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Capillary Action

In this activity, students see capillary action . . . in action! Watch coloured water flow up a celery stalk, showing just how water moves from the roots of plants to their leaves.

All plants need water to survive. Plants use water to carry nutrients to their roots, stems, leaves and flowers and to prevent them from drying out and wilting.

How does a plant move water from the ground up into the rest of the plant?

Just as humans have veins and arteries that move blood around inside the body, plants have xylem to move water. This is the plant’s water transportation system. You’ve probably seen old xylem tissue—the rings on a tree trunk. Xylem tissues live for one year, then die, leaving a ring between the old and new xylem. 

Xylem tissue is made of millions of tiny tubes made of cellulose. Because water molecules like to stick together (cohesion) and like to stick to the walls of the tubes of cellulose (adhesion), they rise up the tubes all the way from the roots to the leaves. Water then evaporates from the leaves, helping to draw up more water from the roots. This process is called capillary action.

Celery is handy for demonstrating capillary action because it has a lot of xylem tubes in the stalk, making for fast water uptake. You’ll see the pale green leaves take on a reddish and bluish colour because the dissolved food colouring moves with the water through the xylem tubes into the celery stalk and leaves. The water evaporates through the leaves (transpiration) and deposits the colour in the plant. This helps pull more water into the plant to keep the xylem tubes filled, continuing the cycle of water uptake. 

Objectives

  • Explain the needs of plants by observing the effect of capillary action.

Materials

  • Per Student or Group:
    celery stalk with leaves attached
    2 tall, straight-walled drinking glasses or jars
    water
    red and blue food colouring
    sharp knife

Key Questions

  • How do plants get water from under the ground?
  • How does water travel into different parts of plants?
  • What kind of plants have stems that we eat?
  • What foods that we eat store a lot of water in them?

What To Do

  1. Cut the bottom of the stem or stalk with a sharp knife to freshly expose the xylem. Do this quickly so that the openings of the xylem tubes aren’t squished shut.
  2. Make a vertical slit about 5 cm (2 in) long in the bottom of the stalk.
  3. Fill two glasses halfway with cold water.
  4. Colour one with red food colouring and the other with blue. Use enough drops to make the water a deep colour to get the best effect.
  5. Put the two glasses of coloured water next to one another and put half of the stalk into one glass, and half into the other.
  6. Wait to see your celery transform! (This could take between 30 minutes (to see a small transformation) and overnight (to see colour all the way to the leaf tips.)
  7. Carefully examine the stalk and the leaves to identify the flow of water up the stalk.
  • Cut the celery stalk in half to see the xylem dyed with the food colouring.
  • Carefully remove the bulk of the stalk pulp. You should be able to observe the xylem tubes from a different angle.

Extensions

  • Try this with a carnation to see which flower petals are fed by each part of the stem. Leave the flower overnight for best results. This can be done as a Make & Take activity by providing supplies to each student.
  • Many coastal First Nations groups, like the Haida, have used the bark of trees (which contains xylem) to make objects like cups and even canoes. How do you think they could remove some of the bark and xylem of a tree without damaging the whole tree?

Other Resources

Science World | YouTube| Explore a Flower’s Vascular System