transform() on selective names. Is it possible?

Hi all,
I am whitening my data:

# code begins
N <- 300
M <- 2
x <- matrix(data=rnorm(N*M, 0, 3)-10, ncol=M, nrow=N)
y <- matrix(c(1,-2,-2,1), ncol=M, nrow=M)
z <- data.frame(x %*% y)
colnames(z) <- c('x','y')
par(mfrow=c(1,3))
plot(z, pch=5, col="blue")

whiten <- function(x) { (x-mean(x))/sd(x) }

zz <- transform(z, x=whiten(x), y=whiten(y))
plot(zz, pch=3, col="red")

#code ends

And everything looks fine enough.
But now I want to withen just one of the columns and I won't know
which one until my script is running, hence I can't hard code it in
the script.
Then I though, well maybe if I define some convenient f...

#begin code

f <- function(a) { paste(a,"=withen(",a,")", sep='') }
a <- 'x' # or a <- 'y' depending on user input.....
f(a)

[1] "x=withen(x)"

# so I could try....
zzz <- transform(z, eval(f('x')))
# which of course doesn't work
plot(zz, pch=3, col="green")

head(z, n=2)

        x         y
1 17.167380  6.884402
2  8.234507 13.940932

head(zzz, n=2)

        x         y
1 17.167380  6.884402
2  8.234507 13.940932

#end code

Could someone provide me with some hint on whether the attempted trick
above is possible and how to proceed further?
Thanks in advance.
jcb!