By the end of this lesson
- Describe plant responses to stimuli.
- Explain how growth can produce movement.
- Relate responses to survival.
BIG QUESTION
How does a plant respond without nerves, muscles or a brain?
Plants coordinate chemical and electrical signals that alter growth, turgor and development.
Plants detect many environmental signals
Plants respond to directional light, gravity, water, touch, temperature and day length. Detection occurs in specialised cells and tissues rather than animal sense organs.
Chemical signals coordinate responses between detection and growth regions.
Growth responses are directional
A tropismA growth response of a plant part towards or away from a directional stimulus, such as light or gravity. is a directional growth response. Growth toward a stimulus is positive; growth away is negative.
Unequal cell elongation on opposite sides bends a root or shoot. Because growth changes structure, many tropic responses are slow and not readily reversible.
Responses improve resource capture and protection
Shoots growing toward light can increase photosynthesis. Roots growing with gravity and toward water improve anchorage and resource uptake.
Touch responses can support climbing or reduce damage; stomatal responses alter gas exchange and water loss.
Examples of plant responses
Not every plant response is a tropism.
| Stimulus | Response example | Possible value |
|---|---|---|
| Directional light | Shoot phototropism. | Improves light capture. |
| Gravity | Root positive geotropism. | Anchorage and soil access. |
| Water gradient | Root hydrotropism. | Improves water uptake. |
| Touch | Tendril coiling/thigmotropism. | Supports climbing. |
| Water stress | Stomatal closure. | Reduces water loss; not a growth tropism. |
Language that earns precision
- tropism
- Directional growth response to a directional stimulus.
- positive tropism
- Growth toward a stimulus.
- negative tropism
- Growth away from a stimulus.
- turgor response
- Movement caused by changes in water pressure inside cells rather than growth.
Common misconceptions
Claim: Plants cannot respond because they lack nerves.
Correction: They use chemical, electrical, growth and turgor responses.
Claim: All plant movement is a tropism.
Correction: A tropism is specifically directional growth.
Claim: Positive means beneficial and negative means harmful.
Correction: They describe direction toward or away from the stimulus.
Match the answer to the command word
Name stimulus, organ, direction and whether the response is positive or negative.
Connect unequal growth to bending and a survival advantage.
Recall, apply, then justify
Explain how a directional growth response can bend a plant organ.
Hint
Compare opposite sides.
Suggested marking points
- The directional stimulus is detected.
- Cells on one side elongate faster than cells on the other.
- Unequal growth bends the organ toward or away from the stimulus.
Give two plant responses and explain their value.
Hint
Match each response to a resource or risk.
Suggested marking points
- A correctly named response to light/gravity/water/touch.
- A linked benefit such as resource capture or support.
- A second correctly named plant response.
- A second linked survival/growth benefit.
Distinguish a tropic response from a rapid turgor response using mechanism, direction and reversibility.
Planning hint
Growth versus water movement.
What a strong design includes
- A tropism is directional relative to the stimulus.
- It involves differential growth/cell elongation.
- Growth responses are relatively slow and often irreversible.
- A turgor response depends on water movement and pressure changes.
- It can be rapid and reversible.
- A turgor response need not depend on stimulus direction.