Question

Explain the reason of the following with the help of diagrams:
Why do the apical parts of stem bend toward light and root towards earth?

Answer 1

When a plant organ curves in response to a unilateral light stimulus, it is called phototropism. Some parts of the plant move towards the light which is called positively phototrophic. For example, stem.

The bending is due to the accumulation of the plant hormone Auxin on the shaded side which causes cell elongation and so there is growth at the shaded part due to which the stem bends towards the light.

So stem bends towards light due to phototropism.

When the growth movements are induced by the stimulus of gravity it is called geotropism. Growth curvature towards the gravitational force is known as positive geotropism. For example, the root which bends towards the Earth due to the stimulus of gravity. The explanation for this was given that when the roots are placed horizontally, the curvature is towards the ground. There is the accumulation of Auxin on this site which causes cell elongation and it inhibits growth on the other side. Therefore the root grows from the lower side and bends down.