Compare plant adaptations using context and normalized records
Keep organ position, water availability and rate units visible when comparing mechanisms.
Section B17.2 · Selection
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall inherited features, phenotype, diffusion, photosynthesis and transpiration. Diagrams are original schematics; comparison records are invented and use only the stated assumptions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is the stated normalized rate calculated?
What you will learn
- Compare dry floating and submerged mechanisms
- Normalize invented records without making causal overclaims
Original schematic and invented comparison records. Features, environment, inheritance information and measurement limits are stated separately.
| Feature or record | Interpretation |
|---|---|
| A record | 12 mg / 2 h / 4 cm² |
| B record | 15 mg / 3 h / 5 cm² |
| Normalized rates | A 1.5 / B 1.0 mg cm⁻² h⁻¹ |
| Difference | 0.5 mg cm⁻² h⁻¹; A 50% above B |
Compare dry floating and submerged mechanisms
Begin a comparison with the environmental problem. The dry-habitat leaf model limits water loss; the floating leaf model balances buoyancy, surface light and air-facing exchange; the submerged model exchanges dissolved gases across underwater surfaces. A waxy barrier and a thin surface barrier can both have useful roles in different settings. They are not contradictory answers when the species, organ position and function are specified.
Two invented water-loss records use different observation times and measured leaf areas. Leaf A loses twelve milligrams over two hours with four square centimetres of measured area. Leaf B loses fifteen milligrams over three hours with five square centimetres. Divide loss by time and area to obtain 1.5 and 1.0 milligrams per square centimetre per hour. Raw total loss is larger for B, but the normalized rate is larger for A. State the measured-area convention consistently rather than silently doubling it for two leaf sides.
Normalize invented records without making causal overclaims
The rate difference is 0.5 milligrams per square centimetre per hour. Relative to B’s normalized rate, A’s is fifty per cent higher. These values are area- and time-normalized comparisons, not whole-plant drought tolerance or reproductive success. For a real experiment, the mass-loss method would need to isolate the intended loss route, account for evaporation outside the leaf, keep conditions comparable and include repeats. A sealed-water-reservoir setup is one planning possibility, not a completed practical assessment.
Normalization removes the stated differences in time and measured area from the comparison; it does not remove every confounding variable. Leaf age, light, air movement, temperature, humidity and stomatal state can still differ. Nor does a rate identify which drawn feature caused it. Use the numbers to describe the record and the diagrams to explain potential mechanisms in their own environments. Teacher review, real practical work and broader assessment remain distinct from these self-checked written tasks.
Sources for this lesson
Pause and explain
Which rate is larger after normalization?
Worked example
Calculate area- and time-normalized rates for leaves A and B and explain the reversal from raw total loss.
Show the worked solution
- A: 12 ÷ 2 ÷ 4 = 1.5 mg cm⁻² h⁻¹; B: 15 ÷ 3 ÷ 5 = 1.0 mg cm⁻² h⁻¹.
- B’s larger total used both a longer time and a larger measured area, so total loss alone does not rank the normalized rate.
Answer A has the higher normalized rate, 1.5 versus 1.0 mg cm⁻² h⁻¹, despite B’s larger raw loss. The records do not alone identify a causal feature or whole-plant performance.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate A’s normalized loss rate from 12 mg, 2 h and 4 cm² of measured area.
Give me a hint
Divide by both denominators and keep the units.
Compare my reasoning
- 12/2 = 6 mg per hour for the measured leaf area.
- 6/4 = 1.5 mg per square centimetre per hour.
A’s normalized rate is 1.5 mg cm⁻² h⁻¹ under the stated area convention.
Look for these in your work
- Both divisions
- Rate units retained
Calculate B’s normalized rate and compare A-minus-B and A’s relative increase over B.
Give me a hint
B uses 15 mg, 3 h and 5 cm².
Compare my reasoning
- B = 15/3/5 = 1.0 mg cm⁻² h⁻¹; difference = 1.5 − 1.0 = 0.5.
- 0.5/1.0 × 100 = 50% relative increase over B.
B is 1.0 mg cm⁻² h⁻¹. A is higher by 0.5 mg cm⁻² h⁻¹, or 50% relative to B; these are invented record comparisons, not causal proof.
Look for these in your work
- B and difference correct
- Reference and units stated
A report says normalization proves that the cuticle alone caused the difference and establishes drought survival. Evaluate the report.
Give me a hint
Normalization addresses only the denominators supplied.
Compare my reasoning
- Light, temperature, humidity, age and stomatal state may still differ.
- The dataset measures loss, not survival or reproductive outcome.
The claims are unsupported. Appropriate controls and feature-isolating evidence are needed for causation, while survival and reproduction require their own measurements.
Look for these in your work
- Confounding retained
- Outcome measure distinguished
Common mistakes
- Comparing raw loss despite unequal time and area
- Doubling measured area without a stated convention
- Treating normalization as proof of experimental control
What can you explain now?
Which rate is larger after normalization?
Compare with the explanation
A: 1.5 versus B: 1.0 mg cm⁻² h⁻¹
The time and area denominators reverse the raw-total order.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, compare dry floating and submerged mechanisms and normalize invented records without making causal overclaims. Explain the mechanism and keep the stated environment and evidence limits clear.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.