[R] no doubt a dumb question, but..

[Dr. John R. Vokey]

Yes, I am a newbie at R, but it is not the complex commands in R that 
have me baffled, but simple data commands.  For example, why does 
something like:

 > plot(Girth ~ Height)

*not* work after a command that allegedly loads the data:

 > data(trees)

with the error message:

Error in eval(expr, envir, enclos) : Object "Girth" not found

but does work after the command:

 > attach(trees)

?

- JRV
--
There are 10 kinds of people:  those who understand binary, and those 
who don't