Plant growth and light lab
01 // Question
How does the amount of daily light a seedling receives change how tall it grows over two weeks?
A testable question names one thing you change (light exposure) and one thing you measure (height over time).
02 // Variables
Independent
Hours of light per day- No light (dark closet)
- Partial light (4–6 hrs, indirect)
- Full light (8+ hrs, sunny window)
Dependent
Seedling height- Height in cm, measured every 2 days
- Number of leaves at day 14
03 // Hypothesis
IF a seedling receives more hours of light per day, THEN it will grow taller over two weeks than a seedling that receives less light, BECAUSE plants use light energy to photosynthesize and build new plant tissue, so more usable light generally supports more growth.
04 // Procedure
1. Plant identical seedlings
Plant the same type of seed in identical pots with the same amount of soil, timed to sprout together.
2. Set the light groups
Place one group in full light, one in partial light, and one in darkness, keeping every other condition the same.
3. Water on a schedule
Water every pot with the same amount on the same schedule, regardless of light group.
4. Measure every 2 days
Measure each seedling's height from soil to tip every 2 days and record it.
5. Graph the growth
Plot each group's height over time on the same graph to compare growth patterns.
05 // Data table
| Light group | Day 2 (cm) | Day 6 (cm) | Day 10 (cm) | Day 14 (cm) |
|---|---|---|---|---|
| No light | ____ | ____ | ____ | ____ |
| Partial light | ____ | ____ | ____ | ____ |
| Full light | ____ | ____ | ____ | ____ |
06 // What to expect
Full-light seedlings usually grow into short, sturdy plants with green leaves, since they have enough light energy to photosynthesize efficiently and build strong tissue.
No-light seedlings often grow tall and spindly with pale, yellowish leaves in the first week — a response called etiolation, where a plant stretches upward searching for light — before running out of stored energy and stalling.
07 // Lab notes
- Measure at the same time of day each check, since some seedlings droop slightly overnight and stand straighter after a few hours of light.
- Keep the 'no light' group's darkness complete — a closet that gets opened often for other things lets in enough stray light to change the result.
- Use a ruler with millimeter marks for more precise height readings on young seedlings.
08 // Remember
A taller seedling in the dark isn't a healthier one. It's stretching for light it can't find, and running out of stored energy to do it.
09 // Questions people ask
Why do seedlings grown in the dark grow taller at first than seedlings in full light?
A plant deprived of light still uses its stored seed energy to grow, and it stretches upward rapidly searching for a light source — a response called etiolation. This growth is weak and unsustainable: the seedling becomes pale, thin, and eventually runs out of stored energy without light to photosynthesize.
What is the independent variable in a plant growth and light experiment?
The amount of light the seedling receives each day is the independent variable — the thing being deliberately changed between groups. The dependent variable is what's measured in response, typically the seedling's height over time.
Why does watering need to stay the same across all light groups?
Water is a separate variable from light. If watering also changed between groups, you couldn't tell whether any growth difference came from the light or the water — controlling it keeps light as the only thing that differs between groups.