Describe phototropism relative to the light source
Define the growth response without equating light direction with up.
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 the stimulus in phototropism?
What you will learn
- Name the directional light stimulus
- Classify growth towards or away from its source
Describe phototropism relative to the light source
| Feature | Meaning |
|---|---|
| Stimulus | Direction of the light source |
| Positive | Growth towards the source |
| Negative | Growth away from the source |
| Not equivalent | Photosynthesis without directional-growth evidence |
Name the directional light stimulus
Phototropism is a response in which a plant part grows towards or away from the direction of a light source. Directional growth is essential: a leaf making glucose in light is photosynthesis, not by itself phototropism. The light source can be at the side of a seedling, so growth towards light need not mean growth vertically upwards.
Positive phototropism means growth towards the source. Negative phototropism means growth away from it. The words describe direction, not whether light is bright or dim, and not whether growth is useful. Draw or identify where the light originates before judging the response. Light rays travel from the source towards the plant, whereas positive growth bends back towards the source.
Classify growth towards or away from its source
A seedling shoot near a window can curve towards that window as its growing region develops. A controlled investigation compares one-sided illumination with a suitable evenly illuminated control and keeps moisture, temperature, species, age and observation time comparable. A single attractive curve without a known source position cannot establish the sign of the response.
Do not confuse a growth response with turning a pot or with every kind of rapid leaf movement. Record the original axis, keep the container fixed and compare later growth. Core learners describe the stimulus and response direction; Supplement learners later explain how unequal auxin distribution causes unequal shoot cell elongation. Both levels use the same clear source-relative direction labels.
Pause and explain
Light originates on the right; growth curves left away from it. Which sign applies?
Worked example
A source is on the left and a shoot grows towards the left. Classify it.
Show the worked solution
- The growth is towards the source.
- Towards the source is positive phototropism.
Answer The shoot shows positive phototropism.
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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.
Define phototropism.
Give me a hint
Include plant growth and the source direction.
Compare my reasoning
- A plant part grows in response to directional light.
- Growth may be towards or away from the source.
Phototropism is a response in which a plant part grows towards or away from the direction of a light source.
Look for these in your work
- Include plant growth
- Specify the light-source direction
Classify growth leftwards towards a source on the left, and rightwards away from that same source.
Give me a hint
Keep the source fixed when comparing.
Compare my reasoning
- Leftwards is towards the source.
- Rightwards is away from the source.
Leftwards: positive phototropism. Rightwards: negative phototropism.
Look for these in your work
- Give both signs
- Relate them to the fixed source
A learner says any photosynthesis in bright light is positive phototropism. Explain the missing evidence.
Give me a hint
Compare a chemical process with directional growth.
Compare my reasoning
- Photosynthesis produces substances using light.
- Phototropism requires growth towards or away from a known light source.
Photosynthesis alone does not show phototropism. Evidence of directional growth relative to the light source is needed.
Look for these in your work
- Distinguish the processes
- Require a directional growth observation
Common mistakes
- Using up as a universal definition of towards light.
- Confusing photosynthesis with directional growth.
What can you explain now?
Light originates on the right; growth curves left away from it. Which sign applies?
Compare with the explanation
Negative phototropism
The curve is away from the source.
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Make it stick
Define phototropism and classify a growth direction using the stated source position.
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.