Why does auxin have different effects on a plants roots and leaves?

Question: Why does auxin have different effects on a plants roots and leaves?
- Keywords:
  auxin,
  plants
Asked by mijeffs to laurenceharwood on 9 Mar 2012.
- Laurence Harwood answered on 9 Mar 2012:
  
  The cells in stems and roots respond differently to gravity under the influence of the auxins. You can buy “rooting hormone” powder from garden centres to encourage plant cuttings to take root. This is usually a substance called indolylacetic acid.

Question: Why does auxin have different effects on a plants roots and leaves?