- Phototropism = light  growth  towards light
- If the light hits the tip of the stem it grows upwards
- If the light is coming from the side (lateral light source) then the plant will bend as it grows
- Light on only one side will cause the other side to bend
- If one side contains the compound Auxin (hormone) then it will bend. This is because it is caused by growth on one side but not on the other.