Auxin

Over a century ago, Charles Darwin and his son, Francis, conducted a series of experiments on plants and light. They came to the conclusion that something internal was causing the plant stems to turn toward the sun. Later, another scientist discovered that this “something” was a chemical substance. This chemical is called auxin, after the Greek word auxein, meaning “to increase.”

This hormone’s name is perfect, because its main function is to promote plant growth. Plant tissues with auxin grow larger and faster than plant tissues without auxin.

Auxin is the chemical that makes phototropism and other tropisms happen. If only one side of a plant gets sunlight, the auxin in the plant moves over to the other, shady side of the plant. The auxin makes the shady side of the plant’s stem grow faster than the sunny side, so the stem starts to curve in the direction of the light. Instead of bending toward the light source, the plant actually grows toward the light! Once a stem grows in one direction, it can grow back in the other direction, but it can’t “unbend.”

Gibberellins and Cytokinins

Gibberellins are hormones that promote plant growth. Gibberellins make stems grow longer, and they help plants to mature and produce flowers or fruit. These hormones can also make fruits grow larger. They can even increase the number of grapes that form in a cluster! Gibberellin doesn’t perform these tasks on its own, however: it usually works together with auxin.

Cytokinins also work with auxin to help plants grow. While auxin helps plants grow taller, cytokinins help plants to branch out. These two hormone groups work together to produce large, healthy plants.