Answer: auxin
Tropism can be defined as the response shown by the plant towards an external stimuli. Phototropism is the response of different parts of the plant towards light. The plant roots shows negative phototropism as they grow away from the sunlight but the plant shoots show positive phototropism as they grow towards the direction of sunlight. Geotropism is the response of different parts of the plant towards the gravity. The plant roots shows positive geotropism as they grow underground towards gravity whereas the plant shoots show negative geotropism because they grow away from the gravity and stand erect.
Plant hormone auxin is responsible for the phototropism and geotropism shown by the plant. Auxin help the plant parts which grow in low sunlight condition to bend towards the light, hence plant shows positive phototropism. Auxin also stimulates the growth and elongation of the root cells in plant which helps in penetration of deeper layers of soil hence, supports positive geotropism.
