Regression tree

Hi everybody.
I'm a new R user and i've been searching a tool for construction of
regression tree...
I found function "tree()" written by a certain Mr. Ripley, and seems to be
just
what i'm looking for, but when i try to use it in R replies me:"Object not
found".
So I was wandering if I should include one special library or something
like that?
Thanks a lot,
ana