Plants are non-motile and cannot move towards a stimulus. Instead they grow towards the stimulus.A tropism is a directional growth response to a stimulus.Plant shoots exhibit positive phototropism. They grow towards light. They also exhibit negative gravitropism and grow away from gravity. This ensures their leaves capture as much light as possible for photosynthesis.Positive gravitropism and negative phototropism increase the probability of roots growing into the soil so they can absorb water and mineral ions.