The Barbershop Window Growth Puzzle

A Question Between Haircuts

Imani owns a barbershop on a busy neighborhood street. People waiting for haircuts notice the coleus plants in the front window getting larger. One customer suggests that the plants turn soil into leaves. Another points to the water Imani adds. A neighborhood science team prepares practice records from an eight-week study to help customers investigate these ideas.

The team starts with matched batches of six rooted coleus cuttings. Extra, closely matched cuttings provide an estimated starting dry plant mass of 12 grams per batch. Dry mass is what remains after an adult dries harvested plants. This removes stored water that could make a recently watered plant seem larger without showing new dry plant material. Adults collect the roots, stems, and leaves.

For the soil study, each batch starts with 1,500 grams of dry soil. The team adds no soil or fertilizer during the study and prevents spills. Batches receive the same light, temperature, normal air, and enough water to keep roots moist. Separate matched batches are harvested at four weeks and eight weeks. Drying ends a plant's growth, so the team does not return dried plants to their pots.

A second study uses two more matched batches without soil. Holders support their roots in water containing mineral nutrients. Each batch receives a total of 0.8 gram of minerals during eight weeks. The Window group receives normal air. Adult-operated equipment gives the Test group air containing much less carbon dioxide, a gas in air. Both groups receive the same light, temperature, water supply, and mineral supply. The team measures their increases in dry plant mass.

Scientists have also used tracing methods to follow materials into plants. They found that material from water taken in by roots and from carbon dioxide taken in by leaves becomes part of newly built plant parts. Roots take in mineral nutrients too. Plants need light, but light transfers energy rather than supplying building material. The barbershop records do not measure the separate amounts of material contributed by air and water.