Investigate gravitropism in seedling roots
Plan a horizontal-start root investigation with timed measurements.
Section 14.5 · Tropic responses
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Identify the root and shoot of a seedling; recall growth, cell structure and the variables in a controlled investigation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must remain comparable between root groups?
What you will learn
- Set the root orientation and control other conditions
- Record directional root growth with repeats
Investigate gravitropism in seedling roots
| Feature | Meaning |
|---|---|
| Change | Starting root orientation relative to gravity |
| Measure | Tip curvature and direction after a fixed interval |
| Controls | Species, age, moisture, temperature and time; exclude directional light |
| Reliability | Several seedlings; retain readings and compare mean and spread |
Set the root orientation and control other conditions
For a teacher-supervised root investigation, use similar germinated seedlings supported on evenly damp material in a transparent container. Keep the roots visible and avoid flooding the seedlings or letting them dry out. Record the initial root axis, then secure the container so the growing roots begin approximately horizontal. The changed factor is the root orientation relative to gravity, not the amount of water.
Keep the experimental and comparison seedlings in darkness at the same suitable temperature for the same observation period. Use a comparison group whose roots remain in their original downward orientation. Match species, age, starting size, moisture and handling as closely as possible. Darkness excludes a directional light cue; uniform moisture reduces a competing water-gradient influence.
Record directional root growth with repeats
Photograph or trace the root axes at the start and after a stated interval without rotating the container again. Measure the change in growing-tip direction from the starting axis using a consistent angle reference. Record the direction as well as the angle magnitude. Repeat with several seedlings and calculate a mean, retaining unusual results rather than silently discarding them.
The usual prediction is that initially horizontal roots curve down towards gravity, showing positive gravitropism. This is growth over time, not roots simply falling. Keep specimens and damp material away from electrical equipment, handle classroom materials under teacher guidance and wash hands afterwards. Designing or interpreting the written model does not certify that a learner has carried out the practical accurately.
Pause and explain
What prediction matches a horizontal root in this dark model?
Worked example
Name the changed factor and recorded response in a root-gravity investigation.
Show the worked solution
- Change the starting root orientation relative to gravity.
- Measure later root-tip curvature and direction after a fixed time.
Answer Changed factor: starting orientation relative to gravity. Recorded response: root-tip curvature and direction over a fixed interval.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Identify the independent and dependent variables for the root-gravity investigation.
Give me a hint
Separate the starting condition from the measured response.
Compare my reasoning
- Vary starting root orientation relative to gravity.
- Measure root-tip curvature and direction after the same time.
Independent variable: starting orientation relative to gravity. Dependent variable: root-tip curvature and direction after a fixed time.
Look for these in your work
- Name the changed factor
- Name a measurable response
Give four controls and a way to improve reliability.
Give me a hint
Control growth conditions and use several seedlings.
Compare my reasoning
- Keep species, age, moisture and temperature comparable.
- Use the same interval and repeat with several seedlings.
Control species, age, moisture and temperature, and use the same interval. Repeat with several seedlings and compare the mean and spread.
Look for these in your work
- Name meaningful controls
- Include repeats and spread
A trial rotates the container again before the final photograph and reports a downward root. Why is the conclusion weakened?
Give me a hint
The reference must remain fixed after the start.
Compare my reasoning
- Later rotation changes the apparent direction.
- The method cannot cleanly separate growth from repositioning.
The changed reference confounds the observed direction. Keep the container fixed and compare timed growth with the original axis.
Look for these in your work
- Identify the confounding movement
- Repair the method
Common mistakes
- Changing water and orientation together.
- Measuring only immediately after moving the container.
What can you explain now?
What prediction matches a horizontal root in this dark model?
Compare with the explanation
New root growth curves down towards gravity
That is positive gravitropism.
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Make it stick
Plan a controlled root-gravity investigation and distinguish growth from later container rotation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.