PREFACE 



The writers acknowledge indebtedness to numerous individuals 

 and institutions for assistance in the preparation of this work. 

 Particular gratitude is expressed to the staff of the Arnold Arbor- 

 etum of Harvard University for their kindness in furnishing mater- 

 ial for study. Appreciation is also extended to Dr. P. D. Straus- 

 baugh. Professor Emeritus of Botany, West Virginia University 

 for his helpful suggestions. The cover photograph, of trees in 

 winter, was made by Dr. John A. Gibson, Professor of Chemistry, 

 West Virginia University. 



THE AUTHORS 

 June, 1958 



VI 



