I don't have any data to document this, but I suspect nutrition also plays a role. I know my garden plants receive much more NPK and minors than those growing wild. Boyce Tankersley Director of Living Plant Documentation Chicago Botanic Garden 1000 Lake Cook Road Glencoe, IL 60022 tel: 847-835-6841 fax: 847-835-1635 email: btankers@chicagobotanic.org