no doubt a dumb question, but..

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)

I have wondered this myself, but one reason may be that more than one data 
table can be loaded at a time.  Quite often data tables will use the same 
names for fields.  It would inconvenient for instance, if you had two tables 
loaded, say one for trees and one for melons with a field named "Girth", and 
tried a plot(Girth ~ Height) command.  So data() more or less accesses that 
table, and attach explicit identifies the table of interest.

JWDougherty