Four Tins of Maize: What the Germination Data Can and Cannot Prove
Four JSS3 pupils in Jos investigate what maize grains need in order to germinate. They label four identical tins A, B, C and D and put 10 healthy maize grains on cotton wool in each. Tin A is kept moist, in the classroom at about 27 °C, in daylight. Tin B is kept dry, in the same place. Tin C is kept moist and placed in a closed cupboard in the same room. Tin D is kept moist and placed in a refrigerator at 4 °C. After five days they count the grains that have sprouted: A = 9, B = 0, C = 8, D = 0. One grain in A has gone soft and is covered with mould. The seedlings in C are taller but pale yellow, while those in A are shorter and green. The pupils must present one conclusion their data supports and one claim they cannot make, and they must explain what went wrong with the single mouldy grain.
1. State which variable each of the comparisons A vs B, A vs C and A vs D is testing, and give the conclusion supported by each comparison.
Read the worked answer
A vs B (moist vs dry). Only the water differs; temperature, light and grain type are the same. Result 9 against 0. Conclusion: water is necessary for germination.
A vs C (daylight vs dark cupboard). Only light differs. Result 9 against 8 - essentially the same. Conclusion: light is not necessary for germination; maize grains will sprout in the dark.
A vs D (27 °C vs 4 °C). Only temperature differs. Result 9 against 0. Conclusion: a suitably warm temperature is necessary; 4 °C is too cold for the enzymes of germination to work.
The design is sound because each pair changes exactly one factor while the other conditions are held constant. That is what makes each conclusion attributable to the variable named and not to something else.
2. The seedlings in tin C are taller but pale yellow. Explain what this shows about the difference between germination and healthy growth, and why the pupils still cannot write 'light is needed for germination' in their conclusion.
Read the worked answer
Two different processes. Germination is the sprouting of the embryo using food already stored in the grain; it needs water, warmth and oxygen, but not light. Growth after sprouting is different: the young plant must make its own food by photosynthesis, and that requires light. In the cupboard the seedlings used stored food and grew tall and spindly to reach light - this stretching is etiolation - but with no light they could not make chlorophyll, so they stayed pale yellow. In tin A the seedlings were shorter, greener and sturdier.
Why the claim fails. The pupils measured sprouting, and C sprouted just as well as A (8 out of 10). Their own data therefore contradict the statement that light is needed for germination. What their observation actually supports is that light is needed for the seedling to become green and healthy after germination. Those are two separate conclusions, and mixing them is the commonest error in this practical. A good report states both: germination does not require light, but continued healthy growth does.
3. Why did the pupils use 10 grains in each tin instead of 1, and how should the mouldy grain in tin A be treated when they report and interpret the results?
Read the worked answer
Why 10 grains. A single grain gives a result of either 0 or 1 - no sense of how typical that is, and one bad grain would wreck the whole experiment. With 10 grains the pupils get a proportion, so 9 out of 10 can be compared meaningfully with 8 out of 10. Individual grains vary in size, age and vigour, so replication averages out that variation and makes the comparison between tins fair. One grain per tin would also make a single contaminated grain look like a treatment effect.
The mouldy grain. It should not be quietly deleted from the table. The honest report is 9 out of 10 sprouted in A, with a note that one grain went soft and grew mould - evidence of fungal infection, probably from cotton wool that was too wet or not clean. That is a failure of one grain, not evidence against the conclusion, because the other nine sprouted. The pupils should record it, state it as an anomaly, and say what they would change next time: clean cotton wool, less water, and a repeat run to confirm that 9 out of 10 is typical rather than a one-off.