Tropic Responses
- Plants don’t have a nervous system, so they rely on chemical growth responses to interact with their environment. These growth responses are called tropisms.
- Gravitropism is a response in which parts of a plant grow towards or away from gravity.
- Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source.
- Positive tropism is growth towards a stimulus.
- Negative tropism is growth away from a stimulus.
Investigating tropic responses
| Plant part | Phototrophism | Gravitropism | How it helps |
|---|---|---|---|
| Shoot | Positively | Negatively | Ensures leaves get maximum light for photosynthesis. |
| Root | Negatively | Positively | Anchors the plant firmly in the soil and helps absorb water and mineral ions. |
In the lab
- Phototrophism experiment:
- First, grow two sets of seedlings on wet cotton wool in Petri dishes in the dark.
- Second, cover the seedlings with a cardboard box with a slit in the side that allows the light to enter from only one direction so the seedlings are exposed to unilateral light.
- Third, place the seedlings on a clinostat arranged in a vertical position. The clinostat rotates four times every hour, exposing each side of the seedlings to the light.
- Fourth, after two days you can see that the seedlings exposed to unilateral light grew towards the light source.
- In conclusion, they were positively phototrophic.
- Gravitrophism experiment:
- Place a seedling horizontally in a dark room. The shoot will grow upwards and the root will grow downwards.
- To eliminate the effect of gravity as a control, a clinostat is used.
Auxin
- A plant hormone called auxin manages the growth of a plant.
The mechanism
- Production: Auxin is made continuously in the shoot tip.
- Diffusion: Auxin diffuses down through the plant from the shoot tip.
- Unequal distribution:
- In response to unidirectional light, auxin moves away from the light and accumulates on the shaded side of the shoot.
- In response to gravity (horizontal shoot), auxin settles due to gravity and accumulates on the lower side of the shoot.
- Cell elongation: Auxin stimulates cell elongation.
- The result is that since there is more auxin on the shaded/lower side, the cells on that size elongate much faster than the cells on the lit/upper side. This unequal growth causes the shoot to bend toward the light or upward away from gravity.