Biology IA · B · Form and function · topic 11 of 25

Leaf thickness along a light gradient · IB Biology IA idea: guide, sample graph and step-by-step plan

no lab needed, but plan the sampling!

A plant has no legs, so it adjusts its leaves to where it grows. Two leaves of the same species, one in sun and one in shade, can be built very differently.

Difficulty●○○ Gentle
KitLight meter or phone sensor, balance, scanner or phone camera, image software
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like being outdoors
  • ✓ you can find one species in sun and shade
  • ✓ you want clear data without a lab

The big idea

← this is your RQ hook!

Leaves in deep shade tend to be thinner and larger for their mass, to catch scarce light. Leaves in full sun tend to be thicker and denser. Specific leaf area (SLA, area divided by mass) turns that into a number. Your job: choose sites that give a real range of light, measure the light and the leaves with a sampling plan you can defend, and ask what shape the relationship has. Fieldwork is messy, so the thinking is the mark-winner.

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 →

Dry mass instead of fresh

Dry the leaves in an oven to constant mass before weighing.

→ Removes the water-loss problem; you must say how you decided 'constant mass' and your DV is area per dry mass.
Add leaf thickness

Measure thickness with a micrometer or from a cut section under a microscope.

→ A second DV, so you can test whether SLA and thickness agree; keep your RQ focused on one.
Compare two species

A shade-tolerant and a sun-loving species along the same gradient.

→ Two lines on the graph; you can ask if one species adjusts more.
Sun leaves and shade leaves on one plant

Sample from the top and from the inside of the same canopy.

→ Reduces genetic variation between plants, since the same individual is compared. Needs a large enough plant.
Stomata or pigments too

Add a simple stomatal count or chlorophyll estimate in the same leaves.

→ A richer story, but only if the question stays narrow. It may be better as a second investigation.
Change the context

A woodland edge, an urban green space or a school garden, with a reason for the gradient.

→ Gives your sampling a purpose; analysis stays the same.

Your Biology IA research question: run the check

write it FIRST, then check it

Write your own question first. Then make sure it ticks every box:

  • Names the species, the independent variable (light level, and how it is measured) and the range you will sample.
  • Names the dependent variable (leaf area per unit mass) and says whether mass is fresh or dry.
  • Says how many sites, how many leaves per site, and how leaves will be chosen (age, position on the plant).
  • Says what you hold the same (species, leaf age, time of day of light readings, season).
  • Is narrow enough to complete with the sites and time you actually have access to.

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The set-up

One way to plan the sampling. Draw your own version, with your real sites and distances, for the Method section.

open sitepart shadedeep shadephoto of each leaf, flat, witha ruler in the shotweigh at once in asealed bag, or dry toconstant masslight meter at leaf height,same time of daysame species, sameleaf age and position

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.

Research question and context~250
Your question, where and why you chose this species and gradient, and what you expect the trend to be.
Background biology~400
Why light affects leaf structure; what specific leaf area measures; the trend your graph should show if your prediction holds (cite your source).
Variablestable
Independent, dependent and controlled variables, with how each is measured or kept the same, and why it matters.
Method~450
Where the sites are (map or sketch), how you chose leaves, how you measured light, area and mass, and enough detail that someone could repeat it at another site.
Safety, ethics, environment~100
Permission to sample, how many leaves you took, a protected-species check, terrain and plant hazards.
Raw datatable
One table per site or one combined table: light reading, area and mass of every leaf, units and uncertainties in the headers.
Processing~250
SLA for each leaf with its uncertainty, site means and a measure of spread, and one worked example of each calculation.
Graphs and analysis~400
Site means against light, with error bars (say SD or SE) and a justified fit; a rank correlation (Spearman) or Pearson on the site means, with n = the number of sites, not leaves.
Conclusion~400
Answer the question with numbers, compare with a published trend, and say how far your result applies.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on your result (few sites, leaf age, light readings), limits of fieldwork, and improvements linked to each.
Referencesnot counted
Every source for the method and the published trend.

What your Biology IA graph could look like

error bars on EVERY point ✓

Made-up data, to show you the form of a strong graph, not the answer. Your sites, species and numbers will differ. example only

0204060801004080120160200240280Light level / % of full sun (± 5%)Specific leaf area / cm² g⁻¹ dry mass (mean ± SE)curve that levels offstraight linesite means (10 leaves per site), with SEstraight line or curve?let the residuals decideILLUSTRATIVE · MADE-UP DATA
  • One point per site, using the mean of all the leaves from that site. Say how many leaves, and show the spread as SD or SE (say which).
  • Light is a field measurement, so give its uncertainty too: readings change with cloud and time of day. Use a reading in the open at the same moment as a reference.
  • Plotting leaves as separate points would suggest you have far more independent sites than you do. A site is the unit that varies in light.
  • Both a line and a curve are drawn so the reader can judge which one the data follows. Do not choose the curve just because it looks nicer.
  • Another graph to consider: plot the inverse, leaf mass per area (g cm⁻²), which is closer to leaf thickness and density. Does it tell the same story?

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result (mean specific leaf area at one site, assumed 150 cm² g⁻¹ on dry mass), not on raw readings. Here the spread between leaves is the biggest, so taking more leaves per site is the cheapest fix.

Leaf-to-leaf spread (SD 25, n = 10)5.3%Not fully dried (assumed 4% water left)4%Mass on a 0.001 g balance (0.050 g leaf)2%Area from photo (±1.5%)1.5%biggest effect: discuss this first!% uncertainty in mean SLAILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

don't skip the trial run! it saves you weeks

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind out what specific leaf area means and how light changes leaf structure. Write down your prediction, and what shape you expect, before you collect anything.
  2. Scout the sitesFind one species that grows across a light gradient. Check that you have permission, that it is not protected, and that you can reach at least five very different light levels.
  3. Trial run (1 lesson)Do the whole procedure on a few leaves: light reading, photo, weighing. How fast does a picked leaf lose mass? Does the photo give a sharp outline? Fix the problems now.
  4. Decide samplingChoose the number of sites and leaves per site, and a rule for picking leaves (age, position). Write down why. A rule you can repeat beats 'whatever looked healthy'.
  5. Measure the lightTake a reading at each site at leaf height, with a reference reading in the open at the same moment. Repeat, and record the weather.
  6. Collect the leavesTake only the few leaves you need. Seal them in labelled bags at once, and record the time of picking.
  7. Measure area and massPhotograph every leaf flat with a scale and measure the area in software. Weigh fresh at once, or dry to constant mass, and say which you did.
  8. Process and graphCalculate SLA for each leaf, then site means with spread. Plot site means against light with error bars and decide what fit is justified.
  9. EvaluateWhich weakness mattered most for your answer? Use your own spread and readings to rank them. Make every improvement specific.
  10. Write, then checkWrite in the order of the layout. Run the checklist below before your teacher's draft.

Biology you'll need

Traps to avoid in this Biology IA

  • Treating every leaf as an independent data point for the effect of light, when many leaves come from one site.
  • Weighing leaves after a long walk back: fresh mass falls fast, and SLA rises for the wrong reason.
  • Mixing leaf ages and positions, so you compare young leaves in shade with old leaves in sun.
  • A single light reading in the morning used for a site that is in full shade by noon.
  • Only two or three sites, so there is no real gradient to analyse.
  • Leaf area measured by eye or from a tilted photo, or with a curled leaf.
  • Picking more leaves than you need, or from a protected plant or without permission.

More Biology IA ideas like this

Make sure it's yours

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.

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