how to construct tree under R

Hello.
I have a hudge problem, don't know how to deal with it...
I'm supposed to implement a tree structure in R, without using
rpart library...
I don't have any clue about appropriate approach...
Anyway, I can't use pointers under R...so how to to it?
lists, vectors...
Anyway, does anybody knows how the regression tree in rpart was
implemented...
Someone please help!
Thanks,
ana