This is a picture of a tree root that has come up. It isn’t very often when one sees a tree root. This is because of tropism, but it isn’t a light stimulus that causes a tree to grow down. A root should never see the light of day, and because the root should never see the sun it is impossible for the root to respond to the sun’s light. A plant instead relies on a different kind of tropism, gravitropism. Gravitropism is when a plant grows either towards or away the center of gravity. The roots experience positive gravitropism, which means they grow towards the center of gravity. The preliminary shoot that hasn’t seen the sun yet also relies on gravitropism. It relies on negative gravitropism, which means it grows away from center of gravity. This allows the preliminary shoot to grow towards the sun, even though it doesn’t know where it is.