Answer:
Tropism refers to the response a body system of the organism produces according to an external stimulus. Plants shows tropism depending upon the type of stimuli. The stimuli can be sunlight, gravity, water or even touch. The tropism related with the sunlight is phototropism. Phototropism is a typical response of a plant shoot towards the sunlight. The plant shoots grow in the direction of sunlight, hence they show positive phototropism, whereas plant roots grow in direction against the sunlight, they show negative phototropism. Phototropism helps in availing sunlight to plants which is a necessary ingredient required for photosynthesis.