How plants respond
Plants cannot move from place to place, but they still respond to stimuli by growing in a certain direction. A directional growth response to a stimulus is called a tropism. If a plant grows towards the stimulus the tropism is positive; if it grows away from the stimulus it is negative. These slow growth movements are controlled by a plant hormone called auxin, which makes cells on one side of the shoot or root grow longer.
Key idea
Name a tropism after its stimulus: phototropism (light), geotropism or gravitropism (gravity), hydrotropism (water) and thigmotropism (touch).
Common tropisms
- Shoots grow towards light — positive phototropism — so leaves catch more sunlight for photosynthesis.
- Roots grow towards gravity — positive geotropism — anchoring the plant and reaching water and minerals.
- Roots also grow towards water (positive hydrotropism) and away from light (negative phototropism).
- Climbing plants coil around a support when they touch it — thigmotropism.
Example
Place a potted plant near a window and its stem soon bends towards the glass. Turn the pot around and, over a few days, the stem bends back the other way to face the light again.
Why it matters
Tropisms help a plant survive without moving from place to place. Growing its shoots towards light lets the leaves trap more sunlight and make more food by photosynthesis. Growing its roots downwards and towards water anchors the plant firmly in the soil and gives it a steady supply of water and minerals. In this way a fixed plant still reacts sensibly to its surroundings.
Remember
- Positive = grows towards; negative = grows away.
- Auxin controls the uneven growth that bends the plant.