For experimental command-of-evidence questions, identify the distinct prediction made by each competing explanation and prioritize comparisons in which those predictions diverge. When treatments are reversed, also check whether the outcome follows the manipulated condition or remains tied to the same subject, since that distinction can separate a responsive effect from a stable preexisting difference.
Hints
- Hint 1
Identify the competing predictions
Determine which measured variable should predict phosphorus absorption under each of the two proposed explanations.
- Hint 2
Focus on opposing rankings
Consider why trials in which one plant supplies more carbon but transpires less are especially informative.
- Hint 3
Interpret the reversal
Ask what should happen after the treatments switch if phosphorus allocation responds to current conditions rather than to a stable difference between the plants.
Step-by-step
- Step 1
Translate the hypothesis into a prediction
If the fungus allocates phosphorus according to current carbon supply, the plant supplying more carbon should receive more tagged phosphorus.
- Step 2
Distinguish the competing explanations
The most informative trials are those in which carbon contribution and transpiration favor different plants. In those trials, the phosphorus recipient can indicate whether phosphorus tracks carbon supply or water loss.
- Step 3
Use the treatment reversal
The reversal provides an additional test. If the phosphorus advantage shifts when the carbon-supply advantage shifts, the result is difficult to explain as a stable difference between the two plants.
- Step 4
Evaluate the complete finding
The correct finding satisfies both tests: phosphorus tracks greater carbon supply when transpiration points toward the other plant, and the phosphorus advantage changes plants along with the carbon-supply advantage.