How to show tree structure indentation?

We can show hierarchical relationship either using simple spaces, or using dash lines:

Spaces

  parent
      child

Dashes

  parent
  --- child

I have seen both of them.

But now that I have to design a hierarchical UI in an application, I wonder if we have any standard or research on it. Which one is the better way to show it?

Tags: , , , , , ,