CBSE Class 10 Answered

This is due to the plant hormone auxin which causes cell elongation. Plants detect light with photoreceptors. The light absorbed by photoreceptors stimulates auxin to move laterally across the stem and leaves of plants to the side which is away from the light. Due to this skewed distribution of auxin, the cells on the dark side of the stem grow longer at a quicker rate than the cells on the same side as a light source. The elongation of cells on one side of the plant creates unequal growth which then bends the plant towards the smaller cells on the side receiving light.