The eu and opsis forms may exhibit the remarkable phenomenon of heteroecism, i.e.
2
This is the lifehistory of the most complicated forms, of the so-called eu forms. In the opsis forms the uredospores are absent, the mycelium from the aecidiospores producing directly the teleutospores.
3
We find a 3 thus that in the eu and opsis forms the association of nuclei takes place at the base of the aecidium which produces the aecidiospores.