This is a picture of a sunflower. It is an example of an organism that exhibits tropism. Tropism is when an organism turns or grows in a specific direction because of an external stimulus, such as light. Most organisms that exhibit tropism are plants. There are many types of tropism, including chemotropism (in response to chemicals), hydrotropism (in response to water), and phototropism (in response to light). The sunflower pictured exhibits phototropism, as it turns to face the direction of the sun as it moves across the sky.