Plants respond to their surroundings by changing the direction in which they grow. They are controlled by plant hormones, which are chemicals made in one part of the plant that travel to another part and change how it grows. The plant hormone auxin is made in the tips of shoots and roots, and it spreads back through the plant. When auxin spreads unequally, with more on one side than the other, the two sides grow at different rates and the shoot or root bends.
Phototropism is growth in response to the direction of light. When light shines on a shoot from one side, auxin moves to the shaded side. Auxin makes the cells in a shoot elongate, so the shaded side grows faster than the lit side and the shoot bends towards the light. This helps the leaves absorb more light for photosynthesis.
Gravitropism is growth in response to gravity. If a shoot is lying on its side, auxin collects on the lower side, which grows faster, so the shoot bends upwards, away from gravity. In a root, auxin also collects on the lower side, but there it slows elongation. The upper side of the root grows faster, so the root bends downwards, in the direction of gravity. This anchors the plant and takes the root towards water.
