Transpiration: How Much Water Does a Tree Transpire in One Day?

Plants absorb water primarily through their roots. They evaporate water through openings in their leaves in a process called transpiration. Plants transpire vast quantities of water - only one percent of all water a plant absorbs is used in photosynthesis; the rest is lost through transpiration. In one growing season for example, one corn plant transpires over 200 liters of water.

Transpiration, along with evaporation of moisture on land, provides almost two-thirds of the atmospheric moisture that falls as precipitation on land surfaces, powerfully affecting global and local climate. Surprisingly, evaporation from the vast ocean surfaces only accounts for one-third of atmospheric moisture.

In this activity, you will make a small terrarium that will allow you to observe and measure the water given off through transpiration.

Materials (per team)

Procedure

  1. Using the scissors, make a small hole (just big enough for the plant stem) in the center of the piece of cardboard.

  2. Pull the plant stem through the hole and seal around the hole with petroleum jelly.

  3. Fill the bottom cup with water and place the stem with the cardboard collar into the cup. Cover with the clear plastic cup as shown.

  4. Put the small terrarium in the sun or under a lamp.

  5. In fifteen minutes, you should begin to see droplets of water on the sides of the clear inverted cup. More moisture will accumulate with time.

  6. If possible, leave the terrarium cups set up in the classroom for several days and measure the total amount of water transpired.

  7. Calculate the water loss per square centimeter of leaf area (you can estimate the surface area of a leaf by tracing it onto a piece of graph paper that you have marked into square centimeters and then counting the number of squares the leaf covers).


Observations and Questions

Answer in your lab book or on a separate piece of paper.

  1. Where does the moisture come from that accumulates along the sides of the top cup?

  2. How do you know the water is coming from the plant and not just evaporating from the water in the cup?

  3. Imagine that your small plant was a large tree with a thousand times as many leaves. Assume that this tree transpires just like your plant. Calculate how much water it would transpire over the time you ran your experiment.

  4. Now imagine a small forest with 1000 such trees. How much water would it transpire?

  5. Do you think that the amount of water coming from the forest would affect local climate in any way? If so, how?

When you're finished with the activity, click on Back to Teacher Guide at the top of the page.