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

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

Background

Trees absorb water primarily through their roots. They evaporate water through openings in their leaves in a process called transpiration. As with human respiration, trees tend to transpire more with increased temperatures, sunlight intensity, water supply, and size. When it gets too hot, though, transpiration will shut down.

Many factors influence transpiration rates, including leaf shape, size, pores (stomata), and waxiness of the leaf surfaces. Where a particular tree species grows often depends upon how it has adapted its transpiration rate to a particular climate. Conifer needles are more efficient at retaining moisture than broadleaf trees because they have stiff, waxy leaves (needles) with small stomata that are recessed in the leaf surface. Because they are efficient in retaining water, conifers are found in drier and colder climates where water supplies are limited.

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, one corn plant transpires over 200 liters.

Transpiration, along with evaporation of moisture on land, provides almost two-thirds of the atmospheric moisture that falls as precipitation on land surfaces. The remaining one-third comes from the evaporation of the vast oceans.

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

Learning Goals

  1. Students will be able to recognize transpiration and explain its value to the plant.

  2. Students will be able to explain how transpiration affects climate.

Alignment to National Standards

National Science Education Standards

Benchmarks for Science Literacy, Project 2061, AAAS

Grade Level/Time

Materials For Each Team of Students

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 amount of water transpired.

  7. Ask students to calculate the water loss per square centimeter of leaf area.

Observations and Questions

  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?

Assessment Ideas

Modifications for Alternative Learners

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