Observation is the simplest activity to teach elementary students about plant transpiration. Start with a clear container, such as a sandwich bag, and secure it over the leaves of a plant. The container does not need to be airtight, but should be secure enough not to let moisture escape. Place the covered plant near a sunny window. For a week, have the students observe the plant each day. Over time, moisture will collect on the inside of the bag in the form of condensation. This moisture is evidence of the process of transpiration.
By creating a controlled environment, students can measure the amount of water plants give off during transpiration. Create a terrarium for this experiment by placing a plant clipping through a hole in a small piece of cardboard, sealing the hole with petroleum jelly and placing it between two clear plastic cups. Fill the bottom cup with water and make sure the stem of the plant clipping is inside the water. Have the students record the amount of water placed in the cup. Create a second terrarium with the same amount of water and no plant clippings to act as the control. Place the terrariums near a sunny window or heat lamp to facilitate the transpiration process. After a few days, have the students observe the moisture on the inside of the top cup. Have students calculate the amount of water missing from the bottom of both cups and subtract the difference from the control cup to account for evaporation. This final figure represents the volume of water transpired by the plant clipping.
Take the volume measurement activity one step farther and have students calculate the rate of transpiration under various conditions. The rate of transpiration depends on air temperature and solar radiation, so place multiple terrariums with equal amounts of water in different locations, such as by a window, inside a closet or on a desk. After a 24-hour period, have the students measure the transpiration for each terrarium using the method from the volume measurement activity. Ask students to compare the results and explain why certain conditions increase the rate of transpiration.
You may choose to conduct these experiments with one sample for the class to observe or you can provide plants and materials to each student or small groups of students so they can set it up on their own. In addition to teaching how to observe and measure transpiration, it is also a good opportunity to discuss the purpose of the scientific method, develop students' presentation skills and demonstrate how math skills can be applied to other subjects.