Why Do Stems Grow Up and Roots Grow Down?

What happens

There are chemicals inside a plant that control its growth. The one that makes the stem grow is a very light chemical. It rises to the tip of the stem and pulls the tip up (like helium in a balloon). It keeps on growing up. If you turn the tip so it is not pointing up, the chemical again pull it until it is growing up.

The chemical in roots is just the opposite. It is heavier than the root cells. It sinks to pull the roots down (like a weight).