Plant and Soil Science
Unit 2
The Carbon Cycle
Testing the Gases Plants Emit
Will plants emit more Oxygen during the day or night?
During the process of respiration, plants in the dark emit carbon dioxide. Plants in the
light emit oxygen as a by-product of photosynthesis. Oxygen is a key component
of fire, while carbon dioxide puts out fires.
• Two sprigs of Elodea or other water plant
• Two test tubes
• water
• two wide-mouthed jars
• matches or lit candle
1. Place a sprig of Elodea into each of two test tubes full of water. Be certain that
the two sprigs are of equal length. Place a couple of inches of water in each.
2. Place your finger over one test tube so that you do not lose any water, invert it, and
place inside one jar.
3. Repeat with the other test tube. Now you have two identical setups.
4. Place one in a dark closet, the other in the sun or under a grow lamp.
5. After a couple of days, some gas should have collected at the upper end of the
test tubes. Carefully invert each of the test tubes keeping them closed with your
6. Holding a burning match or candle near each test tube, let the gas escape. The
gas from one test tube will glow brightly, while the gas from the other might
extinguish the flame.