What Builds the Snapdragons by the Tricycle Bay?

Snapdragons for the Tricycle Parking Bay

A fifth-grade class is helping care for snapdragons beside the playground's tricycle parking bay. Before planting them, the class investigates what supplies the material for new roots, stems, and leaves. They use a protected growing area away from playground traffic. Their question is: Does most new plant material come from the solid material around the roots, or from other supplies?

Step 1: The class sets up three groups of eight matched snapdragon seedlings. Each group grows with its roots supported by clean glass beads instead of soil. The beads do not dissolve. Each group starts with 800 grams of dry beads. Extra, closely matched seedlings are dried by an adult to estimate each group's starting dry plant mass.

Step 2: The class gives the groups different supplies. The usual-care group receives normal air and enough water to keep its roots moist. The low-carbon-dioxide group receives moist roots but much less carbon dioxide, a gas in air. The low-water group receives normal air but less water, so its roots sometimes dry out. Adult-operated equipment controls the air conditions.

Step 3: The class keeps the plant kind, number of seedlings, starting size, light, temperature, bead mass, and six-week growing time the same. An adult prepares nutrient doses containing a total of 3 grams of mineral nutrients for each group. These doses are included in the groups' water supplies. The amount of water differs, but the amount of minerals supplied does not.

Step 4: At the end, an adult collects the plants, rinses their roots, and dries them. Dry plant mass is what remains after stored water is removed. The class subtracts the estimated starting dry mass from the final dry mass to find the gain. After the beads are cleaned and dried, each group's beads still weigh 800 grams.

Step 5: The class repeats the investigation with new, matched seedlings. Run 2 uses the same conditions as Run 1. The graph shows dry plant mass gained in each run. The results are classroom practice data. Each group in each run received 3 grams of minerals, and its bead mass stayed unchanged.

Science notes: Roots take in water and mineral nutrients. Leaves take in carbon dioxide from air. Water and carbon dioxide can provide material used to build plant parts. Light supplies energy rather than building material. Mineral nutrients are also needed. These records do not separately measure how many grams of new plant material came from air and how many came from water.