Substituted arguments surviving multiple function calls

f2n(a1)

I am using R 2.1.1 and have written a function that will retrieve a
named column from a data frame:

d = data.frame(a1=c(4, 2), a2=c(6, 7))
f1 = function(x)
{
  do.call("$", list(d, substitute(x)))
}

So this works:

f1(a1)

[1] 4 2

However, I want to make another function, f2, which also accepts a
column name as an argument and then calls the first function with it:

f2 = function(x)
{
  f1(substitute(x))
}

However, this does not work:

f2(a1)

Error in list(a1 = c(4, 2), a2 = c(6, 7))$substitute(x) :
        invalid subscript type

It works if I take the substitute() out of f1(), but then I can only
call f1() through f2() or something that does the substitution for it.
Is there a better way to do this?

Thanks for any help you can offer.
-- 
Michael Hoffman