Floating Leaf Disks and Ancient Coal Forests

The Coal Forest Room

1The fossil hall of the Granite Bluff Museum of Natural History has a room called the Coal Forest. Its cases hold fossil leaves, bark and stems from swampy forests that grew more than 300 million years ago. Some of the plants were tree ferns and giant horsetails, and some were scale trees that grew more than 30 meters tall. Many of these fossils were found in or next to layers of coal.

2A sign in the room explains how the coal formed. When plants in those swamps died, many of them fell into still water that held little oxygen, so they did not fully decay. Layer after layer of plant matter piled up and was buried under mud and sand. Over millions of years, heat and pressure underground slowly changed the buried plant matter into coal, a black rock that is mostly carbon.

3On a Saturday visit, seventh grader Kwame Asante watched the museum educator, Ms. Annika Varela, run a test with fresh spinach leaves. She used a paper punch to cut 40 small disks from the leaves. She pulled the air out of the disks with a plastic syringe (no needle), so that they sank, and placed 10 disks in each of four clear cups. Cups A, B and D held water with a little baking soda stirred in. Baking soda adds carbon dioxide to the water in a form that leaves can use. Cup C held plain water, which has very little carbon dioxide.

4Cups A and C sat 10 cm from a lamp, and Cup B sat 40 cm away. Cup D sat 10 cm from the lamp too, but it was wrapped in foil so that no light reached the disks. The lamp was the kind that gives off very little heat, and the water in every cup stayed at 22 °C. A sunken disk floats back up when enough gas builds up in the tiny spaces inside the leaf. Kwame counted the floating disks every 5 minutes (Table 1).