The movement of plant part in response to gravity is called geotropic movement and the phenomenon involved is called geotropism.When the tip of the stem grows away from the earth's gravitational forces, it is known as negative geotropism and when the root tips grow towards gravity, it is known as positive geotropism. Stem shows negative geotropism Roots show positive geotropism.
Geotropism : Plants show tropism in response to other stimuli as well. The roots of a plant always grow downwards while the shoots usually grow upwards and away from the earth. This upwards and downwards growth of shoots and roots, respectively in response to the pull of earth or gravity is, called geotropism.
Thus in the figure below roots are positively geotropic while shoot part is negatively geotropic
