Identify xylem and phloem in sections of roots, stems and leaves, using a light microscope.

Learning Goal

Students will be able to identify xylem and phloem in transverse sections of roots, stems and leaves using a light microscope and to explain how their structures relate to their functions, as required by Cambridge IGCSE 0610 (Transport in Plants).

Learning Objective

Identify xylem and phloem in sections of roots, stems and leaves, and relate each tissue’s structure to its function.

Quick‑Check Checklist

• Xylem – dark‑staining, thick lignified walls, located towards the centre of the organ.
• Phloem (sieve tubes) – lighter‑staining, thin walls, contains sieve plates, located towards the periphery of the organ.

Functions Linked to Structure

Typical Arrangement of Vascular Bundles

Microscopic Identification (IKI or Safranin‑Fast Green stain)

• Xylem
  • Long, narrow cells with thick, lignified secondary walls.
  • Vessels appear as empty tubes with perforation plates; tracheids are tapered and have bordered pits.
  • Stains dark (red‑purple) because lignin binds the dye.
• Phloem (sieve tubes)
  • Elongated cells with thin walls, lacking nuclei; sieve plates visible as rows of pores.
  • Companion cells are smaller, densely cytoplasmic and contain a nucleus.
  • Stains lighter (pink‑orange) than xylem.

Structure‑Function Comparison Table

Pathway of Water Uptake (Root to Leaf)

Water moves from the soil into the plant as follows:

1. Root hairs (increase surface area)
2. Cortex – apoplastic (cell walls) and symplastic (through plasmodesmata) routes
3. Endodermis – Casparian strip forces entry into the symplast
4. Stele – enters xylem vessels
5. Stem – upward through continuous xylem columns
6. Leaves – exits via stomata (transpiration pull)

Quick‑Draw Activity (AO 1 – Identify)

1. On a blank sheet, draw three separate transverse sections:
   • Root
   • Dicot stem
   • Dicot leaf midrib
2. Label the xylem (dark) and phloem (light) in each drawing.
3. Indicate the orientation (e.g., “adaxial” vs. “abaxial” for leaf).
4. Check your sketches against the tables above.

Step‑by‑Step Observation Procedure

1. Collect fresh samples from a healthy dicot (e.g., bean) **and** a monocot (e.g., wheat).
2. Using a hand microtome or a very sharp razor, cut thin transverse sections (~10 µm).
3. Place the section on a clean slide, add a drop of IKI (or Safranin‑Fast Green) stain.
4. Lower a cover slip gently to avoid air bubbles.
5. Locate the vascular region at low power (×40), then increase to ×100–×400 for detail.
6. Record:
   • Which tissue is darker (xylem) and which is lighter (phloem).
   • Relative position to the centre of the organ.
   • Distinctive cell types (vessels, tracheids, sieve plates, companion cells).

Mini‑Experiment: Investigating Transpiration

Purpose: Observe how temperature, wind and humidity affect water loss from leaves.

1. Punch out equal‑size leaf discs (≈1 cm diameter) from the same plant.
2. Place each disc in a separate pre‑weighed petri dish.
3. Expose the dishes to different conditions:
   • Room temperature (control)
   • Warm lamp (higher temperature)
   • Fan (increased airflow)
   • Humid chamber (high humidity)
4. After 30 min, re‑weigh the dishes and calculate mass loss (≈ water loss).
5. Discuss results in terms of transpiration rate and its influence on the cohesion‑tension mechanism in the xylem.

Investigation of Factors Influencing Water Uptake (Diffusion/Osmosis)

Goal: Test how temperature and solute concentration affect the rate of dye uptake in a root segment.

1. Cut three 2‑cm root tips from the same plant and place each in a separate beaker of 0.01 % methylene‑blue solution.
2. Treat the beakers as follows:
   • Ice bath (≈5 °C)
   • Room temperature (≈22 °C)
   • Warm water bath (≈35 °C)
3. After 10 min, remove the roots, blot gently, and measure the length of coloured zone with a ruler.
4. Repeat the experiment using 0 %, 0.5 % and 1 % sucrose solutions at a constant temperature (22 °C) to illustrate the effect of a concentration gradient.
5. Analyse the data: higher temperature → faster diffusion; higher external solute concentration → reduced net water uptake (osmotic inhibition).

Pressure‑Flow Mechanism (Phloem Translocation) – Extension

At a **source** (e.g., mature leaf) sucrose is actively loaded into sieve tubes, raising the osmotic pressure. Water follows osmotically, increasing turgor pressure and driving the sap toward a **sink** (e.g., root, growing tip). At the sink, sucrose is unloaded, water exits, and pressure drops, allowing continuous flow.

Guided Question: *If a leaf is covered with aluminium foil so that photosynthesis stops, what will happen to the pressure gradient in the phloem of that leaf and why?* (Answer: loading of sucrose ceases, osmotic pressure falls, turgor pressure drops, and the pressure gradient reverses or disappears, stopping the flow of sap away from that leaf.)

Comparison of Xylem and Phloem in Different Organs

Common Mistakes to Avoid

• Reversing xylem/phloem orientation in leaves – remember xylem is always towards the **upper (adaxial)** surface.
• Confusing sclerenchyma fibres with phloem – fibres are lignified, lack sieve plates, and are usually external to the phloem.
• Using sections that are too thick – they obscure cellular details and hinder identification.
• Neglecting monocot examples – the inner‑xylem/outer‑phloem rule still applies, but bundles are scattered.

Suggested Diagrams for the Notebook

Summary

By preparing thin sections, applying a suitable stain, and observing under a light microscope, students can reliably locate and differentiate xylem and phloem in roots, stems and leaves. The consistent positional pattern—xylem towards the centre of the organ and phloem towards the periphery—directly reflects their functions: thick‑walled, lignified xylem resists the tension generated by transpiration and conducts water upward; thin‑walled sieve tubes with sieve plates allow rapid, low‑resistance movement of sugars bidirectionally. Complementary activities—quick‑draw sketches, investigations of transpiration and water‑uptake factors, and a pressure‑flow question—reinforce the required knowledge, skills and application (AO1 & AO2) for the Cambridge IGCSE 0610 syllabus.