot tip) builds up on the shaded side and makes those cells grow longer, so the shoot bends towards the light.
You've got it
Key idea
- tropism = growth response to a direction; gravitropism (gravity), phototropism (light)
- (Supplement) auxin from the shoot tip spreads unequally and makes cells elongate
- the side with more auxin grows faster, bending the shoot toward the light