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Bagged Branches

Time: 2 Class periods Setting: Schoolyard Type of Organism: Plants Habitat: forest grassland urban
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Guiding Questions

What factors determine how much water plants lose through transpiration? How do two species differ in the amount of transpiration that takes place from their leaves over the course of 1 week?

    Leaves are the "food factories" of plants. Unfortunately, studying photosynthesis directly requires fancy equipment that is not easily available to school teachers. However, making sugars through photosynthesis is not the only way leaves interact with the environment. In this study you will have a chance to investigate first hand the process of transpiration, or the loss of water from plants via evaporation.

    What do you wonder about transpiration? Brainstorm for a minute with your teammates and then decide on a question you'd like to answer. You'll have enough materials to compare water loss from the leaves on several twigs between now and next week. You might want to compare leaves in different micro-environments, (e.g., sunny vs. shaded) of two kinds of plant (e.g., oak vs. maple). You also will have to consider how many replicates you will need so that you will have confidence in your results.

    • 1-qt or 1-gal clear plastic bags (at least 4 per group)
    • masking tape for sealing bags on twigs
    • twist ties or masking tape for marking twigs
    • permanent marker for marking twigs
    • ruler for measuring leaves and twigs
    • graph paper for tracing and measuring leaf area
    • graduated cylinder or marked beaker (for final day only)
    • hand lopping sheers or strong scissors (for final day only

    Decide on what comparison you want to make, then select and bag branches.

    • Select areas, species, and branches etc. to study based upon your question, e.g., if your question "How does transpiration from leaves differ in sunny versus shaded conditions?", then find branches from the same species that have similar leaves (number, size, etc.). Or if your question is, "How do two species differ in the amount of water loss that takes place over the course of 1 week?", then find twigs from two species that are similar in all other respects (e.g., exposure to sun, size of leaves, etc.)
    • Choose at least two branches approximately 15cm long (as replicates) in each of the two+ situations you've decided to focus on. All branches should be as similar as possible, and each should be able to fit into the plastic bag.
    • Mark each branch with a twist tie or tape, and label them in some way (e.g., 1 to 4). Record which number corresponds to which branch.
    • Carefully place the branch with all its leaves in the bag and seal the bag onto the branch at tightly as you can without damaging it.

    2. Collecting data about transpiration.

    • Carefully remove the plastic bag from the branch, making sure you don't spill any water.
    • Pour the water into a marked beaker or graduated cylinder to determine the volume of water that was lost from the twig. Record this amount.
    • Repeat this procedure for each branch.
    • Calculate the amount of water that transpired per square cm of leaf:
        • (1) Clip the part of the branch that was in the bag, or just its leaves.
        • (2) Lay the clear cm grid over each leaf and estimate its area.
        • (3) Add up the areas of all of the leaves to get a total amount of leaf area that was in each bag.
        • (4) Divide the volume of water by the total leaf area to express transpiration as ml water per square cm of leaf.

    Teacher's Study Plan


    1. Write the question you will answer with your study.


    2. Write the "null" hypothesis you are testing.
    3. Write an alternative hypothesis you are testing.


    4 Describe how you will address your question. Give specific details about your methods.
    5. Draw a map showing the location of your sampling sites and/or a diagram showing what your sampling scheme looks like.

    Anticipated Results

    6. What do you expect to find? You might want to draw a graph of your predictions on the back of this sheet.