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Hudson Elementary School Fourth-Grade Student:
Pothos in double-2-liter bottles

Christie ShumwayChristie Shumway needed to do a science project for her fourth-grade class at Hudson Elementary School in Tempe, Arizona.  Her father is a personal friend of ours, and he asked us if we had any ideas.  We told him to check out the Global Change Laboratory in the Experiments section of our web site; and now his daughter's smiling face introduces this student experiment.

After looking at how our first two center experiments were progressing, Christie and her Dad decided they would try something just a little less ambitious than what we were doing.  They chose to work with 2-liter pop bottles instead of aquariums or the larger 3-liter bottles we employ; and they opted for only three CO2 treatments instead of the six we are currently using.  In a further simplification, they didn't use any auxiliary lighting but merely placed their three experimental units on a dresser in Christie's bedroom near a window where they could get some natural light (see Figure 1).

Figure 1Perhaps their greatest simplification, however, was the fact that they didn't measure any CO2 concentrations within their poor man's biospheres.  They reasoned that if one of their biospheres was completely sealed and exchanged no air with its surroundings, the two pothos leaves they planted in it would ultimately draw down the CO2 content of its internal airspace (by removing it from the trapped air via photosynthesis) to the point where the original parent leaves would be unable to acquire enough CO2 from the small amount remaining to meet their continuing needs, which would result in their consequent death.

From our experience with pothos plants, we could assure them that if the plants in their sealed biosphere did indeed die, it would mean that the CO2 concentration of its internal airspace was approximately 100 parts per million (ppm).  At the other extreme, we could also assure them that for a household of their size (Mom, Dad and five children) and circumstance (winter, when they would be drawing no outside air into their home), the CO2 content of the air inside their house would likely be somewhere between 1,000 and 1,500 ppm.  Hence, if they had several openings in the tape that held the top and bottom of their high-CO2 biosphere together (see our Experiment # 2 Setup Directions), they could be confident of its airspace CO2 concentration being somewhere within this range.  Then, with just a couple of tiny holes in their remaining biosphere, they could achieve an intermediate CO2 concentration.  Although they would not know this intermediate concentration with any precision, it would nevertheless provide an example of what happens to plants somewhere between the very low and very high CO2 concentrations they were investigating with their other two biospheres.

Figure 2At the conclusion of her experiment, Christie's three ecosystems appeared as shown in Figure 2.  Both of the original leaves she had planted in the low-CO2 experimental unit had died.  In the intermediate-CO2 unit, one original leaf had died, but the other one had produced a "daughter" plant with a new vine and root system.  The parent leaf of this biosphere, however, was clearly in the process of dying.  In the high-CO2 unit, on the other hand, both parent leaves were still in good shape, and each had produced a daughter plant with vigorous vine and root growth.

Upon dismantling the biospheres and removing the plants, their differences were even more apparent.  Only one plant could be extracted from the intermediate-CO2 unit; while from the low-CO2 unit only a single dead leaf could be removed intact.  From the high-CO2 unit, however, both of the plants that had developed from the original parent leaves were recovered; and both were found to be in a vigorous state of development (see Figure 3).

Figure 3When Christie reported to her class on her experiment, she asked everyone present to hold their breath for about ten seconds.  "Now you know how you feel when you don't get enough oxygen," she said.  Then she asked her classmates how they thought a plant feels when it doesn't get enough of the carbon dioxide that it needs.  The results of her experiment provided a striking visual answer that could be readily appreciated by everyone in her school.

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