Locate xylem and phloem in dicot sections
Xylem and phloem have characteristic positions in typical herbaceous dicot roots, stems and leaves, but the specimen remains the evidence.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In a typical herbaceous dicot stem bundle, where is xylem relative to phloem?
What you will learn
- Describe the distribution of xylem and phloem in transverse sections of dicot roots, stems and leaves
- Use organ orientation and image evidence when identifying vascular tissue
Compare the three transverse arrangements
In a typical herbaceous dicot stem, vascular bundles form a ring; within a bundle, xylem is on the inner side towards the pith and phloem is on the outer side. In a typical root, xylem occupies the central region, often in a star-like pattern, while phloem lies between the xylem arms.
In a typical leaf vein, xylem is nearer the upper surface and phloem nearer the lower surface. These are useful patterns, not permission to ignore a specimen’s orientation or variation. State the organ and the feature that supports the identification.
Read position together with tissue structure
Thick-walled vessel elements support xylem identification; smaller living conducting cells and associated companion cells support phloem identification. Position alone can be ambiguous in a close-up or damaged specimen, so combine location, cell structure and the image key.
Do not call every cell outside xylem phloem: supportive fibres and other tissues may be present. A photomicrograph can also show only one vascular bundle, not the entire ring of a stem.
Pause and explain
Where is phloem typically found in a dicot root with central star-shaped xylem?
Worked example
A transverse root section has a central star-shaped region with smaller vascular patches between its arms. Identify the tissues.
Show the worked solution
- The central star-shaped region is xylem.
- The patches between the arms are phloem in the typical dicot-root arrangement.
Answer The central star-shaped region is xylem; phloem occurs between the xylem arms in a typical herbaceous dicot root.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the typical positions of xylem and phloem in a dicot stem and leaf vein.
Give me a hint
Keep the organ orientation clear.
Compare my reasoning
- Stem bundle: xylem faces the pith; phloem faces outward.
- Leaf vein: xylem is nearer the upper surface; phloem nearer the lower surface.
In a typical dicot stem bundle, xylem is on the inner side towards the pith and phloem is outside it. In a leaf vein, xylem is typically nearer the upper surface and phloem nearer the lower surface.
Look for these in your work
- Stem relationship correct
- Leaf orientation correct
A transverse dicot root has xylem arms from its centre. Where would you look for phloem?
Give me a hint
Use the intervals between the xylem arms.
Compare my reasoning
- Identify the central xylem first.
- Look for phloem patches between its arms.
In the typical dicot-root pattern, phloem is found between the arms of the central xylem region.
Look for these in your work
- Root xylem located
- Phloem position correct
A close-up shows thick red-stained vessels near the inner side of a stem bundle and small cells outside them. Explain how you would identify xylem and phloem without relying on stain alone.
Give me a hint
Use position and visible cell features as separate evidence.
Compare my reasoning
- Thick vessel walls and inner bundle position support xylem.
- The conducting phloem is outside xylem; exclude a distinct fibre cap if present.
The thick-walled vessels on the inner side support identification as xylem. Conducting phloem lies outside the xylem, but I would use the image key and cell features to distinguish it from any supporting fibre cap; stain colour alone is insufficient.
Look for these in your work
- Xylem evidence stated
- Phloem distinguished from support tissue
- Stain limit stated
Common mistakes
- Putting stem phloem on the inner side of the bundle
- Calling the central root xylem a stem pith
- Using colour alone when the image key shows that other tissues share the stain
What can you explain now?
In a typical herbaceous dicot stem bundle, where is xylem relative to phloem?
Compare with the explanation
Xylem is towards the pith, inside the phloem
In a typical stem bundle, xylem is inner and phloem outer.
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Make it stick
Where are xylem and phloem typically located in a dicot root, stem bundle and leaf vein?
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