Investigate phototropism in seedling shoots
Compare one-sided and even illumination with controlled growth conditions.
Section 14.5 · Tropic responses
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
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
Which is a suitable light comparison?
What you will learn
- Vary the direction of illumination
- Measure source-relative shoot curvature
Investigate phototropism in seedling shoots
| Feature | Meaning |
|---|---|
| Change | One-sided versus evenly distributed illumination |
| Measure | Lateral tip curvature and direction |
| Expected response | Towards the one-sided source |
| Check | Temperature, light comparability, repeats and fixed source position |
Vary the direction of illumination
For a teacher-supervised shoot-light investigation, use similar upright seedlings and a safe, cool light source at one side. A screened arrangement can provide directional illumination; a comparison group receives illumination distributed evenly around its shoots. Keep the seedlings fixed, and record the starting shoot axes and exact source position. Gravity acts on both groups throughout the experiment.
The changed factor is directional versus evenly distributed illumination. Keep light quality, exposure time and total illumination as comparable as the apparatus allows, and acknowledge any unmatched intensity. Control species, age, initial shoot size, moisture and temperature. A hot lamp can change temperature as well as light, so use suitable classroom equipment and check temperature rather than assuming it is identical.
Measure source-relative shoot curvature
After a fixed interval, measure the lateral curvature from each shoot’s starting axis and record whether growth is towards or away from the source. Repeat with several seedlings and compare the mean and spread with the evenly illuminated control. The usual prediction is that shoots in one-sided light curve towards the source, while the even-light group has little consistent lateral curvature.
A light source moved halfway through the investigation changes the treatment and complicates the interpretation of the final shape. Maintain the stated direction unless that change is the purpose of a separate trial. Keep damp materials away from electrical equipment and do not stare into bright sources. This lesson develops investigation design and data interpretation; classroom execution and teacher assessment remain pending.
Pause and explain
Why measure temperature near both groups?
Worked example
State the prediction for a one-sided shoot-light treatment and its control.
Show the worked solution
- One-sided light commonly gives curvature towards the source.
- Even light predicts little consistent lateral curvature.
Answer One-sided group: positive phototropism. Even-light control: little consistent lateral curvature.
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.
Name the independent and dependent variables for the shoot-light comparison.
Give me a hint
Identify the treatment and a directional growth measurement.
Compare my reasoning
- Vary illumination direction: one-sided or even.
- Measure lateral shoot curvature after the fixed interval.
Independent variable: illumination direction. Dependent variable: lateral shoot curvature and direction after a fixed interval.
Look for these in your work
- Name direction of illumination
- Name measured curvature
Give a prediction, four controls and a reliability improvement.
Give me a hint
Use the common shoot response and repeated seedlings.
Compare my reasoning
- Predict curvature towards the one-sided source.
- Control species, age, moisture and temperature; repeat with several seedlings.
Predict positive phototropism in the one-sided group. Control species, age, moisture and temperature, keep time fixed and use several seedlings to compare mean and spread.
Look for these in your work
- State a directional prediction
- Include controls and repeats
A report compares the groups without knowing which side the lamp was on. What can and cannot be concluded?
Give me a hint
Curvature magnitude does not supply the missing source reference.
Compare my reasoning
- A bend may be measured.
- Towards or away from the source cannot be established without its position.
The report can describe measured curvature, but cannot establish positive or negative phototropism without the source position. Record the source direction from the start.
Look for these in your work
- Separate magnitude from response sign
- Repair the missing reference
Common mistakes
- Treating a hot lamp as only a light change.
- Forgetting to record the source position.
What can you explain now?
Why measure temperature near both groups?
Compare with the explanation
The lamp could change temperature as well as illumination
Temperature is a possible confounding variable.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Design a shoot-light comparison that isolates directional illumination and records the source reference.
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.