Plant stems are positively phototropic while plant roots are negatively phototropic. Plant stems are negatively geotropic while plant roots are positively geotropic. Therefore, stem and root show unilateral growth towards light and gravity of earth respectively. The growth hormone like auxin, plays a major role in bringing such tropic movements. When a plant receives light, auxin in stem cause cell elongation and therefore stem grows upwards toward light. In the root, auxin build up in the cells of upper surface of root that induces re-orientation of the cell walls and the root grows downwards i.e., towards earth.
