The directional movement of a plant part plant in response to light is called phototropism. The shoot responds by bending towards Iight while roots responds by bending away from the light. We know that the plant stem responds to light and bends towards it due to the action of auxin hormone.
This happens as follows.
(i) When sunlight falls on the plant, the auxin hormone present at the tip of the stem spreads uniformly down the stem. Due to the equal presence of auxin, both the sides of the stem grow straight and with same rapidity.
(ii) When the light falls only on the right side of the stem, then the auxin hormone collects in the left side of the stem, away from the light. This is because auxin hormone prefers to stay in shade, i.e., moves away from the light.
(iii ) Thus, more auxin hormone is present in the left side of stem as compared to the right. The left side of stern, grows faster than its right side and therefore, the stem bends towards the right side (direction of light). The effect of auxin on the growth of a root is exactly opposite to that on a stem. Auxin hormone increases the rate of growth in stem but it decreases the rate of, growth in a root. -the side of root away from light will have all the auxin concentrated in it. Due to this, the side of root which is away, from light will grow slower than the other side and make the root bends away from light.
