Tropism is a growth response between a plant and an external stimulus. The stimulus could be weather, touch, time, gravity or light. A positive response is indicated by growth toward a stimulus and a negative response is indicated by growth away from the stimulus.

Light is a stimulus that plants respond to. This is called phototropism (photo= light). Plants usually display a positive phototropic response to light, which means they grow toward a light source. Plant hormones called auxins play a part in phototropism. Auxin is a plant growth hormone. When light is shined on one side of a plant the auxins move to the dark side of the plant. The hormones stimulate the cells on the dark side of the plant to elongate, while the cells on the light side of the plant remain the same. This elongation on one side and staying the same on the other causes the plant to bend in the direction of the light. This bending allows more light to reach more cells on the plant that are responsible for conducting photosynthesis.

Biology of Plants, 6th ed. Raven, Peter H., Everert, Ray F., Eichhorn, Susan E. Worth Publishing 1999.