[R] units of a biplot

[Mathias Ditzen]

Hello everybody,
I am afraid this might be a stupid question, but after scanning a little 
bit through the help archive and the internet, I didn't come up with an 
answer myself.
I have done a principle component analysis on a data set, which yields 
the mean response of 14 olfactory neurons to 16 different odors. When 
plotting the result in a biplot, it nicely shows me the odors in 
different positions in the principle component space. Additionally, the 
neurons are shown as vectors within this plot. My question now is: what 
are the units of the axes? When doing the principle component in IDL, 
the relative positions of the odors are the same, but units on the axes 
span a much wider range. Additionally I couldn't figure out what the 
units in the variable space (right and upper border in the biplot) do 
represent.
I really hope somebody can helpme with this,
Mathias Ditzen