Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is a "positive phototropism", while growth away from light is called "negative phototropism". Most plant shoots have positive phototropism. Roots usually have negative phototropism, though gravitropism may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them.