2.5.4 Practical: The Xylem

2.5.4 Practical: The Xylem: Describe how a coloured dye can be used to show the pathway of water in a plant stem and what the results reveal about xylem.

Principle of the Investigation

Continuous system

Water moves up through a plant in a continuous system of xylem vessels from the roots to the leaves.

Use of a dye

A coloured dye carried in the water stains the xylem vessels, which makes their position easy to see.

Suitable plant

A fresh celery stalk with leaves is commonly used because its vessels are easy to see in a cut section.

Aim

Show which tissue in the stem transports water and where that tissue is located in the stem.

Add a few drops of coloured dye, such as food colouring or eosin, to water in a beaker and mix gently.

Cut the base of the celery stalk at an angle so that a larger surface is available for taking up liquid.

Stand the stalk in the dyed water and leave it until colour shows in the leaves or the veins.

Remove the stalk and blot it dry, then use a scalpel on a cutting tile to cut a thin cross-section of the dyed stem.

Look at the section with a hand lens or microscope and note where the dye has appeared.

Observation

The dye appears as small coloured spots arranged in a ring or pattern within the stem section.

Conclusion

The stained spots are the xylem vessels, so water and dissolved substances travel up the plant in the xylem.

Leaf veins

Coloured veins in the leaf show that the xylem continues from the stem into the leaf.

Evidence

Only the xylem is stained, which suggests that other tissues do not carry the water upwards.

Safety and Control

Sharp blades

Cut away from your body on a cutting tile, keep fingers clear of the blade and use a clean sharp scalpel.

Staining

Dyes can stain skin and clothing, so wear a lab coat and wash your hands afterwards.

Fresh material

Use a freshly cut stalk because old, wilted material takes up water more slowly and gives poor results.

Control idea

Using the same time, volume of dye and type of plant for each trial allows results to be compared fairly.

2.5.4 – Key Points for the Exam

Xylem: The tissue that carries water and mineral ions up from the roots.
Dye: A coloured substance used to make the pathway of water visible.
Cross-section: A thin slice cut across a stem to show its internal structure.
Transpiration stream: The continuous upward movement of water through the xylem.

Common Mistakes

Saying that the dye travels in the phloem; the stained tissue is the xylem.
Cutting a thick section, which makes the stained vessels hard to see.
Cutting towards the hand holding the stalk instead of away from the body.
Using a dry, old stalk that has stopped taking up water.