UPSC Civil Services Model Paper 4

When growing plants detect light, a hormone called auxin , synthesized at the shoot tip, helps the cells to grow longer. So, option 1 is correct.
When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot.
This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light.
Thus, the plant appears to bend towards the light.