Practical skill 8: The pathway of water through a plant

Aim

To show where water travels in the above-ground parts of a plant by placing cut stems in water containing a stain.

Background

Water enters a plant through the root hairs, crosses the root and moves into the xylem. The xylem vessels carry water up the stem to the leaves. Water evaporates from the leaves in transpiration and the loss pulls more water up the stem.

A coloured stain (dye) such as red food colouring or eosin is carried in the water. Where the stain is found shows the route that the water has taken, and how far it has gone in a given time shows the speed.

In a stem, the xylem is found in the vascular bundles, together with the phloem. In a transverse section, the vascular bundles form a ring of small areas near the outside. Only the xylem will be stained. A celery stalk is really a leaf stalk, but it carries water to the leaves in the same way as a stem.

Hypothesis

The stain will be found in the xylem in the vascular bundles of the stem, and will rise further in the stem the longer the stem is left, because water is pulled up the xylem as water is lost from the leaves.

Variables

Equipment

• 6 fresh celery stalks with leaves, cut to the same length (about 20 cm)
• 6 beakers (100 cm³)
• Water and red food colouring or eosin stain, and a dropping pipette
• Scalpel or sharp knife and white tile
• Paper towels
• Ruler graduated in mm
• Hand lens
• Lamp, to speed up transpiration
• Stop-clock
• Marker pen
• Eye protection

Risk assessment

Method

1. Put on eye protection. Label five beakers 10, 20, 30, 40 and 50 min, and a sixth beaker ‘overnight’.
2. Put 50 cm³ of water in each beaker and add the same number of drops of stain (for example 10 drops) to each, so that the stain is the same strength in every beaker.
3. On a white tile, cut about 1 cm from the base of each celery stalk with a sharp scalpel, to give a fresh cut end. Stand each stalk in its beaker straight away.
4. Use a marker pen to mark each stalk at the surface of the stained water. Start the stop-clock.
5. Place all the beakers close together under the same lamp, so that all the stalks are in the same conditions.
6. After 10 minutes, take the stalk out of the 10 min beaker and blot the base dry with a paper towel.
7. On the tile, cut thin slices (transverse sections) across the stalk, starting at the water mark and working upwards, until you reach a slice with no stain.
8. Measure the distance from the water mark to the highest slice with stain, to the nearest mm. Record it in cm, to one decimal place.
9. Use a hand lens to look at a stained transverse section. Draw it and show which parts are stained.
10. Repeat steps 6 to 9 for the other stalks at 20, 30, 40 and 50 minutes.
11. Leave the sixth stalk overnight. Next lesson, look for stain in the veins of its leaves, and cut a thin longitudinal section of the stalk to see the stained xylem as lines.
12. Clear away, wash your hands and dispose of the plant material.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Line graph. Plot the height of stain above water level / cm on the y-axis and the time in stain / min on the x-axis. Use more than half the grid in both directions, mark each point with a cross (×) and draw a single thin best-fit straight line. Use the gradient of the line (change in height ÷ change in time) to find the speed of water movement in cm per min.