If young plants are exposed to unidirectional light, the stem tip grows towards the source of light and the root tip grows away from the source of light. The stem, thus, shows a positively phototropic and the root a positively geotropic movement.
The curvature in the both cases is obviously due to unequal growth in the illuminated and shade halves of the tips. In case of the stem tip, growth is more in the shaded half while in the root tip; growth is more in the illuminated half.
The unequal amount of growth in two halves of the stem and the root tip is due to light induced unequal distribution of growth hormone auxin. The amount of auxin is more in the shaded half.
The effect of gravity on the growth can be studied with the help of an apparatus called clinostat. It consists of a clock-work which rotates a clocklike container. If a potted plant is fitted horizontally to the clinostat and is rotated slowly, the effect of geotropic stimulus is eliminated. As a result, the root and shoot do not show any curvature. But when the plant is not rotated, the root bends downward and the shoot bends upwards. This is because a particular side of the plant organ gets greater gravitational stimulus than other sides.
Geotropic movements are stimulated by unequal distribution of auxins in roots and stems caused by the force of gravity. In the experiment, the horizontally placed shoot had greater accumulation of auxins on the lower side due to gravitational stimulus.
This caused an accelerated growth on the lower side resulting in the curvature of shoot upward. On the other hand, the horizontally placed root exhibited a curvature in downward direction even though there is greater accumulation of auxin on the lower side.
This happens because roots are sensitive to auxins and at higher concentration; the cell elongation is rather inhibited on the lower side resulting in poor growth compared to the upper side.