IB Biology HLTransport in Animals & PlantsPaper 1 & 2~10 min read
Drawing Root & Stem Structure
A root and a stem contain the same tissues arranged in completely different patterns. Learn the two patterns and you can identify either one instantly – and draw a plan diagram that picks up every available mark.
📚 What you need to know
A dicotyledonous stem contains epidermis on the outside, parenchyma forming the cortex and pith, and vascular tissue arranged in a ring of vascular bundles.
In a stem bundle, xylem lies on the inner side, phloem on the outer side, with cambium between them.
In a dicotyledonous root, the xylem is central and X-shaped, and phloem sits between the arms of the X.
The root epidermis has root hairs for absorbing water and mineral ions; the endodermis forms the boundary between the cortex and the vascular tissue.
Xylem carries water and dissolved mineral ions upwards; phloem carries organic solutes from leaves to the rest of the plant.
A plan diagram shows tissue outlines only – no individual cells, no shading, clear continuous ruled lines.
Add a scale bar or estimated size, and annotate the labelled tissues with their functions.
The stem: a ring near the outside
Cut a young dicot stem across and the vascular bundles form a ring just inside the cortex. That position is not random: it puts the strengthening tissue near the outside, where it does the most to stop the stem bending, in exactly the way a hollow tube is stiffer than a solid rod of the same mass.
Notice what is missing: no shading, no individual cells, and every label line ruled and horizontal.
The root: a cross in the middle
A root has the same list of tissues in a different arrangement. The xylem sits in the centre, shaped like an X, with phloem between the arms. A root is constantly being pulled as the stem is blown about, and a central core of lignified xylem resists that pulling far better than a ring would.
The four small circles are phloem, sitting in the gaps between the arms of the xylem cross.
🧠
X marks the root
An X in the centre means you are looking at a root. A ring of bundles means a stem. That one glance answers most identification questions.
What each tissue does
Tissue
Where it is
Function to annotate
Epidermis
Outer layer of both root and stem
Protection; prevents water loss in the stem; bears root hairs in the root
Cortex (parenchyma)
Under the epidermis
Stores starch and other substances; in the root, water passes through it to the xylem
Pith (parenchyma)
Centre of the stem
Storage and support
Xylem
Inner side of stem bundles; centre of root
Transports water and mineral ions upwards; provides support
Phloem
Outer side of stem bundles; between the xylem arms in a root
Transports organic solutes from sources to sinks
Endodermis
Ring around the vascular tissue of a root
Controls which substances enter the xylem
Notice the difference between a label and an annotation. A label just names the tissue. An annotation names it and says what it does. Root and stem questions specifically ask for annotations, so a bare set of labels is only half an answer.
Plan diagram rules, in short
🧩 Before you hand it in, check
Only tissue outlines are drawn – no individual cells anywhere.
Lines are clear and continuous, drawn with a sharp pencil, not sketched.
Nothing is shaded or coloured in.
Proportions match the micrograph – the cortex really is that thick, the stele really is that small.
The drawing fills at least half the space available.
Every tissue is clearly labelled with a ruled line that stops at the structure, and labels are horizontal and do not cross.
A scale bar or estimated size is included.
Labels are annotated with the function of each tissue.
Worked examples
WE 1
Root or stem?
A micrograph shows a transverse section of a plant organ with a star-shaped mass of thick-walled tissue in the centre and small groups of cells in the gaps around it. Identify the organ and justify your answer. (3 marks)
Step 1: identify the central tissue
The star-shaped, thick-walled tissue in the centre is xylem.
Step 2: identify the small groups
The groups of cells between the arms of the X are phloem.
Step 3: conclude
Central X-shaped xylem with phloem between the arms is the arrangement found in a root; a stem would show a ring of separate vascular bundles instead.
It is a rootjustify by describing the arrangement, not just by naming the organ
WE 2
Why the positions differ
Suggest why xylem is arranged centrally in a root but in a ring near the outside in a stem. (3 marks)
Point 1: forces on a root
A root is mainly subjected to pulling forces as the plant above is moved by wind.
Point 2: why central works
A central core of lignified xylem resists being pulled apart and anchors the plant.
Point 3: forces on a stem
A stem is mainly subjected to bending, and strengthening tissue placed near the outside resists bending far better, in the same way a hollow tube is stiffer than a solid rod of the same mass.
Central for pulling, near the edge for bending“suggest” wants reasoning from the forces involved, not memorised facts
WE 3
Annotating your drawing
A student has drawn a labelled plan diagram of a stem but has not annotated it. Write suitable annotations for xylem, phloem and epidermis. (3 marks)
Xylem
Transports water and dissolved mineral ions from the roots to the leaves, and provides support.
Phloem
Transports organic solutes such as sucrose from sources to sinks.
Epidermis
Forms the outer boundary, reducing water loss and protecting the tissues from herbivores and damage.
Each annotation names one clear functionkeep annotations short – one function per tissue is enough for the mark
💡 Exam tips
Learn the two arrangements as pictures: X in the centre = root, ring of bundles = stem.
In a stem bundle, put phloem outside and xylem inside, with cambium between.
Draw outlines only. Cells belong in a high-power drawing, not a plan diagram.
Use a ruler for label lines, keep them horizontal, and never cross them.
Include a scale bar and annotations – both are specifically asked for here.
Check your proportions against the micrograph before you start labelling.
⚠ Common mistakes
Drawing cells. The single most common error in plan diagram questions.
Shading the vascular tissue in. Never shade.
Putting a ring of bundles in a root or an X of xylem in a stem.
Labelling without annotating when the question asks for functions.
Forgetting the scale bar or estimated size.
Drawing the diagram too small to label clearly.
Up next: Tissue Fluid – back to animals, and the liquid that actually touches your cells.
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