IB Biology SLTopic 2 — Transport in Animals & PlantsPaper 1 & 2Practical skill~11 min read
Drawing Root & Stem Structure (Skills)
Same plant, same two tissues, arranged in two completely different ways. Learn where the xylem sits in each, and you can identify any transverse section in seconds — and draw one that scores full marks.
📚 What you need to know
Stem: vascular bundles are arranged in a ring, with a large pith in the middle.
Root: xylem sits centrally in a cross shape, with phloem tucked between the arms.
Both have an epidermis on the outside and a cortex of parenchyma beneath it.
Parenchyma cells store starch and other substances. Pith is the central parenchyma of a stem.
The root also has an endodermis, the boundary between the cortex and the vascular tissue.
A plan diagram shows tissue outlines only — no individual cells, no shading.
The one thing that tells them apart
Forget everything else for a moment and look at the middle of the section. If the centre is a big open area of storage cells, you are looking at a stem. If the centre is filled with a cross or star of xylem, it is a root.
Red marks the xylem and blue the phloem in both sections. Notice the phloem sits between the arms of the cross in the root.
🧠 The trick everyone remembers
x = a cross = xylem. If the centre of the section looks like a cross or a star, it is xylem, so it is a root.
🤔 Why are they arranged differently?
A root is pulled. As the plant is tugged by wind, or as it anchors itself, the root has to resist being stretched lengthways. Material bunched in the middle resists pulling well, so a solid central core of lignified xylem is the sensible design.
A stem is bent. Wind pushes it sideways, so the stress is greatest near the outside. Material placed in a ring near the edge resists bending far better than material in the middle — the same reason a scaffolding pole is a hollow tube rather than a solid rod. So the stem puts its stiff xylem in a ring.
What is in each section
Tissue
In the stem
In the root
Epidermis
Outer layer; reduces water loss and gives protection
Outer layer; some cells extend into root hairs that absorb water and ions
Cortex
Parenchyma just under the epidermis; stores starch
Wide band of parenchyma; stores starch
Xylem
Inner side of each bundle in the ring
Central, in a cross or star shape
Phloem
Outer side of each bundle in the ring
In patches between the arms of the xylem cross
Pith
Large central region of parenchyma
Not present; the centre is taken up by xylem
Endodermis
Not usually shown
Ring forming the boundary between cortex and vascular tissue
Drawing a plan diagram that scores
A plan diagram is a map of the tissues, not a picture of the cells. Examiners mark it against a short list of rules, and most lost marks come from breaking one or two of them without realising.
🧩 The rules, in the order you will use them
Draw big. Fill at least half the space you are given. A small drawing is hard to label and costs marks.
Outlines only. Show the boundary of each tissue. Never draw individual cells.
Clear, continuous lines. One confident line per boundary, not a sketchy set of overlapping strokes.
No shading. Not even light shading to show a difference. Use labels instead.
Keep the proportions. If the cortex is a third of the radius in the micrograph, make it a third in your drawing.
Label every region with a straight, ruled line that touches the tissue it names.
Add a scale bar or state the size of the specimen or the magnification.
Annotate. Add a short note of what each labelled tissue does — this is expected at SL.
If you find yourself drawing cell walls, stop. That is a high-power detail drawing, which is a different task entirely.
Students lose more marks for drawing too small than for anything else on this skill. Give yourself a big circle before you start adding anything inside it.
Working out the magnification of your drawing
If a question asks for the magnification of a drawing rather than of a microscope image, it is the same idea flipped round.
Magnification of a drawing
magnification = drawing size ÷ actual size
Both measurements must be in the same unit before you divide, otherwise the answer is out by a factor of 1000. Convert first, divide second, and write the answer as a × number with no units.
Worked examples
WORKED EXAMPLE
A transverse section shows a cross-shaped region of tissue at the centre, with small patches of a second tissue between its arms. Identify the organ and the two tissues, giving a reason. [3 marks]
Step 1: read the centre
A cross or star shape in the middle means xylem, and central xylem is a root feature.
Step 2: name the second tissue
The patches sitting between the arms of the cross are phloem.
Step 3: say why it is not a stem
In a stem the vascular bundles would be arranged in a ring with a large pith in the centre, which is not what is shown.
Root; central cross = xylem, patches between arms = phloem
WORKED EXAMPLE
A student’s plan diagram of a root is 90 mm across. The real root is 3.0 mm across. Calculate the magnification of the drawing. [2 marks]
Step 1: check the units match
Both are already in millimetres, so no conversion is needed.
Step 2: divide drawing by actual90 ÷ 3.0 = 30Magnification = ×30No units on a magnification — the mm cancel out. Write it with the multiplication sign.
WORKED EXAMPLE
A student shades the cortex grey, draws in about twenty individual cells, and fits the whole diagram into a corner of the page. State three improvements. [3 marks]
Improvement 1
Remove the shading — plan diagrams should not be shaded, and the tissue should be identified with a label instead.
Improvement 2
Draw the outlines of the tissues only, not individual cells, since a plan diagram shows the distribution of tissues.
Improvement 3
Redraw it large enough to fill at least half the available space, so the regions can be labelled clearly and accurately.
3 rules, 3 marks — each fix paired with its reasonAdding a scale bar and annotations would also be credited if a fourth mark were available.
💡 Exam tip
Check the centre of the section before anything else. Pith means stem; a cross of xylem means root.
Use a sharp pencil and a ruler for label lines. Freehand arrows and shaded regions cost easy marks.
Label lines must touch the tissue, not stop somewhere near it.
Write annotations as short function statements: “xylem — carries water and mineral ions upwards”.
Magnification has no units. Actual size always keeps the unit you divided in.
If the question says “plan diagram”, the word “plan” is doing the work: outlines and nothing else.
⚠ Common mix-up
Drawing individual cells. The single most common way to lose marks on a plan diagram.
Shading tissues to tell them apart. Use labels, never tone.
Putting the pith in a root. Roots do not have one — the xylem occupies the centre.
Xylem and phloem swapped in the stem bundle. Xylem is on the inner side, phloem on the outer side.
Multiplying instead of dividing when finding the magnification of a drawing.
Drawing too small. Half the space is the minimum, and it also makes labelling much easier.
That completes Transport in Animals & Plants. Up next in Topic 2: Gas Exchange — the same exchange-surface rules you met in the capillaries, this time in the alveoli.
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