Picture of the setup for the transpiration demo experiment


Students conduct a transpiration experiment using a plant in their schoolyard. They will set up the experiment, make a hypothesis and observe and explain the results.



  • For the quickest and most dramatic results, do this experiment on a sunny day.
  • Locate a plant in your schoolyard that has live leaves, is in the sun and won’t be disturbed by irrigation or precipitation during your experiment.
  • Create a sign that says, “Do not disturb. Science experiment in progress” to place near the experiment.


Animated graphic showing how water flows from bodies of water and plants into the air, condenses into clouds and then falls back down to the surface.

This graphic shows the various parts of the water cycle, which is what moves Earth's water around the planet to places where plants, animals and humans can use it. Image credit: NOAA SciJinks | + Expand image

Transpiration, which is part of the water cycle, is the process by which water is carried through plants’ roots to their leaves, then changes to vapor and is released into the atmosphere. Transpiration is difficult to observe with the naked eye because we can’t see the internal plant processes, nor can we see the water vapor being released from the leaves. As a result, transpiration is often forgotten when discussing the water cycle. However, approximately 10 to 15 percent of water vapor in our atmosphere comes from transpiration. The rest comes from evaporation, the process by which Earth’s surface water is lost to the atmosphere in the form of water vapor. The sum of surface water evaporation and plant transpiration is known as evapotranspiration.

The transpiration process is vital to plant health. Transpiration in plants is akin to humans exhaling water vapor when they breathe – but it works a little differently. During transpiration, more than 95 percent of the water entering a plant passes through the plant and transpires – primarily through stomata, tiny pores on the underside of leaves – and enters the atmosphere as water vapor.

If water is scarce, such as in drought, the transpiration process can be disrupted. Also, if the air temperature is too high, plants close their stomata to conserve water and the transpiration process is disrupted. The same stomata that release water to the atmosphere through the transpiration process take in carbon dioxide, or CO2, from the atmosphere. While the stomata are closed, CO2 is not processed by the plant and photosynthesis comes to a halt. When this happens, Earth’s carbon cycle is disrupted and excess carbon in our atmosphere isn’t processed as it would normally be by plants.

Excess carbon in our atmosphere contributes to warming temperatures, which can then shut down even more plant stomata, creating a vicious cycle. In a changing climate, the resulting warmer temperatures and increased opportunities for drought put plant health at risk. When plants are at risk, the health of the entire Earth system is at risk.

Satellite image of Costa Rica with a heat map overlayed. Most of the country is shown in red, indicating high planet water stress.

This image shows the ECOSTRESS evaporative stress index for the Guanacaste region of Costa Rica (in red on inset map, left) a few months after the onset of a major Central American drought. Red indicates high plant water stress, yellow is moderate stress and greens/blues are low stress. Light gray is cloud cover. Image credit: NASA/JPL-Caltech | › Full image and caption

NASA’s ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station, or ECOSTRESS, is studying plant health from space. ECOSTRESS uses a thermal infrared radiometer to measure the temperature of plants. Plants regulate their temperature by releasing water through stomata. If they have sufficient water, they can maintain their temperature, but if there is insufficient water, their temperatures rise. ECOSTRESS can measure this rise in temperature.

The images acquired by ECOSTRESS are the most detailed temperature images of Earth’s surface ever acquired from space. These images are so accurate, they can be used to measure the temperature of an individual farmer’s field and thus be used to inform irrigation practices.

Data from ECOSTRESS are also used to determine the evaporative stress index, or ESI, a leading drought indicator. ESI describes changes in evapotranspiration over time. Tracking ESI can provide early warning signs of drought by indicating plant stress before the stress is visually apparent. ESI can be used to estimate current drought conditions, providing the opportunity for decision-makers to take action when drought is imminent.

