I certify that I have read this study and that in my 

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in scope and quality, as a 

 dissertation for the degree of Doctor of Philosophy. 



Thomas J^Sheehan 

 Professor of Ornamental 

 Eorticulture 



This dissertation was submitted to the Graduate Faculty of the 

 College of Agriculture and to the Graduate School and was 

 accepted as partial fulfillment of the requirements fox" the 

 degree of Doctor of Philosophy. 



May 1985 



Dean, 



liege of Agriculture 



Dean for Graduate Studies and 

 Research 



