Plant Hormones and Uses 

Auxin

AUXIN is a PLANT HORMONE that controls GROWTH in plants.

It is always found near the TIPS of the SHOOTS and ROOTS of a plant.

The effect of auxin depends on whether it is found in the SHOOTS or ROOTS:

In SHOOTS:

Auxin PROMOTES CELL ELONGATION, which causes MORE GROWTH.

In ROOTS:
Auxin INHIBITS CELL ELONGATION, which causes LESS GROWTH.

External factors such as LIGHT and GRAVITY can lead to unequal distributions of auxin in shoots and roots which causes UNEQUAL GROWTH rates. This can lead to the DIRECTION of growth to change. 

This type of response is known as TROPISM.

Phototropism

This is the response of the growth of a plant to LIGHT.

Whenever light shines on one side of a plant shoot or root, the opposite side is always SHADED, and this is the side where the AUXIN ACCUMULATES.

This means that in a SHOOT, the SHADED side will experience MORE CELL ELONGATION than the light side, which will cause it to grow FASTER on the shaded side.

This will cause the shoot to grow TOWARDS THE LIGHT. This is known as POSITIVE PHOTOTROPISM.

In a ROOT however, the opposite will happen as auxin INHIBITS growth. This means the LIGHT side will grow FASTER than the SHADED side.

This will cause the shoot to grow AWAY FROM THE LIGHT. This is known as NEGATIVE PHOTOTROPISM.

Gravitropism (Geotropism)

This is the reponse of the growth of a plant to GRAVITY.

Whenever a shoot or root grows sideways, the AUXIN ACCUMULATES on the LOWER SIDE.

This means that in a SHOOT, the LOWER side will experience MORE CELL ELONGATION than the UPPER side, which will cause it to grow FASTER on the LOWER side.

This will cause the shoot to grow AWAY from GRAVITY- UPWARDS. This is known as NEGATIVE GRAVITROPISM.

In a ROOT however, the opposite will happen as auxin INHIBITS growth, which means the UPPER side will grow FASTER than the LOWER side.

This will cause the shoot to grow TOWARDS GRAVITY- DOWNWARDS. This is known as POSITIVE GRAVITROPISM.