(a) Definition of Phototropism: It is directional growth movement of curvature which occurs in plants in response to stimulus of unilateral light.

Definition of Geotropism: It is directional growth movement of curvature which occurs in response to force of gravity. Main root shows positive geotropism while main stem shows negative geotropism.

Activity: Phototropism and Geotropism- Take two potted plants. Place one plant near a window. Keep the other pot tilted horizontally in the open. Water the plants on alternate days. Observe after a week. Potted plant kept near the window shows bending of young stems towards the window. They are positively phototropic. In the horizontal pot, the stem bends upward as it is negatively geotropic. Its root if taken out, shows downward bending indicating its positive geotropic nature.

(b)

• Role of Auxin. It promotes cell enlargement, fruit growth, apical dominance, rooting of cuttings, prevention of abscission and differential growth during tropic movements.
• Role of Abscisic Acid. It checks excessive activity of auxin and gibberellins, closes stomata in water deficiency, induces dormancy of buds and seeds.