R help - May 2009 - stem's scale

What is the model behind stem's scale? How the number of bins are
determined? In the following example session, scale=1 and scale=2 has
the same result.
> stem(c(11,21,31),scale=2)
  The decimal point is 1 digit(s) to the right of the |

  1 | 1
  1 |
  2 | 1
  2 |
  3 | 1
> stem(c(11,21,31),scale=1)
  The decimal point is 1 digit(s) to the right of the |

  1 | 1
  1 |
  2 | 1
  2 |
  3 | 1