By the end of this lesson
- Describe root and stem phototropism.
- Describe root and stem geotropism.
- Interpret tropism experiments.
BIG QUESTION
Why do shoots usually rise toward light while roots grow down into soil?
The same directional stimuli can produce opposite growth responses in different organs.
Shoots respond positively to light
A stem or shoot usually shows positive phototropismGrowth of a plant part, such as a shoot, towards light., bending toward a directional light source. This positions leaves for improved light capture.
Roots are commonly negatively phototropic, although gravity and water can dominate their growth in soil.
Roots and shoots respond differently to gravity
Roots show positive geotropismGrowth of a plant part, such as a root, towards gravity. by growing in the direction of gravitational pull. Shoots show negative geotropismGrowth of a plant part, such as a shoot, away from gravity. by growing upward, away from it.
These responses orient organs after germination even in darkness.
Fair tests isolate direction
Seedlings can be placed horizontally in darkness to test geotropism without a directional-light cue. A rotating clinostat changes the direction of gravity relative to the organ and can serve as a comparison.
Phototropism tests use one-sided light while controlling temperature, water, age and initial orientation. Repeats reduce biological variation.
Root and shoot tropisms
State both stimulus and direction.
| Organ/stimulus | Response | Growth direction |
|---|---|---|
| Shoot + light | Positive phototropism. | Toward light. |
| Root + light | Usually negative phototropism. | Away from light. |
| Root + gravity | Positive geotropism. | With gravitational pull. |
| Shoot + gravity | Negative geotropism. | Against gravitational pull. |
Language that earns precision
- phototropism
- Directional growth response to light.
- geotropism
- Directional growth response to gravity; also called gravitropism.
- clinostat
- Device rotating a plant to prevent a constant one-sided gravity stimulus.
Common misconceptions
Claim: Roots grow down because gravity pulls them like falling objects.
Correction: Gravity is detected and causes differential growth.
Claim: Shoots are always negatively geotropic and therefore ignore light.
Correction: Several stimuli interact; both responses can influence growth.
Claim: A clinostat removes gravity.
Correction: It continually changes its direction relative to the organ, reducing a directional response.
Match the answer to the command word
Use positive/negative plus photo-/geo- and the named organ.
Exclude directional light when investigating gravity, and keep temperature constant near lamps.
Recall, apply, then justify
Describe the phototropic and geotropic responses of a typical root and shoot.
Hint
One correct pairing per mark.
Suggested marking points
- Shoot is positively phototropic.
- Shoot is negatively geotropic.
- Root is usually negatively phototropic.
- Root is positively geotropic.
Describe a fair investigation of shoot phototropism.
Hint
Directional light, a control group and measurement.
Suggested marking points
- Use similar seedlings with the same initial orientation.
- Provide light from one measured direction.
- Keep water, temperature, age and duration controlled.
- Measure direction/angle or curvature after a fixed time.
- Repeat with several seedlings and compare with an evenly lit/dark control group.
Explain how a clinostat helps test whether curvature is a response to gravity.
Planning hint
Compare rotating and stationary seedlings.
What a strong design includes
- Use matched horizontal seedlings in darkness.
- Keep one stationary and rotate another slowly on a clinostat.
- The stationary seedling receives a constant directional gravity stimulus.
- Rotation continually changes the stimulus direction relative to the organ.
- Reduced curvature in the rotating group supports a geotropic explanation.
- Temperature, water, age and rotation effects still need to be kept the same.