ESI, evapotranspiration (ET), and water use efficiency (WUE) are measured by ECOSTRESS at 70-meter resolution every few days. Scientists have determined that these are strong predictors of wildfire burn severity, or damage to soils and vegetation associated with wildfires. Their research provides insights on how plant water stress in advance of a wildfire predicts burn severity and can inform pre‐fire season monitoring and fuel management.


Discuss the water cycle and transpiration

  1. After discussing the water cycle with students, ask them what parts of the water cycle are easily observable. Answers will vary, but most students will agree it is pretty easy to observe precipitation and condensation. Though evaporation isn’t directly observable, it’s easy to observe the effects of evaporation (water in a cup “disappearing” over time). Most students will agree that they have never directly observed transpiration. Tell them that in this demonstration, they will attempt to observe the effects of transpiration.
  2. Remind students that the water cycle is powered by the Sun. Ask them how the Sun affects plants. Answers will vary, but most students will say that the Sun helps plants grow.
  3. Ask students how the Sun helps plants grow. Less-experienced students may describe a simple process of plant growth. More-experienced students should mention photosynthesis.
  4. Ask students what plants need besides sunlight to grow. Answer: water and nutrients.
  5. Ask students how plants take in water and nutrients. Answer: through their roots and through their leaves. The bulk of water and nutrients are taken up through the roots, but the leaves absorb carbon dioxide, or CO2, gas, and – under certain conditions – some water.
  6. Ask students where the water and nutrients go once they are in the plant. Most students will be able to explain that the water and nutrients cause the plant to grow, and produce flowers or fruit.
  7. Explain that some of the water passes through the plants and is moved into the atmosphere as water vapor. Water vapor is invisible, so we can’t see it.
  8. Explain that we will demonstrate this process of water passing through a plant and back into the atmosphere, a process known as transpiration, with a simple experiment.

Set up the experiment

  1. Take students outside to the pre-determined location and show them the selected plant. Be sure to take along the bags, twist ties, pencil and sign, but keep them hidden from student view. Explain that transpiration is happening right now and that the plant is giving off water vapor. Ask students if there is any evidence of this water vapor being given off. Answer: No.
  2. Ask students how we might be able to observe the water vapor. Various answers may be acceptable.
  3. Ask students if we might be able to trap some of the water vapor. Astute students will point out that even trapped water vapor will still be invisible.
  4. Explain that the experiment will entail attempting to trap the water vapor and observe it. Ask students to suggest ways to trap the water vapor, then show them the materials. Students should be able to suggest tying the bag around a plant stem with leaves inside.
  5. Set up the experiment by placing one bag over a group of leaves (the more the better) and using the twist tie to seal the bag around the plant stem as well as possible.
    Picture of the step described.

    Place a bag around a plant or group of leaves and seal the bag around the plant. Image credit: NASA/JPL-Caltech | + Expand image

  6. Remind students that it’s always a good idea to have a control when performing an experiment. Ask how they might set up a control. Show them the remaining materials.
  7. Set up the control by placing the remaining bag over the pencil and securing it tightly with the twist-tie. Make sure the control bag is tied off at approximately the same place on the bag as the experiment bag.
    Picture of the step described.

    Set up a control by placing a bag around a pencil and sealing it around the pencil. Image credit: NASA/JPL-Caltech | + Expand image

  8. Place the control as close to the experiment bag as possible.
  9. Post the “do not disturb” sign, set a timer for 30 minutes and return to the classroom.
    Picture of the step described

    Place a "do not disturb" sign in front of your experiment. Image credit: NASA/JPL-Caltech | + Expand image

Form a hypothesis

  1. Ask students to make a hypothesis about what they will see in each bag after 30 minutes.
  2. Show students these visible-light images of two houseplants, one under water stress and the other not under water stress. Tell students that one plant is healthy and well watered, but the other plant is stressed because it needs water. Ask students to determine which plant is stressed and provide reasons for their choice.
    Two identical healthy-looking plants with tear-drop shaped leaves.

