Lab / experiment Grade 8 Science

The Three-Bag Transpiration Test: Leaves, Stems, and Air

A two-session grade 8 science lab in which students bag a leafy shoot, a stripped stem, and empty air, then graph class data on leaf count against water collected.

Grade: Grade 8
Subject: Science
Material: Lab / experiment
Time: 2 sessions, ~60 min total
Prep: Moderate prep
Materials: One leafy potted plant or two fresh leafy shoots standing in cups of water per group, Clear zip-top bags, 3 per group, Twist ties or masking tape, Digital scale reading to 0.1 gram, or a 10 mL graduated cylinder, Scissors, Permanent marker and labels, Graph paper

Standards

MS-ESS2-4 NGSS

Develop a model to describe the cycling of water through Earth's systems driven by energy from the sun and the force of gravity.

How this material aligns: The three bags are a physical model of one pathway in the water cycle, and analysis items 6 and 8 have students trace the water from soil to leaf to vapor and name the sun as the energy that drives it.

All materials for MS-ESS2-4 →

8.SP.A.1 Common Core Math

Construct and interpret scatter plots for bivariate measurement data to investigate patterns of association between two quantities, describing clustering, outliers, and positive, negative, linear, or nonlinear association.

How this material aligns: Analysis item 4 has students build a scatter plot of class leaf counts against water collected and describe the association they see.

All materials for 8.SP.A.1 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will measure the water a leafy shoot releases into a sealed bag over 24 hours.
Students will compare a leafy shoot, a stripped stem, and an empty bag to isolate the source of the water.
Students will graph class data on leaf count against water collected and describe the association.

Vocabulary

Transpiration: Transpiration is how plants release water into the air through tiny holes in their leaves. A tree pulls water up from the soil through its roots and lets it escape from its leaves as vapor.
Evapotranspiration: Evapotranspiration is the total amount of water that leaves the land and goes into the air, both from wet surfaces and from plants. Scientists combine evaporation and transpiration into one word.
Condensation: Condensation is when water vapor in the air cools down and turns back into tiny liquid water droplets. It is how clouds form and why a cold glass gets wet on the outside on a warm day.

Also: stomata, control, scatter plot

Teacher overview

Plants move an enormous amount of water from the soil back into the air, and this lab lets students collect and measure some of it. Each group seals three bags overnight: one over a leafy shoot, one over a shoot with the leaves stripped off, and one holding only air. The differences among the three bags show where the water came from and rule out condensation from the room as the explanation. The investigation supports the water cycle as an energy-driven system, and it also sets up the reason a farmer irrigating a leafy field must replace far more water than a bare field loses.

The Three-Bag Transpiration Test: Leaves, Stems, and Air — Grade 8 Science

Name: ____________________________ Date: ______________

Procedure

Choose the shoots (Day 1, 8 min). Working in groups of three, select two similar shoots on a potted plant, or take two fresh cut stems and stand them in cups of water. Count the leaves on the first shoot and write the number on your data table. Strip every leaf from the second shoot, leaving the stem the same length. Use scissors carefully and only on plants the teacher provides, since some plants irritate skin.

Seal the three bags (Day 1, 12 min). Slide a clear bag over the leafy shoot and close it around the stem with a twist tie, keeping the bag off the soil so no soil water evaporates into it. Bag the stripped stem the same way. Seal the third bag with nothing but air inside and set it beside the others. Label the bags A, B, and C, and record the starting mass of each empty bag before you use it if your group is measuring by mass.

Wait 24 hours (Day 1 to Day 2). Leave all three bags in the same light and the same part of the room, since warmth and light change how fast leaves release water. Do not open them.

Measure the water (Day 2, 20 min). Open each bag carefully over a cup so no drops are lost. Pour the collected water into a graduated cylinder and read the volume to the nearest 0.1 mL, or weigh the bag and subtract its starting mass. Record all three results. Wash your hands after handling plant material, and do not taste anything from the bags.

