A germination experiment changes one condition at a time, keeps everything else the same, and compares the number of seeds that germinate. The conditions being tested are water, oxygen and temperature. Your job in an exam is to say what the data show, and what they do not show.
This lesson follows seed and fruit formation. It also practises the habits from biology data and investigations.
What makes a fair test?
The independent variable is the one you change. The dependent variable is what you measure. The control variables are everything else that must stay the same.
For a germination test, the dependent variable is usually the number or percentage of seeds that germinate. Use the same species, the same number of seeds and the same time for each set-up. Only one condition is different.
Worked example
The data below are invented for this lesson. A student places 20 cress seeds in each of five set-ups and counts how many germinate after 5 days.
| Set-up | Conditions | Seeds germinated (out of 20) |
|---|---|---|
| A | Damp cotton wool, 25 °C, light | 18 |
| B | Dry cotton wool, 25 °C, light | 0 |
| C | Seeds under boiled water with oil on top (little dissolved oxygen), 25 °C | 1 |
| D | Damp cotton wool, 4 °C, light | 0 |
| E | Damp cotton wool, 25 °C, dark cupboard | 17 |
Which set-up is the control, what do B, C and D show, and does the data prove that light is not needed?
Step 1, control: A has water, oxygen, a suitable temperature and light. A is the control.
Step 2, percentages: A is 18/20 = 90%. B is 0/20 = 0%. C is 1/20 = 5%. D is 0/20 = 0%. E is 17/20 = 85%.
Step 3, B, C and D: B lacks water, C lacks oxygen and D is too cold. Each fails to germinate almost completely, so water, oxygen and a suitable temperature are each needed.
Step 4, E versus A: 90% and 85% differ by 5 percentage points, which is 1 seed in 20. That is a small difference and is well within what seed-to-seed variation could cause.
Answer: A is the control. B, C and D show water, oxygen and warmth are needed. E suggests light is not required for germination, although repeating the test would make this more reliable.
The mistake to watch for
A common slip is to claim a difference from one seed.
Mistaken answer: “Germination was better in the light, because 18 is more than 17, so seeds need light.”
The student treated a 1 seed gap as a real effect and ignored that 17 seeds still germinated without light.
The correction is to look at the size of the difference and to ask whether the result was repeated. A difference of 1 in 20 is not strong evidence. The key observation is that most seeds germinated in the dark.
Check yourself
Answer on paper, then open each answer.
1. In a new experiment, 25 seeds are damp at 25 °C and 23 germinate. What percentage is this?
Show answer
23/25 = 0.92, so 92%.
2. A student compares germination at 10 °C and 25 °C, but uses wheat seeds at 10 °C and cress seeds at 25 °C. Explain the flaw.
Show answer
Two variables changed: temperature and species. The test is not fair, because a difference could be caused by either. Use the same species in both.
3. Why do seeds with no light still germinate?
Show answer
The seed contains a food store, so the seedling has energy from respiration before it can make food by photosynthesis. Light is not needed at this stage.
Where this leads next
Bring the five lessons together in the plant reproduction practice set, and log your slips with the mistake log and retest queue. Reading data accurately is a skill our teachers train in online one-to-one Biology tuition.