Two Seedlings in a Cabinet

What cannot be concluded: that light "makes plants grow," and certainly not the reverse (that darkness stops growth).

Why the result is not paradoxical. Seedlings grown in darkness typically elongate - the stems stretch, become pale and spindly (etiolation). So B being taller than A is a well-known pattern, not a contradiction. Height is therefore a poor proxy for "growth" here: it fails to distinguish a stretched, weak stem from a healthy plant. If the class had measured leaf area, leaf colour, or dry mass, the result would likely have pointed the other way.

Design weaknesses to name explicitly:

• n = 1 per treatment. One seedling each. Any difference could be seed variation, potting, or accident. There is no way to separate treatment effect from individual variation.
• Uncontrolled variables: temperature and humidity inside a closed cabinet differ from a windowsill; light brings heat. Two variables changed, not one.
• Watering by fixed volume rather than by need: the cabinet pot dries more slowly, so it may be waterlogged - another confound.
• One dependent variable, badly chosen: height alone. No dry mass, no leaf area, no colour score.
• No baseline measurement of height at Day 0, so "18 cm" and "24 cm" cannot be converted into growth at all.

What the data do support: only the descriptive statement that, under these conditions, the cabinet seedling grew taller in stem length. That is an observation, not an explanation. The scientifically honest move is to say the experiment is inconclusive as designed, and that the dependent variable must be changed before the question can be answered.

Respond by taking the idea seriously - it is a historically respectable hypothesis, not a silly one. Van Helmont's willow experiment was designed precisely to test it: a potted willow gained tens of kilograms while the dried soil lost only a few hundred grams. The learner's intuition (mass must come from somewhere) is correct; only the source is wrong.

Where the mass actually comes from:

• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, catalysed by light and chlorophyll.
• The carbon - the bulk of the plant's dry mass - comes from atmospheric CO₂, not soil.
• The hydrogen comes from water; the oxygen in carbohydrates comes from CO₂, while the O₂ released comes from water.
• Soil supplies minerals and water in small quantities relative to carbon gain. Roots also respire and release organic acids, and soil mass can change slightly through decomposition and drainage - so "exactly zero" is not the claim, "small compared with the plant's gain" is.

How to settle it with a 0.1 g balance and one week:

1. Weigh the soil (dried to constant mass - that is the hard part in a week, so drying can be done in the sun or over low heat with the teacher) before planting and after removing the plant, washing the roots, and drying.
2. Compare the change in soil mass with the change in plant dry mass (harvest both, dry in the sun or an oven if available, weigh pooled seedlings - see the next task for why pooling matters).
3. Discuss: the plant's mass gain will not be explained by soil loss. Then ask where the carbon came from, and let them connect it to the air.

Teaching point: the learner's error is not a knowledge gap to be corrected by assertion; it is a testable model, and the correction must be empirical. Telling them "it's from photosynthesis" before they have tested the soil idea leaves the misconception intact underneath the correct vocabulary.

The highest-value revision is to change the measured outcome to dry mass and to replicate it - grow several seedlings per treatment and weigh them pooled - rather than to keep measuring stem height.

Reasoning for choosing it over the alternatives:

• It fixes the validity problem. The current design measures a variable (height) that responds in the opposite direction to what the class expects, so no amount of care with height will answer the question. Dry mass is the standard proxy for net growth because it integrates photosynthesis minus respiration; leaf area or chlorophyll content would also work, but dry mass is the one the learner's own "soil is being eaten" question is really about.

• It fixes the precision problem. Monggo seedlings are light. A single seedling's dry mass may be under a gram, and the 0.1 g balance will not resolve small differences between individual plants. Pooling five to ten seedlings per treatment multiplies both the signal and the sample and brings real differences within the balance's resolution. This is a deliberate trade-off: fewer measurable variables, but a measurable one.

• It fixes the n = 1 problem at the same time, at low cost. More pots, same balance. Replication is nearly free compared with the cost of getting the wrong answer confidently.

• It keeps the other variables controlled but does not prioritize them over validity. Temperature and humidity in the cabinet still differ, and Ms. Villamor should record them (a thermometer in each place, checked daily) - but perfect control of a confound still cannot rescue a dependent variable that does not measure the question.

What she should tell the class: "Our measurement was the weak point, not our curiosity. We measured how tall, when the real question was how much plant. Here is what we will do differently, and here is why." The lesson is about the logic of experimental design, and the design flaw is the most instructive object in the room.