Begonia 'Immense' is a named variety of Begonia ricinifolia Kent, possibly a good selection. I linked to one above ..
http://www.peacetreefarm.com/i...
There's another called Begonia manicata which is a little different ..
http://commons.wikimedia.org/w...
There's a list of other hybrids crossed with B. x ricinifolia which I just found ..
http://tech.dir.groups.yahoo.c...
The clue to being a 'variety' is probably given in " 'Fred Moore' ('Ricinifolia ' v. 'Immense' x 'Leo Rowan')'
B. x ricinifolia is a cross between B. heracleifolia and B. peponifolia ..
http://bdei2.cs.umb.edu:8080/e...
So, if there is another cross involved and the name is not know then B. x ricinifolia might still cover it.
This list of named Begonias gives their parentage, it might be useful!
http://www.begoniahouston.org/...
