Tropism is when a plant is affected by its environment and grows a certain way in response to external stimuli. This picture is of a cactus that exhibits tropism. This cactus was on the edge of the shadow of an overhang so it started growing to the side, to grow out of the overhang’s shadow in order to get more light for photosynthesis. This change in how the plant grows due to sunlight is called Phototropism. The growth rate of the plant’s cells causes the plant to bend toward the sunlight.