Conclusions

The current PSU definitions for Asteraceae evolved from highly milled samples of the original eight species in the AOSA Rules for Testing Seeds.  For these species it was only practical to consider intact whole fruits to be the pure seed unit. 

However, there are now at least 90 species of Asteraceae listed in the current (2004) AOSA Rules.  In addition AOSA, ISTA, and Canada have differing pure seed unit definitions for Asteraceae  Samples of many native species currently being tested in the laboratory are generally not highly milled and pose special purity problems. 

Several groups have recently been working on problems with one species in particular (Artemisia tridentata). 

Ideas for new Asteraceae PSU need to be tested by setting up referees on native species, especially those listed in the AOSA Rules.