control and coordination
Plant roots are positively geotropic and negatively phototropic whereas plant shoots are usually negatively geotropic and positively phototropic. The plant hormones are responsible for bringing about such tropic movements.
(When growing plants detect light, auxin synthesised at the shoot tips, help the cells to grow longer. Also, if light comes from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light.)