K327 / K328 / 2027

Lesson 2 of 12 / Nutrition and transport in flowering plants

Finding xylem and phloem

Which transport tissue is which in a stem or leaf section?

I can identify xylem and phloem positions and functions in herbaceous dicot leaf and stem sections.

  • I can identify xylem/phloem positions and functions in leaf and stem sections.Syllabus

    Syllabus K327 / K328, Topic 8(b). Identify xylem/phloem positions and functions in leaf and stem sections.

About 6 min

Make a guess. You are not marked.

Xylem and phloem are both transport tubes in a plant. Are they both made of living cells?

  1. Yes. All plant tubes are living cells.
  2. No. Xylem vessels are dead; phloem sieve tubes are living.
  3. No. Both are dead, empty pipes with no contents.
Show the answer

No. Xylem vessels are dead; phloem sieve tubes are living.

Mature xylem vessels are hollow and dead. Phloem sieve-tube elements are living and need companion cells.

The key idea

Xylem carries water and mineral ions; phloem carries food, mainly sucrose. Their positions help identify them in a dicot section.

Where is each transport tissue in this section?
CentreBundles form a ring

Blue: xylem. Gold: phloem. Heavier outline: selected tissue. Original herbaceous dicot schematics, not to scale; a real microscope section may show only part of a structure.

Read a real microscope section

Find the ring of vascular bundles. Within a bundle, which large open vessels face the centre of the stem?

Light micrograph of a young sunflower stem cross-section with vascular bundles arranged around a large central pith.
Tap the image to see it full size. Image: Berkshire Community College Bioscience Image Library. CC0 1.0. Used without further changes.

Image size changes with your screen. Use a stated scale bar for measurements; do not calculate from display size alone. Practise specimen observation and focusing in your laboratory.

Explanation

In a typical herbaceous dicot stem cross-section, vascular bundles form a ring. Within each bundle, xylem is nearer the centre and phloem is nearer the outside. The central space is a reference for orientation, not a transport tissue label.

In a leaf vein, xylem is normally towards the upper surface and phloem towards the lower surface. First identify whether you are viewing a leaf or a stem, then use the correct reference directions. The same words inside and outside do not describe both sections clearly.

Xylem vessels transport water and dissolved mineral ions from the roots towards the leaves. Their hollow interiors provide a continuous route, and thickened walls provide support. Phloem transports food, mainly sucrose, to places where it is used or stored.

Compare these positions in the real microscope sections as well as the schematic. Xylem commonly has larger open vessels with thick walls. Use position and visible tissue patterns together, rather than assuming every circular opening is a xylem vessel or that artificial colours are present in all specimens.

Step by step
  1. 1

    Identify the organ

    Stem and leaf sections have different reference directions.

  2. 2

    Use position

    Compare centre/outside in the stem or upper/lower in the leaf.

  3. 3

    Confirm with structure and cargo

    Match thick hollow xylem vessels with water/mineral ions and living phloem with food transport.

Worked example

A stem bundle

In a herbaceous dicot stem, tissue A lies on the inner side of a vascular bundle and has large thick-walled vessels. Identify it and its transported substances.

One way to explain it

A is xylem. It transports water and dissolved mineral ions from roots towards the leaves, and its lignified walls also provide support.

Why this answer works
  • Inner position is relative to the stem centre.
  • Large thick-walled vessels support the identification.
  • State water and mineral ions, not sucrose.

Watch out for this

A student says: "Xylem and phloem carry the same substance in opposite directions." What is wrong with this?

Show the answer

Xylem supplies water and mineral ions; phloem carries food, mainly sucrose. Their main transported materials differ.

Check your understanding

Which identifies a typical herbaceous dicot stem bundle correctly?

  1. Xylem is on the inner side and phloem on the outer side.
  2. The bundles join in the centre to form a single tube.
  3. Phloem lies nearer the centre and carries water upwards.
Show the answer

Xylem is on the inner side and phloem on the outer side.

Name both the position within a bundle and the different transported materials.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which transport tissue is which in a stem or leaf section?

You can return to this lesson any time.