Explain leaf features that promote transpiration
Connect moist internal surface, air spaces and stomatal openings with evaporation and diffusion.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall evaporation, diffusion and guard cells.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where are the leaf’s intercellular air spaces?
What you will learn
- Explain the internal-surface contribution
- Connect stomatal size/number with potential vapour movement
Large moist internal surfaces
Many mesophyll cells border internal air spaces, creating an extensive moist surface from which water can evaporate. The spaces interconnect and lead towards stomata, allowing vapour to move from internal surfaces to outside air.
A large exposed internal area can support evaporation, while connected spaces provide a diffusion route. A broad external leaf outline is not the same measurement as its internal mesophyll surface area.
Openings influence the route
Stomatal number and the size of open pores affect the total opening available for vapour diffusion. More or larger open pores can permit more loss when other conditions are matched. Guard cells regulate aperture, so the number of stomata alone does not fix the actual loss rate.
Compare leaf area and conditions before interpreting counts. A leaf with twice as many stomata because it is twice as large need not have twice the stomatal density or the same behaviour per unit area. Avoid treating closed pores as identical to equally numerous open pores.
Pause and explain
Why is stomatal count alone insufficient to predict loss?
Worked example
Why might many wide-open stomata support more vapour loss than the same number of narrowly open ones?
Show the worked solution
- Wider pores provide greater total open area.
- With comparable gradients and conditions, the diffusion route is less restricted.
Answer Greater open pore area can permit greater vapour diffusion, with other conditions matched.
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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.
Explain how moist surface and air spaces each support transpiration.
Give me a hint
Give one role for the surface and another for the spaces.
Compare my reasoning
- Moist mesophyll surface supplies evaporation area.
- Interconnecting air spaces provide a vapour route towards stomata.
Moist surface supports evaporation; air spaces connect the resulting vapour to stomatal diffusion.
Look for these in your work
- Explain evaporation area
- Explain the connected route
Leaf A has 100 stomata in 1 mm² and B has 200 in 2 mm². Compare stomatal density.
Give me a hint
Divide count by area.
Compare my reasoning
- A: 100 / 1 = 100/mm².
- B: 200 / 2 = 100/mm².
Both have density 100 stomata/mm²; the raw counts differ because sampled areas differ.
Look for these in your work
- Calculate both densities
- Reject a raw-count density inference
Two leaves have equal stomatal density, but one closes most pores. Why can their loss rates differ?
Give me a hint
Consider the actual opening rather than the anatomical count.
Compare my reasoning
- Closed pores reduce available open area.
- Equal density does not guarantee equal aperture or vapour gradient.
Different aperture can restrict vapour diffusion despite equal density; match other conditions before comparing rates.
Look for these in your work
- Use aperture
- Retain the condition limits
Common mistakes
- Counting stomata without considering leaf area or aperture.
- Calling internal air spaces water-conducting xylem tubes.
What can you explain now?
Why is stomatal count alone insufficient to predict loss?
Compare with the explanation
Aperture, leaf area and conditions also matter
Actual open area and the gradient influence vapour loss.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Link each of moist surface, air spaces and stomata to a different stage of the loss route.
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.