Question 2 — Microscopy and stereology of a plant stem section (slide J1)
Draw a large low-power plan of the whole stem section on J1 (labelling the xylem), then a high-power drawing of four adjacent epidermal cells (labelling the waxy cuticle). Compare J1 with a printed photomicrograph of a different stem, then use a sector count and an angle from a second photomicrograph to estimate its vascular-bundle density.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 2 — microscope and slide
- microscope(1 between 2)
- slide J1(1 between 2)
SLIDE J1 — LOW POWER, WHOLE STEM SECTION (GENERATED REFERENCE)
J1 is a real, confidential Cambridge specimen — this is a plausible stand-in stem cross-section, not a photograph of J1 itself. Use it to practise the drawing, not to copy it: your actual mark in 2(a)(i) is for a large, ruled, correctly-proportioned plan diagram of the real slide on your bench, which only a human examiner can judge.
EPIDERMIS — FOUR ADJACENT CELLS, HIGH POWER (GENERATED REFERENCE)
Again, a stand-in — not J1. Your real mark in 2(a)(ii) is for a large hand drawing of four adjacent epidermal cells from the actual slide, each touching at least one other, with a ruled label line to the waxy cuticle. Eye-marked only.
FIG. 2.2 — STEM SECTION WITH SECTOR P MARKED (GENERATED)
Click every vascular bundle you judge to be a whole bundle, entirely inside sector P (shaded). Unlike the two drawings above, this diagram is generated for this sim, so there is a real correct answer — and the sim can check it.
Why count one sector and scale up, instead of counting the whole section?
Counting every bundle across a whole stem section is slow and, on a real crowded slide, error-prone. Sampling a known fraction — here a known fraction of the angle, θ — and multiplying up is the same logic behind quadrat sampling in ecology: trade a little precision for a method you can actually finish, and repeat, inside the exam.
Why does the answer depend on 360 ÷ θ specifically?
Sector P is one slice of a pie cut into equal wedges, each spanning θ = 15°. If the bundles are roughly evenly scattered around the ring, the whole section should hold about 360 ÷ 15 = 24 times as many as any single 15° slice — so total ≈ 24 × (count in P).
Why divide by area, and not just report the total?
A raw count of "96 bundles" doesn't tell you how tightly packed they are unless you also know the size of the section holding them. Dividing by the area (44 mm²) turns a count into a density — a number you could compare against a stem of a completely different size.
YOUR TASKS · 0/4
- ✓Study the low-power plan view and find the xylem
- ✓Study the four epidermal cells and the waxy cuticle
- ✓Count the whole vascular bundles in sector P
- ✓Check your count against the real judgement calls
YOUR SECTOR-P CALCULATION
Density is given to 2 significant figures, as the question asks. Your count above is reported as cell p.count — the exact table cell question 2(b)(i) expects, when this sim runs inside a full paper attempt.
A different candidate counted 5 whole bundles in sector P. What would their extrapolated total for the whole section be?
YOUR ANSWERS
J1 is a slide of a stained transverse section through a plant stem. Draw a large plan diagram of the whole section on J1. Use one ruled label line and label to identify the xylem.
Observe the epidermis of the stem on J1. Select a group of four adjacent epidermal cells, each touching at least one other. Make a large drawing of this group of four cells and the waxy cuticle. Use one ruled label line and label to identify the waxy cuticle.
Fig. 2.1 is a photomicrograph of a stained transverse section of a stem from a different type of plant from J1. Identify three observable differences, other than colour, between the stem section on J1 and the stem section in Fig. 2.1, and record them in Table 2.1.
Fig. 2.2 is a photomicrograph of a stained transverse section of a stem from a different type of plant. The circle represents the area of the stem. To calculate the density of vascular bundles you will first need to count the number of whole vascular bundles in sector P.
| sector P | |
|---|---|
| number of whole vascular bundles |
Use your answer to (b)(i) to estimate the total number of vascular bundles in the stem section shown in Fig. 2.2. The angle θ = 15°. Show your working.
The stem section shown in Fig. 2.2 has an actual area of 44 mm². Use your answer in (b)(ii) to calculate the density of vascular bundles in the stem section. Give your answer to two significant figures. Show your working.