What is a tropism?
Plants respond to their environment by growing towards or away from a stimulus. This growth response is called a tropism. It is positive if the plant grows towards the stimulus and negative if it grows away from it.
Key idea
Phototropism = a growth response to light.
Gravitropism (geotropism) = a growth response to gravity.
Shoots and roots
- Shoots are positively phototropic (grow towards light) and negatively gravitropic (grow up, away from gravity).
- Roots are positively gravitropic (grow down, towards gravity) and negatively phototropic.
Remember
- Growing towards light helps a shoot capture more light for photosynthesis.
- Growing down helps roots reach water and anchor the plant.
The role of auxin
Tropisms are controlled by a plant hormone called auxin, made at the tip of a shoot. Auxin causes cells to elongate (get longer). When light shines evenly from all sides, auxin spreads evenly and the shoot grows straight up. When light shines from one side, however, auxin moves to the shaded side of the shoot, so growth becomes unequal.
Because a tropism is a growth response, it is slower than an animal's nervous response, but it still lets a fixed plant place its leaves and roots where they work best. Unlike a nervous response, the bending is permanent, as it results from growth rather than movement.
Worked example
A seedling is lit from the left. Auxin builds up on the right (shaded) side. The right-side cells elongate more than the left-side cells, so the shoot bends towards the light — positive phototropism. This is why plants on a windowsill lean towards the window and why farmers space crops so each plant gets enough light.