Flexi Says: Tropisms are growth toward or away from a stimulus. Types of tropisms include gravitropism (gravity), phototropism (light), and thigmotropism (touch). Plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin.