Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.
A tropism is a growth in response to a stimulus and an auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weed killers, rooting powder and to control fruit ripening.

The tropisms of shoots mean that the shoots are likely to grow into the air, where there is light for photosynthesis.
The tropisms of roots mean that the roots are likely to grow into the soil, where there is moisture.

In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light.

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the lit side. This causes the root to bend away from the light.

Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity.
In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.

Selective weed killers work on some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat. Selective weed killers contain growth hormone that cause the weeds to grow too quickly and then die. The weed killer is absorbed in larger quantities by the weeds than the beneficial plants, which stay healthy.