I assume such a law doesn't really exist. However, I just realized something. There's "lots" of hybrids between Manfreda and Agave. And there's also hybrids between Manfreda and Polianthes. So are there any hybrids between Agave and Polianthes? Google seems to say that Manfreda and Polianthes used to be lumped into Agave. But I am talking about Polianthes and Agave as they exist today. Are there hybrids between the two? (Like P. tuberosa and A. americana, or whatever)