The articles mentioned by BobbyZ, can be downloaded (PDF) here:
http://opuntiads.com/O/zpdf/Opuntia-phylogeny2.pdf" onclick="window.open(this.href);return false;
http://opuntiads.com/O/wp-content/uploa ... counts.pdf" onclick="window.open(this.href);return false;
Another great article, "The Opuntias of Mississippi" by Lucas C. Majure and Gary N.Erwin, here
http://opuntiads.com/O/wp-content/uploa ... Majure.pdf" onclick="window.open(this.href);return false;