Plant Hormones

When movements are the result of a stimuli from 1 direction, the response is called a tropism. 

The tropism is positive if it grows towards the stimulus and negative if away. 

Phototropism: Response from light stimulus. 

Geotropism: Response from gravity.

Plant hormones like auxin are responsible for tropisms. 

In shoots, auxin is more concentrated on the side where the light is less intense. This makes it grow faster than the other side and bend towards the light. 

By taking cuttings of a plant and dipping them in rooting powder that contains plant hormones, you can encourage them to grow their own roots. 

Herbicides with pant hormones stimulate stem growth. The rate is too fast for the rest of the plant so it dies. Only effects weeds as they're broad leaved and absorb more.