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 Eoctor of Philosophy. 



StepheiyR. Kostewicz 

 Associate Professor of Horticulture 

 Science 



This dissertation was submitted to the Graduate Faculty of the 

 College of Agriculture and to the Graduate Council, and was accepted 

 as partial fulfillment of the requirements for the degree of Doctor 

 of Philosophy. 



March I98O 



Dean, Graduate School 



