Tropisms are directional growth responses to stimuli. Examples are phototropism in response to light and gravitropism in response to gravity. If plants are exposed to light on one side only, shoots will grow towards the light (positively phototropic) and roots will grow away from the light source (negatively phototropic). If roots emerge from the soil due to heavy rain etc., they will rapidly return to the soil.If a plant is exposed to light on one side only, auxin accumulates on the side of the plant which is not illuminated. This causes cell elongation on the shaded side so the shoot bends in the direction of the light.