From Cork Boxes to Living Mats

Living Mats in the Fossil Hall

1Rafael Quist's sixth-grade class spent a morning at the Larkspur Museum of Natural History. In the fossil hall, beside a cut slab of layered rock about 3.5 billion years old, educator Ingrid Oduya stopped at a glass tank. The rock is a fossil stromatolite, a mound built in layers as tiny living things trapped mud and sand. The tank holds a living green mat of cyanobacteria from a shallow, salty bay, the same kind of mat that builds stromatolites today. Like plants, cyanobacteria use light to make their own food.

2Next to the tank is a history wall about how scientists learned what living things are made of. In 1665, Robert Hooke looked at a thin slice of cork, which is the bark of a kind of oak tree. His microscope magnified about 50 times. He saw rows of tiny empty boxes and called them cells. The boxes were the walls left behind by cells that had died. In the 1670s, Antonie van Leeuwenhoek used tiny lenses that magnified up to about 250 times. In drops of pond water he saw tiny living things swimming, each one a single cell. Later scientists looked at many plants and animals with better microscopes.

3At the microscope station, the class put a drop of the green mat on a slide and looked at it at 400×. Each cyanobacterium was a single round green cell with no nucleus. Some cells were pinched in the middle, splitting in two. The class counted the cells in one field of view at 10:00 each morning for four days. The dish stayed under the same lamp at the same temperature. For comparison, the museum keeps a jar of very salty water with a string hanging in it. A salt crystal on the string gets larger every day. Chips of the crystal showed flat faces and sharp edges, with no membrane and nothing inside them that split.

4Finally, the class looked at a thin slice of a ginkgo leaf. Ginkgo trees are still planted along city streets, and their fan-shaped leaves look much like fossil leaves in the hall. At high power, the slice showed a top layer of flat, clear cells, long green cells packed below it, pairs of bean-shaped cells around tiny pores and long tube-shaped cells in the veins.