The Mustard Seeds in the Almirah
Class 6 has 34 students, five small pots of mustard seed, one window sill, one closed steel almirah, and three weeks before the school's science exhibition. The teacher wants the class to find out whether light affects the growth of a seedling. Four groups set up pots. Group A: window sill, watered daily. Group B: inside the almirah, watered daily. Group C: window sill, watered twice a day, sharing a tray with A. Group D: inside the almirah, not watered at all. By day ten, Group B's seedlings are yellow, tall and thin, while Group D's have not sprouted. The children conclude, 'Plants need light to grow.' May in Telangana brings 42 °C afternoons, and the window sill receives direct sun until 2 p.m. One parent telephones to say the almirah plant died because the air inside was 'stale', not because of the dark.
1. State which of the four group comparisons supports the conclusion and which do not, and specify the smallest set of pots that would have answered the question properly.
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Valid, with a caveat: A versus B. Same seed, same pot, same watering; only light differs. This is the comparison the conclusion rests on - but it is weakened because the almirah differs from the window sill in more than light: it is closed, still, and probably warmer.
Not valid: A versus C (watering amount differs, and a shared tray means water moves between pots); B versus D (two variables change at once - light and water - so neither can be credited); D alone shows only that unwatered seeds do not germinate.
A further problem: at 42 °C with direct sun until 2 p.m., the window sill is also a heat treatment, so A may be stunted by heat rather than thriving in light.
Smallest adequate set - three pots:
- Bright spot, shaded from harsh afternoon sun, fixed measured watering.
- Same room, same watering, inside an opaque ventilated cardboard box (dark but with air movement).
- Same bright spot, same watering, inside a closed steel container with air holes - this isolates 'still, warm, enclosed air' from 'dark', and it directly answers the parent's objection.
2. Group B's seedlings were tall and pale rather than short and dead. Explain what this does to the sentence 'plants need light to grow', and how you would reword the class's finding.
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What the data actually show: the dark seedlings grew - taller than the lit ones. They were pale (no chlorophyll) and thin (weak stems). This is etiolation: the seedling stretches upward using stored food from the seed, searching for light.
Effect on the sentence: 'Plants need light to grow' is contradicted by the class's own pots, and a class that has been told a sentence their data refutes learns to distrust their data - or to hide it. The sentence is also imprecise about what light does: it is not a fuel for extension but the condition for making food and for normal, compact, green structure.
Reworded finding: 'In the dark, mustard seedlings grew taller but were pale, thin and weak-stemmed. In the light they were shorter, greener and sturdier. Light is needed for the plant to make chlorophyll and to grow normally.'
Practical step: have each group measure height in centimetres and score colour on a 1–5 scale, so the result lives in numbers rather than adjectives. 'Tall and pale' argued about is a quarrel; two columns of figures are evidence.
3. Write your reply to the parent that respects the objection and shows children how a claim about 'stale air' could actually be tested.
Read the worked answer
Reply to the parent: 'That is a fair objection. The almirah is not the same as a dark room - it is also closed and warm, so two things changed at once. We can test your idea: one pot in a cardboard box with air holes, kept in the same room as the almirah, watered the same amount. If your explanation is right, the box plant should suffer too; if ours is right, it should stay green.'
Then actually build it. It costs one pot and one carton, and it converts a complaint into the best lesson available.
To the class: present it as a second explanation that fits the same evidence. Two explanations fitting the same evidence is the normal situation in science; it is resolved by finding a case where they predict different outcomes. Record the parent's hypothesis in the same results table as the children's, marked 'to be tested', not 'wrong'.
The wider point to draw out at the exhibition: the class is not displaying 'what plants need' but 'how we found out, what we got wrong, and what we still cannot tell'. That is a more honest poster and a better answer to a visiting parent.