    Two identical houseplants, Plant A and Plant B, are shown in visible light. One plant was watered regularly over five days, while the other received no water. Can you guess which one is which? Image credit: NASA/JPL-Caltech | + Expand image

  3. Explain that NASA has a mission that is studying Earth from space and is able to determine whether plants are stressed before they show any signs of stress to the human eye.
  4. Show students this video or this more simplified video about how NASA’s ECOSTRESS mission functions. Explain that ECOSTRESS uses infrared, or heat-sensing, imagery to determine if plants are stressed.
  5. Show students these infrared images of the two houseplants. Explain that these images were captured by an infrared camera and can measure the difference in plant temperature, as indicated by the different colors.
    Colors representing temperature are show in infrared images of the houseplants. The temperature scale shown on the infrared view of each plant ranges from 78.7 degrees at the max to 71.3 degrees at the min (represented by a color scale going from white to red to yellow to green to blue to purple to black, respectively). The temperatures on Plant A, which are mostly yellow and orange, are much warmer than those on Plant B, which is a mix of blue, purple, and black on the leaves closest to the base of the pot.

    The same houseplants as above are shown in infrared light. Now, can you guess which one was watered regularly? Image credit: NASA/JPL-Caltech | + Expand image

  6. Ask students to determine which plant is in need of water. Ask them to provide reasons for their statements. If students need help, note the temperature scales for each image. The yellow/red/orange colors mean it’s hotter, the purples/blues/blacks in plant B are cooler temperatures.
    Both plants look the same on Day 1 and 5 of the experiment when viewed in visible light. The temperature scale shown on the infrared view of each plant ranges from 78.7 degrees at the max to 71.3 degrees at the min (represented by a color scale going from white to red to yellow to green to blue to purple to black, respectively). While Plant A went to the warmer end of the temperature scale on Day 5 compared with Day 1, Plant B appears to be much cooler, with some edges of the leaves completely black in infrared.

    Two identical houseplants, Plant A and Plant B, are shown in visible light and infrared light on Day 1 and Day 5 of the experiment. One plant was watered regularly during the experiment while the other received no water. While both plants look healthy in visible light, it's clear in the infrared view that one of the plants is under water stress on Day 5. Image credit: NASA/JPL-Caltech | + Expand image

Review the results of the experiment and make connections

  1. After 30 minutes, return outside and observe the two bags. Ask students to describe what they see. There should be a lot of condensation in the experimental bag and less or no condensation in the control bag.

    Condensation has started to develop on the experimental bag. Image credit: NASA/JPL-Caltech | + Expand image

  2. Ask students where the water condensation came from. Answer: In the experimental bag, the water vapor excreted from the plant leaves was trapped and then condensed on the bag, forming water droplets. In the control bag, if there is any condensation, it is from humidity in the air, which can also equally contribute to the condensation in the experimental bag.
  3. Return to the classroom and explain to students that stomata – tiny pores on the underside of plant leaves – open and allow water vapor to be released as part of the process of transpiration.
  4. Explain to students that the combined processes of transpiration and evaporation are called “evapotranspiration.” Evapotranspiration over farmland includes evaporation from the land surface and transpiration from plants.
  5. Explain that if a plant is stressed because of heat or drought, the stomata will close and not allow evapotranspiration to occur, raising the temperature of the plant. Prolonged stress can cause damage to the plant.
  6. Explain to students that ECOSTRESS is able to observe plant and crop health by measuring evapotranspiration from space. ECOSTRESS, by detecting evapotranspiration rates, can identify whether a crop is stressed before it is detectable by farmers. Farmers could use data from ECOSTRESS to improve their irrigation practices and thus reduce plant stress, use water more efficiently and increase crop yield.


  • Assess that students can describe the mechanism of transpiration.
  • Assess that students can explain the difference between evaporation and transpiration.
  • Assess that students can explain what evapotranspiration data can tell us and why it’s important.