Interpret daily stomatal rhythms
Describe daily changes in stomatal opening without treating them as a fixed universal schedule.
Section 14.2 · Homeostasis in plants
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes stomatal behaviour a daily rhythm?
What you will learn
- Explain that stomata have daily rhythms of opening and closing
- Relate observed daily patterns to light and water conditions
Build the explanation
Stomata show daily rhythms of opening and closing. In many plants, aperture tends to increase in daylight and decrease in darkness, but timing and extent vary with species and conditions such as water availability, humidity and carbon dioxide. A daily pattern is a repeated response over a cycle, not proof that every stomata follow the same timetable.
Apply the limits
When interpreting a graph, describe its measured time pattern first, then relate it cautiously to conditions. Do not infer a universal opening time from one species or one data set.
Pause and explain
One experiment records opening at 07:00 in one species. What can and cannot be concluded?
Worked example
A graph shows larger aperture in daytime than at night. Give a supported description.
Show the worked solution
- Compare the plotted aperture values across day and night.
- State the trend without claiming every plant behaves identically.
Answer The stomata in the measured sample were more open during daytime than at night.
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.
What evidence would support a daily stomatal rhythm?
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- Measure aperture repeatedly across a day-night cycle.
- Look for a recurring pattern rather than one isolated observation.
Repeated measurements showing a patterned change in aperture across the day and night support a daily rhythm.
Look for these in your work
- Correct mechanism
- Direction of response explained
A plant's stomata remain relatively closed through a hot dry afternoon. Does this disprove a daily rhythm? Explain.
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- Water stress can modify aperture.
- Daily rhythms interact with environmental conditions.
No. Water stress can modify stomatal aperture, so a plant may close stomata during a dry afternoon even if it shows a broader daily rhythm under other conditions.
Look for these in your work
- Evidence interpreted
- Control step identified
One experiment records opening at 07:00 in one species. What can and cannot be concluded?
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- The measured species opened by that time under the test conditions.
- One species and data set cannot establish a universal opening time.
The sample showed opening by 07:00 in those conditions. It does not show that every plant species has the same daily schedule.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Claiming identical rhythms in all plants
- Confusing a single opening event with a daily rhythm
What can you explain now?
What makes stomatal behaviour a daily rhythm?
Compare with the explanation
A repeated pattern of opening and closing over the day
The pattern recurs, although its timing depends on conditions.
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Make it stick
Why should daily stomatal patterns be described with their conditions?
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.