Tropism is very important for a plant. Tropism is when something from the outside causes a different direction of growth. There are many types of tropism. One type is phototropism. Phototropism is when a part of the plant grows in a certain direction towards the sun or any other light source. These leaves are experiencing positive phototropism, which means the leaves grow towards the sun. If the leaves had grown away from the sun, they would be experiencing negative phototropism.