← Responses of plants and animals to their external environment
3 Plant responses
Watch: Plant responses Opens a YouTube search: pick a video that clicks for you Plants respond via growth (slow, often permanent) driven by hormones such as
auxin. Two categories: directional
tropisms and non-directional
nastic responses.
Tropisms (directional growth responses)
| Tropism | Stimulus | Example / survival value |
|---|
| Phototropism | Light | Shoots grow toward light (positive) → more photosynthesis |
| Gravitropism | Gravity | Roots grow down (positive), shoots up (negative) → anchorage & water |
| Thigmotropism | Touch | Tendrils coil around supports → climb toward light |
| Hydrotropism | Water | Roots grow toward moisture → water uptake |
The auxin mechanism (phototropism)
- Auxin is made at the shoot tip and moves to the shaded side.
- Auxin promotes cell elongation, so the shaded side grows faster.
- The shoot therefore bends toward the light: maximising light capture for photosynthesis.
Nastic responses (non-directional)
- Independent of stimulus direction: e.g. the speed/opening is fixed, not toward the stimulus.
- Thigmonasty: Venus flytrap snapping shut, Mimosa leaves folding when touched → defence / prey capture.
- Nyctinasty: flowers/leaves opening by day, closing at night → protect pollen, reduce water/heat loss.