auxin
Plants may not be able to move to another location, but they are able to change how they grow in response to their environment. Growth toward or away from a stimulus is known as a tropism Table below . Auxins, a class of plant hormones, allow plants to curve in specific directions as they grow. The auxin moves to one side of the stem, where it starts a chain of events that cause rapid cell growth on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.