Biology IA · B · Form and function · topic 9 of 25
Stomata: sun leaves vs shade leaves · IB Biology IA idea: guide, sample graph and step-by-step plan
good use of a school microscopeCount the pores a leaf breathes through, and find out whether light changes how many it grows.
- ✓ you like microscopes
- ✓ you can reach sunny and shaded spots
- ✓ you like counting and comparing carefully
The big idea
← this is your RQ hook!Stomata let carbon dioxide in and water vapour out. Plants of one species can grow leaves with different numbers of stomata depending on conditions. Compare leaves from full sun and deep shade from the same species, and count stomata per mm² using impressions under a microscope. Your design is a sampling one: fields from the same leaf are not independent, so you need several leaves per environment and a fixed rule for which leaf and which part to sample. A stomatal index adds a second angle, because it removes the effect of cell size.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
Swipe for more twists →
Measure light at each sampled leaf with a meter or phone app and plot density against light, not sun and shade.
→ Turns two groups into a graph with many points and a fitted line, with light uncertainty on the x-axis.Take impressions from both surfaces of the same leaves.
→ Adds a within-leaf comparison, which is a paired design and suits a paired test.Choose one sun-loving and one shade-tolerant species and compare the response.
→ Asks whether the effect is general or species specific.Measure leaf area alongside density and ask if leaves differ in size or just in density.
→ Asks whether the total number of stomata per leaf changes, not just the density.Count epidermal cells too and calculate the stomatal index in every field.
→ Adds counting and a second calculation. Tests whether density changes simply because cells grow differently.Your Biology IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names the species, the independent variable (the light environments, ideally with a measured value) and the dependent variable (stomatal density per mm², and the index if used).
- Says which leaf, which position and which surface are sampled, and why.
- Says how many leaves per environment, and how many fields per leaf.
- Says how the field of view area is calibrated.
- Says which statistic will compare the two environments, and is clear that leaves, not fields, are the replicates.
The set-up
One way to set it up. Draw your own version, with your real leaves and field of view, for the Method section.
How your Biology IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question; why leaves in sun and shade face different conditions; what you expect and why.
Stomata, gas exchange and water loss; what is known about light and stomatal development (cite); your prediction.
Independent, dependent and controlled variables, each with how you measure or hold it, and why it matters.
Species and sites; light measurement; which leaves and surface; how impressions are made; microscope calibration; fields and counting rules; enough detail to repeat.
Specific: nail varnish is flammable and gives off fumes, take few leaves, permission, avoid protected plants.
Counts per field for every leaf, with the field area and the leaf and site recorded.
Density per mm² for each field; mean per leaf; mean and SD per environment, using leaves as replicates; one worked example per calculation.
A graph comparing environments with SD error bars; a t-test on leaf means for two environments (or Spearman's rank against measured light), with degrees of freedom, p-value and sample size.
Answer with values and the test result; compare with published work (cite); keep your claim to the species and sites sampled.
Weaknesses ranked by effect (leaf-to-leaf variation, counting rules, small sample, uncontrolled conditions); linked improvements.
Every source for theory and method.
What your Biology IA graph could look like
Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- Replicates are leaves. Fields from one leaf are averaged first, then leaves are compared.
- Error bars are the SD of leaves, with the number of leaves stated.
- A flat line is drawn so the reader can see 'no response' as well as the trend.
- A measured light value on the x-axis makes a graph with many points, instead of just two columns.
- Another graph to consider: plot stomatal index against light, to see whether density and index tell the same story.
Where does the scatter come from?
pilot first, THEN choose nBefore you decide how many leaves and fields to sample, ask where the variation sits. A quick pilot with your numbers, shown like this, decides your sampling design.
Your game plan
calibrate the microscope earlyTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictFind out how stomatal density is known to respond to light and write down your prediction. Decide what difference you would count as important.
- Choose species and sitesChoose one species that grows in sun and shade near you. Get permission, and check that other conditions such as soil and water are similar.
- Trial run (1 lesson)Make a few impressions and count them. Can you see stomata and cells? How many fields does a leaf give? Which fields are worth counting? Fix your method now.
- Calibrate and fix counting rulesCalibrate the field of view with a stage micrometer or ruler. Write rules for counting stomata at the edge and for partial cells.
- Decide sample sizesChoose the number of leaves per environment and fields per leaf. Write down why, using your pilot variation.
- Collect the dataTake leaves at a fixed time of day from fixed positions. Record light, leaf and site IDs straight into your table.
- ProcessConvert counts to density per mm², average fields to a leaf mean, and calculate the SD across leaves.
- Graph and testPlot with error bars and choose a test suited to your design. State the sample sizes, the p-value and what it means.
- EvaluateRank the weaknesses by effect on your answer. Make every improvement specific to what you found.
- Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.
Biology you'll need
- Uncertainties and error bars (physics examples, same ideas) →
- Biology past-paper analysis (HL) →
- IB command terms explained →
Traps to avoid in this Biology IA
- Treating every field as a separate sample: fields from one leaf are not independent.
- Sampling leaves from different positions on the plant, or different ages.
- Counting different parts of the leaf in sun and shade, such as near the midrib in one and edge in the other.
- No calibration of the field of view, so density has no real unit.
- Using nail varnish without ventilation, or taking leaves from protected plants.
- Concluding light causes the change, when temperature or water also differ between sites.
More Biology IA ideas like this
- Transpiration and the air's drying power●●○
- Leaf thickness along a light gradient●○○
- Does metabolic rate really scale with mass^0.75?●○○
This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.