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 K224 / K225, 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?
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.
Xylem carries water and mineral ions. Its mature vessels are hollow and dead with lignified walls. In a dicot stem bundle it is nearer the centre; in a leaf vein it is towards the upper surface.
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?
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 cross-section of a young stem, the vascular bundles form a ring. In each bundle, the xylem is on the inside, nearer the centre, and the phloem is on the outside.
In a leaf vein, the xylem is usually towards the upper surface and the phloem towards the lower surface. First decide whether you are looking at a stem or a leaf.
Xylem vessels carry water and mineral ions from the roots up to the leaves. They are hollow tubes with thick walls, which also help support the plant. Phloem carries food, mainly sucrose, to the parts of the plant that use or store it.
In a real microscope section, xylem vessels usually look like large, open tubes with thick walls. Use both the position and the look of the tissue to identify it.
Step by step
- 1
Identify the organ
Stem and leaf sections have different reference directions.
- 2
Use position
Compare centre/outside in the stem or upper/lower in the leaf.
- 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?
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
Which transport tissue is which in a stem or leaf section?