Question

why does stem always grow against the gravity and roots towards the gravity

Answer 1

In stem, the presence of asymmetric light, auxin will move towards the shaded side and will promote elongation of the cells on that side and this will cause curvature of plant towards the light source or positive phototropism. In case of roots, they show positive gravitropism that is roots bend in response to gravity. This occur due to plant hormone auxin. The polar transport of auxin from shoot tip to the roots inhibits cell expansion. So increase in concentration of auxin towards the root side causes differential growth resulting in root curvature towards gravity.