Microscopy and stereology: a plant stem cross-section (slide J1)
A fixed, confidential, pre-mounted stem section — not one you prepare yourself. Draw a large low-power plan (labelling the xylem) and a high-power group of four epidermal cells (labelling the waxy cuticle), compare the slide against a printed photomicrograph of a different stem, then use a sector count and a known angle to estimate the section's vascular-bundle density.
WHAT YOU'LL LEARN
- ✓Large-format biological plan and cell drawing to strict conventions: continuous sharp lines, no shading, correct proportions
- ✓Comparing two specimens by describable, observable differences rather than by guessing at identity
- ✓Stereological sampling: counting a representative sector and scaling up by its angle fraction of 360°
- ✓Turning a raw count into a density by dividing by the section's known area
ON YOUR BENCH
- □Light microscope — ×10 eyepiece; ×10 (low-power) and ×40 (high-power) objectives only, no other lenses fitted
- □Slide J1 — stained transverse section through a plant stem (confidential specimen, supplied already mounted)
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Set the scope to low power before J1 goes on the stage
J1 arrives already mounted — there is no wet-mounting, staining or cover-slipping to do here. Start on the ×10 objective so you can see the whole section at once before hunting for detail.
- 02
Draw a large low-power plan of the whole section
Use most of the space available. Show the tissue layers as continuous outlines only — no individual cells at this magnification — and get the relative sizes of the layers right. Add one ruled label line to identify the xylem.
- 03
Switch to high power and find four adjacent epidermal cells
Move to the ×40 objective and refocus with the fine adjustment only. Choose a group of exactly four epidermal cells, each touching at least one other. Draw them large, with the cell wall as two clean lines, and add one ruled label line to the waxy cuticle.
- 04
Compare J1 with the printed photomicrograph (Fig. 2.1)
Fig. 2.1 shows a stem from a different plant. Look for real, describable differences other than colour: how thick the cuticle looks, whether trichomes (hairs) are present, how large the cortex cells appear, and how large the central pith looks. Record any three in Table 2.1.
- 05
Count whole vascular bundles in sector P (Fig. 2.2)
Fig. 2.2 is a second photomicrograph, shown as a circle with one sector, P, marked out at a stated angle θ. Count only bundles that are whole and fall entirely inside P — a bundle merely touching or crossing P's boundary line doesn't count.
- 06
Extrapolate to the whole section and calculate density
Scale your count in P up to the full circle using 360 ÷ θ, then divide that total by the section's given area (44 mm²) to get a density in bundles per mm², to 2 significant figures.
What this task actually rewards
- →The two drawings (2(a)(i) and 2(a)(ii)) are marked by a human examiner against a real specimen — nothing here can substitute for the observation and drawing skill itself.
- →The comparison (2(a)(iii)) rewards any three of: cuticle thickness, presence/absence of trichomes, cortex cell size, central pith size.
- →The sector count and calculation (2(b)) is the one genuinely mechanical part: count in P, multiply by 360 ÷ θ, then divide by the section area for density.
- →With θ = 15° (so 360 ÷ 15 = 24) and a section area of 44 mm², a sector count of 4 gives a total of 96 bundles and a density of 96 ÷ 44 ≈ 2.2 bundles/mm² — the kind of order of magnitude a real stem section should show.
⚠ BEFORE YOU START
- •Carry the microscope with both hands, one under the base.
- •J1 is a shared, confidential specimen on a rota between two candidates — handle the slide only by its edges and never force the objectives down onto it.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.