Post the class data (Day 2, 15 min). Write your group's leaf count and the volume from bag A on the class chart, then copy the whole class table into your notes for the scatter plot in the analysis questions. Return plants and cups to the cart and wipe down the table.

Wrap up (Day 2, 5 min). Answer the analysis questions using your own group's numbers and the class chart.

Data & Analysis

Record your group's results, then copy the class results for the scatter plot.

Bag	What was inside	Leaf count
A	Leafy shoot
B	Stripped stem, no leaves	0
C	Air only	0

Class chart: one row for each group, listing leaf count and the water collected in bag A.

Which of your three bags collected the most water? Give the volume you measured.
How much more water did bag A collect than bag B? Show the subtraction using your own numbers.
Explain why the group sealed bag C with nothing inside it.
Make a scatter plot of the class data, with leaf count on the horizontal axis and water collected in bag A on the vertical axis. Describe the association you see.
Use your scatter plot to predict how much water a shoot with 12 leaves would collect in 24 hours.
Trace the path the water in bag A took, starting in the soil or the cup and ending on the inside of the bag.
A farmer irrigating a field of leafy plants must replace both the water that evaporates from the soil and the water the plants release. Use your bag A and bag B results to explain why a leafy field needs more irrigation water than bare ground.
Name the energy source that drives transpiration, and explain where transpiration fits in the water cycle.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: Bag A, the leafy shoot, with the group's own measured volume.
Nearly every group finds bag A well ahead of the other two. Accept the group's recorded volume in mL or grams as long as it matches their data table.
Answer: Accept the group's own difference, with the subtraction shown.
The value is bag A's volume minus bag B's volume and depends on the plant, the leaf count, and the room, so there is no single correct number. Most groups find bag A ahead of bag B by several milliliters. Any difference that matches their recorded volumes is correct.
Answer: Bag C is the control.
It shows how much water, if any, condenses inside a sealed bag from room air alone. If bag C stays dry, students can rule out the room as the source and credit the leaves instead.
Answer: A positive association.
Points generally rise to the right, because shoots with more leaves release more water. Accept "positive" or any wording describing water collected increasing as leaf count increases, along with a note that the points scatter rather than sitting on a perfect line.
Answer: Accept any prediction consistent with the class trend.
Students should read up from 12 leaves to the pattern of their own class points and give a volume in that range, with a sentence explaining how they used the graph. A prediction far outside the class data without justification is not acceptable.
Answer: Roots or cut stem, up the stem, out the leaves as vapor, then condensing on the bag.
Water entered through the roots or the cut end of the stem, traveled up the stem, and left through tiny openings in the leaves called stomata as water vapor. The vapor cooled on the plastic and condensed into the drops the group measured.
Answer: The leafy field loses water two ways instead of one.
Bag B, with no leaves, collected little water, while bag A collected much more from the same length of stem. Bare ground loses water only by evaporation, but a leafy field loses that same soil evaporation plus everything the leaves transpire, so the farmer has to replace a larger total.
Answer: Energy from the sun, and transpiration returns water from land to the atmosphere.
Sunlight warms the leaves and supplies the energy that turns liquid water into vapor. Transpiration is one of the paths, along with evaporation from soil and open water, that moves water from the land back into the atmosphere, where it later condenses and falls as precipitation.

Differentiation

Support

Set up one demonstration group's bags a day early so these students can see a finished result before building their own. Give them a data table with the units already filled in and the subtraction for item 2 written as a frame, so they only insert their two volumes.

Extension

Have students run a second leafy bag in a darker or cooler part of the room and compare its volume with the first, then write a testable question about how light or temperature changes transpiration rate that another group could investigate.

ELL supports

Label each bag with both a word and a small drawing, so bag A shows leaves, bag B shows a bare stem, and bag C shows only air. Provide sentence frames such as "Bag ___ collected ___ mL because ___." Allow students to trace the water's path in item 6 with arrows on a diagram before writing it in sentences.

Extensions

Have students estimate how much water a single street tree returns to the air on a hot day by scaling up their bag A result, state every assumption they make in the estimate, and explain which assumption they trust least.