Auxin

Auxin is a crucial plant hormone with a multitude of functions.
It is classified as a growth-promoting hormone,
with two key roles being cell elongation and phototropism.

Auxin is chemically an indole-3-acetic acid. It is found in the growing regions of the plant.

Cell elongation is responsible for making the plant taller.

Phototropism is the process by which a plant reorients its new growth towards a light source. Auxin, a plant hormone, tends to move away from the light source.

This property causes Auxin to concentrate in the shady side of the plant. The concentration of Auxin in the shady region causes the plant to grow taller on that side than the side exposed to the light source. This uneven growth, caused by the concentration of Auxin, causes the plant to bend towards the light source.

After auxin effect the plant is pended towards the light

See also:

Tropism

Ethylene

Cytokinin