Evaluate the quality of a tropism investigation
Separate direction, growth, controls and evidence strength.
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 is essential before classifying a response sign?
What you will learn
- Repair confounded root and shoot comparisons
- Use repeats and suitable reference measurements
Evaluate the quality of a tropism investigation
A valid comparison needs growth over time, known stimulus and starting directions, controlled conditions, repeated measurements and an appropriately limited conclusion. Original teaching model.
| Feature | Meaning |
|---|---|
| Four combinations | Root–gravity; shoot–gravity; root–light; shoot–light |
| Controls | Light, gravity orientation, moisture, temperature and time as appropriate |
| Reliability | Repeats and spread; correct systematic reference errors |
| Practical limit | Written planning is not supervised practical competence |
Repair confounded root and shoot comparisons
A useful tropism investigation has a clearly stated stimulus direction, a growing plant part and a measured change over time. Begin by identifying whether the question concerns gravity or light and whether it concerns roots or shoots. An angle without a starting reference, or a final photograph without the source position, may be insufficient to classify the response.
Change one relevant treatment while keeping other growth conditions comparable. Gravity investigations use starting orientation and exclude directional light. Light investigations keep the gravity orientation fixed and compare one-sided with even illumination. Root investigations also need uniform moisture so a water gradient does not change with the treatment. Temperature near a light source should be checked, not assumed.
Use repeats and suitable reference measurements
Use several similar seedlings, the same observation interval and a consistent photographic perspective. Compare means and spread, retain genuine unusual results and explain any measurement uncertainty. Repeats make a conclusion less dependent on one seedling, but do not automatically remove systematic errors such as an incorrectly labelled source side. More readings with the same wrong reference are still misleading.
A slowly rotating clinostat is a possible supervised extension for gravity comparisons because the plant is repeatedly reoriented relative to gravity. It does not eliminate gravity and its mechanical effects need consideration. Similarly, a curve consistent with a tropic response does not alone identify every chemical step. The written activities support investigation reasoning; supervised handling, measurement skill and teacher review remain separate requirements.
Pause and explain
What does repeating an incorrectly labelled source direction fix?
Worked example
Repair a light test in which the illuminated group is also much warmer.
Show the worked solution
- Temperature is a second changed growth condition.
- Use suitable cool lighting and compare temperatures while varying direction.
Answer Keep temperature comparable and checked while varying illumination direction; otherwise light and heating effects are confounded.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
List the four plant-part and stimulus combinations required in this section.
Give me a hint
Cross two plant parts with two stimuli.
Compare my reasoning
- Roots and shoots each respond to gravity.
- Roots and shoots can each be investigated for responses to light.
Root–gravity, shoot–gravity, root–light and shoot–light.
Look for these in your work
- Include both parts
- Include both stimuli
Give three improvements to a single final photograph of a root with an unknown starting angle and source side.
Give me a hint
Restore references and repeated timed observations.
Compare my reasoning
- Record the starting axis and source position.
- Use timed measurements and several controlled seedlings.
Record the starting axis and source side, take timed photographs with the container fixed, and repeat with several controlled seedlings.
Look for these in your work
- Restore directional references
- Add timed repeated measurements
Evaluate: a clinostat turns gravity off, and completing this written worksheet proves practical competence.
Give me a hint
Correct both the physical claim and the evidence claim.
Compare my reasoning
- Rotation changes orientation relative to gravity rather than removing gravity.
- Written planning does not demonstrate supervised execution.
A clinostat continually changes orientation relative to gravity; gravity remains. Written planning alone does not prove practical competence.
Look for these in your work
- Correct the clinostat mechanism
- Distinguish written work from practical evidence
Common mistakes
- Treating more repeats as a cure for a wrong reference.
- Calling written planning a completed practical.
What can you explain now?
What does repeating an incorrectly labelled source direction fix?
Compare with the explanation
It does not fix the systematic reference error
The reference must be corrected.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Evaluate a tropism method and repair its directional references, confounds and unsupported claims.
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.