(a) Define phototropism. Give one example of phototropism.
(b) How does phototropism occur in a plant stem (or shoot)? Explain with the help of labelled diagrams.
(c) What is meant by positive phototropism and negative phototropism? Give one example of each type.
(a) Phototropism: The movement of a plant part in response to light is called phototropism.
Example: Stem bends towards the light is positive phototropism.
(b) Phototropism in a plant stem: The bending of a plant stem towards light is an example of phototropism. The plant stem responds to light and bends towards it due to the action of auxin hormone. The auxin hormone is present at the tip of the stem of the growing plant. Auxin prefers to stay in shade away from light so when sunlight falls on the stem from one side, it gets concentrated on the opposite side. Due to more auxin, the shady side of the stem grows to be longer than the side of stem which is facing light, and makes the stem bend towards light.
(c) The movement of a plant part towards light is called positive phototropism.
Example: the stem of the growing bends towards light. The movement of a plant part away from light is called negative phototropism.
Example: The roots of a plant move away from light.